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Characterization, origin, and evolution of one of the most eroded mafic monogenetic fields within the central Andes: The case of El País lava flow field, northern Chile
Abstract
El País is a monogenetic lava flow field located 16 km southeast of Peine town in the Altiplano-Puna area, Antofagasta region, Chile. It is a monogenetic lava flow field with lava flows of small-to-moderate volumes, which are fed from individual point sources unassociated with a central volcano of large-volume (stratovolcano) typical of the Andean arc. This lava flow field was emplaced in an area dominated by thin-skinned deformation under a compressive tectonic regime, where multiple volcanic centers (monogenetic and polygenetic volcanoes) have been developed. The volumetric and chemical evolutions were determined by fieldwork, stratigraphic, morphometric, textural (density and vesicularity), petrographic, and geochemical analyses. El País can be divided into three principal mafic lava flows such as Northern-, Main-, and Western-lava flows, which display an E-W orientation. Besides, at least eight ephemeral vents were identified as small erosion remnants of lava emitting point-sources. The identified vents are principally aligned in an ENE-WSW direction, east of the Main lava flow. It is inferred that El País was emplaced near-simultaneously above two ignimbrite sheets: Tucúcaro Ignimbrite (3.2 Ma) and Patao Ignimbrite (3.1 Ma). Stratigraphically, the Main lava flow has a surface texture with strong similarities to an ʻaʻā lava flow, composed of three unique layers well distinguishable in the field such as i) Basal, ii) Top auto-breccia, composed of sub-angular scoriaceous and dense clasts in an oxidized matrix, and iii) A single core between both auto-breccias, composed of dense lavas, exhibiting multiple cooling jointing, such as vertical to platy jointing. The advanced stage of erosion since the formation of El País is evidenced by the poorly preserved lava flows (flat morphology), upper auto-breccia layer, and the presence of large sub-angular blocks of lava (vesicular and dense) along the axis of the lava flows. The eruptive products are geochemically basaltic andesite (Main lava flow and ephemeral vents) and andesite (specifically in the Northern lava flow), making the lava flow field one of the most mafic in the broader region. The lava presents a glomeroporphyric texture, showing at least three different glomerocrysts: i) Plagioclase + clinopyroxene having reabsorption boundaries, ii) Clinopyroxene with reabsorption boundaries, and iii) Plagioclase with sieve-texture. The groundmass is principally composed of glass, plagioclase microlites with a low orientation preference, and opaque minerals (magnetite). This study helps understand the development of dispersed-monogenetic-volcanism (e.g., Tilocálar complex and Cerro Tujle maar), and its relationship with neighboring polygenetic volcanism (e.g., Toloncha stratovolcano) in the Central Volcanic Zone of the Andes, northern Chile.
... Bivariate plots of selected major elements versus SiO 2 (wt%) are shown in Fig. 7 for the NEN, CEN, and SEN lava flows, and compared with other monogenetic centers within the Salar de Atacama sector of the CVZ (Cerro Tujle, Cerro Overo, La Albondiga, El Mani, Tilocálar lava field, and El País lava flow; Ureta et al., 2020a, 2021b, 2021a, and Torres et al., 2021. The three clusters exhibit a decrease in MgO ( Fig. 7B; 2.1-4 wt% for NEN; 1.9-7.9 ...
... Remarkably, the CEN cluster, which has the largest number of eruptive phases by erupted center, shows the widest compositional range in most major elements; L1 from Blanca corresponds to the most mafic sample of the whole field, and L2 from Cuernecillo to the most evolved one. More than 50% of the samples from El Negrillar present MgO, FeO T and CaO contents below the average concentrations of the other monogenetic centers in the Salar de Atacama sector (Fig. 7B, C, and E; 4.3 wt%, 6.6 wt% and 7.0 wt%, respectively; Ureta et al., 2020a, 2021a, 2021b, and Torres et al., 2021, where the lowest contents of the three cluster belong to the NEN lavas. Likewise, most samples from the three clusters show higher P 2 O 5, Al 2 O 3 , and Na 2 O contents than the average concentrations of the other monogenetic centers in the Salar de Atacama sector (Fig. 7H, D and G; 0.3 wt%, 15.8 wt%, 3.6 wt%), setting an upper limit for Al 2 O 3 and Na 2 O concentrations in the area. ...
... The calculated morphometric parameters of the lava flows emitted by the eruptive centers from the three clusters are shown in Table 3 Bas et al., 1986) diagram for El Negrillar lava field. Grey circles correspond to other maficintermediate small eruptive centers located <80 km to the north of the field (i.e., Cerro Tujle, Cerro Overo, La Albondiga, El Mani, Tilocálar lava field and El País lava flow; Ureta et al., 2020a, 2021a, 2021b, and Torres et al., 2021. Ureta et al., 2020a, 2021a, 2021b, and Torres et al., 2021. ...
El Negrillar volcanic field is one of the most voluminous monogenetic volcanic fields so far identified in the Central Volcanic Zone (CVZ) of the Andes: its lava field comprises 84 eruptive phases from 35 small eruptive centers. The study of this volcanic field offers a unique opportunity to assess the volumetric, morphometric, rheological, and compositional evolution of the effusive activity in an entire monogenetic cluster; the great extent and exposure of these small eruptive centers is difficult to observe in other monogenetic volcanoes from the Central Volcanic Zone, as they normally appear as isolated vents or in small clusters (< 3 eruptive centers) with less associated eruptive phases. Our methodology utilizes GIS mapping tools to outline the different eruptive phases of El Negrillar to create a detailed lava flow map, allowing us to estimate the volume emitted by each eruptive center. This analysis was also possible through the paleoreconstruction of the buried lava flows, allowing the determination of the variation in the magma supply. This yielded a total bulk volume of ~7.6 km³ (6.8 km³ DRE, considering the low vesicle abundance of the eruptive products), which exceeds by more than one order of magnitude the volume emitted by other monogenetic centers from the Salar de Atacama region (e.g., El País lava flow, Tilocálar Norte, Tilocálar Sur, La Albondiga and Cerro Overo). We also determined the composition of the entire effusive activity of El Negrillar; andesitic magmas represent more than 46 vol% of the lava field. We identify three main clusters (i.e., Northern El Negrillar (NEN), Central El Negrillar (CEN), and Southern El Negrillar (SEN)), that revealed a compositional variation together with a change in the calculated morphometrical and rheological parameters, implying the existence of a local trend within the monogenetic lava field. The NEN cluster represents the most differentiated magmatism in the field, characterized by amphibole-rich lavas with almost absent olivine, and the highest crystallinity and vesicle content, which led to an effusive activity sustained for more than 3 years, with lava accumulation predominating over its transport, leading to the shortest and thickest flows of the field. The presence of “cauliflower shape bombs” in an early stage of one of its eruptive centers indicates that the NEN cluster may have been influenced by environmental factors leading to an early explosive episode during its activity. On the contrary, the SEN cluster represents the less differentiated magmatic activity of the field, characterized by olivine-rich lavas with almost absent amphibole, low crystallinity, and poor vesicles content, which along with a large magma supply led to effusive activity sustained for approximately two years where the lava flows achieved the longest distances in the field. The CEN cluster is compositionally closer to the SEN, but morphometrical and rheologically resembling to the NEN, suggesting that it represents an intermediate behavior between the other two clusters.
... The analytical methods used for As determinations mainly included inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectrometry (ICP-AES), induced neutron activation analysis (INAA), and X-Ray Fluorescence (XRF). Most of the publications provide information on analyses quality (e.g., Torres et al., 2021;Ureta et al., 2021) in which data on used Certified Reference Materials and analytical precision are given. In other cases, authors only mention the name of the commercial/university laboratory and the analytical technique used (e.g., Mattioli et al., 2006;Salado Paz et al., 2018). ...
... A contrasting example to As-enriched magmas is provided by the Asdepleted monogenetic volcanoes whose magmas are connected to mantellic-lower crust sources (Fig. 6) and experienced rapid ascents with low crustal contamination such as El País lava flow field (Torres et al., 2021). The mean As content of this monogenetic unit is of 2.8 mg/ kg (No. 38, Table 2). ...
Volcanic rocks are a common, worldwide source of geogenic arsenic (As) that can affect water quality detrimentally. Nonetheless, variations of As concentration within different types of volcanic rocks and questions related to the original source of As in magma are not yet fully understood. We compiled published As data from the abundant Cenozoic volcanic rocks of the Altiplano-Puna plateau, considered the main source of As for waters in the region. These data indicates that volcanic rocks in the Altiplano-Puna have a mean As concentration of 9.1 mg/kg, which is almost 2 times higher than the upper continental crust. Arsenic increases within the more silicic compositions such as dacites and rhyolites as well as from mafic monogenetic volcanic edifices to stratovolcanoes of intermediate composition, silicic calderas and ignimbrite fields. The rate and composition of crustal assimilation by the magmas (enhanced by the crustal thickness up to >70 km) strongly influence the As concentration in volcanic rocks. The assimilation of argillaceous shale-type sediments represents the most important source of As in the magmas. The eruption dynamics has an enormous effect on As concentration. The higher the explosivity of an eruption, the higher the amount of ash that will be formed. Arsenic can be sorb onto ash particles so that fall deposits produced during explosive volcanic eruptions are prone to high As concentrations. Large volume ignimbrites formed after caldera collapses in the Altiplano-Puna are often the result of sustained explosive eruptions related to low, dense pyroclastic fountaining eruptions where highly concentrated pyroclastic density currents (PDC, flowing pyroclastic mixtures of particles and gas) are able to maintain high temperatures for long periods. Then As released from volatiles may remain for longer time in the emplacing PDC and can be sorbed onto ash particles, resulting in deposits with a higher As concentration. Post eruption hydrothermal alteration and sulfide mineralization produce As enrichment up to 100 times in the volcanic rocks (957 mg/kg) of the region, but can be very heterogeneous. Volcanism and its related products are the main source of As in the region. Once As is released from the volcanic sources, the semiarid climate, the occurrence of Na-HCO3 water types, and alkaline pH further enhance As concentrations in surface water and groundwater.
... Risse et al. (2008) determined that mafic magmatism in the back arc developed during the late Miocene to Pleistocene, reaching a peak in the early Pliocene. On the other hand, within the main arc, the dominant compositions range from andesitic to dacitic (e.g., Godoy et al. 2014Godoy et al. , 2017González-Maurel et al. 2019;Torres et al. 2021;Ureta et al. 2020aUreta et al. , b, 2021a, with a continuous development of stratovolcanoes (since the Miocene) and with the youngest monogenetic centers in the CVZ (Fig. 8). The ages compiled in previous works and the ones obtained in this study suggest that monogenetic mafic magmatism in the main arc began in the Miocene, as exemplified by Morro Punta Negra, with its peak activity occurring during the Pleistocene. ...
El Negrillar volcanic field has the largest extent and erupted volume (~ 6.8 km3 DRE) of all the monogenetic centers of the Andean Central Volcanic Zone (CVZ). The volcanic field comprises 51 eruptive centers and 98 lava flows distributed in three clusters: Northern El Negrillar (NEN), Central El Negrillar (CEN), and Southern El Negrillar (SEN). Here, we present a geological map of El Negrillar, with detail of effusive and explosive volcanic deposits not previously mapped in the southern sector of the CEN and SEN clusters. Ten samples of El Negrillar’s deposits associated with effusive and phreatomagmatic activity were dated using 40Ar/39Ar geochronology, establishing, along with previously published dates, a geochronological characterization of the development of El Negrillar’s monogenetic field. The collected age data yields a range of 0.982 ± 0.008 to 0.141 ± 0.072 Ma, compared to previously published K–Ar ages for the same deposits range from < 1.5 Ma to 0.6 ± 0.4 Ma. The new ages presented here indicate that the effusive activity at El Negrillar (NEN, CEN, and SEN), and the phreatomagmatic activity in the CEN (dated for first time) occurred quasi-simultaneously (within error). The end of the volcanic activity within the monogenetic field occurred in the eastern sector of the CEN at 0.141 ± 0.072 Ma, which represents the youngest eruption ages of El Negrillar. If these new ages are revisited within the regional context of the SW sector of the Altiplano-Puna Volcanic Complex (APVC), the monogenetic volcanoes appear to be the result of a migration of mafic vents along a southwest-northeast trend, as shown by their age variation from the oldest to the youngest volcanic center: Morro Punta Negra, La Negrillar, El Negrillar, Tilocálar Sur, Tilocálar Norte, Cerro Tujle, El País, Puntas Negras, La Albóndiga Grande, and Cerro Overo. These results highlight the structural control on the emplacement of monogenetic mafic volcanism in the APVC.
... In general, rocks from the Yura Monogenetic Field may represent the mafic end-member of the Pleistocene-Holocene volcanic products in the Arequipa basin and surrounding areas (e.g., Aguilar et al., 2022). At a regional scale, these magmas show one of the least differentiated magmas in the Central Volcanic Zone (e.g., Delacour et al., 2007;Mamani et al., 2010;Torres et al., 2020). ...
Arequipa (Peru) is an area where volcanic activity has been persistent during the Quaternary. Studies carried out in this area have highlighted the emplacement of ignimbrite deposits, large volcanic clusters and stratovolcanoes. Monogenetic volcanism is also present, although poorly explored and studied. Due to its location over an ignimbrite plain and poor state of preservation, the only identified monogenetic cone in the Arequipa basin was the Nicholson volcano, while other monogenetic centers remained unknown. This lack of information about the recent volcanism can lead to inadequate definition of scenarios in a hazard assessment in the region. The present study has investigated monogenetic volcanism in the northwestern edge of the Arequipa basin based on geological survey, geochronology and geochemical data. Here, we report for the first time five small volcanic centers such as Yura Viejo, Ccapua, Uyupampa, El Chiral and Patacocha, which together with the Nicholson volcano form the Yura Monogenetic Field. Stratigraphic considerations and new ⁴⁰Ar/³⁹Ar ages allow us to place the eruptive activity in the Middle–Upper Pleistocene (c. 195–54 ka). Phreatomagmatic, Strombolian and effusive eruptions characterize the monogenetic activity of the field. As a result of these eruptions, small scoria cones, maars, and lava flows/coulées were generated. The eruptive products show ubiquitous olivine phenocryst-rich (<10 vol%) set in a fine pilotaxitic groundmass, suggesting rapid ascent of basaltic magmas to the surface controlled by the tectonic setting. The analyzed rocks lie in a narrow range of basaltic-andesite composition (50.9–55.6 wt% SiO2) being the most mafic Pleistocene - Recent volcanic products identified in the Arequipa basin, along with the least differentiated magmas from the nearby Chachani volcanic cluster. This work shows how monogenetic volcanism can occur contemporaneous and closely spaced to larger volcanic clusters and active stratovolcanoes. We hope the information provided here will contribute to improve the risk management by highlighting the scenario of monogenetic eruptions that should be considered in the hazard assessment.
... The authors conclude that the change from effusive to explosive eruptive style is related to a decrease in the magma flux and a withdrawal of magma below the groundwater table, which has fostered magma-water interaction and produced phreatomagmatic eruptions. Torres et al. (2021) carried out a study in El País monogenetic lava flow field, placed in the Altiplano-Puna area, Chile, that helps understand the development of dispersed-monogenetic-volcanism and its relationship with neighboring polygenetic volcanism in the Central Volcanic Zone of the Andes. The authors suggest that El País was emplaced near-simultaneously above two ignimbrite sheets: Tucúcaro Ignimbrite (3.2 Ma) and Patao Ignimbrite (3.1 Ma). ...
The Latin American Association of Volcanology (ALVO) arose in 2010 as a regional network, promoting regional cooperation, the exchange of experiences and the strengthening of local capacities in the field of volcanology. In 2020, ALVO organized its first scientific congress under the slogan ‘Volcanology in and for Latin America’ (1er Congreso ALVO: Volcanología en y para Latinoamérica). The Special Issue, Volcanism in Latin America, hosts several works presented at this Congress. The articles that constitute this Special Issue cover a broad spectrum of topics studied by many research groups and institutions working on volcanology in the region. Such topics include disciplines like physical volcanology, geochemistry, seismology, remote sensing, volcano observatories, instrumentation, and volcanic hazards and risks, among others. These papers represent good examples of the state-of-the-art of volcanology research in Latin America.
... Ramírez, C., Gardeweg 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 sustained column related to explosive lava fountain activity at Volcán Villarrica (Chile) Struct Geol, 107, 73-92. 953 Torres, I., Németh, K., Ureta, G. and Aguilera, F., 2021. Characterization, origin, and evolution of one of the 954 most eroded mafic monogenetic fields within the central Andes: 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 Ureta, G., Németh, K., Aguilera, F., Zimmer, M. and Menzies, A., 2021b. ...
Monogenetic volcanoes are small-volume landforms (< 1 km3) which can develop complex eruption histories. Their genesis may include variations in their primary magma sources and petrogenetic evolution, eruption styles transitions, changes in their emplacement conditions. The Central Andes (Northern Chile) contains more than 900 small-volume volcanoes, most of which lack a detailed knowledge of their eruption histories due to the limited volcanological studies available. In this paper, we present a detailed volcanostratigraphic, morphometric and petrologic study are used to reconstruct the geology and eruptive history of Cerro Negro volcano. It is a Pleistocene pyroclastic cone which stands out from all monogenetic volcanoes of this region due to its larger size (~1 km3) and location along the axis of the orogen. The activity of Cerro Negro began with a phreatomagmatic phase that produced breccias and a lithic-rich tuff, followed by magmatic activity with lava fountaining and Strombolian styles. This latter magmatic phase produced short lava flows, but mostly spatter agglutination and tephra fallout layers. The composition of erupted materials spans from calk-alkaline basaltic andesite to dacite. The least evolved magmas of Cerro Negro are derived from mantle wedge’s partial melts without significant crustal assimilation. However, changes in the composition through the eruption suggest crystal fractionation and shallow magma storage, followed by mixing and crustal contamination during the late stage of the eruption. The absence of significant repose period between the deposition of the distinct products and their total volume together with the observed compositional variations imply that Cerro Negro represents a large volume polycyclic monogenetic volcano. These results are important to understand the mafic magma evolution in thickened crusts and the eruptive styles of long-lasting monogenetic eruptions.
... The initial viscosity was estimated using the grdViscosity calculator (http//:11www.eos.ubc.Ca/~krussell/VISCOSITY/grdViscosity.html.) of Giordano et al. (2008) using the content of major elements, amount of water, and temperature (e.g., Vona et al., 2011;Russell and Giordano, 2017;Torres et al., 2020). ...
Effusion rate is the instantaneous lava flow output by a vent. It is one of the most important factors that govern the emplacement and dynamics of lava flows and can be determined by direct measurements or estimated through modeling. The Negros de Aras monogenetic volcanic field, located in the Central Volcanic Zone of the Andes, is the largest volcanic field in northern Chile. It is situated north of the active Socompa volcano and south of the Salar de Atacama basin, consisting of scoria cones and lava flows. Here, we estimate the effusion rates of three overlapping and representative solidified lava flows from this volcanic field applying an iterative procedure using the Q-LavHA GIS plugin, where the effusion rate is treated as an unknown parameter and the solution is reached by comparing the simulated results with the real extension of the flows. The pre-eruptive surface was reconstructed using the 12 m resolution TanDEM-X digital elevation model. Other required input parameters for the modeling (e.g., channel ratio, viscosity, temperature) were estimated through geomorphometric, petrographic, and geochemical analysis. The estimated effusion rates vary between 14 and 113 m³/s, comparable with rates measured elsewhere. The method was validated by applying it to two directly measured (Nyamuragira 2006; Mount Etna 2001) and one calculated effusion rate (Lentiscal 2450 BP, Canary Islands). The estimated effusion rates gave an average underestimation of 10%. Considering this percentage of adjustment, the effusion rates for the three studied lava flows from the Negros de Aras monogenetic volcanic field vary from 15 m³/s to 124 m³/s. Our results provide valuable information for mitigating volcanic risk in the event of future lava flow eruptions, which could affect nearby villages and mining operations.
... As indicated by Stein & Sella (2002in Cañón-Tapia, 2016, it has been observed that most of the volcanic fields are in, or very close to, diffuse deformation zones or plate boundaries. The local environment of volcanic fields can be extensional, transient, or compressional and its regional distribution seems to be controlled by the regional structure of the lower crust and the upper mantle (Le Torres et al., 2020;Ureta et al., 2020). ...
Small–volume basaltic volcanoes in Colombia are located on the eastern flank of the Central Cordillera and in the Upper Magdalena Valley, in a rear–arc position with respect to the active arc front. These are mainly small volcanoes that form predominantly scoria cones with associated lava flows, pyroclastic rings, and isolated lava flows, which have a composition that varies from highly subsaturated nephelinites, basanites, and alkaline basalts to basalts and subalkaline andesites. In rear–arc position to the segments of the active volcanic front, three main groups are recognized with this type of volcanoes. The first group, located in the south, has been recognized mainly through photogeological studies in Nariño, Putumayo, and Caquetá Departments. From this group, only the nephelinic Sibundoy Volcano (Muchivioy) has been directly studied through field–based geological observations. A second group in the Huila Department has been denominated in the literature with the name of “Alkalibasaltic Volcanic Province”, in which three monogenetic volcanic fields are defined: Moscopán, Isnos–San Agustín, and Acevedo, taking into account their geographical position, geochemical characteristics, and structural setting. However, the interaction of the tectonic and structural factors give rise to differences and similarities in the products emitted by these volcanoes, making this volcanic province a clear example of the difficulty of defining a monogenetic volcanic field. In addition, in this group, for the first time four scoria cones with associated lava flows are reported, that were previously known as “Basaltos de Acevedo” of ultramafic character. The third group, called in this work the Metaima Monogenetic Volcanic Field, is located in the Tolima Department and corresponds to basalt and calc–alkaline high–magnesium (Mg# = 65–70) basaltic andesites that may represent primary magmas.
The presence of this volcanism inferred to be related to the complex tectonic configuration of the Andean North Volcanic Zone due to the rupture of the Farallón Plate and the formation of the Panamá Basin in the Miocene. Despite the lack of radiometric dating, this volcanism likely spams an age range from the Pliocene – Pleistocene (?) to the Holocene based on their morphometry, preservation, or stratigraphic considerations. The geomorphology and the preliminary morphometric analysis of the volcanic centers suggest a very recent age for some of them, even with the possibility of having been formed by historical eruptions, according to some reports that should be analyzed in greater detail.
Monogenetic volcanism is the most common manifestation of volcanism on Earth and many planets and moons of the Solar System. They are variously organized volcanic forms reflecting volcanic eruptions that are normally short lived and produce small volume of eruptive products. Monogenetic volcanism can occur in any geological environment hence their physical appearance and volcanic geoforms is great despite that their types have high relative abundance in the whole geosystem. Conceptual framework of monogenetic volcanism revealed that individual volcanoes are sensitive for the external conditions which may influence the course of the eruption due to the low volume of magma involved in most of such volcanism. In extreme cases, the external forces that are mostly governed by the availability of surface and ground waters can overtake the shaping of the volcanic eruption styles hence the resulting eruptive products and geoforms. Intuitively, geodiversity of monogenetic volcanism is expected to be great, however the way to measure it faces with numerous challenges. One of the most obvious issues is the small size of such volcanoes that are in magnitude smaller from the surface manifestation of many other types of geological formation, creating a challenge to be able to “detect” and effectively distinguish geological varieties in normal geological mapping scales. This scale problem poses significant challenges to apply modern technologies such as GIS to create geodiversity maps. In addition, it has been recognized in many cases that within individual volcanic fields “overlapping” high value geoheritage sites of monogenetic volcanoes can exist hence to select the one form, as a geosite requires complex evaluation methods to apply. In this chapter, after basic introduction to monogenetic volcanism, a summary is given for the recently identified challenges and their potential solutions.
The Tilocálar volcanic complex comprises four monogenetic volcanoes located at the southeastern edge of the Salar de Atacama Basin, northern Chile. These monogenetic volcanic centers are the Tilocálar Norte lava, the El Maní dome, the Tilocálar Sur small–lava shield, and the Tilocálar Sur maar, respectively, and were formed between 460 ± 50 ka to < 1 Ma ago along the north–south-striking Tilomonte ridge that underlies the Tucúcaro Ignimbrite (3.2 ± 0.3 Ma). Their formation is the result of an intermediate magma rising to the surface during a local transtensional regime, which is observed at El Maní, under a dominantly compressional tectonic setting. The Tilocálar monogenetic centers provide a rare opportunity to examine the variability from individual and small–volume magma batches to the development of a monogenetic volcanic field. These volcanoes and the control on their eruption styles were studied by geological mapping and stratigraphic considerations, morphometric analysis, textural studies (density and vesicularity), petrographic observations, and geochemical analyzes. The Tilocálar Norte lavas have an intermediate composition (mostly andesite, 62.02 wt.% SiO2) that reach a maximum distance of 3.9 km to the north and 1.2 km to the southwest from their vent, with a dense–rock equivalent (DRE) volume of 8.73×10⁷ m³. The El Maní lava dome is small (covering 750 m² area and consisting of 1.06×10³ m³ DRE volume) that presents mingled mafic (63.89 wt.% SiO2) and felsic (73.73 wt.% SiO2) products of the original magma source, related to shallow crustal melts. The Tilocálar Sur small lava–shield is characterized by a pyroclastic fall deposit and lavas. The pyroclastic fall deposit is characterized by agglutinated andesite scoria–spatter fall units, covering 2.81 km² and consisting of 7.75×10⁵ m³ DRE volume. In contrast, the lavas are characterized by an andesitic composition (57.64 wt.% SiO2), which is 1.8 km in length to the northwest and 1.4 km to the east of their source. These lavas cover an area of 3.78 km² with 9.15×10⁷ m³ DRE volume. Tilocálar Sur maar crater has an elliptical shape in map view (363 m diameter east–west and 288 m diameter north–south), surrounded by individual conglomerates, sandstones, granitoids, and tuff fragments that lie on the present–day surface around the crater rim. The Tilocálar volcanic complex is interpreted as a small monogenetic volcanic field that shows an eruption history defined by a spectrum of eruptive processes determined by internal and external parameters at any given time from the four independent volcanic centers.
Monogenetic volcanism produces small eruptive volumes with short eruption history, different chemical compositions, and relatively simple conduit. The Central Volcanic Zone of the Andes is internationally known as a natural laboratory to study volcanism, where mafic and felsic products are present. In this contribution, the spectrum of architectures, range of eruptive styles, lithological features, and differ- ent magmatic processes of the mafic and felsic monogenetic Neogene to Quaternary volcanoes from the Central Volcanic Zone of the Andes in northern Chile (18°S-28°S) are described. The major volcanic activity occurred during the Pleistocene, where the most abundant activity corresponds to effusive and Strombolian eruptions. This volcanism is characterized by external (e.g., magma reservoirs or groundwater avail- ability) and internal (e.g., magma ascent rate or interaction en-route to the surface) conditions, which determine the changes in eruptive style, lithofacies, and magmatic processes involved in the formation of monogenetic volcanoes.
Cerro Tujle is an isolated Quaternary maar located 21 km south-east of the Salar de Atacama basin. It is situated at 3,554 m a.s.l., on the top of the Cordón de Tujle ridge forming a north-south striking morphological element in the Central Volcanic Zone of the Andes, in northern Chile. The material erupted at Cerro Tujle lies over the Tucúcaro Ignimbrite (3.2 ± 0.3 Ma), covering the whole area. The origin of the magma source, as well as the evolution and control of the volcanic eruptive styles, were determined by fieldwork, stratigraphic, morphometric, textural (density and vesicularity), petrographic, and geochemical analyses. The Cerro Tujle maar features an elliptical crater (333 × 279 m wide in east-west and north-south directions, respectively) surrounded by lava flow and tephra deposits. The total Dense Rock Equivalent volume of the products was calculated in 1.53 × 105 m3, and mainly correspond to andesites (58.62–59.80 wt% of SiO2). The lava flow can be categorized in at least two types: i) brown-red and green-to-gray andesites located on the north-northwestern flank of the crater, and ii) black andesites with felsic rock clasts distributed around the crater underlying the other lavas. In contrast, the pyroclastic deposit displays breccias of juvenile pyroclasts with xenoliths of rhyolitic composition. These felsic clasts suggest magma assimilation with a shallow magma reservoir during a relatively fast ascent of the magma. This is evidenced by disequilibrium textures in mineral levels, such as reaction rim, and skeletal and sieve textures of the phenocrysts. The field evidence and results of the different analyses suggest a change in the eruptive style from effusive to explosive. This transition is associated with a decrease of the magma flux and a withdrawal of magma below the groundwater table, which has produced a phreatomagmatic eruption by magma-water interaction.
The Tilocálar volcanic complex comprises four monogenetic volcanoes located at the southeastern edge of the Salar de Atacama Basin, northern Chile. These monogenetic volcanic centers are the Tilocálar Norte lava, the El Maní dome, the Tilocálar Sur small–lava shield, and the Tilocálar Sur maar, respectively, and were formed between 460 ± 50 ka to < 1 Ma ago along the north–south-striking Tilomonte ridge that underlies the Tucúcaro Ignimbrite (3.2 ± 0.3 Ma). Their formation is the result of an intermediate magma rising to the surface during a local transtensional regime, which is observed at El Maní, under a dominantly compressional tectonic setting. The Tilocálar monogenetic centers provide a rare opportunity to examine the variability from individual and small–volume magma batches to the development of a monogenetic volcanic field. These volcanoes and the control on their eruption styles were studied by geological mapping and stratigraphic considerations, morphometric analysis, textural studies (density and vesicularity), petrographic observations, and geochemical analyzes. The Tilocálar Norte lavas have an intermediate composition (mostly andesite, 62.02 wt.% SiO2) that reach a maximum distance of 3.9 km to the north and 1.2 km to the southwest from their vent, with a dense–rock equivalent (DRE) volume of 8.73×10⁷ m³. The El Maní lava dome is small (covering 750 m² area and consisting of 1.06×10³ m³ DRE volume) that presents mingled mafic (63.89 wt.% SiO2) and felsic (73.73 wt.% SiO2) products of the original magma source, related to shallow crustal melts. The Tilocálar Sur small lava–shield is characterized by a pyroclastic fall deposit and lavas. The pyroclastic fall deposit is characterized by agglutinated andesite scoria–spatter fall units, covering 2.81 km² and consisting of 7.75×10⁵ m³ DRE volume. In contrast, the lavas are characterized by an andesitic composition (57.64 wt.% SiO2), which is 1.8 km in length to the northwest and 1.4 km to the east of their source. These lavas cover an area of 3.78 km² with 9.15×10⁷ m³ DRE volume. Tilocálar Sur maar crater has an elliptical shape in map view (363 m diameter east–west and 288 m diameter north–south), surrounded by individual conglomerates, sandstones, granitoids, and tuff fragments that lie on the present–day surface around the crater rim. The Tilocálar volcanic complex is interpreted as a small monogenetic volcanic field that shows an eruption history defined by a spectrum of eruptive processes determined by internal and external parameters at any given time from the four independent volcanic centers.
Interactions between volcanic and tectonic processes affect the distribution, morphology, and volume of eruptive products in space and time. The Queréndaro area in the eastern Michoacán-Guanajuato Volcanic Field affords an exceptional opportunity to understand these relationships. Here, a Pleistocene lava plateau and 20 monogenetic volcanoes are vented from an active ENE-striking segment of the Morelia-Acambay fault system. Thirteen scoria cones are aligned along this structure, vented from an extensional gap in between two rotated hanging wall blocks of a listric fault. A new geological map, volcanic stratigraphy, and 40Ar/39Ar dating indicate that this lava plateau and volcanic cluster were emplaced from 0.81 to 0.25 Ma by 11 intermittent eruptive epochs separated by ca. 0.05 Ma, emplacing a total magma volume of 5 km3. Petrography and chemistry of rocks suggest that all volcanic structures were fed by three different magma batches but vented from independent feeder dikes. Our results indicate that preexisting faults exert a strong influence on volcanic spatial and temporal distribution, volcanic morphology, magma volume, and eruptive dynamics in this area. ENE-breached and ENE-elongated scoria cones indicate parallel subsurface fissure and feeder dikes. Additionally, points of maximum fault dilation at depth related to a transtensive state of stress coincide with less fragmented deposits and larger magma volumes. Furthermore, this study raises important questions on the geodynamics of volcano-tectonic interactions possible in similar monogenetic volcanic alignments worldwide.
The Neogene history of the Central Andes records one of Earth's most productive periods of high-flux silicic magmatism. Subduction of an aseismic ridge, the Juan Fernández Ridge (JFR), led to changes in mantle melt productivity that initiated a transcrustal magmatic system culminating in massive caldera- and ignimbrite-forming eruptions. This volcanism is time transgressive, tracking the southward passage of the JFR beneath the Central Andes. The volcanic field is underlain by a composite, arc-long mid- and upper-crustal granodiorite batholith that represents extensive processing of the continental crust by mantle-derived magmas. This batholith stabilized the upper crust and contributed to the extreme elevations despite a net crustal loss beneath the Puna region.
Active continental margins are shaped by subduction-related magmatism,
and the Central Andes of South America are a prime example. The
Central Andean orogen has evolved over the past 25 My via magmas
ascending from the mantle and interacting with increasingly thickened continental
crust. This process is reflected in the volumes and compositional variations
of the magmas that erupt at the surface. These compositional variations
can be traced in time and space, and, herein, we provide explanations for
their cause and explore the nature of the Central Andes transcrustal magma
systems that feed the iconic stratovolcanoes today.
The orbicular granodiorite of the Cordón de Lila is emplaced within the Ordovician pluton El León, in the southeastern part of Cordón de Lila. This forms a small pipe emplaced in an aplitic band, near the contact with a roof-pendant of amphibole bearing gabbro. The presence of allanite, considered a tracer mineral in magmatic crystallization processes, in the three facies that constitute the pipe (core, margin and matrix) suggests that the orbicules of this granodiorite are the result of magmatic crystallization that affected the magma that originated the orbicular granodiorite.
A continental magmatic arc (the Famatinian magmatic arc) was developed on the western margin of Gondwana during the Early to Middle Ordovician. This has a northwestern orientation in the northern Chile-Argentina Andes between 21° and 26° south latitude with a northeastern directed subduction zone and developed on a continental crust represented by a metamorphic basement. A paleogeographical scheme for the Ordovician magmatic arc is proposed and two tectonic environments can be recognized from our own data and data from the literature: forearc and arc. The Cordón de Lila Complex can be assigned to a forearc position. Here the turbiditic flows become paralell to the northwestern elongation of the magmatic arc. The sedimentation in the frontal-arc high platform of the forearc is represented by stromatolitic limestones and a zone of phosphate production. The internal structure of the arc can be inferred from the petrographic composition of the turbidites: basaltic and andesitic lavas, dacitic and/or rhyolitic lavas and ash fall tuffs. Also the Quebrada Grande Formation was developed on the forearc. Plutonic Ordovician rocks testify the continuity of the magmatic arc. The data about the basement exposed in the present paper do not support the existence of the Arequipa-Antofalla Terrane.
The role of magmatic processes as a significant mechanism for the generation of voluminous silicic crust and the development of Cordilleran plateaus remains a lingering question in part because of the inherent difficulty in quantifying plutonic volumes. Despite this difficulty, a growing body of independently measured plutonic-to-volcanic ratios suggests the volume of plutonic material in the crust related to Cordilleran magmatic systems is much larger than is previously expected. To better examine the role of crustal magmatic processes and its relationship to erupted material in Cordilleran systems, we present a continuous high-resolution crustal seismic velocity model for an ~800 km section of the active South American Cordillera (Puna Plateau). Although the plutonic-to-volcanic ratios we estimate vary along the length of the Puna Plateau, all ratios are larger than those previously reported (~30:1 compared to 5:1) implying that a significant volume of intermediate to silicic plutonic material is generated in the crust of the central South American Cordillera. Furthermore, as Cordilleran-type margins have been common since the onset of modern plate tectonics, our findings suggest that similar processes may have played a significant role in generating and/or modifying large volumes of continental crust, as observed in the continents today.
Small-scale volcanic systems are the most widespread type of volcanism on Earth and occur in all of the main tectonic settings. Most commonly, these systems erupt basaltic magmas within a wide compositional range from strongly silica undersaturated to saturated and oversaturated; less commonly, the spectrum includes more siliceous compositions. Small-scale volcanic systems are commonly monogenetic in the sense that they are represented at the Earth's surface by fields of small volcanoes, each the product of a temporally restricted eruption of a compositionally distinct batch of magma, and this is in contrast to polygenetic systems characterized by relatively large edifices built by multiple eruptions over longer periods of time involving magmas with diverse origins. Eruption styles of small-scale volcanoes range from pyroclastic to effusive, and are strongly controlled by the relative influence of the characteristics of the magmatic system and the surface environment.
Monogenetic volcanism produces small-volume volcanoes with a wide range of eruptive styles, lithological features and geomorphic architectures. They are classified as spatter cones, scoria (or cinder) cones, tuff rings, maars (maar–diatremes) and tuff cones based on the magma/water ratio, dominant eruption styles and their typical surface morphotypes. The common interplay between internal, such as the physical–chemical characteristics of magma, and external parameters, such as groundwater flow, substrate characteristics or topography, plays an important role in creating small-volume volcanoes with diverse architectures, which can give the impression of complexity and of similarities to large-volume polygenetic volcanoes. In spite of this volcanic facies complexity, we defend the term “monogenetic volcano” and highlight the term’s value, especially to express volcano morphotypes. This study defines a monogenetic volcano, a volcanic edifice with a small cumulative volume (typically ≤1 km3) that has been built up by one continuous, or many discontinuous, small eruptions fed from one or multiple magma batches. This definition provides a reasonable explanation of the recently recognized chemical diversities of this type of volcanism.
SUMMARY: Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs.
In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (≤1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (≤2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by
The basaltic Al-Du’aythah volcanic cones lie in the northern part of the extensive lava field of Harrat Rahat, and only 13 km from the centre of Al-Madinah City, in the Kingdom of Saudi Arabia. Historical records indicate they may have erupted in AD 641. The four cones are formed by deposits that record a transition from phreatomagmatic to magmatic explosions followed by minor lava effusion. Three cones display elongated tuff rings at the base, and two produced late-stage lava flows. The cones themselves are symmetrical and constructed mostly by the accumulation of ballistically ejected pyroclasts. Spherical bombs and lapilli (cannonball bombs/lapilli), occasionally with country-rock fragments inside (both cored and loaded bombs/lapilli) are common within the tuff ring deposits. LiDAR data show a total volume of 1,664 × 10−6 km3 for the four cones (418 × 10−6 km3 DRE). Whole-rock chemical analyses indicate alkali-basalt compositions (SiO2 44.7-45.9 wt%), with little compositional variation and no relationship between chemistry and eruptive styles. Small differences in composition may reflect variations in fractional crystallisation of clinopyroxene and olivine. A magnetotelluric 2D cross-section shows that the cones are located adjacent to a buried sediment-filled alluvial channel along a NNW-SSE fault dipping to the east. The Al-Du’aythah eruption was related to the ascent of magma through this structure, with the first phase of the eruption triggered by the interaction of the magma with water from the northern Harrat Rahat aquifer that exists in the Al-Madinah basin. This initial water source was rapidly exhausted, while the eruption progressed roughly from north to south and from west to east, the latter motion probably along the fault-controlled feeding dyke. Our work draws attention to the existence of recent explosive phreatomagmatic eruptions in the Al-Madinah basin, which, despite the hyperarid climate of the area, must be considered a potential future eruption hazard.
The effusion of lava flows from the base of volcanic cones is a common process in continental monogenetic basaltic fields. However, there are no descriptions in the literature of the structures that channel the lava output through the cone. The Pliocene-Quaternary Las Herrerias volcano (Calatrava, Spain) was constructed from the superposition of a nephelinite spatter and scoria cone and related lavas over a maar. Different eruptive styles contributed to the construction of the current volcanic cone: phreatomagmatic, Hawaiian, Strombolian and violent Strombolian. Quarrying has exposed a lava pond inside the volcanic cone, the intrusion of lava through cone-forming pyroclastic deposits and the ponding of associated lava flows within the maar crater. Low fire-fountaining activity formed a lava pond that stagnated within the crater protected by the cone's highly welded spatter deposits at its base and overlying scoria deposits. Once the pressure exerted by ponding lava and overlying pyroclastic deposits exceeded the yield strength of the damming rock walls, the melt oozed out through fractures to form an intricate network of dikes and sills and extruded through the lower part of the volcanic cone, forming lava flows. We propose that lava intrusions similar to those here described may represent the final stage of feeding systems for lava flows associated with scoria and spatter cones.
Monogenetic volcanic fields, such as the Auckland Volcanic Field (AVF), New Zealand, are common on the Earth's surface and are typically dominated by basaltic lava flows up to 10 s of km long. In monogenetic volcanic fields located in close proximity to human population and infrastructure, lava flows are a significant threat. In this study, lava flow emplacement conditions for some basaltic eruptions of the AVF were reconstructed using the thermo-rheological MAGFLOW model. Eight existing lava flows in the AVF were simulated using MAGFLOW and eruptive volumes measured from Light Detection and Ranging (LiDAR)-derived digital terrain models (DTMs). Fitting the simulations to the dimensions of actual lava flows provides insight into their emplacement mechanisms and conditions, such as effusion rate, and probable eruption durations. By looking at emplacement in different settings, the likely magma ascent rate for studied AVF eruptions is calculated to have been on the order of 0.1 m/s. In the AVF, the typical estimated duration of past lava flows was from a minimum of 2 days for small-volume flows, such as Little Rangitoto (0.0015 km(3)), up to 83 days for large volume flows, such as Three Kings (0.078 km(3)). The three best-fitting simulations were used to establish eruption scenarios for future volcanic hazard mapping for the AVF. Inferences of eruption duration that will be useful for developing realistic emergency management plans and recovery scenarios for this densely populated volcanic field are also provided.
Distributed "monogenetic" volcanic eruptions commonly occur in continental settings without obvious structural alignments or rifting/extensional structures. Nevertheless, these may develop as fissures, representing the surface expression of dykes with a range of orientations, especially when stress regimes vary over time and/or older crustal features and faults are exploited by rising magmas. Dykes reaching the surface as fissures can last hours to months and produce groups of closely aligned vents, hiding the true extent of the source fissure. Grouped or aligned vents in a distributed volcanic environment add complexity to hazard modelling where the majority of eruptions are single-vent, point-source features, represented by cones, craters or domes; i.e. vent groups may represent fissure events, or single eruptions coincidently located but erupted hundreds to tens of thousands of years apart. It is common practice in hazard estimation for intraplate monogenetic volcanism to assume that a single eruption cone or crater represents an individual eruptive event, but this could lead to a significant overestimate of temporal recurrence rates if multiple-site and fissure eruptions were common. For accurate recurrence rate estimates and hazard-event scenarios, a fissure eruption, with its multiple cones, must be considered as a single multi-dimensional eruptive event alongside the single-vent eruptions. We present a statistical method to objectively determine eruptive events from visible vents, and illustrate this using the 968 vents of the 10 Ma to 0.6 ka volcanic field of Harrat Rahat, Saudi Arabia. A further method is presented to estimate the number of hidden vents in a thick volcanic pile. By combining these two methods for Harrat Rahat, we determined an updated spatial recurrence rate estimate, and an average temporal recurrence rate of 7.5 × 10-5 events/year. This new analysis highlights more concentrated regions of higher temporal hazard in parts of Harrat Rahat, which has significant implications for the city of Al-Madinah and surroundings.
Monogenetic basaltic volcanism is characterised by a complex array of behaviours in the spatial distribution of magma output and also temporal variability in magma flux and eruptive frequency. Investigating this in detail is hindered by the difficulty in evaluating ages of volcanic events as well as volumes erupted in each volcano. Eruptive volumes are an important input parameter for volcanic hazard assessment and may control eruptive scenarios, especially transitions between explosive and effusive behaviour and the length of eruptions. Erosion, superposition and lack of exposure limit the accuracy of volume determination, even for very young volcanoes. In this study, a systematic volume estimation model is developed and applied to the Auckland Volcanic Field in New Zealand. In this model, a basaltic monogenetic volcano is categorised in six parts. Subsurface portions of volcanoes, such as diatremes beneath phreatomagmatic volcanoes, or crater infills, are approximated by geometrical considerations, based on exposed analogue volcanoes. Positive volcanic landforms, such as scoria/spatter cones, tephras rings and lava flow, were defined by using a Light Detection and Ranging (LiDAR) survey-based Digital Surface Model (DSM). Finally, the distal tephra associated with explosive eruptions was approximated using published relationships that relate original crater size to ejecta volumes. Considering only those parts with high reliability, the overall magma output (converted to Dense Rock Equivalent) for the post-250 ka active Auckland Volcanic Field in New Zealand is a minimum of 1.704 km3. This is made up of 1.329 km3 in lava flows, 0.067 km3 in phreatomagmatic crater lava infills, 0.090 km3 within tephra/tuff rings, 0.112 km3 inside crater lava infills, and 0.104 km3 within scoria cones. Using the minimum eruptive volumes, the spatial and temporal magma fluxes are estimated at 0.005 km3/km2 and 0.007 km3/ka. The temporal–volumetric evolution of Auckland is characterised by an increasing magma flux in the last 40 ky, which is inferred to be triggered by plate tectonics processes (e.g. increased asthenospheric shearing and backarc spreading of underneath the Auckland region).
The last eruptions of the monogenetic Bakony-
Balaton Highland Volcanic Field (western Pannonian Basin,
Hungary) produced unusually crystal- and xenolith-rich alkaline
basalts which are unique among the alkaline basalts of the
Carpathian–Pannonian Region. Similar alkaline basalts are
only rarely known in other volcanic fields of the world. These
special basaltic magmas fed the eruptions of two closely
located volcanic centres: the Bondoró-hegy and the Füzes-tó
scoria cone. Their uncommon enrichment in diverse crystals
produced unique rock textures and modified original magma
compositions (13.1–14.2 wt.% MgO, 459–657 ppm Cr, and 455–564 ppm Ni contents). Detailed mineral-scale textural
and chemical analyses revealed that the Bondoró-hegy and
Füzes-tó alkaline basaltic magmas have a complex ascent
history, and that most of their minerals (∼30 vol.% of the
rocks) represent foreign crystals derived from different levels
of the underlying lithosphere. The most abundant xenocrysts,
olivine, orthopyroxene, clinopyroxene, and spinel, were incorporated
from different regions and rock types of the subcontinental
lithospheric mantle. Megacrysts of clinopyroxene
and spinel could have originated from pegmatitic veins/sills
which probably represent magmas crystallized near the crust–
mantle boundary. Green clinopyroxene xenocrysts could have
been derived from lower crustal mafic granulites. Minerals
that crystallized in situ from the alkaline basaltic melts (olivine
with Cr-spinel inclusions, clinopyroxene, plagioclase, and Fe–
Ti oxides) are only represented by microphenocrysts and overgrowths
on the foreign crystals. The vast amount of peridotitic
(most common) and mafic granulitic materials indicates a
highly effective interaction between the ascending magmas
and wall rocks at lithospheric mantle and lower crustal levels.
However, fragments from the middle and upper crust are
absent from the studied basalts, suggesting a change in the
style (and possibly rate) of magma ascent in the crust. These
xenocryst- and xenolith-rich basalts yield divers tools for estimating
magma ascent rate that is important for hazard forecasting
in monogenetic volcanic fields. According to the estimated
ascent rates, the Bondoró-hegy and Füzes-tó alkaline
basaltic magmas could have reached the surface within hours
to few days, similarly to the estimates for other eruptive centres
in the Pannonian Basin which were fed by “normal” (crystal
and xenoliths poor) alkaline basalts.
Kelut volcano, East Java, is an active volcanic complex hosting a summit crater lake that has been the source of some of Indonesia’s most destructive lahars. In November 2007, an effusive eruption lasting approximately 7 months led to the formation of a 260-m-high and 400-m-wide lava dome that displaced most of the crater lake. The 2007–2008 Kelut dome comprises crystal-rich basaltic andesite with a texturally complex crystal cargo of strongly zoned and in part resorbed plagioclase (An47–94), orthopyroxene (En64–72, Fs24–32, Wo2–4), clinopyroxene (En40–48, Fs14–19, Wo34–46), Ti-magnetite (Usp16–34) and trace amounts of apatite, as well as ubiquitous glomerocrysts of varying magmatic mineral assemblages. In addition, the notable occurrence of magmatic and crustal xenoliths (meta-basalts, amphibole-bearing cumulates, and skarn-type calc-silicates and meta-volcaniclastic rocks) is a distinct feature of the dome. New petrographical, whole rock major and trace element data, mineral chemistry as well as oxygen isotope data for both whole rocks and minerals indicate a complex regime of magma-mixing, decompression-driven resorption, degassing and crystallisation and crustal assimilation within the Kelut plumbing system prior to extrusion of the dome. Detailed investigation of plagioclase textures alongside crystal size distribution analyses provide evidence for magma mixing as a major pre-eruptive process that blends multiple crystal cargoes together. Distinct magma storage zones are postulated, with a deeper zone at lower crustal levels or near the crust-mantle boundary (>15 km depth), a second zone at mid-crustal levels (~10 km depth) and several magma storage zones distributed throughout the uppermost crust (<10 km depth). Plagioclase-melt and amphibole hygrometry indicate magmatic H2O contents ranging from ~8.1 to 8.6 wt.% in the lower crustal system to ~1.5 to 3.3 wt.% in the mid to upper crust. Pyroxene and plagioclase δ18O values range from 5.4 to 6.7 ‰, and 6.5 to 7.6 ‰, respectively. A single whole rock analysis of the 2007–2008 dome lava gave a δ18O value of 7.6 ‰, whereas meta-basaltic and calc-silicate xenoliths are characterised by δ18O values of 6.2 and 10.3 ‰, respectively. Magmatic δ18O values calculated from individual pyroxene and plagioclase analyses range from 5.7 to 7.0 ‰, and 6.2 to 7.4 ‰, respectively. This range in O-isotopic compositions is explained by crystallisation of pyroxenes in the lower to mid-crust, where crustal contamination is either absent or masked by assimilation of material having similar δ18O values to the ascending melts. This population is mixed with isotopically distinct plagioclase and pyroxenes that crystallised from a more contaminated magma in the upper crustal system. Binary bulk mixing models suggest that shallow-level, recycled volcaniclastic sedimentary rocks together with calc-silicates and/or limestones are the most likely contaminants of the 2007–2008 Kelut magma, with the volcaniclastic sediments being dominant.
In order to investigate the mechanics of magma fracture, the driving and resisting pressures in a propagating dyke are estimated and the dominant physical balances between these pressures are described. It is shown that the transport of magma in feeder dykes in characterized by a local balance between buoyancy forces and viscous pressure drop, that elastic forces play a secondary role except near the dyke tip and that the influence of the fracture resistance of crustal rocks on dyke propagation is negligible. -from Authors
The 163 k.y. history as well as the chemical and 46 km(3) volumetric evolution of Volcan Parinacota are described in detail by new mapping, stratigraphy, and 57 Ar-40/Ar-39 ages determined from groundmass or sanidine crystals in basaltic andesitic to rhyolitic lavas. A more precise chronology of eruptions and associated eruptive volumes of this central Andean volcano, which was built upon 70-km-thick crust, provides a more complete view of how quickly volcanic edifices are built in this setting and how their magmatic systems evolve during their lifetime. Development of the complex involved initial eruption of andesitic lava flows (163-117 ka) followed by a rhyodacite dome plateau (47-40 ka) synchronous with the onset of the building of a stratocone (52-20 ka), which was later destroyed by a debris avalanche similar to 3 times larger than that at Mount St. Helens in 1980. Dome plateau emplacement occurred faster and later than has previously been published, implying a compressed duration of cone building and introducing a preceding 65 k.y. hiatus. Debris avalanche timing is refined here to be older than 10 but younger than 20 ka. Rapid postcollapse rebuilding of the volcanic edifice is delineated by 16 groundmass and whole-rock Ar-40/Ar-39 ages, which include some of the youngest lava flows dated by this method. Increase in cone-building rate and a continued trend toward more mafic compositions following collapse imply an inter-relationship between the presence of the edifice and flux of magma from the feeding reservoir. Cone-building rates at Parinacota are similar to those at other well-dated volcanoes on thinner crust; however, the distributed basaltic volcanism prevalent in those other arcs is virtually absent both at Parinacota and elsewhere in the Central Volcanic Zone. This suggests that while the hydrous, calc-alkaline magmas that make up the central volcanoes are not significantly retarded by thick crust, primitive, dry basalts might be.
In this contribution we examine the relationship between active compression and construction of Pleistocene volcanoes in the present-day magmatic arc of the central Andes (23°S-24°S). Deformation produced several N-S striking, ˜40 km long subparallel ridges. These ridges formed by folding of Pliocene ignimbrites and upper Pliocene and Pleistocene lavas; they are asymmetrical in profile and have a gentle back limb and steeper frontal limb. Andesitic monogenetic volcanoes show a close spatial relationship with the ridges; some volcanoes are on the hinge zone, whereas others lay on the sides of the ridges. We interpret this spatial pattern as a result of magma storage and migration along a system of subhorizontal reservoirs and reverse faults. Magma reservoirs probably formed along flat portions of reverse faults between ramp structures that serve as episodic magma transport pathways.
P-to-S converted waves at the continental Moho together with waves multiply reflected between the Earth’s surface and the Moho have been used to estimate the Moho depth and average crustal Vp/Vs variations in the central Andes. Our analysis confirms and significantly complements the Moho depth estimates previously obtained from wide-angle seismic studies and receiver functions. The resulting crustal thickness varies from about 35 km in the forearc region to more than 70 km beneath the plateau and thins (30 km) further to the east in the Chaco plains. Beneath the Andean plateau, the Moho is deeper in the north (Altiplano) and shallower in the south (Puna), where the plateau attains its maximum elevation. A non-linear relation exists between crustal thickness and elevation (and Bouguer gravity), suggesting that the crust shallower than 50–55 km is predominately felsic in contrast to a predominately mafic crust below. Such a relation also implies a 100 km thick thermal lithosphere beneath the Altiplano and with a lithospheric thinning of a few tens of kilometers beneath the Puna. Absence of expected increase in lithospheric thickness in regions of almost doubled crust strongly suggests partial removal of the mantle lithosphere beneath the entire plateau. In the Subandean ranges at 19–20°S, the relation between altitude and crustal thickness indicates a thick lithosphere (up to 130–150 km) and lithospheric flexure. Beneath a relative topographic low at the Salar de Atacama, a thick crust (67 km) suggests that the lithosphere in this region is abnormally cold and dynamically subsided, possibly due to coupling with the subducting plate. This may be related to the strongest (Ms=8.0) known intra-slab earthquake in the central Andes that happened very close to this region in 1950. The average crustal Vp/Vs ratio is about 1.77 for the Altiplano–Puna and it reaches the highest values (1.80–1.85) beneath the volcanic arc, indicating high ambient crustal temperatures and wide-spread intra-crustal melting.
Volcanic eruptions are amongst the most spectacular of natural phenomena. During the past few years, our knowledge of the basic physics and chemistry of magmatic and volcanic processes has been transformed. We now understand how rocks melt and how the resulting magmas migrate upward and eventually accumulate to form magma chambers. We can model a range of physical and chemical processes in these chambers, and we recognize why chamber walls fracture and how magma flows away from chambers through fractures in the crust. Experimental and theoretical work on bubble nucleation and growth has improved models of magma ascent and explosive volcanic eruption mechanisms. Detailed geophysical, geochemical and field research has led to major advances in understanding the mechanisms that trigger changes between lava-forming (or effusive) eruptions and explosive eruptions. Numerical models that simulate processes involved in explosive and effusive eruptions are at an advanced stage but are capable of further refinement. Many of these advances have been made through interdisciplinary collaboration between geologists and colleagues in other areas, especially physics and mathematics. In this contribution, we review the results of some of this work, and draw attention to areas where future collaboration will help to unravel outstanding problems in volcanology.
The Negros de Aras monogenetic volcanic field in the Central Volcanic Zone of the Andes displays the highest concentration of scoria cones in northern Chile (0.23 per km²). It contains 66 vents, 22 of which are emission points of lava flows without an associated recognizable volcanic edifice, whereas 44 are typical scoria cones. Most vents are associated with basaltic andesite and andesite lava flows of up to 4.6 km in length. A few scoria cones show signs of either initial or final phreatomagmatic eruptions recorded in their pyroclastic successions. A geomorphological, morphometric, and spatial distribution analysis of the scoria cones of this volcanic field was carried out with the purpose of identifying the factors and processes that governed their emplacement, growth, and final morphology. The morphometric analysis was performed on 16 well-delimited scoria cones applying the MORVOLC algorithm using the TanDEM-X 12 m resolution digital elevation model. Analysis of the distribution and elongation of the craters of the scoria cones defines four alignments inferred to be related to the local and regional structural elements of the area. Volcanic activity seems to have been related to a N-S fault system with an E-W maximum stress orientation, favoring magma ascent and scoria cone emplacement following inherited fractures in these preferential orientations. The scoria cones show a wide range of morphologies and morphometries that can be related to different syn- and post-eruptive processes. Cones with large craters and high crater width / basal width ratios show clear evidence of phreatomagmatic eruptions (within their pyroclastic successions), whereas horseshoe-type cones are related to cone breaching or coeval lava flow effusion, and more pristine (i.e., relatively younger) cones are generally steeper.
Cerro Tujle is an isolated Quaternary maar located 21 km south-east of the Salar de Atacama basin. It is situated at 3,554 m a.s.l., on the top of the Cordón de Tujle ridge forming a north-south striking morphological element in the Central Volcanic Zone of the Andes, in northern Chile. The material erupted at Cerro Tujle lies over the Tucúcaro Ignimbrite (3.2 ± 0.3 Ma), covering the whole area. The origin of the magma source, as well as the evolution and control of the volcanic eruptive styles, were determined by fieldwork, stratigraphic, morphometric, textural (density and vesicularity), petrographic, and geochemical analyses. The Cerro Tujle maar features an elliptical crater (333 × 279 m wide in east-west and north-south directions, respectively) surrounded by lava flow and tephra deposits. The total Dense Rock Equivalent volume of the products was calculated in 1.53 × 105 m3, and mainly correspond to andesites (58.62–59.80 wt% of SiO2). The lava flow can be categorized in at least two types: i) brown-red and green-to-gray andesites located on the north-northwestern flank of the crater, and ii) black andesites with felsic rock clasts distributed around the crater underlying the other lavas. In contrast, the pyroclastic deposit displays breccias of juvenile pyroclasts with xenoliths of rhyolitic composition. These felsic clasts suggest magma assimilation with a shallow magma reservoir during a relatively fast ascent of the magma. This is evidenced by disequilibrium textures in mineral levels, such as reaction rim, and skeletal and sieve textures of the phenocrysts. The field evidence and results of the different analyses suggest a change in the eruptive style from effusive to explosive. This transition is associated with a decrease of the magma flux and a withdrawal of magma below the groundwater table, which has produced a phreatomagmatic eruption by magma-water interaction.
The Altiplano-Puna Volcanic Complex is a province of the Central Andes mainly associated, since the Miocene (26 Ma), with the eruption of voluminous silicic magmas (>65 wt % SiO2). The result of these eruptions is an extensive ignimbritic plateau which covers an area >50,000 km2. Major eruptions (i.e. flare-up events) of these magmas occurred with a cyclic periodicity, on which each cycle last 2–3 Ma, and steady-state and/or waning periods occurred between each cycle. After the last flare-up cycle of evolution of the Altiplano-Puna Volcanic Complex (ended at ca.1 Ma), low to intermediate silica (<59 wt % SiO2; commonly referred as “mafic”) extrusive products have erupted in a scattered way throughout the magmatic province. This mafic volcanism is exposed mainly as monogenetic centers (scoria cones or maar-like structures), isolated flows, or as part of more differentiated stratovolcano complexes (e.g., San Pedro, Sairecabur, Licancabur, and Lascar). The origin of these mafic products has been related to partial melting of the mantle wedge and an evolution affected by the presence of different MASH-type zones at lower (>40 km depth) and upper (<40 km depth) crustal levels, with limited low-pressure fractional crystallization. Nevertheless, dacitic magma chambers (4–8 km depth) may have benefited from mafic magma inputs to be remobilized and erupted as silicic domes <0.2 Ma (e.g. Chao Dacite). In fact, widespread enclaves hosted in these young domes represent a snapshot of the pre-eruptive magma mingling-mixing processes at the dacite-mafic interface. We are going to deal with the mafic magmas erupted after the last major flare-up ignimbritic phase - i.e. during the last 1 Ma - when the Altiplano-Puna Volcanic Complex entered in a steady-state stage. Such mafic products, mainly erupted near the border or outside of the well detected Altiplano-Puna Magma Body located in the upper crust, can help to unravel the evolution of the present-day Arc Magmatic Stage of this large volcanic silicic province.
La Poruña (21º53’S; 68º30’W) is a 140 m high scoria cone composed of pyroclastic material and an extensive basaltic-andesite to andesite lava flow that is up to 8 km in length. Automated mineralogical analysis describes a suite of porphyritic mafic samples, comprising olivine- and pyroxene-bearing rocks. Well-defined major element compositional trends, as well as trace and rare earth element characteristics (e.g. Sr/Y < 47; Sm/Yb < 4), likely reflect magmatic differentiation at middle-upper crustal levels. Additionally, magma mixing and assimilation and fractional crystallization processes act on these La Poruña magmas within the thickened continental crust, which is typical in Andean volcanic systems. A remarkable compositional feature is the unusual reversed isotopic behaviour of increasing silica with decreasing 87Sr/86Sr compositions. In a process of crustal assimilation during turbulent magma ascent (ATA), the least differentiated rocks are the most contaminated ones since the turbulent hottest magmas effectively assimilate the crustal material. We relate the inverse Sr isotope trend to latter magmatic evolution involving ATA at shallow crustal levels prior to eruption, therefore differing from the broadly accepted Central Andean magmatic model. The older volcanics (> 96 ka) from San Pedro volcano exhibit similar isotopic characteristics, therefore evidence of similar magmatic processes. This new dataset clearly defines magma compositional changes during the La Poruña eruption (ca. 100 ka), revealing an increase in crustal contamination at shallow crustal levels for the younger San Pedro lavas (< 96 ka), likely controlled by increasing amounts of deep-sourced basaltic input over time.
The evolution of the magma plumbing system of the Pleistocene Apacheta-Aguilucho Volcanic Complex area (Altiplano-Puna Volcanic Complex, northern Chile) was investigated through petrographic, geochemical and isotopic studies of representative lavas and related enclaves. Updated available dates of these products, both from the Apacheta and Aguilucho stratovolcanoes and nearby domes, allow us to define the activity during the last 1Ma. This investigation shows that the andesitic magmas were affected by processes of Assimilation plus Fractional Crystallization (AFC, with a significant role played by amphibole fractionation) during their ascent through the upper crust, presumably by the interaction with the Altiplano-Puna Magma Body (15–20km). These andesitic magmas were erupted with no or minor additional contamination at shallower levels, or experienced plagioclase-dominated Fractional Crystallization (FC) to dacite within shallower crustal magma chambers (4–8km depth). The constructional phase of the Apacheta and Aguilucho stratovolcanoes (≥1 to ca. 0.6Ma) reflect a transition from high-flux (i.e. flare-ups) to steady state magmatism, as also documented
in other Pleistocene volcanic complexes of the Altiplano-Puna Volcanic Complex. During this stage the mafic magma recharge was high enough to permit a large spectrum of hybridization of the resident magmas in the upper crust to form the abundant andesites and dacites lavas. In contrast, at ∼150–100ka, the magmatism turned to a new stage of recharge (waning stage?) and the episodic intrusion of small-volumes of andesitic magmas permitted the remobilization of the crystal-rich dacites, triggering the extrusions of the Chanka, Chac-Inca and Cerro Pabellón domes. The andesitic enclaves in the domes studied here represent a snapshot of the magmatic processes of interaction that occurred in the shallower reservoir at the interface of the resident dacite with the ascending andesitic magma. Nevertheless, as there are no dated volcanic products from the area between 0.6 and 0.1Ma, the youngest dacitic domes could also be interpreted as the beginning of a new magmatic pulse of the Altiplano-Puna Volcanic Complex. Independently from the significance of the mafic recharge at 150–100ka (waning stage vs. new pulse) the youngest investigated domes share similar geochemical features and crustal depth constraints (4–8km) with the ignimbrites of the Altiplano-Puna Volcanic Complex, therefore suggesting that the remobilized magmas erupted as the domes are possibly remains of older plumbing systems left over from the last magmatic flare-up of the Altiplano-Puna Volcanic Complex.
Three distinct alkaline magmas, represented by shonkinite, lamprophyre and alkali basalt dykes, characterize a significant magmatic expression of rift-related mantle-derived igneous activity in the Mesoproterozoic Prakasam Alkaline Province, SE India. In the present study we have estimated emplacement velocities (ascent rates) for these three varied alkaline magmas and compared with other silicate magmas to explore composition control on the ascent rates. The alkaline dykes have variable widths and lengths with none of the dykes wider than 1 m. The shonkinites are fine- to medium-grained rocks with clinopyroxene, phologopite, amphibole, K-feldspar perthite and nepheline as essential minerals. They exhibit equigranular hypidiomorphic to foliated textures. Lamprophyres and alkali basalts characteristically show porphyritic textures. Olivine, clinopyroxene, amphibole and biotite are distinct phenocrysts in lamprophyres whereas olivine, clinopyroxene and plagioclase form the phenocrystic mineralogy in the alkali basalts. The calculated densities [2.54–2.71 g/cc for shonkinite; 2.61–2.78 g/cc for lamprophyre; 2.66–2.74 g/cc for alkali basalt] and viscosities [3.11–3.39 Pa s for shonkinite; 3.01–3.28 Pa s for lamprophyre; 2.72–3.09 Pa s for alkali basalt] are utilized to compute velocities (ascent rates) of the three alkaline magmas. Since the lamprophyres and alkali basalts are crystal-laden, we have also calculated effective viscosities to infer crystal control on the velocities. Twenty percent of crystals in the magma increase the viscosity by 2.7 times consequently decrease ascent rate by 2.7 times compared to the crystal-free magmas. The computed ascent rates range from 0.11–2.13 m/sec, 0.23–2.77 m/sec and 1.16–2.89 m/sec for shonkinite, lamprophyre and alkali basalt magmas respectively. Ascent rates increase with the width of the dykes and density difference, and decrease with magma viscosity and proportion of crystals. If a constant width of 1 m is assumed in the magma-filled dyke propagation model, then the sequence of emplacement velocities in the decreasing order is alkaline magmas (4.68–15.31 m/sec) > ultramafic-mafic magmas (3.81–4.30 m/sec) > intermediate-felsic magmas (1.76–2.56 m/sec). We propose that SiO2 content in the terrestrial magmas can be modeled as a semi-quantitative “geospeedometer” of the magma ascent rates.
The Neogene mafic volcanism of the Northern Puna region in the Central Andes is represented by scoria cones and lava flows dispersed over a wide region (c. 9150 km2) as isolated or poorly clustered centres. Although all the products are basaltic andesites to andesites, the behaviour of these magmatic systems resembles that seen in basaltic monogenetic fields. These centres were studied with the aim of defining the main volcanic lithofacies and evaluating the eruptive styles. The results suggest that the eruptions developed under a dry strombolian dynamic, with brief periods of lava fountaining and hydrovolcanism, the latter usually restricted to the early stages of cone construction. Changes in eruptive style are thought to be caused by variations in both the internal (e.g. magma ascent) and external (e.g. surficial water availability) conditions. The transitions do not reflect compositional changes, as evidenced by the small chemical differences observed among the products of the studied eruptive centres. Stratigraphic analysis, in addition to a few pre-existing radiometric dates, suggests that this volcanic activity occurred during the Late Miocene to Early Pliocene. This information supports the inference that these eruptions occurred before the peak of Southern Puna mafic volcanism and that they were coeval with eruptions of some of the most important silicic calderas of the Altiplano-Puna Volcanic Complex. The good preservation of volcanic edifices reveals that erosion rates were extremely low, in agreement with the high aridity conditions that have prevailed in the Puna region since the Mid- to Late Miocene.
The Salar de Atacama in northern Chile is a sedimentary basin containing a 900 m thick salt crust (nucleus), about 1.100 km2 in area, surrounded by a 2.000 km2 fringe of saline muds. The salt crust is filled with a sodium chloride interstitial brine rich in Mg, K, Li, B. The main inflows to the salar drain volcanic formations of the Andean Highlands at the east side of the basin. The salts dissolved in inflow waters have a double origin. The weathering of volcanic rocks supplies K, Li, Mg, B and, to a lesser extent, Na and Ca. The leaching of ancient evaporites beneath the volcanic formations provides additional amounts of Na, Ca, Cl, SO4 in the most saline inflow waters. The mass-balance of the upper nucleus shows a strong excess of NaCl with respect to the bittern solutes Mg, K, Li, B, which suggests that the nucleus did not originate from inflow waters similar to the present ones. The excess of NaCl is likely to be due to NaCl-rich inflow waters that formerly drained the Cordillera de la Sal, a Tertiary evaporitic ridge at the western rim of the salar. The average sedimentation rate of halite has been estimated at 0.1 mm/year from the date of an ignimbrite interbedded in the nucleus. The same rate is obtained from the present inflow waters, which suggests that the halite of the nucleus was deposited from ancient inflows similar to the present ones. This is in contradiction with the mass-balance which indicates that the former inflows were much more concentrated in NaCl. The discrepancy may be solved assuming an intermittent activity of the salar. Long dry periods of inactivity were alternating with short wet periods during which large amounts of salt were deposited. The lack of lacustrine deposits and the high purity of the salt suggest that the nucleus is not the remnant of an ancient deep saline lake, but originated mostly from sub-surface and subterraneous saline inflows.
Lava flow hazard modelling requires detailed geological mapping, and a good understanding of emplacement settings and the processes involved in the formation of lava flows. Harrat Rahat, Kingdom of Saudi Arabia, is a large volcanic field, comprising about 1000 predominantly small-volume volcanoes most of which have emitted lava flows of various lengths. A few eruptions took place in this area during the Holocene, and they were located in the northern extreme of the Harrat Rahat, a close proximity to critical infrastructure and population living in Al-Madinah City. In the present study, we combined field work, high resolution digital topography and morphometric analysis to infer the emplacement history of the last historical event in the region represented by the 1256AD Al-Madinah lava flow field. These data were also used to simulate 1256AD-type lava flows in the Harrat Rahat by the MAGFLOW lava flow emplacement model, which is able to relate the flow evolution to eruption conditions. The 1256AD lava flow field extent was mapped at a scale of 1:1000 from a high resolution (0.5m) Light Detection And Ranging (LiDAR) Digital Terrain Model (DTM), aerial photos with field support. The bulk volume of the lava flow field was estimated at 0.4km3, while the source volume represented by seven scoria cone was estimated at 0.023km3. The lava flow covered an area of 60km2 and reached a maximum length of 23.4km. The lava flow field comprises about 20.9% of pahoehoe, 73.8% of 'a'a, and 5.3% of late-stage outbreaks. Our field observation, also suggests that the lava flows of the Harrat Rahat region are mainly core-dominated and that they formed large lava flow fields by amalgamation of many single channels. These channels mitigated downslope by topography-lava flow and channel-channel interactions, highlighting this typical process that needs to be considered in the volcanic hazard assessment in the region. A series of numerical lava flow simulations was carried out using a range of water content (0.1-1wt.%), solidification temperature (800-600°C) and effusion curves (simple and complex curves). These simulations revealed that the MAGFLOW code is sensitive to the changes of water content of the erupting lava magma, while it is less sensitive to solidification temperature and the changes of the shape of effusion curve. The advance rate of the simulated lava flows changed from 0.01 to 0.22km/h. Using data and observations from the youngest volcanic event of the Harrat Rahat as input parameters to MAGFLOW code, it is possible to provide quantitative limits on this type of hazard.
Figures Tables Albert Streckeisen Foreword to 1st edition Chairman's Preface Editor's Preface 1. Introduction 2. Classification and nomenclature 3. Glossary of terms 4. Bibliography of terms Appendices.
A late Permian 2 km thick volcaniclastic succession is well exposed in the area between the Cerros de Cas and Cerro Lanquir at the eastem margin of!he Salar de Atacama in northem Chile. From base to top it consists of 1he three members of the Peine Formation and !he three members of the Cas Formation. The terreslrial succession is dom naled by producls of andesitic to rflyodacitic effusive and explosive volcanism. Only in the Middle Member of!he Peine Fonnation sedimentary deposits are predominan!. Here, lacustrian, flood and braid plain facies interfinger late rally in a basin-basin margin setting. Deposition was accompanied by extensional lectonics. From sedimentary and volcanic paleocurrent indi::alors of!he Peine and Cas Formations, and geochemical dala of magmatic rocks of the region, it is inferred that the succession was formed al the easlem margin of an exlensional inlra-arc basin. Volcanic facies analyses reveal that, at least, some of the basic and silica-rich volcanic centres were located close to or even within the field area. The thick ignimbriles and proximal fall out deposits of the Middle Member of the Cas Formation are supposed to belong, together with ring dyke and central intrusions, to a late Permian caldera complex, which is localed in the Cerro Lanquir area (Lanquir Caldera Complex). Key words: Late Permian, Peine Forma/ion, Cas Formation, Stra/igraphy, Volcanism, In/ra-are extens/on, Northem Chile. RESUMEN Las formaciones Peine y Cas del Pérmico tardío en el margen oriental del Salar de Atacama, norte de Chile: estratigrafía, facies volcánica y tectónica. Una sucesión volcaniclástica del Pérmico tardío, de 2 km de espesor, aflora excepcionalmente en el área entre Cerros de Cas y Cerro de Lanquir, en el margen oriental del Salar de Atacama. De la base al techo, la sucesión consiste en los tres miembros de la Formación Peine y los tres miembros de la Formación Caso Las rocas terrestres se componen, principalmente, de productosandesíticos a riodacíticosde un volcanismo efusivo y explosivo. Sólo en el Miembro Medio de la Formación Peine predominan depósitos sedimentarios, con facies lacustre, de planicies de inundación, y de planicies de ríos anastomosados, que engranan lateralmente en un ambiente de cuenca y de margen de cuenca. La deposición estuvo acompañada por una tectónica exlensional. En base a indicadores de paleocorrientes sedimentarias y volcánicas en las formaciones Peine y Cas, además de datos geoquímicos de rocas magmáticas de la región, se presume que la sucesión se formó al borde oriental de una cuenca intra-arco exlensional. El analisis de facies volcánicas indica que por lo menos, algunos de los cenlros volcánicos tanlo de bajo como de alto contenido de snice, estuvieron situados en, o cerca del área estudiada. Se supone que los depósitos ignimbríticos de gran espesor, los depósitos proximales de carda de pómez del Miembro Medio de la Formación Cas, así como de las intrusiones centrales y anulares forman parte de un complejo de caldera pérmico tardío, en el área de Cerro Lanquir (Complejc Caldera Lanquir).
Chemical and isotopic data from 12 volcanic centers of the southern Central Volcanic Zone (CVZ) in Chile, whose ages of 20, 16, 11, 8, 5, 2 and <1 Ma bracket the peak of shortening and crustal thickening in the mid-Miocene, show systematic differences with age. The composition of andesites erupted before and after crustal thickening are similar in terms of most major and trace elements, but the post-Miocene andesites show enrichments in Th, U, Cs and Rb, as well as high 87Sr/86Sr and 206Pb/204Pb ratios coupled with low εNd values which indicate greater crustal contamination compared with the older equivalents. Comparison of contamination indicators with age shows that contamination was low from 20 Ma to 8 Ma, increased sharply between 8 and 5 Ma, and remained at a high level into the Quarternary. Constant ratios of fluidmobile vs immobile elements (Cs/Th or Ba/Zr) in even the most contaminated rocks indicate that fluid interaction was negligible. The contaminated andesites display disequilibrium textures and contain phenocrysts with a mixed population of melt inclusions. We suggest that the main process of crustal contamination was mixing with crustal melts. This is supported by geophysical evidence for a zone of partial melting in the mid and lower crust under the magmatic arc and by the presence of late Miocene to Pliocene crustal-derived felsic ignimbrites in the CVZ.
A tectonic reinterpretation is reported for the southeastern margin of the Salar de Atacama basin of northern Chile. Detailed structural mapping revealed the presence of an east vergent thin-skinned fold and thrust belt affecting Oligocene-Miocene Paciencia Group rocks and the overlying Plio-Pleistocene volcanic rocks. Along-strike segmentation of the main fold implies local foreland influence on footwall ramp geometry leading to local thrust sheet rotation. To the east the adjacent western slope of the Western Cordillera displays two different structural domains, probably controlled by preexisting basement structures. The southern domain comprises two N-S oriented sigmoidal belts of linear arranged pressure ridges, indicating left-lateral transpression. In contrast, the northern domain is characterized by east vergent fold and thrust belt structures and reactivated NW-SE striking sinistral strike-slip faults, governing clockwise block rotations. An indenter-driven deformation model is proposed to explain sinistral transpression and clockwise block rotations around vertical axes. This variant of a small-block rotation mechanism is discussed in the context of oroclinal bending of the central Andes, emphasizing the significance of ancient structures in controlling rotations.
The estimates of average magma ascent rates for silicic to intermediate composition lava extrusions are fairly well constrained in the range of 0.001 m/s for very slowly ascending magmas, to 0.015 m/s for more rapidly ascending extrusive magmas (Table 2). These estimates come from a variety of observations made at different volcanoes, and it is important to remember that they are average rates. Ascent rates are likely to vary over the vertical extent of the conduit carrying the magma, because the density and viscosity of the magma will vary considerably over this length, particularly in water-rich magmas undergoing crystallization and volatile exsolution. Additionally, the effective cross section area of the conduit at depth is generally not known. One topic for future study is the nature of changes in flow within volcanic conduits. It has also been suggested that convection may occur in conduits (Witter et al. 2005); in such a case, the magma ascent would be very difficult to assess, but a record should be preserved in some of the phenocryst and groundmass minerals such as the Fe-Ti oxides. It is rather interesting that observations made for basaltic composition magma extrusions give essentially the same range of average ascent rates as obtained for more silica-rich magmas. The assessment of magma ascent rates during explosive eruptions is more complex because there is almost certainly a much greater variation in the ascent rate from the base of the conduit to the surface. This is particularly true for eruptions where there is evidence for alternating effusive dome-forming eruptions and conduit-clearing explosive events (e.g., 1980 Mount St. Helens and 1996-2000 Soufriere Hills, Montserrat). The magma leaves the top of the conduit at essentially sonic velocities for truly explosive eruptions (Papale et al. 1998; Sparks et al. 2006). This high rate of ascent is achieved as a result of the low density and rapid expansion of the gas phase in these gas-rich magmas. Deep in the conduit, very little of the gas is exsolved, the magma is dense, and depending on the size and shape of the conduit, flow rates are calculated to be much lower. In our view, some of the more profitable areas for study of magma ascent lie with investigations of the behavior of the various magmatic volatiles; the rates of their diffusion in melts, and the processes by which gas can be lost from magmas during ascent. Additionally, since so many eruptions appear to be staged from a magma storage zone in the earth's crust, a better understanding of storage zone processes (e.g., magma injection, convection and mixing) would greatly improve our understanding of the magma ascent process.
This review and personal account of basaltic volcanism concentrates on the geological aspects and physical controls, unlike most others which concentrate on geochemistry. It serves as an update to reviews by Wentworth & Macdonald 20 to 40 years ago. The concept of volcanic systems is developed, a system including the plumbing, intrusions, and other accoutrements of volcanism, as well as the volcanic edifice. Five types of systems are described, namely lava-shield volcanoes, stratovolcanoes, flood-basalt fields, monogenetic-volcano fields, and central volcanoes (having silicic volcanics as well as basaltic). It is postulated that the system-type depends on (a) the magma-input rate and (b) the frequency with which magma-batches enter the system (or modulation frequency). These controls determine whether a hot pathway is maintained from the source to the high-level magma chamber, and hence if eruptions are concentrated in the central-vent system. Reviews are given on many aspects: the different kinds of rift zones that volcanoes exhibit, and their controls; the mostly small intrusions, including coherent-intrusion complexes, that occur in and under the volcanic edifices; the great sill swarms that are an alternative to flood basalts; the origin of the craters and calderas of basaltic volcanoes; high-level magma chambers, their locations, and cumulate prisms; the positive Bouguer gravity anomalies that are attributed to cumulates and intrusion complexes; the structures of basaltic lava flows; the characteristics of basaltic explosive volcanism and the consequences of participation by non-volcanic water; the products of underwater volcanism; the origin and significance of joints, formed either by contraction or expansion, in volcanic rocks; and the distribution of vesicles in basaltic rocks. A fairly extensive bibliography is included.
The Salar de Atacama basin of northern Chile preserves stratigraphic evidence for the evolution of the Andean cycle. It has evolved from a non-arc-related rift, through back-arc and inter-arc stages, to a Neogene fore-arc basin. Accumulation of the sedimentary succession was mainly due to extensional faulting. Important but short-duration contractional episodes do link to known first-order plate-margin changes, but their stratigraphic effect appears to be restricted to uplift-erosion rather than creation of significant flexural subsidence. -from Authors
Parícutin Volcano (Mexico) is situated on a northeast-southwest zone of weakness, which is reflected in the location of the lava vents and a satellite cone. That the lava rises from considerable depths is shown by inclusions of several rock types not known to crop out anywhere in the vicinity.
A detailed 4 month record of activity at the crater and in the area of lava outflow at the side of the cone shows little correlation between them. Only once, when the eruption became unusually vigorous, was there a direct response in the lava movement. No cyclic change was observed in the eruptive behavior of the crater except a vaguely defined alternation of moderate activity and quiet in 3 week periods; no periodicity whatever appeared in the lava movement, and the amount of flowing lava remained roughly constant during the 4 months of observation.
A mechanism of eruption is suggested in which lava stands continuously at a high level in the conduit. Dissolved gas bubbles out through the crater, while the liquid spills over into a network of fissures which connect with lava vents at the side of the cone. This mechanism accounts for the simultaneous emission of huge amounts of gas from the crater and emission of lava with little gas from fissures near the base of the cone. It explains also why SO2 is prominent in gas from the crater, while the small amount of gas accompanying the lava contains practically no sulfur compounds but is rich in HCl. Because the lava vents connect with the main conduit by narrow, winding channels, the lava flows are responsive only to major changes in the activity of the crater.
UPb zircon ages of four ignimbrite samples from a Late Paleozoic volcaniclastic succession in northern Chile are presented. The 2 km thick succession consists of the Peine Fm. and the overlying Cas Fm., which crop out at the eastern margin of the Salar de Atacama near the village of Peine. One sample was collected from the oldest ignimbrite of the succession (Lower Member of the Peine Fm.); the other three samples were collected from the Middle Member of the Cas Fm. and its assumed equivalents. The 206Pb238U ages of all samples overlap within their analytical uncertainties, which results in a best estimate of 248 ± 3 Ma (Late Permian: Harland, 1990) for deposition of the entire succession. The age for the stratigraphically older sample is statistically indistinguishable from the other three samples, so that the depositional interval could have been less than 1 million years; maximum errors in the data allow that the Peine and Cas Fms. could have formed during a time span of 6 Ma. The data provide tight constraints on the age of these units, part of which formed in the Lanquir Caldera Complex. They also have implications for the stratigraphy and paleoclimatic models of the Southern Central Andes during the Late Permian.Although the zircon fractions analyzed here document Late Permian ages, they show varying amounts of inheritance of early Paleoproterozoic (to Late Archean?) zircons and also indicate the incorporation of ca. 2.5 Ga initial common lead to varying degrees. This and results from other studies in the area lead us to infer the existence, at least during the Paleozoic, of Precam-brian basement located below the Salar de Atacama area.
A model is proposed for the opening of ephemeral vents in the solid front of a stationary lava flow. The front is considered to be a viscoelastic shell which deforms due to the pressure exerted on it by the inner fluid lava. The vent opens when the normal stress in the front overcomes the tensile strength of solid lava. Cooling of the front is considered and the increase in crust thickness, as the isothermal surface at the solidus temperature deepens into the lava body, is calculated. The thermal effect is negligible, if the timescale for the opening of ephemeral vents is on the order of days, as is often observed. Considering stationary flow lengths of a few hundred meters, this timescale corresponds to an average viscosity of the crust of 1012 Pa s, assuming reasonable values of the crust strength and initial thickness. Observation of ephemeral vents on Mount Etna is found to be consistent with the model predictions.
Lava flow units from ephemeral vents at Mount Etna are characterized by a cross-flow subdivision into zones with different surface morphology, symmetrically distributed with respect to the centerline of the flow. These zones are: (1) a central zone (cp zone) with a relatively smooth surface near the vent; (2) a lateral zone (lp zone) covered by a carpet of clinkers overlying the massive lava body; and (3) a lateral levée zone (ll zone). In many cases the cp and lp zones are separated by an inward-dipping groove, and the cp zone is from some decimeters up to more than 1 m thicker than the adjacent lp zone. In sections perpendicular to the flow direction, the vesicles corresponding with the cp zone are systematically distributed in a ring-like region and are characterized by a sub-elliptical shape with their minor axes along a direction radial to the center of such region. Vesicles corresponding with the lp zone are still elliptical and their major axes tend to be parallel to the nearest cooling surface. Vesicularity is the lowest at the center of the ring-like region where vesicles approach a spherical shape. In the upper part of the ring-like region, vesicle accumulation and coalescence below the crust produces one or more gas-rich layers which act as levels of preferential detachment for the formation of lava tubes. The observed morphological characteristics and reconstructed vesicle distribution patterns are consistent with a radial distribution of velocity within the lava flow unit and suggest the existence of an inner plug flow region where the velocity gradient is zero. A simple evolution scheme is proposed in which a lava flow unit from ephemeral vent invariably evolves to a lava tube through the downward migration of the plug flow region. The progressive reduction of the cross-flow section due to cooling produces a pressure increase within the flowing lava body and the thickening of the active central portion of the lava flow unit in order to satisfy mass conservation.
A system is presented whereby volcanic rocks may be classified chemically as follows:I. Subalkaline Rocks:A. Tholeiitic basalt series:Tholeiitic picrite-basalt; tholeiite; tholeiitic andesite.B. Calc-alkali series:High-alumina basalt; andesite; dacite; rhyolite.II. Alkaline Rocks:A. Alkali olivine basalt series:(1) Alkalic picrite–basalt; ankaramite; alkali basalt; hawaiite; mugearite; benmorite; trachyte.(2) Alkalic picrite–basalt; ankaramite; alkali basalt; trachybasalt; tristanite; trachyte.B. Nephelinic, leucitic, and analcitic rocks.III. Peralkaline Rocks:pantellerite, commendite, etc.A system is presented whereby volcanic rocks may be classified chemically as follows:I. Subalkaline Rocks:A. Tholeiitic basalt series:Tholeiitic picrite-basalt; tholeiite; tholeiitic andesite.B. Calc-alkali series:High-alumina basalt; andesite; dacite; rhyolite.II. Alkaline Rocks:A. Alkali olivine basalt series:(1) Alkalic picrite–basalt; ankaramite; alkali basalt; hawaiite; mugearite; benmorite; trachyte.(2) Alkalic picrite–basalt; ankaramite; alkali basalt; trachybasalt; tristanite; trachyte.B. Nephelinic, leucitic, and analcitic rocks.III. Peralkaline Rocks:pantellerite, commendite, etc.
Volcanoes change shape as they grow through eruption, intrusion, erosion, and deformation. To study volcano shape evolution we apply a comprehensive morphometric analysis to two contrasting arcs, Central America and the southern Central Andes. Using Shuttle Radar Topography Mission (SRTM) digital elevation models, we compute and define parameters for plan (ellipticity, irregularity) and profile (height/ width, summit/basal width, slope) shape, as well as size (height, width, volume). We classify volcanoes as cones, sub-cones, and massifs, and recognize several evolutionary trends. Many cones grow to a critical height (∼1200 m) and volume (∼10 km3), after which most widen into sub-cones or massifs, but some grow into large cones. Large cones undergo sector collapse and/or gravitational spreading, without significant morphometry change. Other smaller cones evolve by vent migration to elliptical subcones and massifs before reaching the critical height. The evolutionary trends can be related to magma flux, edifice strength, structure, and tectonics. In particular, trends may be controlled by two balancing factors: magma pressure versus lithostatic pressure, and conduit resistance versus edifice resistance. Morphometric analysis allows for the long-term state of individual or volcano groups to be assessed. Morphological trends can be integrated with geological, geophysical, and geochemical data to better define volcano evolution models.