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Pressure Ridge

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It is thought that the Cerberus Fossae fissures on Mars were the source of both lava and water floods two to ten million years ago. Evidence for the resulting lava plains has been identified in eastern Elysium, but seas and lakes from these fissures and previous water flooding events were presumed to have evaporated and sublimed away. Here we present High Resolution Stereo Camera images from the European Space Agency Mars Express spacecraft that indicate that such lakes may still exist. We infer that the evidence is consistent with a frozen body of water, with surface pack-ice, around 5 degrees north latitude and 150 degrees east longitude in southern Elysium. The frozen lake measures about 800 x 900 km in lateral extent and may be up to 45 metres deep--similar in size and depth to the North Sea. From crater counts, we determined its age to be 5 +/- 2 million years old. If our interpretation is confirmed, this is a place that might preserve evidence of primitive life, if it has ever developed on Mars.
Article
Selected Hawaiian volcanic features, ranging from shield volcanoes to strands of volcanic glass, are illustrated in 62 photographs. Each feature is defined in a glossary, with a description as to its location, scale, photographer, and date photographed. -Authors
Article
Lunar volcanic deposits are dominated by areally extensive mare units occurring in regionally low areas predominantly on the lunar near side. Data obtained from lunar orbit and earth-based observations have been used to extend the detailed characterizations derived from Apollo and Luna sample return missions to other parts of the moon. Mare volcanism occurred over a period of about 1.3 billion years (b.y.), from about 3.8 to about 2.5 b.y. ago, although the absolute age of the youngest flows is not known. An early Ti-rich mare phase (Apollo 11 and Apollo 17 type basalts) flooded large areas of the eastern portion of the lunar near side in the early Imbrian Period (about 3.5–3.8 b.y. ago). An intermediate age, less Ti-rich phase (Apollo 12 and Apollo 15 type basalts) flooded widespread areas of the moon predominantly in the middle to late Imbrian Period (about 3.0–3.5 b.y. ago). Finally, a second Ti-rich phase (unsampled by Apollo and Luna) flooded portions of Mare Imbrium and the western maria in the early Eratosthenian Period (about 2.5–3.0 b.y. ago). Features associated with mare deposits and processes include lobate scarps, sinuous rilles, domes, cones, dark halo craters, collapse craters, kipukas, lava terraces, mare ridges, and volcanic complexes. The very fluid nature of lunar lavas, their voluminous and extensive flows, and the dominance of volcanic landforms of nonexplosive origin all strongly indicate that lunar eruptions and deposits are most similar to terrestrial basaltic flood and Hawaiian eruptions. Volcanic morphologic features, deposit volumes, extrusion rates, and association of many sources with major crustal fractures all suggest that mare lavas originated at subcrustal depths and worked their way to the lunar surface through a passive but fractured lunar crust. Lavas preferentially infilled the existing low regions of the moon, apparently in a hydrostatic mode. Since lunar impact basins produce most of the topographic lows, mare deposits are concentrated in and around these features. The lunar crust acted essentially as a passive platform on which mare lavas were emplaced. Far side mare deposits are patchy but are concentrated on the floors of the largest basins. Thicker far side crust and hydrostatic emplacement of mare lavas may explain the lack of extensive far side deposits even in deep basins. Individual maria had a history of filling largely determined by the geometry and state of degradation of the basins in which they occur. Mare deposits are generally less than about 2 km in thickness but probably approach 6–8 km locally in the center of some mare basins. The process of filling took place over extensive periods of time and produced complex deposits. Although some near-surface fractionation of mare lavas must have occurred, there is little photogeologic evidence for environments in which extensive fractionation would take place; lavas appear to have erupted at high rates, to have been very fluid, and to have spread out into thin flows; evidence is lacking for shallow magma reservoirs and for lava lakes on the scale of a mare basin. The portion of lunar mare deposits emplaced on the surface of the moon represents less than 1% of the total volume of the lunar crust. There is little unequivocal morphologic evidence for extensive highland volcanism, although intense bombardment may have erased such evidence from early lunar history. The majority of upland plains appear to be related to impact processes. Morphologic evidence for local highland volcanism is found primarily in the form of spectrally distinct domes.
Article
From the similarity of sea ice pressure ridge between Mars and Earth, we infer that they first started from a ductile material to a single phase while deforming, resulting in brittle behavior near the end of the ridging process.
Article
The recent flood lavas on Mars appear to have a characteristic "platy-ridged" surface morphology different from that inferred for most terrestrial continental flood basalt flows. The closest analog we have found is a portion of the 1783-1784 Laki lava flow in Iceland that has a surface that was broken up and transported on top of moving lava during major surges in the eruption rate. We suggest that a similar process formed the Martian flood lava surfaces and attempt to place constraints on the eruption parameters using thermal modeling. Our conclusions from this modeling are (1) in order to produce flows >1000 km long with flow thicknesses of a few tens of meters, the thermophysical properties of the lava should be similar to fluid basalt, and (2) the average eruption rates were probably of the order of 104 m3/s, with the flood-like surges having flow rates of the order of 105 - 106 m3/s. We also suggest that these high eruption rates should have formed huge volumes of pyroclastic deposits which may be preserved in the Medusae Fossae Formation, the radar "stealth" region, or even the polar layered terrains.
Article
Mercury appears to have a tectonic framework and diastrophic history not found on other terrestrial planets explored to date. On the part of the planet viewed by Mariner 10, only two localized areas show evidence of tensional stresses, both of which are apparently associated with the Caloris basin. Lobate scarps occur in the remainder of the explored region and appear to be primarily reverse or thrust faults which have resulted from compressive stresses acting on a global scale. The period of compression represented by these scarps occurred during the final phase of heavy bombardment on Mercury and was probably caused by crustal shortening due to a small decrease in the planet's radius. Stratigraphic, volumetric, and albedo considerations together with distribution indicate that the majority of smooth plains on Mercury were produced by volcanism which occurred at the close of the period of late heavy bombardment similar to that on the moon and Mars. Several generations of plains are evident; the oldest may have resulted in part from an early differentiation of the planet.
Article
Tumuli are positive topographic features that are common on Hawaiian pahoehoe lava flow fields, particularly on shallow slopes, and 75 measured examples are presented here to document the size range. Tumuli form by up-tilting of crustal plates, without any crustal shortening, and are thus distinguished from pressure ridges which are up-buckled by laterally directed pressure. The axial or star-like systems of deep clefts that characterize tumuli are defined here as lava-inflation clefts; their tips advanced into red-hot lava and they widened as uplift proceeded and while the lava crust was thickening. Flat-surfaced uplifts, formed like tumuli by injection of lava under a surface crust, were previously called pressure plateaus, but lava rise is proposed instead. The pits that abound among lava rises, previously attributed to collapse or subsidence, are generally formed because the lava around them rose, and the name lava-rise pit is proposed. Unique examples of tumuli and lava rises, from which lava drained out under a surface crust 1.5 to 2.5 m thick, are described from Kilauea caldera. These examples show that in tumuli and lava rises the crust floats on considerable bodies of fluid lava, and is able to do so because of its higher vesicle content: the fluid lava loses many of its gas bubbles during residence beneath the crust. The bulk densities of samples from tumuli show a general downward increase. The form of the density profile is consistent with the relationship that for any given crustal thickness the density of fluid lava closely matched the average density of that crust, suggesting that the lava was stably density-stratified. It is inferred that stable stratification was regulated by out-flows of the more vesicular lava fractions, loss of bubbles through the lava-inflation clefts, and entry of injected lava at its level of neutral buoyancy. Below the uppermost meter the downward decrease in vesicularity closely conforms with that expected by compression of a uniform mass of gas per unit mass of lava.
Article
Recently acquired data from the High Resolution Imaging Science Experiment (HiRISE), Context (CTX) imager, and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) spacecraft were used to investigate the emplacement of the youngest flood-lava flow on Mars. Careful mapping finds that the Athabasca Valles flood lava is the product of a single eruption, and it covers 250,000 km2 of western Elysium Planitia with an estimated 5000–7500 km3 of mafic or ultramafic lava. Calculations utilizing topographic data enhanced with MRO observations to refine the dimensions of the channel system show that this flood lava was emplaced turbulently over a period of only a few to several weeks. This is the first well-documented example of a turbulently emplaced flood lava anywhere in the Solar System. However, MRO data suggest that this same process may have operated in a number of martian channel systems. The magnitude and dynamics of these lava floods are similar to the aqueous floods that are generally believed to have eroded the channels, raising the intriguing possibility that mechanical erosion by lava could have played a role in their incision.
Article
The large shield volcano Olympus Mons has an aureole of grooved terrain that has been the subject of much controversy since it was first seen on Mariner 9 pictures. Various origins have been proposed for the aureole of grooved terrain. Each of the proposed origins attempts to explain certain aspects of the aureole; none satisfactorily explains all of the observed relations of the deposits to one another and to Olympus Mons. In connection with the present investigation, the proposal is made that the aureole deposits were formed by a series of great pyroclastic eruptions from several vents, fissures, or cauldrons. They occurred near the site of Olympus Mons but were emplaced prior to its construction. At least six major eruptions of pyroclastic material may have occurred.
Article
The morphological and dimensional similarities of the structures within the wrinkle ridge assemblages observed on terrestrial planets are investigated, including structures that occur in mare basalts on the moon and in smooth plains on Mars and Mercury. These structures can be classified as either arches or ridges on the basis of morphology, and ridges can be subdivided onto first-, second-, and third-order ridges on the basis of dimensions. Using ridge structures on the Columbia Plateau (U.S.) as analogs, a basis for a structural interpretation of the wrinkle ridge assemblages on the terrestrial planets is established. It is shown that the anticlinal ridges of the Columbia Plateau are appropriate analogs to the first-order ridges, supporting tectonic interpretations for the ridges.
Article
It has been found that the wrinkle ridge systems of the Caloris basin on Mercury display many of the traits which are characteristic of ridges in the mare-filled lunar multiring basins. The considered investigation is concerned with the ridge systems within the Caloris basin on Mercury, and implications for the origin of ridges within basins on both Mercury and the moon. The observed features are found to indicate that the early evolution of Caloris was similar to that of lunar mascon basins. The morphology of ridges within Caloris compares favorably with lunar ridges when viewed on similar resolution earth-based lunar photographs. Ridges in Caloris occur from 1000 to 1320 km diameter range, and are situated within the boundary delineated by topographic benches in the northeastern and southeastern parts of the basin. The orientation of Caloris ridges is more dominantly concentric than ridge orientations in lunar basins.
Article
The morphological features of Mare Imbrium and Mare Serenitatis are discussed on the basis of Apollo, Ranger, and Orbiter photography. It is suggested that the mare basins were filled relatively slowly by interdigitating, overlapping lava flows, over an extended period of time. Mare wrinkle ridges were formed by localized compression of a relatively thin crust which is effectively decoupled from underlying topography and structure. Since the deformation took place shortly after the time the mare filling was completed, it is possible that this thin crust was underlain by still liquid lava.
Article
High-resolution images of south and west Arsia Mons, eastern Tharsis, and Memnonia Fossae on Mars are analyzed. Lava flows with different types of features, such as pressure ridges, tumuli, festoon ridges, and ring ridges, are observed. The effects of these small features on the type of eruption, mode of flow emplacement, and characteristics of the terrain over which the lavas flow are examined. The development of terrestrial pressure ridges, festoon, and ring structures is discussed and they are compared to the observed Martian features. The interior viscosity of the features are calculated using the Fink and Fletcher (1978) and Fink (1980a) models; the interior viscosity was estimated as 100 MPa sec in flow margins, ponded areas, and across flow lobes. The data reveal that south of Arsia Mons, flows containing pressure ridges are emplaced as multiple flow units from sporadic low-effusion rate eruptions, and the flows at the other three areas are large sheet flows that represent basaltic flood-type volcanism.
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