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Phreatomagmatic volcanism in complex hydrogeological environments: La Crosa de Sant Dalmai maar (Catalan Volcanic Zone, NE Spain)

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The volcano of La Crosa de Sant Dalmai is a roughly circular asymmetrical maar that forms part of the Catalan Volcanic Zone (Girona Province, NE Spain). The edifice is an example of a maar-diatreme volcano constructed on a mixed basement of hard Paleozoic granites and schists and soft Pliocene and Quaternary deposits. The heterogeneities and differences in these rocks' hydraulic properties and fracturing patterns influenced the way in which the magma-water interaction took place during the eruption and, consequently, the style of the eruption and the resulting deposits. The eruption of La Crosa de Sant Dalmai consisted of four consecutive eruptive phases characterized by alternating phreatomagmatic and magmatic fragmentation. The eruptive sequence and the variety of deposits—mainly fallout with subordinate surges—generated by this single eruption are a stark contrast to the compositional monotony of the magma, which thus highlights the role played by the geological and hydrological characteristics of the substrate in determining the eruptive style and associated hazards in this type of volcanism.
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170
Phreatomagmatic volcanism in complex hydrogeological environments:
La Crosa de Sant Dalmai maar (Catalan Volcanic Zone, NE Spain)
Dario Pedrazzi*, Xavier Bolós, and Joan Martí
Institute of Earth Sciences Jaume Almera, ICTJA-CSIC, Group of Volcanology.SIMGEO (UB-CSIC) Lluis Sole i Sabaris s/n, 08028
Barcelona, Spain
ABSTRACT
The volcano of La Crosa de Sant Dalmai
is a roughly circular asymmetrical maar that
forms part of the Catalan Volcanic Zone
(Girona Province, NE Spain). The edifi ce is
an example of a maar-diatreme volcano con-
structed on a mixed basement of hard Paleo-
zoic granites and schists and soft Pliocene
and Quaternary deposits. The heterogenei-
ties and differences in these rocks’ hy draulic
properties and fracturing patterns infl u-
enced the way in which the magma-water
interaction took place during the eruption
and, consequently, the style of the eruption
and the resulting deposits. The eruption of
La Crosa de Sant Dalmai consisted of four
consecutive eruptive phases characterized by
alternating phreatomagmatic and magmatic
fragmentation. The eruptive sequence and
the variety of deposits—mainly fallout with
subordinate surges—generated by this single
eruption are a stark contrast to the composi-
tional monotony of the magma, which thus
highlights the role played by the geological
and hydrological characteristics of the sub-
strate in determining the eruptive style and
associated hazards in this type of volcanism.
INTRODUCTION
Maar-diatreme volcanoes are typical products
of phreatomagmatism (e.g., Fisher and Waters,
1970; Lorenz, 1973, 1974, 1986; Fisher and
Schmincke, 1984). They represent one of the
most interesting examples of the explosive exca-
vation of geological substrate because the lithic
components in the maar deposits are an excel-
lent source of information that reveals much
about the substrate and the depth of the explo-
sions. These monogenetic volcanoes are created
by comparatively low-volume and low-intensity
eruptions, but this form of volcanism represents
a localized, unpredictable volcanic hazard.
These volcanic explosions are caused by the
interaction of magma with phreatic water, and
their exact nature depends on the substrate and the
proportions and extent to which magma and water
interact (Wohletz and Sheridan, 1983; Houghton
and Hackett, 1984; Kokelaar, 1986; White and
Houghton, 2000; Mastin et al., 2004). The type
of substrate controls the characteristics of the
aquifer(s) in which the external water is stored
(fracture-controlled vs. porous aquifers) and has
an important infl uence on the eruption dynamics
and the characteristics of the resulting pyroclas-
tic deposits. The substrate also affects the result-
ing overall shape of the volcano—for example,
the diatreme and the posteruptive lacustrine
architecture of the maar crater (Lorenz, 2003)—
and gives rise to a wide range of maar types
and maar processes (e.g., Tihany maar vol-
canic complex in Hungary [Németh et al.,
2001]; Balaton Highland, Hungary [Auer et al.,
2007]; Campo de Calatrava, Spain [ Martín-
Serrano et al., 2009]; Atexcac crater, eastern
Mexico [Carrasco-Núñez et al., 2007]; Pali Aike
volcanic fi eld, Argentina [Ross et al., 2011]).
Maars commonly display evidence of com-
plex eruptive dynamics and different phases dur-
ing individual eruptive events that can include
phreatic, phreatomagmatic, and magmatic epi-
sodes (e.g., Houghton et al., 1996; White and
Houghton, 2000; Carrasco-Núñez et al., 2007;
Németh et al., 2001).
The Catalan Volcanic Zone (Martí et al.,
1992), one of the Quaternary volcanic regions
related to the European rift system, exhibits a
wide range of phreatomagmatic episodes that
depend on the stratigraphic, structural, and
hydrogeological characteristics of the subsoil
below each volcano (Martí et al., 2011). Of
these hydrovolcanic edifi ces, La Crosa de Sant
Dalmai offers the most characteristic example
of a maar structure in this volcanic fi eld (Martí
et al., 1986; Martí and Mallarach, 1987) and
reveals how much the resulting volcanic edifi ce
depends on the substrate (Bolós et al., 2012).
La Crosa de Sant Dalmai represents, in fact,
an example of an edifi ce emplaced in a mixed
substrate. These types of edifi ces are so far less
well documented (e.g., White, 1991; Sohn,
1996; Sohn and Park, 2005; Ross et al., 2011)
compared to examples of maars emplaced in
hard substrates (e.g., Lorenz, 1987; Lorenz and
Zimanowski, 2008).
In order to determine how the magma-water
interaction occurred during the eruption of La
Crosa de Sant Dalmai and how it was infl u-
enced by the stratigraphic, lithological, and
hydrological characteristics of the substrate, we
performed a detailed lithological and sedimen-
tological analysis of the stratigraphic succession
of this volcano and interpreted it in terms of its
eruption dynamics. In this paper, we describe
the main characteristics of the deposits in La
Crosa de Sant Dalmai and discuss the infl uence
of the substrate on its eruption behavior. The
results obtained here help to explain changes
in the explosive behavior of a maar volcano
emplaced in a mixed substrate with complex
hydrogeological behavior and can be extrapo-
lated to other phreatomagmatic volcanoes of
similar characteristics.
GEOLOGICAL SETTING
The Catalan Volcanic Zone is situated in the
NE Iberian Peninsula and is one of the Quater-
nary alkaline volcanic provinces that belong to
the European Cenozoic rift system. The Catalan
Volcanic Zone is mainly represented by alkali
basalts and basanites and includes several dis-
tinct volcanic fi elds ranging in age from older
than 12 Ma to early Holocene (Fig. 1; Martí
et al., 1992). The volcanic activity in the Cata-
lan Volcanic Zone is characterized by small sco-
ria cones that were produced during short-lived
monogenetic eruptions associated with widely
dispersed fractures of short lateral extent.
For permission to copy, contact editing@geosociety.org
© 2014 Geological Society of America
*E-mail: dpedrazzi@ictja.csic.es.
Geosphere; February 2014; v. 10; no. 1; p. 170–184; doi:10.1130/GES00959.1; 10 fi gures; 1 supplemental fi gure.
Received 29 June 2013 Revision received 18 October 2013 Accepted 18 December 2013 Published online 14 January 2014
La Crosa de Sant Dalmai maar
Geosphere, February 2014 171
The total volume of extruded magma in each
eruption was small (0.01–0.2 km3 dense rock
equivalent [DRE]), suggesting that the amount
of magma available to feed each eruption was
also very limited. Strombolian and phreato-
magmatic episodes alternated in most of these
eruptions and gave rise to complex stratigraphic
sequences displaying a wide range of pyroclas-
tic deposits (Martí et al., 2011).
With a diameter of 1200 m, the maar of La
Crosa de Sant Dalmai is the largest edifi ce in the
Catalan Volcanic Zone. It belongs to the Garrotxa
Volcanic Field (0.6–0.01 Ma), which includes
the youngest volcanoes in the Catalan Vol canic
Zone (Fig. 1). This volcano is located at the
northern border of La Selva graben, a Neogene
tectonic depression bounded by ENE-WSW–
and NW-SE–oriented normal fault systems that
affect the Paleozoic basement, and it is infi lled
with Pliocene and Quaternary sediments (Fig. 1).
La Crosa de Sant Dalmai is an example of a
maar-diatreme volcano consisting of a circular
tephra ring, 30 m and 50 m high on its eastern
and western sides, respectively. Volcanic deposits
cover an area of 7 km2, extending up to 4 km
eastward but only a few hundred meters west-
ward (Fig. 2). Geophysical studies (Bolós et al.,
2012) have found that La Crosa de Sant Dalmai
developed on a NW-SE–oriented fault through
which deep magmas were transported to the
surface. This maar volcano is mostly composed
of phreatomagmatic deposits with subordinate
Strombolian phases. La Crosa de Sant Dalmai
eruption ended with the formation of a scoria
cone in the northern part of the main maar cra-
ter (Fig. 2). This small edifi ce emitted a basal-
tic lava fl ow that fl owed southward and fi lled
much of the maar crater (Bolós et al., 2012).
Currently, postvolcanic lacustrine sediments
cover this lava fl ow. The age of this volcano is
not well constrained, but stratigraphic relations
and existing U-Th and C14 ages of the lava fl ow
and posteruptive sediments suggest that it dates
from the end of the Quaternary age.
METHODS
An important part of the research was carried
out in the fi eld in an area of ~10 km2 surround-
ing the edifi ce of La Crosa de Sant Dalmai using
as a reference the geological map produced by
Bolós et al. (2012). In total, six stratigraphic
sections were carefully studied. The strati-
graphic criteria used to distinguish the different
units forming the succession of volcanic depos-
its included color, nature and relative content
of the components, and variations in grain size,
texture, and sedimentary structures. Estimates
of grain size were conducted partially in the
eld using a comparative grain size chart and
then completed in the laboratory.
Grain-size analyses consisting of dry-sieving
techniques, and componentry analysis were per-
formed by weighing/counting 47 representative
samples of the identifi ed units. Large boulders
were not considered for sieving but were mea-
sured and considered as part of the stratigraphic
column for comparison with other layers.
Samples were sieved with a set of sieves with a
mesh size ranging from –6φ to +4φ units (64 to
1/16 mm). Grain-size data were used to defi ne
the median diameter (Mdφ) and sorting (σφ)
(Inman, 1952) to help discriminate between
deposits emplaced by fall and fl ow mechanisms.
Clast compositions were characterized immedi-
SD
CVZ
N
200 km
42°N
2°30E
Figure 1. Simplifi ed geological
map of the Catalan Volcanic
Zone (CVZ) and its three sub-
zones, La Selva (7.9–1.7 Ma),
L’Empordà (12–8 Ma), and La
Garrotxa (0.5–0.01 Ma) (modi-
ed from Guérin et al., 1986;
Martí et al., 1992). Dashed
red square and SD indicate La
Crosa de Sant Dalmai location.
Pedrazzi et al.
172 Geosphere, February 2014
ately in the fi eld by hand-sample observation
and then confi rmed in the laboratory using a
binocular microscope and petrographic analy-
sis. Component analysis was carried out on the
–4φ, –3φ, –2φ fractions of the deposits. Clasts
were separated into juvenile and accidental
lithics classes belonging to the Paleozoic base-
ment and La Selva infi ll succession. The main
difference lies in the roundness and alteration
of the clasts; nevertheless, this difference was
not suffi cient to discriminate between clasts in
fractions smaller than –2φ. Maximum juvenile
(scoria and caulifl ower bombs) and lithic clast
sizes were determined by measuring and aver-
aging the long axes of 5–10 of the largest clasts
in each bedset.
In order to establish a qualitative idea of the
different degrees of vesiculation of the juvenile
clasts, comparative petrographic and image
analyses were carried out using a binocu-
lar microscope and ImageJ software (www
.ImageJ .com).
Samples obtained from the lava fl ow and
organic matter from a drill core made on the
northeastern side of the maar crater (Bolós
et al., 2012) were used for dating. A prevolcanic
organic sediment sample was sent for dating to
the Beta Analytics Laboratory (UK). The analy-
sis was performed through accelerator mass spec-
trometry (AMS) (radiocarbon .com /accelerator
-mass -spectrometry .htm). The chemical proce-
dure and mass spectrometry for lava samples are
described in Sigmarsson et al. (1998).
CHARACTERISTICS OF THE
PYROCLASTIC SUCCESSION
In order to reconstruct the complete succes-
sion of deposits, we carried out a detailed char-
acterization of six stratigraphic sections in which
six different facies were identifi ed (Fig. 3). The
lateral correlation of individual beds was pos-
sible using stratigraphic markers (Fig. 3); the
maximum thickness of the observed succession
was ~20 m (column 1, Fig. 3).
Facies Analysis
Facies SDA: Large Lithic Ballistic Deposits
This facies (Fig. 4A) has a maximum thick-
ness of 200 cm (Fig. 3). It is clast supported and
well sorted (e.g., samples SD1–1E, SD1–19E,
SD2–1, SD3–2D; Fig. 5), with block- and lapilli-
sized angular prevolcanic accidental lithic clasts
(up to 70%; Fig. 3), as well as poorly vesicu-
lated scoria fragments (Fig. 6A) and a scarce
interstitial matrix of juvenile coarse lapilli-sized
to coarse ash-sized clasts and the same prevol-
canic accidental lithic clasts. The largest lithic
clasts—up to 70–80 cm in diameter (Fig. 5)—
are horizontally aligned and mainly correspond
to granites and schists; they are subangular in
shape, and some have partly or totally oxidized
surfaces. Subordinate bombs of the same size
are also present.
Facies SDB: Clast-Supported Deposits
The deposits of this facies (Fig. 4B) have a
maximum thickness of ~300 cm (Fig. 3). They
are clast supported and medium to well sorted
(e.g., samples SD1–3CG, SD1–20, SD2–1B,
SD2–5AB, SD2–7A, SD3–4G; Fig. 5), and they
have coarse lapilli-sized fragments of poorly
vesiculated scoriae (Fig. 6A) and granite and
schist lithic clasts, with an interstitial matrix
of lapilli and coarse ash fragments of the same
composition. The largest clasts have a maxi-
mum size of 50 cm (Fig. 5). Facies SDB looks
very similar to facies SDA (Fig. 4B), but it is
characterized by a different percentage of non-
volcanic lithic clasts (up to 50%) compared to
facies SDA (up to 70%; Fig. 3), and by poor
stratifi cation.
Facies SDC: Scoriaceous
Clast-Supported Deposits
This facies (Fig. 4C) has a maximum thick-
ness of 70 cm (Fig. 3). Its deposits are clast
supported and moderately to well sorted (e.g.,
samples SD1–5AS, SD2–15BI; Fig. 5) and have
vesiculated scoria (up to 70%; Fig. 3) the size
of coarse lapilli (Fig. 6C), as well as granite
and schist lithic clasts with a maximum size of
40 cm (Fig. 5) and an interstitial matrix mainly
consisting of juvenile fi ne lapilli and coarse
ash fragments. Impact structures are generally
absent. These deposits are characterized by nor-
mal and reverse grading.
Facies SDD: Scoriaceous Deposits
This facies (Fig. 4D) occurs in the middle of
the sequence, where it reaches a maximum thick-
ness of 250 cm (Fig. 3) and also corresponds to
the last episode of the eruption, which led to the
formation of a Strombolian cone (Fig. 2). No
deposits directly connected to the scoria cone
located inside the crater were present in the stud-
ied sections. The facies mantles the topography
and consists of black-and-red, well-vesiculated
bombs and lapilli scoriae (Fig. 6D) covered by
a subordinate fi ne lapilli and coarse ash matrix.
These deposits are generally well sorted (e.g.,
samples SD1–18E, SD3–2AI; Fig. 5). A few
accidental lithic (granites and schists) clasts with
a maximum size of 10–15 cm are found at cer-
tain levels (Figs. 4D and 5).
5
500 m
1
2
3
4
5
6
N
Quaternary post-volcanic deposits
LEGEND
Phreatomagmatic deposits
Scoria
Spatter
Quaternary pre-volcanic deposits
Paleozoic pre-volcanic deposits
Volcanic crater
Volcanic cone
Village
Road
River
Maar crater
limit
Strombolian
cone crater
limit
1
2
34
5
6
1000 m
Aiguaviva
Sant Dalmai
Salitja
Crosa de Sant
Dalmai
N
Figure 2. Google Earth image and geological map of the volcano of La Crosa de Sant Dalmai
(modifi ed from Martí et al., 2011) showing the main crater and the inner scoria cone, as well
as the extent of the phreatomagmatic deposits and the pre- and postvolcanic deposits. The
studied outcrops are also shown (numbers).
La Crosa de Sant Dalmai maar
Geosphere, February 2014 173
Facies SDE: Thinly Bedded Deposits
This facies (Fig. 4E) consists of thinly bedded,
poorly vesiculated scoria of fi ne lapilli size (Fig.
6E) with subangular accidental lithic clasts (up
to 30%–40%; Fig. 3) having a maximum size of
few centimeters. This deposit occurs overall at
the bottom and top of the stratigraphic sequence
with a maximum thickness of 50–70 cm (Fig. 3).
The deposits are poorly sorted (e.g., samples
SD1–2, SD3–1B; Fig. 5). The bed surfaces have
planar and low-angle cross-stratifi ed laminations
and basal erosional contact (Fig. 4E).
Facies SDF: Diffusely Stratifi ed Deposits
This facies (Fig. 4F) has a maximum thickness
of 50 cm (Fig. 3) and consists of poorly sorted
deposits (e.g., samples SD1–14, SD2–10; Fig. 5)
with coarse, poorly vesiculated scoria lapilli
(Fig. 6F) and accidental lithics (up to 30%–40%;
Fig. 3) with a maximum size of 10 cm. The bed
surfaces have diffuse stratifi cation.
Stratigraphic Units and Facies Associations
Four stratigraphic units (Figs. 2, 3, and 7)
can be described from the study of the facies
associations, each of which represents a suc-
cessive stage in the construction of the volcano
(Fig. 3). Unit I is represented by the lithofacies
association I (facies SDA-SDB-SDC-SDE-
SDF). Its thickness varies from 11 m in the
western section (column 1, Fig. 3) to only 3 m
in the east (column 6, Fig. 3). It is dominated by
clast-supported deposits with relatively minor
intercalated layers of lapilli-size material. The
base of this unit is only visible in a few outcrops
(columns 3, 5, and 6, Fig. 3), and it has thick
layers of lithic-rich, fi ne lapilli (facies SDE)
that correspond to the beginning of the erup-
tion. On the eastern side, this initial deposit is
overlain, in almost all the outcrops, by a series
of thick deposits of coarse angular to subangu-
lar, lapilli-sized clasts (facies SDB) alternating
with thin layers, just a few centimeters thick,
of lapilli-sized clasts (facies SDE and SDF).
The following clast-supported, well-sorted
deposit (facies SDA), with decimetric angular
prevolcanic accidental lithic clasts, is a good
stratigraphic marker found throughout almost
all of these outcrops (Fig. 3). A monotonous
sequence characterized by facies SDB, facies
SDC, facies SDE, and facies SDF completes
unit I. In the eastern section (column 3, Fig. 3),
the sequence is characterized by thin, bedded
deposits of fi ne and coarse lapilli-sized clasts
(facies SDE). Unit II (lithofacies association II,
facies SDD) is represented by deposits that are
almost 3 m thick in section 3 (Fig. 3) but that
overall decrease to 1 m (column 1, Fig. 3) or
less (column 2, Fig. 3). The unit is made up
of highly vesiculated bombs and scoria lapilli
with a certain percentage (up to 30%; Fig. 5)
of accidental lithic clasts in certain levels.
Unit III is somewhat similar to unit I, and it
is composed of the lithofacies association III
(facies SDA-SDB-SDC-SDE-SDF). All the
stratigraphic logs (Figs. 3 and 7) have a simi-
lar pattern. Their thickness varies from 8 m to
less than 1 m (Fig. 3). This unit begins with a
breccia (facies SDA) that has the same grain
size as unit I and large blocks of accidental
lithic clasts up 70 cm (Fig. 5). The following
deposits are domi nated by thick, coarse layers
of lapilli-sized breccia with accidental lithics
up to 30 cm and non vesicu lated (juvenile)
scoria (facies SDB), and occasional deposits
with more vesiculated juvenile scoriae (facies
SDC), intercalated with a small proportion of
1m
6
1
5
10
15
20
I
II
III
3
5
4
5
5
5
10
2
5
10
I
II
III
LEGEND
a
b
c
d
e
f
g
h
1
2
3
4
I
II
III
SDA
SDE/F
SDB
SDC
SDD
SDD
SDA
SDB
SDC
SDE/F
0 20 40 60 80 100
LITHICS
%
Figure 3. Composite stratigraphic column of the deposits at La Crosa de Sant Dalmai show-
ing the main facies: (a) facies SDA—large lithic ballistic deposits; (b) facies SDB—clast-
supported deposits; (c) facies SDC—scoriaceous clast-supported deposits; (d) facies SDD—
scoriaceous deposits; (e) facies SDE—thinly bedded deposits; (f) facies SDF—diffusely
stratifi ed deposits; (g) Paleozoic basement; (h) Pliocene–Quaternary basement. Lithics con-
tents in the various units are reported as well: (1) juvenile clasts; (2) Paleozoic lithic clasts;
(3) Pliocene–Quaternary lithic clasts; (4) altered lithic clasts. Four lithofacies associations
and four units are identifi ed based on the depositional processes and resulting deposits. Unit
IV corresponds to the product of the inner scoria cone shown in Figure 2.
Pedrazzi et al.
174 Geosphere, February 2014
thin fi ne lapilli layers with planar stratifi ca-
tion (facies SDE). Unit IV corresponds to a
small scoria cone with an associated lava fl ow
formed inside the maar (litho facies associa-
tion IV-SDD).
No evidence of stratigraphic discontinui-
ties was observed in the sequence, and in some
cases, slightly diffused contacts were observed
among the different facies (Fig. 4). Mantle-
derived nodules in the juvenile fragments are
present in all of the units.
GRAIN SIZE, MODAL VARIATIONS,
AND LITHIC DISTRIBUTION
Grain Size and Modal Variations
Vertical variations in the grain-size distribu-
tion and modal variations were analyzed by
selecting representative samples from both the
coarse- and fi ne-grained layers (Fig. 5). The
maximum clast size is related to the energy con-
ditions and effi ciency of magma fragmentation,
vent excavation, ejection, and emplacement. In
the lowermost unit (unit I), fi ne layers dominate
in the fi rst part, with a general increase in grain
size up to the fi rst lithic-rich breccia. A general
trend of alternating, well-sorted coarse lithic-
rich lapilli deposits and poorly sorted fi ne lapilli
deposits characterizes the fi rst unit. As shown
in Figure 5, the largest blocks measure up to
50–70 cm. Unit II includes coarse juvenile frag-
ment-rich layers. Unit III is similar to unit I and
is dominated by coarse deposits, particularly in
the lower half of the unit, and a grain size that
gradually decreases toward its upper part. Both
units show the same characteristics and can be
distinguished by being above or below unit II,
which is an important stratigraphic marker of
the eruption. Although the distribution of large
blocks is variable, units I and III clearly include
the largest proportion of blocks of the whole
sequence.
Based on the grain-size data, Inman param-
eters (Inman, 1952) of median diameter (Mdφ)
and sorting (σφ) were obtained and plotted on
frequency histograms (Fig. 8) in order to help
discriminate between fall and surge deposits.
Sorting (σφ) values for La Crosa de Sant Dal-
mai samples range between 1φ and 2.25φ, while
the median diameter Mdφ values generally
range between –4.3φ and –0.2φ (Fig. 8).
Componentry Analysis
La Crosa de Sant Dalmai deposits con-
sist of a mixture of juvenile scoria and acci-
dental lithic clasts in differing proportions
(Figs. 3 and 5 and the Supplemental Figure1).
Juvenile fragments are fresh, black, dense or
vesicular scoria with a basaltic composition.
The lithic fragments found in the different
beds throughout the succession include gran-
ite and schist from the Paleozoic basement,
as well as the same type of fragments—but
with a different grade of roundness—from
the Pliocene–Quaternary sediments that fi ll
La Selva depression. These latter fragments
were formed by the erosion and reworking of
the Paleozoic basement. Although the juve-
nile material is present at all stratigraphic lev-
els, its distribution is variable (Figs. 3 and 5).
Figure 4. Field photographs
of the characteristic facies
of the maar of La Crosa de
Sant Dalmai: (A) Facies SDA:
block- and lapilli-sized angu-
lar prevolcanic accidental
lithic clasts (AG, G, and S)
and poorly vesiculated scoriae
(SC); (B) facies SDB: clast-sup-
ported, medium to well sorted,
with coarse lapilli consisting of
poorly vesiculated lithics (AG,
G, and S); facies SDB resembles
facies SDA closely but has a dif-
ferent percentage of lithics (up
to 50%); (C) facies SDC: clast-
supported deposits with coarse
lapilli consisting of vesiculated
scoriae (SC) and subordinate
lithics (G and S) in a mainly
juvenile matrix with fi ne lapilli
and coarse ash; (D) facies SDD:
angular- to fluidally shaped,
black-and-red, well-vesiculated
bombs and lapilli scoriae,
where I and II represent lithic-
rich levels (delimited by yellow
dotted lines), and III represents the lithic-rich transitional upper part toward the following breccia deposit; (E) facies SDE: thinly bedded,
poorly vesiculated fi ne scoria lapilli with subangular accidental lithics, where the bed surfaces show planar and low-angle cross-stratifi ed
laminations and basal erosional contact; and (F) facies SDF: poorly sorted deposits with coarse, poorly vesiculated scoria lapilli and acci-
dental lithics, where the bed surfaces show a diffuse stratifi cation. AG—altered granite, G—granite, S—schists, SC—scoriae. Dashed red
lines represent the facies limits.
A
C
D
E
G
G
SC
S
AG G
S
PALEOZOIC
BASEMENT
S
SC
SC
I
II
III
G
AG S
B
F
G
S
1Supplemental Figure. Lithics content of addition-
al samples from the composite stratigraphic columns
shown in Figure 5. If you are viewing the PDF of this
paper or reading it offl ine, please visit http:// dx .doi
.org /10.1130 /GES00959.S1 or the full-text article on
www .gsapubs .org to view the Supplemental Figure.
La Crosa de Sant Dalmai maar
Geosphere, February 2014 175
1
5
10
15
20
SD1 1E-
SD1 1F-
SD1 2-
SD1 3B-
SD1 3CG-
SD1 4A-
SD1 5AI-
SD1 5AS-
SD1 5BG-
SD1 9AG-
SD1 9B-
SD1 12G-
SD1 14-
SD1 15B-
SD1 18E-
SD1 18F-
SD1 19E-
SD1 20-
SD1 23-
SD1 25E-
I
II
III
a
b
c
d
e
f
g
h
LEGEND
1
08040
Maximum
diameter ( m)c
Lithics Scoriae
08040
( ) ( ) ( ) ( )
( ) ( ) ( ) ( ) ( )
2
3
4
5
6
Figure 5 (on this and following page). Composite stratigraphic column of the three main outcrops in La Crosa de
Sant Dalmai. The lithics contents of the main samples are shown, and vertical variations in the maximum diameter
of the lithic and scoria clasts are also indicated. (a) Facies SDA; (b) facies SDB; (c) facies SDC; (d) facies SDD;
(e) facies SDE; (f) facies SDF; (g) Pliocene–Quaternary basement; (h) Paleozoic basement. (1) No componentry;
(2) Pliocene and Quaternary lithic clasts; (3) metamorphic lithic clasts; (4) altered lithic clasts; (5) granite lithic
clasts; (6) juvenile clasts.
Pedrazzi et al.
176 Geosphere, February 2014
5
5
SD2 1B-
3
2
III
II
I
III
II
I
SD2 5AB-
SD2 DB-5
SD2 1-
SD2 7A-
SD2 7B-
SD2 9-
SD2 10-
SD2 13A-
SD2 13D-
SD2 15A-
SD2 15BI-
SD2 15BS-
SD2 15C-
SD2 19G-
SD2 21-
SD2 24-
SD2 25M-
SD3 1B-
SD3 2AI-
SD3 2AS-
SD3 2B-
SD3 2D-
SD3 4G-
SD3 5AI-
SD3 5SG-
SD3 7-
08040
Maximum
diameter ( m)c
Lithics Scoriae
08040
08040
Maximum
diameter ( m)c
Lithics Scoriae
8040
( ) ( ) ( )( ) ( )
( ) ( ) ( )( ) ( )
0
Figure 5 (continued).
La Crosa de Sant Dalmai maar
Geosphere, February 2014 177
Small systematic variations in the occurrence
of the lithic fragments can be seen in the strati-
graphic succession. The lower part of unit I is
characterized by a breccia deposit (e.g., sample
SD1–1E; Fig. 5) with angular lithic fragments,
mainly granites and schists (up to 40%), and
Pliocene–Quaternary fragments (around 15%)
with subordinate altered clasts (~10%). The
whole of unit I is dominated by alternating
coarse-grained deposits (e.g., samples SD1–
3CG, SD2–1B, SD2–5AB; Fig. 5) that contain
~45%–50% juvenile lithic clast fragments (10%
of Pliocene–Quaternary fragments, less than 5%
of altered clasts, and around 40% of fragments
from the Paleozoic basement), as well as fi ne
lapilli deposits (e.g., samples SD1–2, SD1–14,
SD3–1B; Fig. 5) with lithic fragment contents
of around 30%–40%. Only a few levels (e.g.,
sample SD1–5AS; Fig. 5) of unit I are domi-
nated by juvenile material with lithic fragments
reaching 20%–30% in abundance (with less than
5% of Pliocene–Quaternary lithics clasts). The
highest proportion of juvenile clasts is found
in unit II, where the accidental lithic fragments
from the Paleozoic basement represent less
than 1% (e.g., samples SD1–18E, SD3–2AI;
Fig. 5). However, some levels in unit II show a
notable increase in granite and schist fragments
(up to 30%) and a very high content of altered
clasts and metamorphic fragments with almost
no Pliocene–Quaternary content (e.g., samples
SD1–18F; Fig. 5). In unit III, the same trends as
in unit I are present. The sequence starts with a
lithic-rich breccia with a lithic content of 50%–
60% (mainly from the Paleozoic basement; e.g.,
samples SD1–19E, SD2–1, SD3–2D; Fig. 5)
and continues with the same alternating succes-
sion as in unit I, with a variable amount of lithic
clasts (20%–40%), which mainly originate from
A
B
C
d
D
d
d
F
E
3 mm 3 mm
3 mm 3 mm
3 mm 3 mm
Figure 6. Photographs comparing the juvenile fragments of different lithofacies: (A) facies SDA; (B) facies
SDB; (C) facies SDC; (D) facies SDD; (E) facies SDE; and (F) facies SDF. Most of the samples are made
of poorly vesiculated scoriae except that in D, which represents a pure Strombolian deposit (facies SDD).
Pedrazzi et al.
178 Geosphere, February 2014
A
I
II
III
CB
II
III
III
SDA
SDE
SDB
SDC
SDD
SDA
SDF
SDC
SDB
SDE
SDF
OUTCROP 1
OUTCROP 2 OUTCROP 3
SDD
SDE
SDA
SDB
1m
50 cm
II
SDD
30 cm
Figure 7. The three main outcrops (1, 2, and 3) showing the main characteristics of the La Crosa de Sant Dalmai
sequence. I, II, and III refer to the different units of the eruption, and the green letters represent the facies as shown
in Figure 4.
Figure 8. Sorting (σϕ) versus
median diameter (Mdϕ) plot
of grain-size data from the
fall and surge deposits. Dotted
line defi nes samples from surge
deposits, while continuous line
shows samples from fall-out
deposits.
La Crosa de Sant Dalmai maar
Geosphere, February 2014 179
the Paleozoic basement (e.g., samples SD1–20,
SD2–7A, SD3–4G; Fig. 5) and with lesser
amounts of Pliocene–Quaternary lithics (~5%).
Unit IV is represented by a scoria cone (Fig. 2)
largely made up of juvenile scoria fragments.
LITHOLOGICAL AND
HYDROGEOLOGICAL
CHARACTERISTICS OF THE
PREVOLCANIC SUBSTRATE
The maar of La Crosa de Sant Dalmai is
located on the northern border of La Selva Basin
on the fault contact between the Paleozoic base-
ment and the Pliocene–Quaternary sediments
that fi ll the depression (Bolós et al., 2012). La
Selva Basin covers an area of 565 km2 and is
located in NE Catalonia (Fig. 9). It has a gra-
ben structure (Pous et al., 1990) and is bounded
on four sides by mountain ranges with greater
relief, including the Guilleries range to the west,
the Transversal range to the north, the massif
of Gavarres to the east, and the Selva Marítima
mountains to the south. This basin was created
during the Neogene extensional period follow-
ing the Alpine orogeny. Two main watersheds
in the area correspond to the basins of the Santa
Coloma and Onyar Rivers (Fig. 9). The Santa
Coloma River Basin extends along the whole
southwestern side of La Selva Basin and part of
its headwaters are in the Montseny-Guilleries
Mountains (Fig. 9). The Onyar River basin, on
the other hand, occupies the northeastern side of
the basin (Fig. 9) and has its headwaters in the
Gavarres and Selva Marítima ranges. As pro-
posed by Menció (2005) and Folch et al. (2011),
three main hydrogeological units are present
in La Selva Basin (Figs. 9 and 10): (1) alluvial
materials, surface Neogene sedimentary layers,
and highly porous and permeable weathered
igneous rocks; (2) layers of arkosic sands, grav-
els, and conglomerates with a low clay content
and Neogene sediment alternating with layers
of low-permeability clays and silt, which com-
pose the main infi lling in this basin (where the
transmissivity and permeability of the Neogene
sediments are both very low; except for the
conglomerate-rich levels); and (3) crystalline
materials (Paleozoic igneous and metamorphic
rocks), which generally have low permeabilities
but also have a set of structural heterogeneities
(fractures, schistosity, presence of dikes and
alteration) that act as zones of preferential fl ow.
As proposed by Menció et al. (2010) (Fig. 9),
based on hydrochemical and isotopic data, the
general model for underground fl ow requires
a local fl ow system generated by the subsur-
face topography of the basin that is related to
the main alluvial aquifers and more superfi cial
Neogene layers. Furthermore, a regional fl ow
system runs across La Selva Basin, and its
recharge zone is located in the adjacent mas-
sifs (the Guilleries and Transversal ranges and,
to a lesser extent, in the Gavarres and Selva
Marítima ranges). Piezometric data proposed
by Folch et al. (2011) indicate the presence in
the La Selva area of unconfi ned aquifers less
than 30 m deep and confi ned or semiconfi ned
aquifers over 30 m deep. Furthermore, based
on hydrochemical and isotopic data, the same
authors proposed a lateral hydraulic connection
between the range-front areas and the basin
aquifers, which would indicate an effective
recharge through fault zones and fracture net-
works within the basement. Similar behavior is
also suggested to occur at the contact between
the sedimentary infi ll of the basin and the
basement, with the magnitude of the recharge
depending on distinct geological features such
as the hydraulic conductivity of the lowest Neo-
gene sediments, the thickness of the weathered
granite on top of the basement, and the fracture
network. At the same time, hydraulic head data
indicate a vertical connection between sedi-
mentary aquifer levels at various depths, which
allows distinct vertical connections between
the Neogene sedimentary aquifer layers (Folch
et al., 2011). Additionally, hydraulic head
records indicate that the regional hydraulic
head decline due to water withdrawal is recov-
ered annually despite the rainfall regime. This
behavior is attributable to the effective recharge
from the aforementioned regional fl ow system
(Menció, 2005).
In general, the distribution of the water table
is consistent with the topography of the area.
The western, eastern, and southeastern areas
of La Selva Basin are characterized by a steep
gradient (coinciding with the mountainous areas
of Guillerias, Gavarres, and Selva Marítima),
while in the central part of the basin, the dis-
tance between the isopieces (equipotential
curves representing the phreatic surfaces of
the aquifer) grows, and the gradient decreases
(Menció, 2005).
DISCUSSION
The eruption of La Crosa de Sant Dalmai
included episodes that were clearly dominated
by a magma-water interaction and magma-poor
phases as shown by fi eld evidence (Fig. 4), the
abundance of lithics fragments (Figs. 3 and 5
and the Supplemental Figure [see footnote 1]),
and the general low vesicularity of the juvenile
fragments (Fig. 6). The lithological and depo-
sitional characteristics (Fig. 4) as well as the
granulometrical analysis (Fig. 8) of the resulting
deposits reveal that most were formed by fall-
out mechanisms of ballistic blocks and bombs
impact sags, and subordinate pyroclastic surges.
The characteristics of the lithofacies and litho-
facies associations, as well as the results of
the componentry analysis, provide the neces-
sary clues for understanding the evolution of
the eruption and the construction of the vol-
canic edifi ce.
The sequence starts with lithic-rich fi ne lapilli
layers (facies SDE) deposited by pyroclastic
surges, as suggested by the presence of cross
laminations (Fig. 4E). This fi rst episode cor-
responds to an initial phreatomagmatic phase
during which the effi ciency in the energy trans-
fer from the magma to the phreatic water was
optimal, as indicated by the characteristic high
degree of fragmentation in the resulting deposit.
At this stratigraphic level, it is very likely that
the locus of the explosions was located between
the weathered surface of the granite basement
and the Quaternary deposits (stage I, Fig. 10), as
shown by the relative abundance of Quaternary
fragments compared to the rest of the sequence
(Fig. 5). The characteristics of these initial
deposits and their radial distribution refl ect the
presence of a base-surge–type explosion (Crowe
and Fisher, 1973; Fisher and Waters, 1970;
Druitt, 1998), as has occurred at the beginning
of other phreatomagmatic eruptions (e.g., Atex-
cac crater [eastern Mexico], Carrasco-Núñez
et al., 2007; Tihany [Hungary], Németh et al.,
2001). The fi rst episode was followed by the
deposition of mainly lithic-rich fallout lapilli-
sized clast layers (e.g., samples SD2–1B, SD2–
5AB; Fig. 5). As proposed by Carrasco-Núñez
et al. (2007), the deposition of these layers could
have been associated with the formation of an
ephemeral eruptive column. This vent-opening
episode was immediately followed by the for-
mation of a thick breccia deposit (facies SDA),
with abundant angular lithic clasts of block and
lapilli size, derived from the mixed (Paleozoic
and Pliocene–Quaternary) substrate rocks, and
poorly vesiculated scoria fragments (Fig. 6A).
This breccia corresponds to the main vent-
enlargement phase caused by a major infl ux of
phreatic water into the eruption conduit. The
largest clasts, mainly granites and schists, are
horizontally aligned and did not generate impact
structures. Martí et al. (1986) suggested that this
breccia originated from a very shallow explo-
sion that generated ballistic trajectories with
an important lateral component. Following this
major explosive phase, a thick sequence formed
the rest of unit I, dominated by poorly strati-
ed, clast-supported deposits (facies SDB; Fig.
6B) alternating occasionally with deposits of
more vesiculated scoriae (facies SDC; Fig. 6C)
and diffusely stratifi ed deposits of lapilli-sized
clasts (facies SDF; Fig. 6F) and subordinate
thinly bedded deposits (facies SDE; Fig. 6E).
Pedrazzi et al.
180 Geosphere, February 2014
4612000 4636000 4658000
Northing (m, UTM 31N-ED50)
Crosa de Sant Dalmai
HO
2
Montnegre
Range
470000
Easting (m, UTM 31N-ED50)
HO
2
HO
2
A
Transversal
Range
Guilleries
LEGEND
495000
HO
2
HO
2
HO
2
HO
2
A
A
Empordà
Basin
Gavarres
Selva
Marítima
Mediterranean Sea
Pre-Gavarres
Vilobí d’Onyar
Girona
Caldes de Malavella
Maçanet
de
La Selva
Santa Coloma de
Farners
Llagostera
Ter
River
Tordera River
Hercynian granites and
metasedimentary rocks
Paleogene sedimentary rocks
Neogene-Quaternary volcanic
deposits
Faults
Neogene-Quaternary
sedimentary deposits
Streams
Watershed boundaries
Tow n s
La Selva
Basin
200 km
N
Onyar River watershed
Santa Coloma River watershed
AA
Figure 9. Geographical and geological setting of La Selva Basin, with the watershed boundaries with the two main subbasins (Onyar
and Santa Coloma Rivers) marked. Arrows indicate the recharging area of the basin (modifi ed after Folch et al., 2011). The A–A
profi le consists of a block diagram showing the general hydrogeological characteristics of the substrate below La Crosa de Sant
Dalmai and La Selva depression and the infi lling of the tectonic graben of La Selva Basin and the crystalline materials (igneous and
metamorphic rocks). The orthophoto was provided by ICC (UTM 31N-ED50 Institut Cartogràfi c de Catalunya, 2013, www.icc.cat).
La Crosa de Sant Dalmai maar
Geosphere, February 2014 181
Paleozoic basement Pleistocene basement Volcanic deposits
Normal faults
Dike
Explosion locus
Fracture zone
Aquifer Phreatomagmatic
activity
Strombolian
activity (fallout)
H
2
O
Weathered Paleozoic rocks
H
2
O
H
2
O
H
2
O
Fragmentation
level
Vesiculation
level
Fragmentation
level
Vesiculation
level
Fragmentation
level
Vesiculation
level
surface
surface
H
2
O
H
2
O
Fragmentation
level
Vesiculation
level
surface
surface
Without
vesiculation
surface
Phreatomagmatic phase
Phreatomagmatic phase
Strombolian phase
Strombolian phase Effusive phase
EW
Stage 0 E
WStage I
E
WStage II E
WStage III
E
W
Stage IV(b)
E
W
Stage IV(a)
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
Figure 10. Four stages of the evolution of La Crosa de Sant Dalmai edifi ce: Stage 0—formation of La Selva
Basin with the associated aquifers; Stage I—interaction of the ascending magma with the shallower Qua-
ternary and Paleozoic altered granite aquifers; Stage II—fi rst magmatic phase, probably due to a general
decreasing of the water content in the shallower aquifer; Stage III—decrease in the fragmentation level in
the conduit and a new phreatomagmatic episode in a deeper aquifer; Stage IVa—pure Strombolian phase,
with the rise and eruption of the magma and no interaction with the probably almost exhausted aquifer;
Stage IVb—emplacement of a lava fl ow inside the maar crater.
Pedrazzi et al.
182 Geosphere, February 2014
We suggest that the whole of unit I derived
from explosions occurring in the weathered
granitic basement, which would have contained
abundant water (stage I, Fig. 10). This idea is
supported by the large proportion of basement-
derived granite and schist clasts in the beds that
form this part of the succession (Fig. 5). Pre-
sumably, the rising magma would have occu-
pied existing fractures in the granite and schist
that would have probably fi lled with water. The
lack of interaction with the fi rst aquifer could
be related to a high and rapid input of magma,
as suggested by the presence of large mantle-
derived nodules in the deposits, which would
not have allowed the required energy transfer
effi ciency to permit magma-water interaction.
Facies SDA, SDB, and SDC suggest fallout
and ballistic emplacement (Fig. 8; see Németh
et al., 2001). In particular, facies SDC includes
some horizons of juvenile scoria lapilli with
few prevolcanic lithics fragments (Fig. 5), prob-
ably indicating episodes involving less water
recharge from the aquifer. Generally, these latter
explosions did not have the same energy transfer
effi ciency during the magma-water interaction
as during the fi rst explosion, as shown by the
abundance of breccia deposits. The stratifi ed
beds (facies SDE and SDF) could be interpreted
as deposits originating from a high-concentra-
tion suspension with little tractional transport
(e.g., Chough and Sohn, 1990).
These deposits, different that facies SDA–
SDC, suggest other transport and depositional
mechanisms, probably related to changes in the
eruption dynamics caused by changes in the
effi ciency of the hydromagmatic fragmentation.
The effi ciency of hydromagmatic fragmenta-
tion and the corresponding eruption dynamics
depend on the pressure differences between
magma and water, the water-magma contact
mode, and magma temperature and viscosity
(Wohletz and Sheridan, 1983), as well as on the
exact nature of the coolant (White, 1996). This
implies that the eruption responsible for the con-
struction of La Crosa de San Dalmai maar was
continuous but included several pulses in which
different types of deposits were formed.
Unit II refl ects an important change in the
eruption dynamics (stage II, Fig. 10). It is made
of well-vesiculated (Fig. 6D) scoria bombs and
lapilli (facies SDD) with a few (less than 1%;
Fig. 5) accidental lithic clasts in some levels.
This facies is the result of fallout deposition
from Hawaiian-style fi re fountains. The fact that
this scoria deposit appears in stratigraphic con-
tinuity (as suggested by the absence of erosional
surfaces) with phreatomagmatic unit I and
immediately precedes a new phreatomagmatic
episode (Figs. 4D and 7) indicates that, at this
stage, the water supply from the aquifer located
between the altered granites and the Quaternary
sediments (stage II, Fig. 10) was not suffi cient
to sustain the phreatomagmatic interaction with
the ascending magma.
Due to the effect of hydromagmatic erup-
tions, a large amount of water is vaporized,
causing a large and almost instantaneous with-
drawal of groundwater from the aquifer. A
lowering of the water table can be expected if
hydromagmatic activity lasts over a period of
several days (Lorenz, 1986). As suggested by
Németh et al. (2001), in a porous media aqui-
fer, with lateral heterogeneities, water might
not fl ow fast enough to the vent area to main-
tain the phreatomagmatic character of the erup-
tion, despite the abundance of groundwater in
the rest of the aquifer. Thus, the conditions for
a purely magmatic eruption might be reached,
and Strombolian explosions may occur.
Unit III (stage III, Fig. 10) started with a new
breccia episode (facies SDA) characterized by
abundant large heterolithologic blocks (up to
almost 1 m in diameter) originating from the
Paleozoic basement, and poorly vesiculated
juvenile scoria (Fig. 6) resulted from the inter-
mittent fallout deposition (facies SDB and SDC)
and the subordinate pyroclastic surges (facies
SDE and SDF). The lithic fragment contents,
which mostly correspond to Paleozoic basement
clasts with lesser amounts of Pliocene–Quater-
nary clasts (Fig. 5), indicate that the locus of the
explosions migrated downward.
A possible water transmissivity is thought to
occur at the contact between the sedimentary
infi ll of the basin and the basement, although it
would depend on hydraulic conductivity of the
lowest Neogene sediments, the thickness of the
weathered granite on the top of the basement,
and the fracture networks. As the eruption pro-
gressed, the fracture-controlled aquifer could
have been disrupted by the initial shock wave,
causing an increase of secondary permeability
and further excavation of the maar crater. This
might have led to a decrease of the lithostatic
pressure and a progressive lowering of the posi-
tion of the fragmentation level in the eruption
conduit during the course of the eruption (see
Papale et al., 1998; Macedonio et al., 2005).
This would have permitted a new phreatomag-
matic episode when the magma interacted with
a deeper aquifer located in the fractured Paleo-
zoic basement, as indicated by the nature of the
lithic fragments included in unit III.
A second line of evidence of the existence of
a deeper aquifer was provided by Menció (2005)
and Folch et al. (2011) with fi eld data, where
multilayer aquifers were recognized in the La
Selva area. Furthermore, investigations carried
out by Folch and Mas-Pla (2008) higlighted
the relevance of fault geometry upon the fl ow
system and the connection between the upper
basin-fi ll formations and the Paleozoic base-
ment. Moreover, the same authors explained
how some deep wells located close to La Crosa
de Sant Dalmai area showed a confi ned type of
behavior according to structural characteristics,
fault geometry, and scaling. This might suggest
a similar behavior for La Crosa de Sant Dalmai,
enhancing the hypothesis of multilayer aquifers
acting at different depth.
The eruption ended with a Strombolian epi-
sode (unit IV) (stage IV, Fig. 10) focused in the
interior of the maar crater, which gave rise to
the formation of a small scoria cone (stage IVa,
Fig. 10) and the emission of a lava fl ow (stage
IVb, Fig. 10) that was subsequently covered
by lacustrine deposits. The transition from wet
to dry conditions might suggest a signifi cant
decrease in the volumetric water content in the
deeper levels of the aquifer as well as a changing
water supply that thus ensured that the eruption
would continue in a pure Strombolian phase. As
suggested by Németh et al. (2001), a fracture-
controlled aquifer might have a very strong
seasonality, with an increasing or decreasing
groundwater supply. Another hypothesis could
suggest a magma conduit able to seal itself off
from the surrounding aquifer, leading to a fi nal
purely magmatic phase.
The eruption sequence deduced for La Crosa
de Sant Dalmai follows the generalized model
proposed by Lorenz (1986). The proportion of
lithic and juvenile fragments in the phreatomag-
matic deposits and the presence of pure Strom-
bolian phases in the middle and at the end of
the eruption suggest that water supply was not
constant. Even assuming that magma supply
was not continuous, the changes observed in the
eruption sequence and the resulting deposits are
better explained by changes in the water supply.
This variation in the amount of water interacting
with the erupting magma could be due either to
the intermittent recharge of the aquifer during
the eruption or to the magma interacting with
a heterogeneous aquifer (in which the levels
had different hydraulic characteristics) at differ-
ent depths in the conduit. The fi rst case would
account for a relatively long eruption in which
seasonal recharges of the aquifer could have
induced this type of pulsating behavior. How-
ever, the eruption of La Crosa the Sant Dalmai
seems to have occurred in a continuous fashion
and over a short period of time, as is suggested
by the absence of discontinuities in the strati-
graphic sequence (Figs. 3 and 4). The intermit-
tent magma-water interaction would thus seem
to result from the interaction of the erupting
magma with different aquifer levels located at
different depths and with different hydrogeo-
logical properties, an explanation that matches
La Crosa de Sant Dalmai maar
Geosphere, February 2014 183
the hydrogeological characteristics of the study
area. Alluvial and weathering materials with
high permeabilities composing the main infi ll-
ing deposits of La Selva Basin would have
allowed the fi rst phreatomagmatic phase, while
the crystalline materials characterized by struc-
tural heterogeneities, enhanced by the presence
of a fault system connected to La Selva Basin,
would explain the second phreatomagmatic
phase. Similar to La Crosa de Sant Dalmai, the
same types of stratigraphic successions can be
observed in other edifi ces of the Catalan Vol-
canic Zone. Martí et al. (2011) explained the dif-
ferences in the eruptive behavior of the Catalan
Volcanic Zone as related to the occasional inter-
action of the ascending magma with ground-
water rather than to changes in magma compo-
sition rheology or magma supply.
The succession of deposits that form La
Crosa de Santa Dalmai has uniform stratigraphy
all around the vent, albeit with smaller, thicker
units and steeper angles in the west, and thinner
units, gentler angles, and a broader distribution
in the east, thereby suggesting a radial but asym-
metrical distribution of the deposits (Fig. 2).
This highlights the importance of differences
in rock strength in mixed substrates, as already
emphasized by Smith and Lorenz (1989), Sohn
and Park (2005), and Auer et al. (2007) in other
maar examples, which, in the case of La Crosa
de Sant Dalmai (Bolós et al., 2012), made it
easier for the phreatomagmatic explosions to
excavate the eastern side where the soft Plio-
cene–Quaternary sediments were found. This is
also suggested by the strong eastward horizontal
component in the fallout deposits, which were
probably infl uenced by this type of rheological
contrast with the host rocks.
CONCLUSION
La Crosa de Sant Dalmai maar formed on
the northern border of the Neogene La Selva
Basin on a NW-SE–oriented normal fault that
was probably used by deep magma to reach the
surface. This maar volcano is an example of the
way in which a tephra ring develops in a mixed
setting characterized by a hard (Paleozoic gran-
ites and schists) and soft (Quaternary fi lling)
basement with heterogeneities and differences
in the hydrogeological structure of the area, and
aquifer levels with different hydraulic proper-
ties and fracturing patterns. These differences
clearly infl uenced the way in which the magma-
water interaction occurred throughout the erup-
tion and, consequently, the style of the eruption
and the resulting deposits. The eruption at La
Crosa de Sant Dalmai included four eruptive
phases with alternating phreatomagmatic and
magmatic fragmentation. As occurs in many
other volcanoes in the same monogenetic fi eld,
the eruptive sequence and resulting deposits that
formed La Crosa de Sant Dalmai contrast with
the compositional monotony of the magma,
thereby emphasizing the role played by the geo-
logical characteristics of the substrate in deter-
mining the eruptive style and associated hazards
in this type of volcanism.
ACKNOWLEDGMENTS
This study was partially funded by the grant
CROSAND and the European Commission
(FT7 Theme: ENV.2011.1.3.3-1; grant 282759:
“VUELCO”). We would like to thank Lluis Motjé
(Consortium of La Crosa de Sant Dalmai: manage-
ment of fi eld geology in La Crosa volcanic area)
for his logistical support during the fi eld work. We
are also grateful to Llorenç Planagumà and Leandro
d’Elia for their help with the fi eld work and Guillem
Serra for his contribution to the discussion of this
paper. We are also grateful to Editor Tim Wawrzyniec,
Associate Editor Dave Hirsch, and Pete Stelling and
an anonymous reviewer for their constructive reviews
of this manuscript. English text was corrected by
Michael Lockwood.
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... The products of these eruptions are mainly alkaline basalts and basanites (Araña, 1983;López Ruiz and Rodríguez Badiola, 1985;Martí et al., 1992;Cebría et al., 2000). The eruptions of most of these volcanoes have alternated between Strombolian and phreatomagmatic phases (Martí et al., 2011;Pedrazzi et al., 2014Pedrazzi et al., , 2016, thus generating a large variety of pyroclastic deposits. The succession is different at each volcano, indicating that there is the absence of an eruptive common pattern for the GVF volcanoes (Martí et al., 2011). ...
... Differences between units 1 and 2 suggest different transport and depositional mechanisms. Unit 1 originated from fallout while Unit 2 indicates abundant country rock fragmentation during the eruption, with deposition mainly from dilute PDCs, similar to other phreatomagmatic deposits of the same volcanic field, such as the Puig d'Adri (PA) or La Crosa de Sant Dalmai (SD) (Fig. 1c; Pedrazzi et al., 2014Pedrazzi et al., , 2016. Unit 3 (Figs. ...
... 2b and 10), all the lithic clasts found in the studied deposits correspond to the latter. This differs from other similar volcanoes of the same sector of the GVF, such as La Crosa de Sant Dalmai (SD) (Bolós et al., 2012;Pedrazzi et al., 2014) and Puig d'Adri (PA) (Pedrazzi et al., 2016), Fig. 8. Petrographic microscope (a, b, e, f) and scanning electron microscopy (c, d) images of PBB volcanic products: a) porphyritic and vesicular texture, CPx-Clinopyroxene, Ol-Olivine; b) Opx-orthopyroxene and Spl-spinel crystals with corona-texture. The former is mainly characterized by olivine crystals and subordinated Fe-Ti oxides, while the latter only by Fe-Ti oxides; c) lava sample, Cpx-Clinopyroxene, Ol-Olivine, Plg-Plagioclase, Fe-Ti Ox-Fe-Ti Oxides; crystalline groundmass in a lava sample; d) interstitial glass in scoria samples; e) scoria vesicular texture f) poorly vesiculated lava sample. ...
Article
Full-text available
The Puig de la Banya del Boc volcano is located in the southern sector of the Garrotxa Volcanic Field in the NE of the Iberian Peninsula and is part of the European Cenozoic Rift System. This monogenetic volcano was constructed on a hard basement of Paleozoic metamorphic rocks and shows a complex eruptive succession with phreatomagmatic, Strombolian, and effusive phases. Similar to other volcanoes of the same volcanic field, the succession of deposits of the Puig de la Banya del Boc volcano reveals the influence of the substrate, upon which the volcano forms, on eruption dynamics. In this case, the Paleozoic basement and its particular hydrodynamic properties controlled the way in which magma/water interactions occurred throughout the eruption. This volcano is coeval with the Clot de l'Omera, a small maar-type edifice that was emplaced on a conjugate fault, which belongs to the same fault system.
... The morphology of a maar depends on several factors, including the structural regime and hydrological environment (e.g. Lorenz, 2003;Son et al., 2012;Jordan et al., 2013;Pedrazzi et al., 2014). An aquifer is both the country rock surrounding the dike and diatreme, as well as the source of water for the MFCI, and its structural and rock mechanical characteristics influence the resultant diatreme and the eruption style (Lorenz, 2003;Auer et al., 2007). ...
... The main crater stratigraphic sequence starts in the first stage, with a juvenile lapilli fallout (facies F1) in unit A. This first eruptive phase corresponds to an initial strombolian activity (e.g. Martí et al., 2011;Pedrazzi et al., 2014;Chako-Tchamabé et al., 2015;Murcia et al., 2015). In this scenario, the ascending magma could be fragmented by dissolved gas decompression (e.g. ...
... In the western section, an increase in accidental lithic clasts occurred (facies F2), mostly several local lavas and sedimentary rocks, possibly associated with a deepening of the eruptive loci, so the crater deepened and widened (e.g. Martí et al., 1986;Carrasco-Núñez et al., 2007;Martí et al., 2011;Pedrazzi et al., 2014;Chako-Tchamabé et al., 2015). Thus, strong and intermittent magma-water interactions were formed at this stage. ...
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The Aljojuca maar is located in the eastern Trans-Mexican Volcanic Belt, within the Serdán-Oriental Basin, which is characterized by Quaternary bimodal monogenetic volcanism. The Aljojuca maar has an irregular shape, with an eastern embayment structure that forms an E-W alignment with at least three older scoria cones to the east. The crater walls expose a sequence of pre-maar volcaniclastic deposits, with a lava flow linked to the older scoria cones and a paleosol at its top; the latter indicates a significant hiatus occurred between the growth of the scoria cones and the eruption of the Aljojuca maar, whose deposits cap this sequence. Changes in the local conditions produced strong interactions of the ascending magma with groundwater, shifting the strombolian eruptive style of the three older scoria cones to intense phreatomagmatic explosions of Aljojuca maar, leading to the emplacement of ca. 30-m-thick maar (tuff ring) sequence. A detailed stratigraphic study indicates that the eruption can be grouped into two main stages, comprising five eruptive phases dominated by phreatomagmatic and ephemeral strombolian eruptions, which are represented by eight stratigraphic units in total. The first stage reflects the development of the main crater and is recorded by six stratigraphic units (from A to F), and the second stage includes the occurrence of a possibly fissural E-W structure, which formed a scallop in the eastern flank of the maar (eastern embayment structure), and formed two stratigraphic units (G and H). To explain the processes involved in that evolution, a sequential model is proposed. At the outset of the Aljojuca eruption, the feeder dike arrived at the main crater, leading to many explosions around the crater, creating a maar sensu stricto. Later, migration of the explosive loci toward the eastern flank of the maar showed a tectonic control by the dominant regional east-trending structural pattern. Several 14C paleosol ages support a late Holocene age (ranging from 4140 30 BP to 2870 30 BP.) of Aljojuca maar. This has important implications for the hazard assessment of volcanism in this area, which had not been considered previously as a potentially active system.
... Several models exist on their formation and evolution (Lorenz, 1975(Lorenz, , 1986Geshi et al., 2011;Valentine et al., 2011;Valentine and White, 2012;Graettinger et al., 2014Graettinger et al., , 2015Ross et al., 2017). Maar eruptions are mostly investigated through the study of the deposits forming their tuff or tephra rings (e.g., Pedrazzi et al., 2014;Valentine et al., 2015). Moreover, intracrater successions also provide valuable information to understand the formation of such particular volcanoes and their diatreme architectures, where most of the studies come from kimberlite pipe exploration (e.g., Sparks et al., 2006;Lorenz and Kurszlaukis, 2007;Brown et al., 2009;Seghedi et al., 2009) and from old exhumed maars or eroded structures (e.g., Suoana maar in Miyakejima Volcano, Japan (Geshi et al., 2011); Hopi Buttes, USA (White, 1991;Vazquez and Ort, 2006;Lefebvre et al., 2013;Ross, 2019, 2020), Missouri River Breaks volcanic field, USA (Delpit et al., 2014); Coombs Hills, Antarctica (McClintock and White, 2006;Ross and White, 2006), etc.). ...
... The tectonic evolution of this volcanic region was controlled by the Neogene extensional fault system, which was responsible for the formation of several fault-bounded basins (Fig. 1b). The CNMD belongs to La Selva Basin, a tectonic depression bounded by ENE-WSW and NW-SE fault systems that also contain a younger maar, La Crosa de Sant Dalmai volcano (Bolós et al., 2012;Pedrazzi et al., 2014). The CNMD was built astride two different substrates (i.e., mixed setting), Paleozoic granites and pre-volcanic unconsolidated Pliocene sediments corresponding to the as much as 200 m thick succession that infills La Selva Basin (Fig. 1d), and which contains the main Pliocene groundwater aquifer in the area. ...
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Camp dels Ninots is a mixed hard-soft maar-diatreme located in the Catalan Volcanic Zone (NE of Iberia), in which intra-maar lake sediments have preserved one of the most remarkable Pliocene fossil records in Europe. Geophysical surveys combined with the geological map and 11 boreholes, including two new continuous intra-crater drill cores, have enabled the construction of a 3D geological model of this maar-diatreme and its basement. The formation of this maar-diatreme started with a vent-opening phreatomagmatic explosion at the intersection between a regional fault and the Paleozoic groundwater level at a depth of 210 m. We infer and calculate the geometry, dip direction and dip angle of this regional fault. During the eruption, mixed Strombolian and phreatomagmatic episodes occurred, forming the tuff ring and filling the diatreme with minimum estimated volumes of 0.012 and 0.004 km³, respectively. The diatreme infill is composed of three main lithofacies that include tuff-breccias, welded scoriae, and mafic intrusions into the phreatomagmatic breccias. Thus, the stratigraphy of the diatreme succession suggests a progression of explosive events from deeper to shallower zones with short lateral migration of explosion vents, which control its final morphology, without evidence for significant deep enlargement of the diatreme during the later phases. This generated a wide funnel-shaped with low diatreme wall angles that differs from kimberlite pipes with great depths and sharp slope geometries. Hence, such 3D geological model helps to understand the complex architecture of maar-diatreme structures, highlighting the lack of geological modeling of this type of monogenetic system.
... The interaction between magma in the conduit and groundwater can produce significant changes in the behavior of explosive eruptions, influencing magma fragmentation, and transport and deposition of pyroclasts Wohletz, 1981, 1983;Lorenz, 1987;Pedrazzi et al., 2014). The dynamics of FIG. 7 Contour plots of crystallinity at conduit top (A), mass fraction of dissolved gas in melt at conduit top (B), exit temperature (C), exit pressure (D), exit velocity (E), and mass discharge rate (F), as a function of the relaxation parameters for gas exsolution (x-axis) and crystallization (y-axis). ...
Chapter
In the world, volcanic systems exhibit a wide range of eruption styles threatening the lives of millions of people. Relatively slow effusive eruptions generate lava flows (low viscosity magma) and lava domes (high viscosity magma) and tend to evolve over days to decades. Alternatively, explosive eruptions can inject very large volumes of fragmented magma and volcanic gas high into the atmosphere over shorter periods (minutes to weeks to months). Mitigation of the associated risk to populations, the built environment, and the cultural heritage relies upon our ability to accurately assess volcanic hazards, and this, in turn, depends on our understanding of the processes that control the style and scale of volcanic eruptions. To this end, technological developments over the last couple of decades have greatly improved our ability to characterize magmatic systems and detect precursors at high spatial and temporal resolution through the use of analytical and observational volcanology, including monitoring-derived data, and volcano geophysics. Numerical modeling of magma ascent can serve to link all of these data and processes to build effective near-real-time strategies. The complexity of the volcanic system, derived from the multiphase, multicomponent character of the magmatic mixtures and from their interaction dynamics with the surrounding host rocks, is however manifested in the complexity of its mathematical representation, and numerical models able to describe several interdependent processes, eventually at disequilibrium conditions, are required to capture the nature of volcanic systems with fidelity. In this chapter, we present the main equations governing magma ascent, highlighting the multiphase and disequilibrium nature of volcanic flows, and the presence of complex feedback mechanisms between gas exsolution, outgassing, and crystallization that are able to influence the most important characteristics of the resulting volcanic events. Then, a suite of numerical simulations is described to show the effect of some parameters and processes in controlling eruption style and scale, and thus the potential eruption hazard.
... For example, scoria cones are one of the most common volcano types on Earth (Wood, 1980b), constructed through magmatic gas segregation driven explosions at shallow depths (Guilbaud et al., 2009;Courtland et al., 2013;Kereszturi and Németh, 2016). On the other hand, hydromagmatic volcanic edifices including tuff rings, maars, and tuff cones are considered to be the second most common volcanic landforms on Earth, often produced by different degrees of magma-water interactions (White, 1996;Lorenz and Kurszlaukis, 2007;White and Ross, 2011;Pedrazzi et al., 2014b). ...
Article
Deception Island is one of the most active volcanoes in Antarctica, with more than 20 monogenetic eruptions during the Holocene. The latest episodes of 1967, 1969, and 1970 have shown that volcanic activity on Deception Island can become a concern for tourists, scientists, and military personnel working on or near the island. The objective of this work is to identify eruptive processes and the geomorphic evolution of post-caldera monogenetic volcanic edifices at Deception Island by morphometric analyses, supported by field observations. Morphometric analysis has been used since the 1970s to analyse scoria cones, but it has rarely been applied to other monogenetic volcanoes, such as tuff cones and tuff rings. Tuff cones and tuff rings represent the most common landforms during Deception Island's recent geological past, with over 70 scattered eruptive vents inside and along the caldera rim. These volcanic landforms have been studied based on field observations and later, Digital Elevation Model analyses. Their geometry ranges from 10 to 100 m in height, and diameters vary between 300 and 2500 m. The morphometric data suggest that tuff cones have a more circular crater than tuff rings, with a significant separation between the two landforms using the height (Hco) and basal width (Wco) parameters. Tuff cones have Hco/Wco ratios with values between 0.04 and 0.16 and outer slopes between 7° and 34°. Tuff rings have lower Hco/Wco ratios that are between 0.01 and 0.04 and lower outer slopes between 3° and 21°. This study shows that basic shape parameters in combination with slope angle analysis can be used to discriminate among different types of monogenetic volcanoes. The initial eruptive-related morphometric diversity, however prevents correlation of eruption ages with morphometric parameters, or constructing a relative chronology of the monogenetic eruptions. This work provides a better comprehension of the potential evolution of a future eruption and a broader understanding of volcanic hazards on Deception Island.
... We then adapted the method of Kereszturi et al. (2017) to determine Pr (PM | X L ), the probability of an eruption beginning with a phreatomagmatic phase. Near-surface geology, hydrology and topography are all recognised as controls on phreatomagmatism (Connor et al., 2000;Kereszturi et al., 2017;Pedrazzi et al., 2014). Using WinBUGS software (Lunn et al., 2000) we performed a logistic regression analysis on the occurrence of a phreatomagmatic phase in past eruptions using as independent variables the environmental factors of vent elevation above sea level, thickness of water-saturated and porous sediments, and distance from known faults. ...
Article
The dichotomy between probabilistic and scenario-based volcanic hazard assessments stems from their opposing strengths and weaknesses. The quantification of uncertainty and lack of bias in the former is balanced against the temporal narrative and communicability of the latter. In this paper we propose a novel methodology to bridge between the two, deriving a pseudo-probabilistic hazard estimate from a suite of dynamic scenarios covering multiple volcanic hazards and transitions in eruptive style, as designed for emergency management purposes, in a monogenetic volcanic field. We use existing and new models for eruption style transitions, which provide probabilities conditional on local environmental conditions, thus obtaining the relative likelihoods of each scenario at every location in the field. The results are interpreted in terms of the probability of various hazards and combinations of hazards arising from various scenarios at critical locations. Conversely, we also demonstrate that it may be possible to optimise the likelihood of the scenario allocations across desired locations for emergency management training purposes.
Preprint
Full-text available
Tephra rings that surround maar craters are typically inferred from field observations to be emplaced rapidly over a time period of days to years and thus monogenetic, which is, however, rarely assessed quantitatively. This paper reports the discovery of polygenetic origin of the Mamiyadake tephra ring (Japan), comparing the paleomagnetic directions obtained from throughout the stratigraphy. The new data show that the paleomagnetic directions change systematically with height through the sections, which is interpreted to record paleosecular variation (PSV) of the geomagnetic field during formation of the tephra ring. The paleomagnetic results, together with using an average rate of PSV in Japan, indicate that the Mamiyadake tephra ring was constructed episodically with five major eruptive episodes, separated by centuries or longer, over at least 1000 years. The findings demonstrate that detailed paleomagnetic characterization can uncover the temporal evolution of tephra rings, providing a useful criterion for identifying time breaks, even where field evidence is lacking, and a minimum estimate of the time interval for their emplacement. The approach used here may be applicable to volcanoes of any type.
Article
Volcanic zones are important geoheritage sites. Active volcanic fields are of special interest because they allow to observe the complex and interesting stratigraphic relationships that often characterise their products and processes, as well as landscapes of unusual beauty. These geological sites enable visitors to appreciate the full complexity of volcanic activity and the importance of the geoheritage values that should be preserved. To ensure a correct preservation of these zones while opening them up to visitors, management plans should establish and consolidate a network of outcrops that allow visitors to get to know the area in question and preserve its geological heritage and the social and economic dynamism. In La Garrotxa Volcanic Field, most of which is protected by a natural park, a new methodology has been created to identify the sites of greatest geological interest and, above all, those that exemplify the diversity of eruptive styles and volcanic products and landforms in the area. Taking into account the most relevant management requirements (conservation and dissemination), in 1994 a total of 60 outcrops were classified as points of geological interest of the area. In this study we further present a specific methodology to select the most suitable sites for the preservation of the main volcanological features of La Garrotxa Volcanic Field. A group of 12 sites were selected from the initially identified outcrops and included in an itinerary as the most representative geological and pedagogical sites while also bearing in mind their ease of access and preservation and the possible impact of visitors (e.g. erosion). The following five criteria were considered when restoring these outcrops: integration into the landscape, consolidation of their geology, regulation of visits, the mitigation of risk, and the participation of the people that live in the area.
Chapter
Neogene-Quaternary alkaline volcanism is widely distributed along an extensive rifts system in central and Western Europe, including the Rhenish Massif and Rhinegraben of Germany, the Massif Central of France, and the western Pannonian Basin in Eastern Europe. This rifts system evolved in the Alpine foreland during late Eocene to Recent times, being its development contemporaneous with the main and late Alpine orogenic phases. In the Iberian Peninsula this rifts system and its associated alkaline volcanism is present along its eastern coast (Valencia Trough and eastern sector of the Betic Cordilleras), and also at the interior of the peninsula (Campo de Calatrava and Volcanic Province). All these volcanic zones share common petrological, structural, and volcanological characteristics with those other zones from the European rifts system. The main characteristics of the Neogene-Quaternary alkaline volcanism are described and discussed.
Article
The El Pozo Volcanic Complex, in the south-central Mendoza province, is formed by three maars, the first to be described in the Payenia Volcanic Province and one of the few examples in Argentina. The maars of El Pozo Volcanic Complex (1, 2 and 3) show large craters (0.97 km, 1.2 km and 0.96 km, respectively) and their depth is between 10 and 57 m below the pre-eruptive surface. Their deposits are described from the only outcrops that have been found, called El Pajarito and El Visitante. It was interpreted that the first maar to be formed would have been No. 2, from which only the deposits corresponding to El Pajarito (3.6 m) and the lower section of El Visitante (28 m) are preserved. We suggest that its shape is the result of lateral vent migration and coalescence of successive vents. Then, after a short break represented by the angular unconformity in the El Visitante section, the active vent would have shifted more to the W and the maar No.3 is thought to have been formed. The activity of this maar is represented in El Visitante upper section, with features resulting from the presence of water in the time of deposition. Maar No. 1 was probably active simultaneously with maar no. 3, with which coalesces but the lack of outcrops does not guarantee this affirmation. Towards the end of the eruptive cycle, depletion of water produced a change in activity from phreatomagmatic to Strombolian and effusive. This gave rise to the nested pyroclastic scoria cone, El Pozo cone, which lies in the maars’ 1 and 3 craters floors. El Pozo cone is related to lava flows with a tumulus and the deposit of a lapilli-bearing tephra plume.
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The Quaternary volcanic province of Catalonia is situated south of the Pyrenees. This volcanic region with strombolian and maar volcanoes is mainly composed by basanite lava with analcime and leucite. Twenty four new analysis confirm the alkaline nature of this volcanism. Eleven new absolute datations by termoluminiscence confirm that the volcanic activity of this area has been sporadic from ca.350,000 to 11,000 years BP. About fifteen volcanic centre would have suffered hydrovolcanic eruptions.
Article
The temporal evolution of fundamental flow conditions in the magma chamber plus conduit system–such as pressure, velocity, mass flow-rate, erupted mass, etc.–during sustained magmatic explosive eruptions was investigated. To this aim, simplified one-dimensional and isothermal models of magma chamber emptying and conduit flow were developed and coupled together. The chamber model assumed an homogeneous composition of magma and a vertical profile of water content. The chamber could have a cylindrical, elliptical or spherical rigid geometry. Inside the chamber, magma was assumed to be in hydrostatic equilibrium both before and during the eruption. Since the time-scale of pressure variations at the conduit inlet–of the order of hours–is much longer than the travel time of magma in the conduit–of the order of a few minutes–the flow in the conduit was assumed as at steady-state. The one-dimensional mass and momentum balance equations were solved along a circular conduit with constant diameter assuming choked-flow conditions at the exit. Bubble nucleation was considered when the homogeneous flow pressure dropped below the nucleation pressure given the total water content and the solubility law. Above the nucleation level, bubbles and liquid magma were considered in mechanical equilibrium. The same equilibrium assumption was made above the fragmentation level between gas and pyroclasts. Due to the hydrostatic hypothesis, the integration of the density distribution in the chamber allowed to obtain the total mass in the chamber as a function of pressure at the chamber top and, through the conduit model, as a function of time. Simulation results pertaining to rhyolitic and basaltic magmas defined at the Volcanic Eruption Mechanism Modeling Workshops (Durham, NH, 2002; Nice, France, 2003) are presented. Important flow variables, such as pressure, density, velocity, shear stress in the chamber and conduit, are discussed as a function of time and magma chamber and conduit properties. Results indicate that vent variables react in different ways to the pressure variation of the chamber. Pressure, density and mass flow-rate show relative variations of the same order of magnitude as the conduit inlet pressure, whereas velocity is more constant in time. Sill-like chambers produce also significantly longer and more voluminous eruptions than dike-like chambers. Water content stratification in the chamber and the increase of chamber depth significantly reduce the eruption duration and volume. Maximum erupted mass fractions of about 0.2 are computed for small water-saturated and shallow chambers.
Article
Numerous measures are used in the literature to describe the grain-size distribution of sediments. Consideration of these measures indicates that parameters computed from quartiles may not be as significant as those based on more rigorous statistical concepts. In addition, the lack of standardization of descriptive measures has resulted in limited application of the findings from one locality to another. The use of five parameters that serve as approximate graphic analogies to the moment measures commonly employed in statistics is recommended. The parameters are computed from five percentile diameters obtained from the cumulative size-frequency curve of a sediment. They include the mean (or median) diameter, standard deviation, kurtosis, and two measures of skewness, the second measure being sensitive to skew properties of the "tails" of the sediment distribution. If the five descriptive measures are listed for a sediment, it is possible to compute the five percentile diameters on which they are based (phi 5 , phi 16 , phi 50 , phi 84 , and phi 95 ), and hence five significant points on the cumulative carve of the sediment. This increases the value of the data listed for a sediment in a report, and in many cases eliminates the necessity of including the complete mechanical analysis of the sediment. The degree of correlation of the graphic parameters to the corresponding moment measures decreases as the distribution becomes more skew. However, for a fairly wide range of distributions, the first three moment measures can be ascertained from the graphic parameters with about the same degree of accuracy as is obtained by computing rough moment measures.