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Supervolcanoes within an ancient volcanic province in Arabia Terra, Mars (Michalski and
Bleacher, 2013)
The purpose of this assignment is to give you an idea of how understanding volcanic landforms
on Earth can be applied to increase our understanding of the geomorphic features, geologic
history, and climatic history of other planets.
After you have read the article, then you should answer the questions below.
1. List any terms that you are unfamiliar with or don’t understand as you read the article
and attempt to define them.
2. What is the hypothesis of this article?
3. Describe the main objectives for this study.
4. What is the evidence for volcanic origin of the features discussed in the article?
5. What are the issues with the authors hypothesis of volcanic origin?
6. Briefly summarize the volcanic history of Mars.
7. Describe Figure 5. What does show? How does it support the author’s hypothesis?
8. Describe the main results of this study.
9. How did the author’s apply their understanding of volcanic geomorphology on Earth to
better understand the formation of the craters in Arabia, Terra, Mars? Describe the
main methods used in this study.
10. After reading this article are you convinced that the features discussed are of volcanic
origin? Why or why not?
11. Did you enjoy this article? Why or why not? Do you have any further questions
regarding this article/research?
Supervolcanoes within an ancient
volcanic province in Arabia Terra, Mars
Joseph R. Michalski1,2 & Jacob E. Bleacher3
Several irregularly shaped craters located within Arabia Terra, Mars, represent a new type of highland volcanic construct
and together constitute a previously unrecognized Martian igneous province. Similar to terrestrial supervolcanoes, these
low-relief paterae possess a range of geomorphic features related to structural collapse, effusive volcanism and explosive
eruptions. Extruded lavas contributed to the formation of enigmatic highland ridged plains in Arabia Terra. Outgassed
sulphur and erupted fine-grained pyroclastics from these calderas probably fed the formation of altered, layered
sedimentary rocks and fretted terrain found throughout the equatorial region. The discovery of a new type of volcanic
construct in the Arabia volcanic province fundamentally changes the picture of ancient volcanism and climate evolution
on Mars. Other eroded topographic basins in the ancient Martian highlands that have been dismissed as degraded impact
craters should be reconsidered as possible volcanic constructs formed in an early phase of widespread, disseminated
magmatism on Mars.
The source of fine-grained, layered deposits1,2 detected throughout
the equatorial region of Mars3 remains unresolved, though the deposits are linked to global sedimentary processes, climate change, and
habitability of the surface4. A volcanic origin has been suggested on
the basis of the stratigraphy, morphology and erosional characteristics
of the deposits5. The case for a volcanic source is further strengthened
by the spectroscopic detection of sulphates in many of these deposits6
and detailed analyses of such rocks at the Meridiani Planum landing
site, which revealed materials altered under water-limited, acidic conditions that were probably governed by volcanic outgassing7. Yet,
although very fine-grained ash can be dispersed globally from a large
explosive eruption on Mars5,8, the currently known volcanic centres
are unlikely to have been the sources for thick, low-latitude layered
deposits in Arabia Terra9.
The lack of identifiable volcanic sources that could have produced
possible volcanogenic sediments in Meridiani Planum or in Gale
crater is not a unique problem. In fact, 70% of the crust was resurfaced
by basaltic volcanism, with a significant fraction emplaced from as yet
unrecognized sources10. Thus, undetected volcanic source regions
must exist within the ancient crust of Mars. Therefore, the following
questions arise: first, is ancient volcanism poorly understood because
higher Noachian erosion rates11 obliterated evidence for source
regions? Second, are ancient volcanoes highland volcanoes of fundamentally different character from the well-recognized, massive,
Hesperian shield volcanoes12,13? We suggest that the answer to the
second question is yes; we propose a new category of ancient volcanic
construct that has escaped detection until now.
Volcanism is the thread binding nearly every aspect of Mars’s
geological evolution. The crust of the planet was built through magmatism and effusive volcanism14, although an early phase of explosive
volcanism might have emplaced a significant amount of fragmented
material across the ancient crust15. Volatiles outgassed16 from volcanoes
controlled atmospheric chemistry17 and strongly affected climate18–20
throughout Martian history. The geochemistry and habitability of
Martian soils and sedimentary rocks are ultimately controlled by the
global sulphur cycle, which is fundamentally linked to volcanism21. It is
therefore critical to understand all styles and phases of Martian volcanism and how they have affected the Martian climate through time.
Evidence for volcanism in Arabia Terra
We present evidence for a new category of ancient volcanic construct
on Mars, ancient supervolcanoes, which together could have produced
vast amounts of lava and pyroclastic materials throughout Arabia
Terra and beyond. The features, which we call ‘plains-style caldera
complexes’, are characterized by the presence of collapse features,
low topographic relief (lower than that of typical paterae), and association with plains-style lavas and friable, layered deposits. Taken
together, the features, each with explosive outputs probably in excess
of terrestrial supervolcanoes, constitute a previously unrecognized
ancient volcanic province in Arabia Terra (Fig. 1).
The best example of a plains-style caldera complex is Eden patera,
which is a large, irregularly shaped topographic depression (dimensions ,55 km northwest–southeast and 85 km southwest–northeast)
located at 348.9u E, 33.6u N within Noachian–Hesperian ridged plains
of probable volcanic origin. The complex, which reaches a maximum
depth ,1.8 km below surrounding plains, includes at least three
linked depressions (Fig. 2) bounded by arcuate scarps and associated
with numerous faults and fractures. Although this feature has never
been differentiated from impact craters in the region, it lacks any
geological indicator of an impact origin, such as the presence of ejecta,
an uplifted rim, nearly circular geometry or the presence of a central
peak22. Its high ratio of depth to diameter is inconsistent with that of
an ancient impact crater that has been modified by erosion23. We
therefore rule out an impact origin for Eden patera.
We interpret Eden patera as a caldera complex on the basis of its
similarity to terrestrial calderas24 and its association with features that
indicate formation by means of collapse and volcanism both within
and outside the depression. The surrounding terrain comprises ridged
plains typical of Hesperian basaltic volcanism on Mars10. Within the
complex are fault-bounded blocks that display surfaces similar to the
adjacent ridged plain lavas (Fig. 2a). These blocks are tilted towards
the crater centre and are unrelated to headwall scarps that would
Planetary Science Institute, Tucson, Arizona 85719, USA. 2Department of Earth Sciences, Natural History Museum, London SW7 5BD, UK. 3NASA Goddard Space Flight Center, Greenbelt, Maryland
20771, USA.
3 O C T O B E R 2 0 1 3 | VO L 5 0 2 | N AT U R E | 4 7
©2013 Macmillan Publishers Limited. All rights reserved
Study area
Semeykin crater
0° N
Oxus patera
Ismenia patera
Euphrates patera
0° E
Dich dary
Oxus cavus
Siloe patera
25 km
Eden patera
Elevation (m)
100 km
Figure 1 | Geographic context of the northern Arabia Terra region. The
dusty nature of Arabia Terra is shown in false-colour TES-derived albedo data
draped over MOLA hillshade data; bright colours correspond to dusty surfaces.
Recently named geographic features discussed in the text are labelled.
suggest a process similar to landslides. Graben associated with the interior fault blocks may have originally been linked with circumferential
graben outside the complex related to older collapses or progressive
formation through ‘piecemeal’, multicyclic evolution24. We interpret
a mound ,700 m high (11 km north–south and 23 km east–west)
within the complex to be a graben-related vent (Fig. 2b). Two sets of
nearly continuous terraces are found ,100 and 150 m above the caldera
floor. These terraces are strikingly similar to the ‘black ledge’ described
during the Ki¯lauea Iki eruption in 1959 (ref. 25), indicating high stands
of a drained lava lake26. A small mound (1 km across) several hundred
metres high and located between the two terraces shows surface cracks
similar to a tumulus27. Although tumuli clefts form during inflation, we
suggest that these cracks formed as the lava lake drained and the sinking
lake crust was draped onto caldera wall rocks.
The presence of volcanic features and significant faulting consistent
with collapse leads us to conclude that these linked depressions represent a large caldera complex formed in the Late Noachian to Early
Hesperian. A lacustrine origin for the terraces is unlikely due to the
60 km
Elevation (m)
Lava lake
Caldera 1
Caldera 2
20 km
Caldera 3
4 8 | N AT U R E | VO L 5 0 2 | 3 O C TO B E R 2 0 1 3
©2013 Macmillan Publishers Limited. All rights reserved
Figure 2 | The geology of Eden
patera. a, MOLA topographic data
are draped over THEMIS daytime
infrared data, showing the
morphology of Eden patera.
b, Geological mapping reveals the
presence of at least three calderas,
indicated by coloured shading.
c–e, Enlargements of the rectangles
in b. The caldera contains evidence
for fault blocks that preserve ridged
plain lavas on the upper surface (c),
a probable vent (d), and a series of
terraces that mark lava high stands of
a once active lava lake (white arrows)
and cracked crust (black arrows) due
to the draping of fragile crust onto
pre-existing surfaces during lava lake
drainage (e).
paucity of channels found in or around the depression that could be
linked to aqueous surface processes. In addition, there is no apparent
evidence for lacustrine sediments within the basin, and the depression
is deeper than expected for a feature of this size that was partly filled by
outside sediment. The sequential development of this feature (calderas 1–3 in Fig. 2) seems to have undergone a transition from surface
sagging (caldera 1 in Fig. 2) to significant disruption of the crust and
subsequent down-dropping of large surface blocks (calderas 2 and 3 in
Fig. 2).
Several other features throughout the region have similar characteristics. Euphrates patera is an irregularly shaped depression that
reaches 700 m in depth below the surrounding lava plains and contains several benches in the interior that might be explained by
sequential episodes of collapse or lava-lake high stands (Fig. 3). The
irregular, rhombohedral form of the depression might relate to shortening in the southwest–northeast direction. Fractured surface textures in the centre of the depression are morphologically similar to
lava surfaces disrupted by collapse caused by the withdrawal of lava.
Other features in northern Arabia Terra contain evidence for collapse associated with volcanic activity. Siloe patera (6.6u E, 35.2u N) is
a set of nested deep depressions that reach ,1,750 m below the surrounding plains (Fig. 3). Similar to Eden patera, the nested craters are
characterized by steep-walled depressions linked by arcuate scarps
and faults. The primary structure is linked to a subtle northeast–
southwest-trending depression to the south that reaches ,700 m
depth, which we interpret as evidence for sagging due to the migration
of a magma body at depth. Although there is no evidence for impact
ejecta around the structure, there is a single set of lobate flows emanating from the southwest portion of the depression rim, which may
represent a single set of lava or pyroclastic flows reaching ,60 km
from the rim. Irregular mounds of friable materials inside the nested
craters are interpreted as pyroclastics from the volcano or as younger
friable deposits of another origin.
Some other depressions in the region contain less well preserved
evidence for volcanism, but in all cases they contain suites of features
Elevation (m)
75 km
that are difficult to explain by other geological processes. Semeykin
crater is a large scalloped depression surrounded by lava plains and
friable deposits; it also contains mounds of friable materials in its
interior and ridged plains along the exterior. A suite of features,
Ismenia patera, Oxus patera and Oxus cavus, are located together near
0u E, 38.5u N. The two paterae have scalloped, breached rims composed
of layered materials. Oxus cavus is an elongated depression within a
broad mound 200–300 m high adjacent to a deep depression indicative
of sagging or collapse. Although none of these structures individually
contains as many pieces of evidence to clearly point to volcanism as are
seen at Eden patera, all of the features contain some evidence for
structural collapse, which is most likely to have occurred through
magmatic activity (although other hypotheses are considered below).
Eden patera and Euphrates patera represent the strongest evidence
for large calderas in Arabia Terra, on the basis of their association with
features that are diagnostic of surface disruption and collapse coupled
with evidence for effusive and explosive volcanism. Some of the other
features with fewer diagnostic features might not all represent caldera
formation, or they could have experienced a range of processes
responsible for the current morphology. Nonetheless, the region does
show strong evidence that several large depressions did not form as
impact craters and are most easily explained as volcanic calderas.
The roles of ice and impact
Some depressions throughout Arabia Terra have previously been
interpreted as thermokarst features28,29. There is no doubt that geological surfaces in and around the Arabia Terra region have been
modified by ice30, but we argue that it is unlikely that ice removal
could have created the collapse features themselves. Scalloped depressions in the Utopia Planitia region of Mars bear a striking resemblance
in size, shape and morphology to thermokarst features found on
Earth31,32; both terrestrial and Martian types form depressions on the
order of metres to tens of metres in depth33,34 (Fig. 4). Thus, those wellaccepted thermokarst features are orders of magnitude smaller than
the collapse features discussed here, whereas the proposed volcanic
10 km
Siloe patera
Elevation (m)
25 km
5 km
Euphrates patera
Figure 3 | The geology of Siloe and Euphrates paterae. MOLA data draped over CTX images show the morphologies of Siloe patera (a; rectangle enlarged in b)
and Euphrates patera (c; rectangle enlarged in d).
3 O C T O B E R 2 0 1 3 | VO L 5 0 2 | N AT U R E | 4 9
©2013 Macmillan Publishers Limited. All rights reserved
A new category of Martian volcanic construct
Taken together, these features constitute a new category of Martian
volcano that can be described as plains-style caldera complexes, of
which Eden patera is the type example. Eden patera is not associated
with a major edifice. Each of the Martian low-slope paterae recognized previously12,13 shows a major edifice related to repeated volcanic
deposition of explosive and effusive products. Thus, Eden patera
seems to be a new class of Martian volcanic feature, formed through
a combination of magma withdraw, subsurface magma migration
(caldera 1) and/or major explosive episodes that would have distributed ash regionally or globally such that they did not accumulate near
the vent (calderas 2 and 3). These geomorphic features are most
analogous to those of a terrestrial supervolcano.
On Earth a supervolcano is defined as a volcano that can produce at
least 1,000 km3 of volcanic materials in an eruption. On Mars it is
generally not possible to link a single volcanic deposit to a particular
eruption event. However, erupted volumes can be constrained from
the volume of void space observed in the caldera itself, if that collapse
ss 2
ss 3
Class 1
Class 2
Class 3
Class 4
Siloe patera
(inner basin)
Rim–floor depth (km)
structures in Arabia Terra are of the same scale and morphology as
terrestrial supervolcanoes35 (Fig. 4). If these proposed volcanic structures
are in fact the result of thermokarst, then they are a new type of thermokarst beyond any that has been definitively recognized previously.
In addition, the large volume of the collapse features is a strong
constraint on the possible origins. If they formed by collapse associated with the removal of subsurface ice, it necessarily implies that a
significant volume of ice was removed from each location, quickly
enough to cause the high strain rates required for faulting. However,
none of the features is associated with outflow channels, which are
typically cited as evidence for the rapid removal of surface or nearsurface ice. Furthermore, the amount of ice that could have existed
below such depressions can be constrained from quantitative models
of Martian subsurface porosity36. For example, if Eden patera had
been formed by the removal of subsurface ice, it would have been
necessary for all of the available void space to be entirely saturated
with ice to a depth of ,10 km (see Supplementary Information). We
therefore conclude that, although ice and thermokarst processes could
have been involved in the modification of the collapse features, it is
unlikely to explain the origin of the collapse or the presence of the
large depressions.
It is also possible that the depressions in Arabia Terra represent
degraded impact craters. However, none of the features described above
contain evidence for impact geology, such as the presence of ejecta,
raised crater rims, central peaks or inverted stratigraphy. It is possible
that erosion has removed such evidence, but the proposed calderas are
found adjacent to ancient impact craters of similar size (and possibly
similar age) that have preserved clear evidence for impact origins (Fig. 5).
Furthermore, impact craters that have been degraded by erosion37 have
much lower depth-to-diameter ratios than those measured in the proposed calderas (Fig. 5). The relations between depth and diameter
among the calderas are only consistent with depth-to-diameter ratios
of impact craters that are only partly modified; such craters have preserved at least some critical aspects of impact geology.
Siloe patera
(whole basin)
Euphrates patera
Eden patera
Martian scalloped terrains
Terrestrial thermokarst features
Terrestrial supervolcanoes
Length of minor axis (km)
Candidate Martian supervolcanoes
(this study: Eden patera, Euphrates
patera, Oxus patera, Siloe patera)
Crater diameter (km)
Siloe patera
(no rim, no ejecta)
Elevation (m)
Length of major axis (km)
Figure 4 | Comparison of thermokarst features, terrestrial supervolcanoes
and the putative supervolcanoes on Mars. A plot of the dimensions of typical
terrestrial and Martian thermokarst features shows that they have roughly
similar sizes32,34. The proposed calderas in Arabia …
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