The planet Mars formed about 4.5 billion years ago, and is thought
to have been highly volcanically active in its early history. However due to
its smaller size it is thought to have cooled much more rapidly than the Earth,
with volcanism ceasing in most areas by the end of the Noarchian Epoch (about
3.7 billion years ago), with activity persisting in areas such as the Hesperian
ridged plains and Tyrrhenus and Hadriacus Montes till about 3.5 billion years
ago. The youngest lavas in the two main Martian volcanic provinces, Tharsis and
Elysium are thought to be low-viscosity basaltic lavas, i.e. lavas that reached
the surface in a hot, runny state then cooled rapidly on exposure to the
atmosphere. More viscous, evolved lavas (lavas that have risen to the surface
slowly, cooling and losing minerals which crystalize at high temperatures) are
thought to be rare on Mars.
In a paper published in the journal Earth and Planetary ScienceLetters on 1 April 2015, Petr Brož of the Institute of Geophysics of the Academy of Sciences of the Czech Republic and the Institute of Petrology and StructuralGeology at Charles University in Prague, Ernst Hauber of the Institute of Planetary Research, Thomas Platz of the Institute of Geological Sciences at the
Freie Universität Berlin and the Planetary Science Institute and Matt Balme also
of the Planetary Science Institute, and of the Open University, describe a
series of small-scale volcanic edifices in the Terra Sirenum region of the
Martian Southern Highlands, which appear to have been formed by the action of
viscous, highly evolved lavas during the mid-Amazonian Epoch, i.e. less than a
billion years ago.
The Terra Sirenum is a highland region within the proposed former
Eridania paleolake, crossed by east-west trending radial graben-systems
(longitudinal depressions caused by stretching and thinning of the Martian
crust) which may be associated with unexposed volcanic dykes (intrusive
volcanic lavas running forming horizontal channels), derived from Arsia Mons
3700 km away, as well a series of wrinkle-ridges interpreted as
fault-propagation folds associated with deformation caused by contraction of
the crust.
Regional map of part of the southern hemisphere on
Mars. The cyan colordelineates the extent of the proposed Eridania Lake
based on the 1100m contour. Position of investigated area is marked by dashed
box and clearly the area lies inside the proposed borders of the former
lake. Brož et al. (2015).
The putative volcanic structures lie within an unnamed depression
measuring approximately 150 km by 30 km, with all bar one of these within a
further depression located inside this. The volcanic structures comprise three
volcanic domes (A, B & C), all located within the inner depression, with A
and B located on different sinuous wrinkle-ridges, which rise tens of meters
above the surrounding plains, and two volcanic cones (T1 &T2), with Cone T1
sitting outside the inner depression on the rim of an ancient, highly eroded
impact crater, which is roughly 110 km in diameter, and Cone T2 within the
inner depression. In addition there are four flows of apparent volcanic
material associated with these structures (Flows 1-4). Flow 1 may originate
from Cone T1, though this area is also intersected with other flows, thought to
be related to a 6 km diameter impact crater rather than any volcanic structure,
making its interpretation complex; Flow 2 is elongate and appears to originate
from Dome A, Flow 3 is roughly circular and surrounds Dome B and Dome C, though
its precise origin is unclear, and Flow 4 is again elongate and appears to
originate from Cone T2.
THEMIS-IR daytime (a), nighttime (b) and
interpretational map of the study area (c). The thermal contrast between the
two upper images (a, b) suggests the presence of flow structures associated with
cones and domes. Note the wrinkle ridges crossing the basin. The cross-section
shows the stratigraphic relations of the investigated edifices with underlying
units. Brož et al. (2015).
Cone T1 is about 3 km in diameter and about 230 m in height. It is
breached to the south, giving rise to a flow apron 8.5 km wide and 100-130 m
thick, which partially covers the lower flanks of the cone. Two small impact
craters, 1.3 and 0.8 km in diameter, are located on this flow apron.
(a)
Cone T1 with associated flow apron. (c) Detail of Cone
T1 central vent from flow aprons material erupted to the surface. Brož et al. (2015).
Cone T2 is about 3 km in diameter and 290 m in height, and breached
to the east, giving rise to a flow apron measuring 12.5 km from north to south,
with four distinct lobes. The margins of these lobes appear to overlap,
suggesting that the apron was formed by multiple eruptive episodes. The source
of Flow 4 lies beneath these aprons; Flow 4 appears to originate from Cone T2
and flows roughly 50 km to the southwest, measuring 1-8 km wide and 30-50 m
thick; it appears to have originally flowed to the west then been deflected
southward when it encountered a wrinkle-ridge.
(b) Cone T2 with associated flow apron. (d) Detail of
Cone T1 central vent from flow aprons material erupted to the surface. (e) Edge
of overlapping flow aprons. Brožet al. (2015).
Dome A is a 2.5 km diameter mound located on top of a wrinkle ridge.
Flow 2, which is about 5 km long and about 300 m wide appears to originate from
this dome and spread to the southeast.
Image of Dome A with marked Mars Orbiter Laser Altimeter Precision Experiment Data Records (MOLA PEDRs) and associated
topographic profiles. Part of the Dome A seems to collapse and propagate in
eastern direction. Brož et al. (2015).
Dome B is approximately 5 km by 7 km and 390 m in height, being
highest in the north. It is located on top of a wrinkle-ridge, and appears to
extend some way to the south along the crest of this ridge. It is irregular in
shape, with four deeply incised valleys dividing the dome into four portions.
It is surrounded by flow features, some of which are shared with Dome C,
superimposed upon surrounding terrain which appears to be a single ancient unit
with a dense covering of impact craters. The western margin of the flow apron
surrounding Dome B has a 3 km impact crater, largely infilled with debris;
several smaller impact craters are also located on this apron, and on Dome B
itself.
Image of Dome B with close up details. (a) Detail of
Dome B and surrounding flow and marked position of HiRISE image ESP_033977_1385
with marked positions of MOLA PEDRs and associated topographic profiles. (b)
Detail of HiRISE image showing the contact between the northwestern edge of the
dome and the underlying unit. Large boulders forming the dome itself and
aeolian deposits at the top of the dome together with modification by gully
activities are clearly visible. (c) Detail of the bedrock on which Dome B is
superposed as exposed by an ∼80mhigh scarp. Pristine fracture morphologies suggest ongoing scarp
erosion. Note that the talus is mainly formed by fine-grained material and
small amounts of boulders less than ∼6metres large (marked by gray arrows), larger blocks are missing.
The tensile fractures parallel to the scarp in the capped unit (marked by white
arrows) are similar to fractures associated with rotational block-fall
landslides known from Earth. (d) Detail of HiRISE image showing another part of
the scarp. Again, no large blocks of fallen rocks are visible and the talus is
composed mainly of finer particles and smaller boulders (marked by gray
arrows). Two white arrows indicate small grooves, which might have formed by
ongoing aeolian erosion. These grooves show that the exposed material is
susceptible to erosion. Brožet al. (2015).
Dome C is roughly 3.5 km by 6 km and 530 m in height, again being
higher closer to its northern margin.
Image of Dome C and surrounding flow. (a) The dome
edifice is clearly surrounded by a flow structure that has steep edges, as
demonstrated by the shadows on the southern margin. White arrow marks scarp on
the edge of Flow 3. Position of MOLA PEDRs marked by lines and HiRISE image
ESP_026474_1385 marked by dashed rectangle. (b) Detail of HiRISE image covering
part of dome flanks (left part of image b) and flow structure (right part).
Note large boulders forming the dome itself and aeolian deposits forming
Transverse Aeolian Ridges (marked by white arrow). (c) The edge of the flow
structure on the border with surrounding older, flat layer. The margin of the
internal flow is formed by large-scale boulders that are partly covered by
aeolian material (marked by white arrow). The older unit contains small
Transverse Aeolian Ridges. Brož et al. (2015).
The ages of deposits on other planets are usually worked out by
measuring the density of impact craters on those deposits, since impacts are
thought to have occurred at a steady rate for much of the history of the Solar
System. However none of the volcanic deposits examined by Brož et al. were extensive enough to use of
this method. Instead the ages of the deposits were worked out by their
stratigraphic relationships with other deposits for which age estimates were
available; this is possible because a rock formation must be younger than a bed
lying beneath it and older than a bed laid down on top of it, enabling
geologists to estimate the age of geological strata in the absence of direct
data.
Using this method it was determined that the flow apron associated
with Cone T1 is approximately 600-800 million years old, while Flow 1 (also
associated with Cone T1) is approximately 560-760 million years old. The flow
apron around Cone T2 is estimated to be 370-570 million years old, while Flow
4, which originates beneath this flow, is 420-520 million years old. It was not
possible to obtain dates for the dome structure, but these are thought to share
a common origin with the cone structures, are therefore thought to be of
similar ages.
Absolute model ages of (a) the crater’s ejecta and
Flow 1, and (b) Cone T2, and Flow 4. (a) The cumulative crater size-frequency
curves indicate an absolute model age 660 ± 100 Ma for Flow 1 and 700 ± 100 Ma for
the crater’s ejecta. (b) The cumulative crater size-frequency curves indicate
an absolute model age of 470 ± 100 Ma for Cone T2 and 470 ± 50 Ma for Flow 4. Note
that for Flow 4 a resurfacing correction was applied to exclude larger craters,
which underlie, and therefore predate Flow 4. Note the panels above the
cumulative crater size-frequency plots represent the randomness analyses. Brož et al. (2015).
A spectrographical analysis of the mineral composition of exposed
boulders on Domes D and C and Flow 3 suggests that these have compositions with
low olivine abundances and moderate levels of pyroxene. Such mineral
compositions are typical for terrestrial lava domes, and are associated with
mature lavas; lavas that have been cooling for some time close to the surface
before being extruded and have a distinctive mineralogical composition because
of this (on smaller, chillier Mars such cooling may have occurred deeper within
the planet). This is quite different from previous small volcanic structures
recorded on Mars, such as scoria cones and tuff rings, which on Earth are
typically formed by the eruption of younger, less viscous lavas. The margins of
the flows are also quite steep, in places up to 20°, which is also indicative
of more viscous mature lavas, and quite different from previously observed
basalt flows in the Tharsis or Elysium volcanic provinces, which show very
gentle flank slopes.
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