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Lecture Notes for the Midterm
First order of relief: Features covering at least a quarter of the
planetary surface
- Tharsis
- See Viewgraphs: "First order dichotomy: Tharsis"
- The Tharsis "lump," with its associated five monster shield volcanoes (and another seven
significant tholi or pateræ), sprawls across about a quarter of the surface of
Mars.
- Huge volcanic rise along the equator at roughly 250° (~110° W),
about 5,000 - 8,000 km across
- Nearly 10 km thick, not counting the volcanoes on and near it
- Would cover most of the United States and portions of Canada and Mexico
- Supports the Tharsis Montes (Arsia, Pavonis, Ascraeus) running along its
central spine, with Olympus Mons and Alba Mons just off the main rise
- There are several other volcanic edifices on Tharsis:
- Jovis Tholus east of Olympus, south-southwest of Alba
- Uranius Tholus southeast of Alba, nearly in a straight line to the northeast of
Tharsis Montes
- Uranius Patera east of Uranius Tholus and directly in a straight line northeast of Tharsis Montes
- Ceraunius Tholus in that same group on Tharsis Montes' northeast trendline, south of Uranius
Tholus and southwest of Uranius Patera
- Tharsis Tholus is on the east side of the Tharsis Rise, north of the west end of Valles
Marineris and west of Kasei Valles
- Biblis Patera, between Olympus Mons and Arsia Mons, west of Pavonis Mons
- Ulysses Patera in the same general location, east-northeast of Biblis
- There are massive lava flows that almost look like lunar maria that sprawl
outward from the three Tharsis Montes, especially to the south and southeast:
- They cover Dædalia Planum to the south of Arsia Mons
- They run over similar, older, fractured flows centered on Alba Mons
- To the east, they extend out into Lunæ Planum to Maja Vallis (border with Xanthe
Terra), and seem gouged out by Kasei Valles and then invade the western edge of Kasei
- To the southeast, they seem to comprise the Thaumasia block surface
- Much of this sprawling flow is pretty young (Amazonian), judging by the sparsity of cratering
- There are these weird, rough-textured features ringing Olympus Mons, called aureole deposits
- These are most obvious and spatially extensive to the north and west
- There's some sign that they may have extended to the east and southeast, except they
seem buried there, at least partially, by lavas associated with Arsia Mons and perhaps
the smaller volcanoes between Olympus and Arsia
- These have been explained as volcanic products, such as massive pyroclastic flows or badly
eroded lava flows, possibly older than the Olympus volcano itself
- Alternatively, they've been interpreted as gigantic mass movements, landslides on steroids, which might explain that odd escarpment surrounding the base of Olympus.
- Tharsis is such a gravitational anomaly that it would affect Mars'
rotation, perhaps determining the axis of rotation itself through the
centrifugal force that creates planets' oblate ellipsoid shape: It is possible that Tharsis functions gravitationally sort of the way our own Moon does, stabilizing the obliquity of Mars' axis of rotation (though not as efficiently as the Moon does for us)
- There is a kind of topographic depression that surrounds Tharsis, again
possibly a geoid compensation for the vast weight of the Tharsis "lump," like
a dip in the bed when you lie on it
- The Chryse Trough running along the east side of Tharsis, including
Argyre Planitia, a series of connected craters to its north and east, the
Margaritifer Terra depression with its distinctive channels and chaos terrain
- The Chryse Planitia embayment of the Northern Lowlands, perhaps itself a
buried crater basin
- Acidalia Planitia to the north and northeast of Tharsis
- Parts of the Vastitas Borealis Basin
- Arcadia Planitia
- Amazonis Planitia
- the local depression is very attenuated or buried by newer material to
the southwest and south of Tharsis' Dædalia Planum and into northern
Terra Sirenum and Aonia Terra
- This depression affected the path of surface water/fluid: Noachian era
dendritic channel networks flow in a direction consistent with today's Tharsis
Rise, suggesting that the rise itself dates back to at least Late Noachian times.
- The Tharsis bulge is comprised of basalts from eruptions
- Valles Marineris' floor is covered with sheets of lava from the Tharsis
activities, as is much of southern Kasei Valles
- The Tharsis magmas are so massive that Roger Phillips estimated that the
amount of carbon dioxide and water that would outgas during Tharsis' early
eruption history back in Noachian times would have been enough to increase the
density of the Martian atmosphere to 1.5 bars and create a global ocean
averaging 120 m above the geoid!
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Signs of stresses and deformations from whatever caused the uplift of Tharsis:
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Radial grabens and fractures (including in a sense Valles Marineris), most of
which converge roughly around 4° S and 253° E (107° W), though
there's some evidence that the central focus of tension has shifted around the
Tharsis vicinity through time by Anderson et al., who mapped these
tensional features and isolated graben and tension crack systems of different
ages.
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Compressional ridges ringing it: Circumferential wrinkle ridges, which are
found in volcanic plains and consist of long arches many kilometers long,
about 100-200 m high and a few kilometers across, which may be themselves
wrinkled. They are believed to represent deformations of the surface by thrust
faults, which are features showing compressional stress and strain. They are
especially common in the eastern part of Solis Planus on the Syria-Thaumasia
block on Tharsis' east side, Lunæ Planum and Xanthe Terra east of
Tharsis and north of Valles Marineris. There are somewhat similar but sparser
features west of Tharsis, too.
Some explanations for the Tharsis rise:
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Dynamic support by an underlying great plume of mantle material, which
depressurizes, expands, and spreads laterally as it approaches the surface.
Can such a plume remain that stable for over 4 billion years?
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Maybe plate tectonics, e.g., a subducting plate originating in the
northern
hemisphere, which can produce uplift, extension, and faulting in the plate
above (much as the Farallon Plate uplifted the western United States) - but 4
billion years is a long time for such a process to be persistent and stable,
and we've already seen that plate tectonics is generally not viewed as
convincing to most Mars researchers.
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Some kind of mantle anomaly in terms of temperatures or chemical composition,
but, again, 4+ billion years is a long time for that not to have reached
equilibrium
- And, while the bulk of Tharsis volcanism may go back to later Noachian and earlier Hesperian times, perhaps in the 4.1 to 3.8 Ga area, there is evidence of more recent volcanism:
- Much of the lava on the Tharsis structure is Amazonian, judging from sparse cratering
- Some of the specific lava flows and caldera lavas may be under 20 million years old (even under 2 millions years around Olympus)
- But what could feed this kind of persistent vulcanism?
- On Earth, plate tectonics generates magma through heating and
decompression, but Mars has had little to no plate tectonics.
- But, then, again, the three Tharsis Montes are aligned along a single
1,500 km long line, and Arsia looks older and more eroded than Pavonis, which
looks older than Ascraeus -- perhaps, like Hawai'i, the bulge is moving over a
stationary hotspot?
- Maybe, though plate tectonics never really advanced, a volcanic hot spot
did develop as one of perhaps two major ventings of magma created in the
mantle to channel lava onto the surface, and it just kept on doing so,
building this huge mound of lava, so heavy it depressed the surrounding
countryside.
- C. Reese and V. Solomatov are not convinced by the core/mantle magma
plume and hotspot theory and think that maybe Tharsis is the result of locally
focussed heat energy from comet/asteroid impact.
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Their work is echoed in Linda Elkins-Tanton's work on massive impacts on Earth
excavating/vaporizing chunks of crust enough to cause the surrounding
lithosphere to rebound through isostacy, creating a bulge and magma as mantle
materials experience a reduction in pressure - the magma may be so copious as
to produce flood basalts.
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Jonathan Hagstrum criticizes the narrow plume model of hot spot vulcanism by
pointing to antipodal pairs of hotspots on Earth and the possibility that
large impacts' seismic energy may be focussed in the antipodal
æsthenosphere,
resulting in heating, melting, rifting, flood basalts, and persistent hot spot
vulcanism. There has been speculation that things like this have happened on
Earth:
- Chixulub impact of ~65.5 Mya at the Cretaceous-Tertiary boundary -- and
the Deccan Traps flood basalts of about the same time -- antipodal
- Bedout High off the northwestern shore of Australia and the Siberian
Traps of ~250 Mya at the Permian-Triassic boundary, which was another massive
extinction event, marking the end of the Palæzoic and the beginning of
the Mesozoic (Age of Dinosaurs).
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There has been some speculation that Hellas is the guilty party, nearly
antipodal to Tharsis (at least longitudinally but somewhat off latitudinally) and
appropriately huge.
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An interesting argument has recently been put forward by Andrea Borgia and
John B. Murray in a 2010 special paper of the Geological Society of America
that the whole Tharsis Rise is itself one ginormous volcano!
- They point out a type of volcano on Earth that might be an appropriate
analogy: spreading volcanoes, which are volcanoes built on a weak rock layer.
As more lava is added to it, the support weakens and the lava can only spread
outward. Such volcanoes tend to develop smaller parasitic cones when lava
escapes the main vent and comes out laterally.
- Such volcanoes also tend to feature a rift zone across the top, a weak rift that can account for the collection of volcanoes running across Tharsis' middle.
- They also include peripheral compression belt and a series of grabens and faults that link the central rift to the compressional periphery. Kind of sounds like Tharsis!
- Mt. Etna in Sicily is of this type, and spreading volcanoes share traits
that scale up readily, so they think Mt. Etna might be an appropriate model
for the much vaster (200 times larger) Tharsis.
- In this view, the largest volcano in the solar system, Olympus Mons,
might just be a parasitic cone on Tharsis, along with Alba Mons and Arsia,
Pavonis, and Ascræus Montes and the other volcanos on Tharsis.
- Of course, this re-inmaging of Tharsis still doesn't help us understand how such a volcanic structure could persist for so long in one place.
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![[ orthographic image of Mars on a black background ]](https://home.csulb.edu/~rodrigue/mars/marsblackbg.jpg)
![[ Olympus Mons seen at oblique angle that gives a 3-d sense ]](https://home.csulb.edu/~rodrigue/mars/olympusmons3d.jpg)
![[ Mars explorer ]](https://home.csulb.edu/~rodrigue/mars/marsexplorerpainting.jpg)
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![[ image of Mars ]](https://home.csulb.edu/~rodrigue/mars/marsatlas.gif)