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Module 3 - Shaping Planetary Surfaces

Mars volcanism

Volcanism on Mars has played a significant role in shaping the surface of the planet. Large areas of the Mars surface are covered by volcanic rock, including lava flows and pyroclastic deposits.

Relative ages determined by crater counting of volcanic areas imaged by spacecraft indicate that volcanic activity spanned nearly the entire history of the planet. The youngest volcanic surfaces found by the crater counting method may be as young as a few to 10s of millions of years old. This may seem like a long time from a human perspective (humans have only been on Earth for about 200,000 years), but dormant periods between volcanic eruption cycles on Mars are thought to be quite long (up to 100 million years). Martian volcanoes are actually considered to be dormant and may awaken. Humans may yet witness a volcanic eruption on Mars.

Most volcanoes on Mars are shield volcanoes, composed of basaltic lavas and thin tephra airfall deposits, similar to shield volcanoes like Mauna Loa on Earth, only much, MUCH bigger. Olympus Mons, for example is the largest volcano in the Solar System and it stands two and a half times the height of Mount Everest above sea level. The diameter of Olympus Mons is over 600 km and is comparable to the entire Hawaiian island chain, which is composed of many overlapping shield volcanoes. (More on Olympus Mons in a later section…)

Why are volcanoes on Mars so big compared to the ones on Earth? It comes down to Mars never having developed plate tectonics. The Mars crust has always been stationary relative to the Mars mantle. But the heat of the Mars' interior still needs to escape somehow as the planet cools to space. Heat flows outward from the Mars interior at focus points called hotspots (kind of like Hawaii on Earth). All of the volcanic materials associated with the Martian hotspots pile up in one place, building a large volcanic plateau. In contrast, hotspots on Earth create volcanic chains because the crust moves relative to its mantle. Another important difference between the two planets is that there are 20-30 different hotspots on Earth, but Mars only has two: 1) Tharsis (includes Olympus Mons) and 2) Elysium. The small number of hotspots on Mars is thought to be due to its very stable and immobile crust.

You can take a tour of the major Mars volcanoes (set to soothing ambient music) in the video below:

Tharsis

Tharsis is the largest volcanic province on Mars, making up roughly 25% of the planet's surface. The broad region is also referred as the Tharsis bulge. It includes:

Three enormous shield volcanoes (Arsia Mons, Pavonis Mons, and Ascreaus Mons) that are collectively known as the Tharsis Montes. The alignment of the three volcanoes is related to tectonic stresses in the Mars crust.

Olympus Mons located at the western edge of the plateau (see below).
Alba Mons, an immense low-lying shield volcano to the north (described further below)
Numerous smaller volcanoes. These include heavily cratered (old) dome-shaped features (called tholi, or tholus singular) with large calderas. Tholi are interpreted to be the summits of old shield volcanoes that have been buried by great thicknesses of younger lava flows.
Valles Marineris, the largest canyon system in our Solar System.

Olympus Mons

Because Olympus Mons is so exceptional for its size and clear volcanic landforms, it is here described in greater detail as an example of the great Tharsis shield volcanoes. The volcano is nearly 22 km tall and 600 km wide.
The summit has six nested calderas, which are collapse craters that form after a magma chamber is drained by large lava flow eruptions. The Olympus Mons calderas (shown as title image for this page) are 60 km x 80 km across and up to 3.2 km deep. The size of the calderas gives you an idea of the approximate size of the magma chamber, located at several to 10s km depth.

A nearly 8 km tall escarpment or cliff makes up the outer edge of the volcano. This feature is unusual for Martian shield volcanoes. Otherwise, Olympus Mons forms a very broad profile with low angle slopes.

Olympus Mons is constructed of innumerable basaltic lava flows. Many features recognizable on terrestrial basaltic lava flows are also seen on Olympus Mons lavas, including levees and collapse pits that hint at the presence of lava tubes. Lava tubes are subsurface caverns that are thought to form from fast-moving lava flows. As the lava channel cools, a hard crust develops over the molten interior. The flow eventually drains and leaves a void space. Lava tubes are protected from the harsh environment of the Mars outer surface and are potential targets for the search for life.

Alba Mons

Alba Mons, located in the northern Tharsis region is an enormous, low-lying volcano (up to 3000 km across and 6.8 km tall) that straddles the dichotomy boundary separating the northern lowlands and southern highlands of Mars. Alba Mons is so large and broad that it falls well outside the norm of a typical shield volcano and may belong to a class of volcano that is unique to Mars.

Regional normal fault scarps running north-northwest form an incomplete ring around the central edifice of Alba Mons. Lava flows make up a broad apron that extends away from the volcano. Alba Mons lava flows must have been very fluid (low viscosity) in order for them extend so far (100s km) away from their vent over a gentle slope (average 0.5˚).

Located on the opposite side of Mars from the Hellas impact basin, Alba Mons volcanism may be the result of crustal weakening caused by intense seismic waves caused by that impact. Magmatism found on the opposite side of a large impact is called antipodal magmatism.

Elysium

The Elysium volcanic province is smaller than Tharsis and lies several thousand kilometers to the west. It is still pretty damn big: 2000 km in diameter and includes three main shield volcanoes Elysium Mons, Hecates Tholus, and Albor Tholus. These volcanoes are thought to be constructed of both lavas and pyroclastic deposits. In particular, Hecates Tholus is heavily dissected with channels, suggesting that the volcano is composed of easily erodible material such as volcanic ash.

Explosive Volcanism on Mars

The products of explosive volcanic activity have been recognized on the Mars surface, but the low gravity relative to the Earth leads to a much greater dispersal (more than 10 times) of pyroclasts. Here’s why:

Magma fragmentation (break up) is driven by the formation of bubbles, which in turn depends on the confining pressure. A good analogy is popping the cork off of champagne. When the cork is removed, the pressure in the bottle decreases and bubbles can form, expanding the volume of the contents in the bottle, and leading to an “eruption” of champagne.
The confining pressure on a magma chamber is a function of gravity, which on Mars is 38% of the Earth’s gravity. Bubbles in Martian magmas therefore form at greater depths (roughly 3 times greater), which leads to greater eruption speeds and a greater degree of fragmentation, resulting in finer pyroclasts.
Upon eruption, finer pyroclasts (ash) rain out of the eruption cloud farther from the vent than coarser pyroclasts. The rate of airfall also depends on gravity. The result is that fine ash will travel greater distances. Large explosive eruptions on Mars have the potential to produce globally distributed airfall deposits.

An example of a globally distributed deposit on Mars that has been interpreted to be an airfall deposit is the Medusa Fossae Formation (MFF). Medusa Fossae is a soft, easily erodible unit that is interpreted to be relatively young. The deposit is recognizable from orbital Gamma Ray Spectroscopy (GRS) by its enhanced chlorine signal. The estimated total volume is 1.4 million km³ (for reference, the largest explosive volcanic eruption deposit on Earth is 5000 km³). Below is a paper (Kerber et al., 2011) that presents evidence that the volcanic vent for the Medusa Fossae Formation may be Appolineris Patera. Other vent sources including the Tharsis Montes volcanoes Arsia Mons and Pavonis Mons have been also considered. Scroll through the figures of this paper and pay attention to Figures 1 and 6, which shows the distribution of Medusa Fossae and chlorine on the Mars surface.

Martian Meteorites: Samples of volcanic rock

A small, but significant number of the meteorites collected on Earth (132 out of the over 61,000) are thought to originate from Mars. (You can buy a piece of Mars on eBay for 200-400 USD per gram.) Textural features, including impact melt pockets (dark blobs in ) within the Martian meteorites point toward a history of impact at the Martian surface. These meteorites were likely ejected from Mars by large impact(s) and eventually crossed paths with the Earth.
Why are they from Mars? The Martian meteorites have elemental and isotopic compositions that are similar to rocks and atmosphere gases analyzed by spacecraft on Mars. In particular, gasses analyzed in the impact melt pockets in the meteorites have the same concentrations of carbon dioxide, nitrogen, and noble gas isotopes that were measured by the Viking Lander (1976).

The Martian meteorites are igneous rocks and many (the largest group called shergottites) are thought to be lavas. Radiometric ages measured in labs on Earth of the Martian meteorites are as young as ~150 million years old. These are remarkably young ages considering that the vast majority of meteorites are 4.55 billion years old.

While we have a lot of information gleaned form laboratory analysis of the Martian meteorites, we really do not know where on the Mars surface they originate. Likely source candidates include Olympus Mons and the Tharsis Montes volcanoes because they are young, at high elevation (making ejection more likely), and have compositional characteristic (low potassium) that may link them to the meteorites.

A really good summary of Martian meteorites is found at this link.