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At its closest Mars is ~55 million kilometres from Earth. The main research into Mars has been propelled by the fascination of finding possible life on Mars, but the geological history and volcanic evolution of Mars could have had a massive influence on the planets habitability. So before we look for life we need to understand whether the Martian surface has ever actually been habitable, and to work this out we need to study the volcanoes! In this article we will take a look at some of the different volcanic features present on our neighbouring planet and the types of eruptions that have been identified, and relate these to volcanic processes here on earth.
The structure of Mars is thought to be similar to that of the Earth; there is a solid outer crust beneath which may lie a ‘rocky, partially molten mantle’. There is evidence that tectonic processes might have allowed plate movement and heat transfer from within the planet in the past, but today the crust of Mars is one solid mass with no active tectonic activity.
Olympus Mons is the tallest volcano in the solar system, rising to about 21km above the surrounding plains, and is around three times the height of Mount Everest . This giant mass takes the form of a large shield volcano, created by successive extrusions of basaltic lava, with a large central complex of overlapping calderas at its summit. It is thought that the magma that supplied this volcano comes from a hot spot or plume, like the magma that supplies the Hawaiian Emperor Seamount chain in the Pacific Ocean . Unlike Hawaii there are no plate tectonics active at Olympus Mons – or anywhere on Mars – so the hotspot does not appear to move and only supplies one area, allowing a great mass of lava to build up and form massive a shield like structures of lava. To support this idea, it has been calculated that the total volume of Olympus Mons is the same as the total volume of basalt contained in the Hawaiian Emperor Seamount chain . If plate tectonics were not active on earth it is quite likely that a large Olympus Mons-like feature might have been produced in the middle of the Pacific Ocean at Hawaii.
There’s no shortage of volcanic features on Mars especially seeing as ~70% of the Martian surface has been ‘resurfaced by basaltic volcanism’ . Some cone and ring like features that have previously been attributed to mud volcanoes have now been suggested as having a magmatic/volcanic origin . These features have been identified as resembling the tuff cones and tuff rings that are seen on earth, formed by hydrovolcanic/phreatomagmatic activity when magma or hot volcanic material comes into contact with water. This is a possible explanation for these structures as it is well known that water was once present on Mars. If these features are hydro or phreatomagmatic then this is further evidence for the presence of ancient water on Mars.
The recent discovery of a ‘supervolcano’ on Mars was published in Nature earlier this year. The discovery came after investigations into an impact crater in the Arabia Terra region of Mars revealed that the crater did not have typical impact features, such as a missing rim, impact ejecta and splashes of melted rock . Volcanic processes were then considered for the formation of this large crater and it was soon realised that the crater could actually be the caldera of a large supervolcano. Using what is known of calderas on earth several volcanic features were identified, for example faults related to ground collapse and ledges of rock that could be associated with the slow draining of a lava lake – like those seen at Nyiragongo, Africa. There were also lava and pyroclastic deposits spread throughout Arabia Terra which couldn’t be linked to any volcano , so this provided an origin for this material, strengthening the argument that this was once an active volcano and not an impact crater. But this type of volcano has not been seen on Mars before, so Eden Patera is considered to be a new type of volcano ! A supervolcano!
On Earth a supervolcano is a volcano which produces more than 1000 km3 of erupted material in one large eruption. These eruptions are usually associated with the formation of a caldera which may form due to the ground above an emptying magma chamber subsiding or collapsing into the void beneath it once large quantities of magma have been ejected during an eruption. No supervolcanic eruptions have been witnessed on Mars but by looking at the size of the caldera at Eden Patera it has been calculated that at least as much as 4600-7200 km3 of material may have been erupted ! In comparison one of the largest caldera forming eruptions on Earth was that of Toba 74,000 years ago, which erupted 2800 km3 of magma .Although, the caldera of Eden Patera may have formed from several eruptions rather than one large eruption that dwarfs all volcanism seen on Earth.
Such a great eruption would have released large amounts of volcanic gases such as volcanogenic sulphur which would have had profound effects on the Martian surface and planetary climate, possibly causing the barren harsh landscape that we recognise on Mars today. The sulphur could have caused acidification of the land and dramatic fluctuations in climate from substantial cooling to dramatic warming . Different atmospheric pressures in the past may have allowed volcanic plumes to reach higher into the atmosphere therefore affecting the climate on a wider and much more prolonged time scale.
So how did this volcano form and why is it so different from all of the other basaltic volcanoes on Mars?
The Arabia Terra region of Mars may have formed by extension of the crust allowing areas beneath the crust to rise to shallower levels and cause ‘rapid ascent of magma’ . This magma would have been basaltic in composition like that elsewhere on Mars so something else is needed to make the magma go explosive! Unlike on Earth the magma doesn’t have to be more silicic (more viscous and sticky) and it would not have to have a higher amount of dissolved volatiles. Instead the effects of Mars’ lower gravity and a different atmospheric pressure may have allowed bubbles to form in the rising magma and grow to greater volumes allowing increased fragmentation of the magma, leading to a more explosive eruption .
On Earth the different styles of eruptions are mainly controlled by the silica or volatile content of a magma, we have much more evolved magmas and a wider range of magma types compared to Mars, which only really produces basalt. Though what we can learn from studying Martian volcanoes could help us to understand the processes and evolution of our own terrestrial volcanoes, many of which have been studied in very little detail. The size of eruptions on Mars can help us to understand the implications of larger eruptions on Earth – eruptions that have not been witnessed or possibly not even identified yet in the deposits that record Earth’s history. Much of the Martian surface has not been studied in great detail, so there may be many more volcanoes just waiting to be found – watch this space!
 Isherwood, R.J., Jozwiak, L.M., Jansen, & Andrews-Hanna, J.C. 2013. The volcanic history of Olympus Mons from paleo-topography and flexural modelling.Earth and planetary Science Letters, 363, 88-96.
 Michalski, J.R. & Bleacher, J.E. 2013. Supervolcanoes within an ancient volcanic province in Arabia Terra, Mars.Nature, 502, 47-53.
 Broz, P. &Hauber, E. 2013. Hydrovolcanic tuff rings and cones as indicators of phreatomagmatic explosive eruptions on Mars.Journal of geophysical research: Planets, 118, 1656-1675.
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