Mars – Land of Megavolcanoes

Mars – Land of Megavolcanoes

Ceraunius and Uranius Tholus, considered 'intermediate-sized' Martian volcanoes - the summit caldera of Ceraunius Tholus (bottom) is 25km diameter! (Source: NASA, MGS)

At the mention of Martian volcanism, chances are the first thing you think of is Olympus Mons. It’s a monster of a volcano: both the highest (22km) and the most long-lived we know of in the Solar System. And though it’s an extreme, it’s not alone: there are four other shields dubbed ‘Mons’ on Mars: Ascraeus, Pavonis and Arsia Mons nearby on the Tharsis rise and Elysium Mons over 4000km to the west. Add to that a slew of intermediate-sized volcanoes (the large-calderaed patarae and the steep-sided tholi) and that’s a lot of volcanism, even if it’s not as widespread as on Venus.

Most of the plains around these edifices are also volcanic: they formed when huge floods of very fluid lava erupted from fissures in the ground. The size of these floods is partly a product of Martian lava geochemistry: they have a higher iron content than basalts on Earth and that breaks up the polymerisation of the lava and makes it flow more easily.  It’s also because of the lower gravity of the planet, which allows the dykes feeding the fissures to be wider and so increases the flux to the surface.  It shares both these characteristics with the Moon and you can see by just looking up at night how large the dark mare flows are there.

Many of the lava plains are very ancient: it’s thought that volcanism on Mars started out delocalised and plains-forming and slowly concentrated into regions like Tharsis, building up the huge central volcanoes.  This is due to a reduction in the energy that needed to be dissipated: the planet cooled faster than Earth because it’s smaller and has a higher surface area:volume ratio.  Over time volcanism has settled down in a way it couldn’t on Venus.

The most exciting recent findings regard the longevity of this activity.  We used to believe volcanism died out hundreds of million years ago, but discovery by discovery we’ve brought that down to just a few million years. Geologically speaking, that’s yesterday: there’s a strong chance the planet’s still volcanically active and may one day show us its fiery side.

Olympus Mons – a dormant giant?

The central part of Olympus Mons with its nested calderas. The volcano actually extends well beyond the scarp you see here - it totals around 600km diameter with flows from it out to 1000km. (Source: NASA, Viking)

The earliest activity to build up Olympus Mons was at the beginning of the Hesperian period, perhaps as long ago as 3800Ma.  Over the millenia, through many phases of eruption, the volcano built up to its present mammoth size. The periodicity of that activity can been seen in its summit, where five different calderas nest within each other. These calderas are likely to show periods of intense eruptive activity: lava escaped from fissures on the flanks of the volcano and the summit sagged down due to the removal of the supporting material. By dating these calderas, we can see that those phases were about 20 million years apart.1 The most recent collapse was around 140Ma2 or 100Ma3, so was that the final outburst of this beast?

Probably not. Although most of the lava deposits on the volcano’s flanks are 170Ma or older4, there are some very young flows: the youngest yet found has a cratering age of 2.4Ma.5 It’s difficult to date such young surfaces using impact craters because there’s a large error margin when you just have a small set of craters to count. All the same, the very sparsity of craters makes it clear: these are very young flows.

A few million years would still be pretty long for a volcano to be dormant on Earth, but as we’ve seen, on Olympus Mons it’s just a short doze.

Cerberus Plains – no sleeping dog

Outflows of lava and/or water forming the Cerberus Plains, Mars (Source: NASA, via Google Mars)

The Cerberus plains are close to my heart: I’ve mapped part of them, finding recent outpourings from a series of fissures. But don’t take my word for it: over the past decade it has been increasingly accepted that this is the youngest surface on Mars and that it formed by outflows of water and lava. Though the exact importance of these two processes is debated, most work suggests that lava flows were the finishing touch.

So how recent are we talking? Again, we come up against the difficulty of dating very young surfaces. However, it looks like the youngest lava flows draping the ends of the Athabasca Valles aqueous outflow channel are 10Ma6, and dates have been given for parts of that outflow channel which are as young as 2-8Ma.7

These aren’t issuing from such an established system of magmatic plumbing as Olympus Mons has, so we can’t be so sure this region is still active. But it does reveal that Mars’ internal energy was still great enough recently for molten magma to form and for it to escape to the surface along  planes of crustal weakness. There may be life in an old dog yet.

  1. Neukum et al. (2004) Nature: 432, pp.971-979 []
  2. Robbins et al. (2011) Icarus: 211, pp.1179–1203 []
  3. Neukum et al. (2004) Nature: 432, pp.971-979 []
  4. Werner (2009) Icarus: 201, pp. 44-68 []
  5. Neukum et al. (2004) Nature: 432, pp.971-979 []
  6. Hartmann & Bermann (2000) J. Geophys. Res.:105, pp. 15,011–15,025. []
  7. Burr et al. 2002: Icarus, 159, pp.53-73. []