Planet Earth Online

Tharsis Rise could be solar system's biggest volcano

29 November 2010, by Tom Marshall

Published in a Geological Society of America Special Paper, the idea is that Tharsis Rise could be what geologists call a spreading volcano – like a much, much bigger version of Sicily's Mount Etna, itself by far the largest volcano in Europe and among the most active on Earth.

The largest volcano found so far is Olympus Mons, a Martian peak that rises 27km high – three times taller than Everest. But Tharsis Rise dwarfs even this; it stretches 7000km across Mars' surface, although it isn't as tall – its central section towers a mere 10km over the lowlands to its north.

Professor Andrea Borgia of Rutgers University in New Jersey and Dr John Murray of the Open University in the UK created a mathematical model of how 'spreading' volcanoes develop and change shape over time, based on long-term studies of how Etna's form is slowly changing.

Borgia and Murray's analysis of Tharsis Rise suggests that it shares many of Etna's characteristics and proportions, and so may have developed in a similar way.

'This is the first suggestion that something of this size could be regarded as a single volcano,' explains Murray. 'But we have shown there's a continuum between massive structures like Tharsis Rise and comparatively small volcanoes like Etna.'

There are plenty of similarities between the two volcanoes, despite the discrepancy in scale – Etna is around 200 times smaller. Tharsis is surrounded by titanic gouges in the landscape radiating outwards for hundreds of miles, known as fossae. The biggest of these structures, Valles Marineris, averages 4km deep and stretches some 4000km.

On a smaller scale, comparable faults radiate into the landscape around Etna. Both Etna and Tharsis show similar patterns of rifting running through the body of the volcano. And the basic proportions of the two structures are surprisingly alike.

Tharsis Rise is so big that it features several smaller but still huge volcanoes on it – Pavonis Mons, Ascraeus Mons and Arsia Mons. If the whole rise does turn out to be a spreading volcano, these will have to be re-assessed as Tharsis' 'summit craters', like the four craters around the top of Etna, rather than volcanoes in their own right. The smaller volcanoes of the region will likewise have to be reinterpreted as something like the flank vents seen on Etna.

It's even possible that the volcano is still spreading, though this is not certain and more research would be needed to confirm it. This could be the result of volcanic activity, although many researchers think Mars is essentially dead in geological terms, with no volcanism or tectonic activity remaining.

If Tharsis Rise is still spreading, Murray believes this is more likely to be because it continues to slump down and outwards simply through the action of gravity on its huge weight, spreading like a jelly on a plate, than because of continuing volcanism.

Proving the theory could prove tricky, though. Detailed information on the long-term development of Martian geological features is thin on the ground. Murray says it may be possible to find a simple experiment that could confirm or refute the various rival theories of Tharsis' origin – these ideas include hypotheses such as that it was created by some kind of upwelling mantle plume from deep within Mars.

'It's the same with many problems about Mars,' Murray says. 'There are very wide discrepancies of opinion, because we don't have the basic information, and we can't just go to the specific places needed to resolve a particular question. We need to all agree on a specific test in a specific location that will at least rule out some possibilities.'

Keywords: Geology,

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