Worlds in Eruption

Again the answer lies with plate tectonics. On Earth, as a crustal plate moves over a hot spot, the rising plume ‘burns’ a series of holes in the crust - each hole being marked by a volcano. A good analogy is moving a piece of paper over a candle. The candle’s heat will burn a path as the paper moves above. The Hawaiian Islands have formed as a chain of volcanoes where the Pacific Plate is moving over a hot spot. Because the crust on Mars is stationary, and has been some time, up-welling plumes simply continued to burn a larger hole in the same area of crust, supplying the resulting volcanoes with more magma that erupted in lava flows. This built up much larger volcanoes with a longer history of activity, including the vast bulk of Olympus Mons.

NASA The dark maria on the surface of the moon are not 'seas' of water as once thought but of volcanic basalt flows.

Dead worlds, fossilised eruptions

While there is some debate as to whether Venus and Mars are volcanically active today, the general consensus is that the Moon and Mercury are volcanically dead. But this wasn’t always so.

Just glance at the Moon, and you can’t fail to notice the large blue-grey patches on its surface. These patches were once considered to be oceans of water, hence the name ‘maria’ (Latin for ‘seas’). We know differently today. In truth they are some of the Solar System’s largest expanses of basalt, the most common volcanic rock. The lunar basalts were erupted thousands of millions of years ago, at temperatures as great as 1400 C. The very high temperatures meant the lavas were as fluid as engine oil, allowing the flows to cover vast distances.

Mercury also shows evidence for past volcanism, but you’ll need to look a lot harder to find it. Masked by craters that pockmark its surface, Mercury hides lava plains similar to the lunar maria. The craters themselves actually give some indirect evidence for past volcanic activity on Mercury. The planet lacks craters less than 50 km in diameter, and the simplest interpretation is that lavas buried them thousands of millions of years ago, when Mercury was a much fierier world.

How long ago did volcanism stop on the Moon and Mercury - and why? Samples of lunar lava returned by the Apollo astronauts have been dated at 3200 million years or older - far more ancient than most rocks on the Earth. The cratering record for both worlds confirms that most volcanic activity ended over 3000 million years ago.