VOLCANOES IN THE SOLAR SYSTEM
Volcanoes can be found throughout the solar system. Some of these volcanoes would look familiar, if only we could visit them. Others have no parallel on Earth. Mercury, Mars, Venus and the Moon are all made of the same materials as Earth, predominantly silicon, iron and magnesium. The melting that occurs within these planets makes lavas like basalt. Further out in the solar system, the planets and their moons are made of frozen ices of water, ammonia or even nitrogen. Many of these bodies may well have melted in the past. The lavas on these are like icy cold slurries, and their eruptions are like spouting geysers rather than fountains of fire.
The Moon is now cold and quiet. Because it is so small, it lost its heat very quickly, and its volcanoes were only active in the distant past when it was still young and hot. Like Earth, most of the melts on the moon were types of basalt. The Moon has much lower gravity than Earth and no atmosphere, so its eruptions would have been spectacular. As the magma rose towards the surface, bubbles of gas would have started to form, as on Earth, but even the tiniest bubbles would have expanded to fill a huge space because of the low pressure at the surface. Even the smallest amount of gas in the melt would have caused a violent eruption. All lava eruptions on the moon would have started as fire-fountains. The lower gravity of the moon would have allowed these fountains to reach a much greater size than on Earth. The dark blotches that we can see on the face of the moon are the ‘lunar maria’. These are seas of lava, that filled up from spraying vents and fissures. During vigorous fire-fountaining, the droplets of lava would still have been hot enough to flow once they had landed, and the sprays of molten rock would have turned into snaking torrents of lava. These eruptions would have been like huge fountains of water cascading into a pond.
Like the Earth’s ancient komatiites, some of the moon’s lavas were erupted at a temperature hot enough to melt their way into deep channels as they flowed across the surface. The long, winding ‘rilles’ that cut across the moon’s surface are certainly old lava channels. If they were formed by melting, they must have been made by very vigorous and long-lasting eruptions.
Of all the planets in the solar system, Venus is the most similar to Earth. It is just a little smaller than Earth, and has more or less the same composition. In other ways it is really very different. Venus is a planet with a ‘runaway Greenhouse effect’. The atmosphere is a choking mixture of carbon dioxide gas and clouds of sulphuric acid. The pressure at the surface is the same as that at a depth of fifty to a hundred meters under water, while the temperature is high enough to melt lead. Since NASA’s Magellan mission to Venus in the early 1990’s, Venus has actually been mapped in more detain than out own planet. The depth of the oceans, combined with military secrecy have held back the mapping of Earth. What the NASA scientists found was extraordinary. Venus is covered with volcanoes, many of which are enormous structures, much larger than those found on Earth.
There are two interesting difference between Venus and Earth. The Magellan mission found no erupting volcanoes. This was unexpected since, if Venus is losing heat in the same way as Earth, it should be erupting tens of cubic kilometers of lave per year. Secondly, the mission found no evidence of tectonic plates like those on the Earth. Like Mars, much of Venus is covered by a single plate. Unlike Mars, though, this plate is young, and torn, or tearing, in places. Most volcanism is due to hotspots in the mantle, rather than due to activity at the edges of the plate. One theory suggests that the plate on Venus survives for about half a billion years, before sinking catastrophically in a flood of volcanism. Then the process begins all over again. Radar images of the surface show a bewildering array of different kinds of volcanoes that have formed above Venus’s hot spots. In some places, clusters of small cones stretch across fields several hundred kilometers wide, just like smaller cone fields on Earth.
‘Arachnoids’ are circular structures criss-crossed with a spider’s web of fractures. These have no parallel on Earth, and are perhaps huge collapsed domes of lava. Several hundred arachnoids have been found each of which is up to a couple of hundred kilometers across. Larger still are ‘coronae’. These enormous circular structures can be as large as a thousand kilometers across, and are usually associated with smaller volcanoes and lava flows. Coronae may form as huge bulges over the top of a rising hotspot, which then collapse after the hotspot dies.
Some of the most remarkable small volcanoes on Venus are domes. Domes on Earth are usually small; less than a few kilometers across. Domes on Venus are considerable larger; as much as fifty kilometers across. The difference may be due to the higher surface pressure on Venus. This means that Venus should have fewer explosive eruptions than Earth, and more large eruptions of lava. One group of domes, ‘pancake’ domes, are only a few hundred meters thick but spread, like pancakes, for tens of kilometers. Another set of domes have scalloped or fractured edges and reminded NASA scientists of huge ticks and anemones.
Venus also has some giant lava flow fields, and long, winding lava channels. The most extraordinary channels are a couple of kilometers wide and stretch for up to seven thousand kilometers. The longest lava flows on Earth rarely approach a hundred kilometers. How such long channels formed is perplexing. If they formed like lava channels on Earth, then the lavas must have been very hot, very fluid and have erupted at an enormous rate.
Mars is another venerable planet that hasn’t erupted for millions of years. It is covered with a single thick plate, so there are no chains of volcanoes around the plate edges as on Earth. The few volcanoes that remain on Mars formed above hotspots, but are long since dead. The most enormous of these, Olympus Mons, rises twenty-five kilometers above the surrounding plains, and is capped with a sixty-kilometer wide crater. This is three times the height, and thirty times the volume, of Hawaii. Olympus Mons was able to grow so big for two reasons. Firstly, the plate that it sits on is static – it does not move away from the hotspot as out plates move on Earth. Secondly the plate on Mars is thicker and stronger than our tectonic plates. If Olympus Mons were on Earth, the plates would sag so much under the weight that it would not be able to survive.
Mars also has some huge plains covered with lava and pyroclastic flow deposits. On Earth, the biggest lava plains form where hotspots arrive under the continents. The largest of these, like the Deccan traps in India, or the Karroo basalts in South Africa, cover up to half a million square kilometers with one or two million cubic kilometers of lava. Alba Patera is the largest field on Mars. It covers over a million square kilometers with many millions of cubic kilometers of rock, and forms a huge flat-lying plain. The eruptions that made Alba Patera needn’t have been any more vigorous than eruptions on Earth. They just lasted an awful lot longer.
Some of the best examples of volcanic landforms on Mars, visible in images returned by the two Viking Orbiter spacecraft
Ý Mars is only about one-half of the size of Earth and yet the red planet has several volcanoes that surpass the scale of the largest terrestrial volcanoes. The four brown spots visible on the northern (upper) portion of the planet are four enormous volcanoes on the Tharsis region of Mars.