Tuesday 26 July 2011

Volcanoes on the far side of the moon.

On 24 July 2011 an article appeared in the journal Nature Geoscience, reporting the discovery of shield volcanoes on an area of the far side of the moon called the 'Compton-Belkovich Thorium Anomaly' (CBTA). The article, by a team led by Bradley L. Jolliff of the Department of Earth and Planetary Sciences at Washington University in St. Louis, used data from the Lunar Reconnaissance Orbiter to build up a picture of the geochemistry and topology of the area, from which they draw the conclusion that the feature is in fact a cluster of shield volcanoes.

Wide angle photograph of the central part the CBTA.

This is not actually news, since the team had already announced the finding at the 42nd Lunar and Planetary Science Conference in March this year, but getting published in a prestigious journal seems to have brought the story to the attention of more of the press.

Shield volcanoes are made up of layers of lava from repeated eruptions, they tend to be dome shaped with low profiles, unlike the more classically cone shaped stratovolcanoes, which are made up of a mixture of lava and ash layers, but grow much larger. The largest known volcanoes are all shield volcanoes, such as Mauna Loa on Earth and Olympus Mons on Mars.

Shield volcanoes are well documented on the moon, where they are commonly referred to as 'lunar domes' due to their shape. Many lunar volcanoes are clustered around impact craters, suggesting that the impact event may have in some way triggered the eruption of the volcanoes. However this does not appear to be the case with the CBTA volcanoes.

Most of the moons surface is of course made up of volcanic material, since the only source of erosion is meteor impacts, which produce a regolith (soil) of rock fragments and dust, but nothing like the sedimentary processes seen on Earth. The lighter lunar terrae (highlands) are probably the result of the original cooling of the surface of the moon after its formation and the darker lunar mare (lowlands or oceans) appear to be later flood basalts. These mares are concentrated on the near side of the moon, though nobody is quite sure why; it was formerly thought that this was a tidal effect caused by the Earth's gravity, but it is now understood that the centrifugal forces caused by the moon's orbit should cancel this out.

The CBTA volcanoes appear to resemble none of these. They are apparently made up of evolved, silica rich volcanic material. This deserves a slight explanation. When lava cools different minerals solidify at different temperatures. Above 1000° basaltic lavas are formed, between 800° and 1000° andersitic Lavas and between 650° and 800° rhyolitic lavas. Thus the large plains of basalt that make up the lunar mare are assumed to be the result of vast amounts of very hot lava being expelled then cooling rapidly when exposed to the vacuum of space. The rhyolitic lavas of the CBTA must presumably have cooled beneath the surface for some time, so that the basaltic and andersitic components had precipitated out (solidified) before the rhyolitic components were ejected.

The location of the Compton-Belkovich Thorium Anomaly (C-B) on the moon; the yellow and green areas on the top left are the Lunar Maria, which are permanently turned towards the Earth.