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Earth scientists discover ‘upside-down volcano’ on the Moon

The discovery was recently published in the journal ‘Nature Geoscience’.

Deep inside the Moon, there are significant quantities of molten rock (magma). But there are no active volcanoes on the Moon – the last volcanic eruption on the surface of the Moon took place over a billion years ago. Research led by Mirjam van Kan Parker, Elodie Tronche, Nachiketa Rai and Wim van Westrenen of the Faculty of Earth and Life Sciences at VU University Amsterdam has shown how this is possible: the magma deep in the Moon is so dense that it cannot rise up towards the surface. In fact, the magma may sink downwards through the surrounding solid rock, the opposite of what happens in a volcano. Their results were published in Nature Geoscience on 19 February.

What is inside the Moon?
We still know relatively little about the formation and geological history of our Moon. Although the Apollo astronauts returned almost 390 kilograms of lunar rock to Earth, and also installed a series of scientific instruments on the Moon’s surface, we actually know very little about the composition and history of the inside of the Moon. Last year, NASA researchers presented a new model of the internal structure of the Moon. The researchers claim that the deepest layers of the Moon’s mantle include a large proportion of magma (up to 30%). On Earth, similar amounts of magma rise upwards to the surface causing the formation of volcanoes. The force that drives these movements is the difference in density between the light magma and the heavier solid rock surrounding it. 

Titanium in lunar magma
There are no active volcanoes on the Moon. This can only mean that the magma in the lunar mantle must be as dense as, or denser than, the rocks above it. The research team from VU University Amsterdam – working with partners in France and Scotland – measured the density of lunar magma at the high pressure and high temperatures that occur inside the Moon. They concluded that there is only one type of lunar magma that has the same density as the solid rocks in the Moon’s lower mantle: magma containing a large amount of the heavy element titanium. All other lunar magma compositions are lighter and would therefore quickly push upwards towards the surface. 

The upside-down volcano effect
Titanium-rich magma can only be formed from rocks that are rich in titanium. This shows that there is titanium-rich rock in the deep lunar mantle, and that this has become partially molten. Earlier work has already shown that this kind of titanium-rich rock can only be made very close to the Moon’s surface. These new findings mean that early in lunar history, large vertical movements must have taken place, causing titanium-rich rocks to be moved from just below the surface (where they formed) to deep inside the Moon. This rock partially melted deep in the lunar mantle. The resulting magma composition is so dense that it remains deep inside, or even sinks further towards the very centre of the Moon – a kind of upside-down volcano.

The findings of this research team provide new parameters for the development of models of the Moon’s early history. In many respects, the Moon can  be considered a miniature Earth, and researchers can use these data to learn more about large-scale movements in our own planet as well.

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