Why the Moon has no active volcanoes | WIRED UK

Why the Moon has no active volcanoes
Unlike Earth, the Moon has no active volcanoes despite the fact that recent moonquake data suggest that there is a lot of magma under its surface.
A team of Dutch earth scientists -- lead by Mirjam van Kan Parker and Wim van Westrenen from VU University Amsterdam -- have now identified the likely reason for the lack of volcanic activity: the hot, molten rock contained within the Moon's interior could be so dense that it is too heavy to rise to the surface. The team came to this conclusion by producing microscopic copies of Moon rock collected by the Apollo missions and melting them at extremely high pressures and temperatures, similar to those found inside the Moon. They then measured their densities with powerful X-rays generated at the European Synchrotron Radiation Facility (ESRF).
50 years after the Apollo missions, the formation and geological history of the Moon still hold many secrets. The astronauts brought back 360 kg of Moon rocks and placed many instruments on the satellite's surface. Last year, Nasa published a new model for the make-up of the interior of the Moon, using moonquake data collected from Apollo-era seismometers.
Planetary scientist Rene Weber and team claim that the deepest parts of the lunar mantle, next to its small metallic core, are partially molten. In Earth, such bodies of magma tend to rise up towards the surface, leading to volcanic eruptions. Until now, it has not been clear why the Moon's magma doesn't do the same.
The reason magma moves to the surface is the density difference between the magma and the surrounding solid material, which causes it to move upwards like a bubble. The lighter the magma, the more violently it rises.
Wim van Westrenen and colleagues sought to determine the density of the lunar magma using synthesised Moon rock. They applied pressures of 4.5 GPa and temperatures of 1,500 degrees Kelvin in order to replicate the conditions found in the Moon's core. They did this by heating them with a powerful electric current while squashing them in a press.
They then used a powerful synchrotron X-ray beam at the ESRF to measure the density of the Moon rock when solid and molten. These measurements were then combined with computer simulations to calculate the magma density at any location in the Moon.
Most lunar magmas were found to be less dense than their solid surroundings, just as they are on Earth. However, some samples from the Apollo 14 mission, which comprised of titanium-rich glass, were found to produce a magma as dense as the rocks found in the deepest part of the lunar mantle today. This magma wouldn't move towards the surface.
This titanium-rich magma could have only been formed by melting titanium-rich solid rocks. Previous experiments have shown that these sorts of rocks were formed shortly after the formation of the Moon at shallow levels. It is thought that vertical movements must have occurred early in the history of the Moon, which caused these titanium-rich rocks to descend into the Moon's core-mantle boundary.
The team hasn't ruled out volcanoes emerging on the Moon's surface in the future. Wim van Westrenen explains: "Today, the Moon is still cooling down, as are the melts in its interior. In the distant future, the cooler and therefore solidifying melt will change in composition, likely making it less dense than its surroundings. This lighter magma could make its way again up to the surface forming an active volcano on the Moon -- what a sight that would be! -- but for the time being, this is just a hypothesis to stimulate more experiments."
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By David Nield
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