Volcanoes capture the imagination like little else on Earth, but did you know that there are also volcanoes on the Moon? Planetary scientists have often thought that the Moon is a “dead” satellite, in that there is no longer any internal heat left to drive surface processes like earthquakes, mountain building, or volcanic eruptions. Although recent evidence has suggested that volcanism on the Moon may have been happening more recently than previously thought, a new study published this week in Geophysical Research Letters shows that scientists still aren’t quite sure what’s happening up there: a new, mysterious volcanic feature has been found on the lunar south pole.

This feature, named the Mafic Mound after the type of lava it’s made from, is 800 meters (2,600 feet) high and a whopping 75 kilometers (47 miles) across, sitting right in the middle of a gigantic impact crater, the South Pole-Aitken Basin. Its composition is vastly different from the lunar rock it is surrounded by, so where did it come from?

^{The enigmatic Mafic Mound, outlined. Image credit: NASA/Goddard/Arizona State University.}

The lunar surface is an entirely volcanic landscape. The mare or “seas” that are most prominent on the near side of the Moon, tidally-locked to Earth, are huge flows of basaltic lava, almost exactly the same type you would see in Hawaii. Major impacts from various space rocks punctured holes in the lunar surface, causing swathes of lava to flood out into basins.

The Moon has experienced bonafide volcanic eruptions too. Between 3 and 4 billion years ago small domes and cones similar volcanoes on Earth formed. The gravitational field strength of the Moon is just a sixth of Earth’s, meaning that volcanic debris was thrown over a very large area even for small explosive eruptions. This meant the cones were not able to successfully build themselves up; consequently, lunar volcanoes are pretty tiny.

So how did this massive southern Mafic Mound form? The authors of the study think the large impact crater is to blame. When an ancient impactor smacked into the south pole, a cauldron of magma 50 kilometers (30 miles) deep was formed. As it cooled, it shrunk, but the core of the cauldron remained molten right until the last minute, and it was this prolonged cooling process that drastically changed the mineral content of the lava. At the last minute, this severely altered – or evolved – sea of lava was squeezed up out of the crust, forming the Mafic Mound.

The Solar System is full of weird and wonderful volcanoes: Cassini has just started its flyby of the icy Saturnian moon of Enceladus, where it will rocket through an erupting ice volcano’s plume. Venus erupts pancakes, Io erupts plumes over 60 times the height of Everest, and it appears even our own little Moon has exhibited novel volcanism in the recent past.

^{Image credit: NASA}