The Earth's moon was formed by a collision between two separate bodies early in the history of the solar system. That collision left the Moon completely molten, and provided it with both a large reservoir of heat and an interior that produced huge outpourings of basaltic rock, which formed the dark maria that can easily be seen on the body's surface. These features are also associated with volcanic domes that indicate a more familiar and subdued form of volcanism also occurred on the Moon, albeit rarely. Now, researchers think they've spotted the most recently formed volcanoes on the Moon, hidden on the dark side and far away from any maria.

Because of the way the Moon's material fractionated as it cooled, many of the large maria on the near side are associated with elevated levels of thorium, a radioactive element (elevated, in this case, means a few dozen parts-per-million). But the Lunar Prospector mission, which mapped the abundance of the element, also found a few isolated hotspots on the dark side, including one at a site near two impact craters, Compton and Belkovich. Figuring out what caused the CBT anomaly (Compton-Belkovich thorium), however, would have to wait for the Lunar Reconnaissance Orbiter, which had some cameras with sufficient resolution to pick out features only a meter across.

Combining images taken from two or more angles, along with radar altimetry data, the authors have reconstructed the three-dimensional landscape at the CBT, and find that it's filled with features we commonly associate with terrestrial volcanism: collapsed calderas and conical peaks with flat tops, some apparently indented with craters. One of the features was over six kilometers across, and rose about a kilometer above its base.

That steepness actually provides an indication of the cone's origin, since the low-density basalts that typify the Moon's volcanic features couldn't possibly pile up that steeply; they'd spread out into a far flatter structure. Instead, the structures are consistent with the eruption of a viscous material, like silica-rich rhyolite. Most of the lower density volcanic material on the Moon is the product of what are called KREEP eruptions (for potassium, rare earth elements, and phosphorous). The authors suggest that, in this case, a large amount of KREEP material made it close to the surface, but then pooled there, with its different components separating as it cooled. This allowed the viscous rhyolite to make it to the surface and form these distinctive volcanic cones.

To figure out when these volcanoes formed, the authors started counting craters, which accumulate with age. They came up short of what they were expecting, saying their count suggests "extraordinarily young ages" for the CBT cones. They don't name them, but an accompanying perspective does: 800 million years old, 200 million years younger than the next most recent feature we're aware of. But the Lunar Reconnaissance Orbiter imaged many other areas of the Moon, so there's still the chance there's something younger lurking in the data.

Nature Geoscience, 2011. DOI: 10.1038/NGEO1212 (About DOIs).