Unique volcanic complex discovered on Moon’s far side

From Washington University in St. Louis: Unique volcanic complex discovered on Moon’s far side


Analysis of new images of a curious “hot spot” on the far side of the Moon reveal it to be a small volcanic province created by the upwelling of silicic magma. The unusual location of the province and the surprising composition of the lava that formed it offer tantalizing clues to the Moon’s thermal history.

The hot spot is a concentration of a radioactive element thorium sitting between the very large and ancient impact craters Compton and Belkovich that was first detected by Lunar Prospector’s gamma-ray spectrometer in 1998. The Compton-Belkovich Thorium Anomaly, as it is called, appears as a bull’s-eye when the spectrometer data are projected onto a map, with the highest thorium concentration at its center.

Recent observations, made with the powerful Lunar Reconnaissance Orbiter (LRO) optical cameras, have allowed scientists to distinguish volcanic features in terrain at the center of the bull’s-eye. High-resolution three-dimensional models of the terrain and information from the LRO Diviner instrument have revealed geological features diagnostic not just of volcanism but also of much rarer silicic volcanism.

The volcanic province’s very existence will force scientists to modify ideas about the Moon’s volcanic history, says Bradley Jolliff, PhD, research professor in the Department of Earth and Planetary Sciences in Arts & Sciences at Washington University in St. Louis, who led the team that analyzed the LRO images.

“To find evidence of this unusual composition located where it is, and appearing to be relatively recent volcanic activity is a fundamentally new result and will make us think again about the Moon’s thermal and volcanic evolution,” he says.

The work is described in the July 24 advance online issue of Nature Geoscience.

Volcanism on the Moon
Lunar volcanism is very different from terrestrial volcanism because the Moon is a small body that cooled quickly and never developed rock-recycling plate tectonics like those on our planet.

The Moon, thought to have been created when a Mars-size body slammed into Earth about 4.5 billion years ago, was originally a hellish world covered by a roiling ocean of molten rock some 400 kilometers deep.

But because the Moon was small and had no atmosphere, the magma ocean cooled quickly, within perhaps 100 million years. Eventually lighter minerals such as feldspar crystallized out of the magma and floated to the top to create huge masses of feldspathic rock that formed the lunar highlands. Denser iron- and magnesium-rich minerals sank when they crystallized, forming the upper part of the Moon’s mantle.

The differentiation of the crust and mantle was followed by a wave of volcanic activity between about 3 to 4 billion years ago, when basaltic lavas erupted on the lunar surface, filling old impact craters and other low spots to form the lunar mare.

One of the mysteries of lunar volcanism is the unequal distribution of these flood basalts. Nearly a third of the Moon’s near side is covered by ancient flood basalts but the Moon’s far side, where the crustal rocks are thicker, has much less.

Moreover, almost all of the volcanism on the Moon is basaltic rather than silicic, enriched in minerals containing the elements iron and magnesium rather than the elements silicon and aluminum.

Earth’s continental crust, which reflects active geological processes such as subduction, magma intrusion and mountain building, includes many rocks whose compositions are intermediate between basalt and silica-rich rocks like granite, which are common on Earth. On the Moon, on the other hand, there are many basaltic rocks and only a small fraction of granite. Rocks of intermediate composition are all but missing.

Procellarum KREEP Terrane
It wasn’t so very long ago, Jolliff says, that scientists talked about the Moon as having two sorts of terrain, the dark maria, or “seas,” and the light terra, or highlands.

This simple picture of the Moon’s geology served for many years, but in 2000, Jolliff and his colleagues in the department of Earth and Planetary Sciences and WUSTL’s McDonnell Center for the Space Sciences introduced a concept in which they distinguished three different “terranes,” or regions of the Moon with distinctive geologic histories.

One of these, which encompasses much of the mare basaltic volcanism on the Moon, is called the Procellarum KREEP Terrane, or PKT for short. This immense lunar “hot spot” contains high concentrations of thorium and other radioactive, heat-producing elements, such as potassium and uranium. [KREEP stands for potassium, (K), rare-earth elements (REE), and phosphorus (P).]

As the magma ocean cooled, Jolliff explains, elements such as thorium were preferentially excluded from crystallizing minerals, forming pockets of KREEP-rich magma sandwiched between the crust and mantle.

A concentration of heat-producing elements under the Procellarum KREEP Terrane may be partly responsible for the intensive mare volcanism there. The maria, Jolliff explains, were formed when the hot radioactive elements melted minerals deep in the Moon’s mantle, forming basaltic lava which erupted through fissures onto the Moon’s surface. Well over half the Procellarum KREEP Terrane was resurfaced by volcanism.

Although most of the volcanism was of the basaltic variety, resulting in the large, dark patches on the Moon visible to the unaided eye from Earth, a much rarer form of volcanism, one that produced lavas rich in silica, also occurred in the PKT. These volcanic deposits are known as “red spots” because of their spectral characteristics, and recent results from the LRO spacecraft confirmed their silica-rich compositions. The red spots include some with distinctive dome shapes, some quite large, and all within the boundaries of the PKT.

A new volcanic province
Ever since the Lunar Prospector mission first revealed the thorium-rich bull’s-eye isolated on the far side of the Moon and distant from the Procellarum KREEP Terrane, Jolliff’s group has been curious as to what it was. “When the Lunar Reconnaissance Orbiter was launched in 2009, we were finally able to image it at high resolution,” he says.