Huge granite 'body' on far side of the moon offers clues to ancient lunar volcanoes

Found underneath a lunar volcano that most likely erupted 3.5 billion years ago are layers of cooled magma.

Another indication that the far side of the moon formerly sparkled with volcanic eruptions is a huge granite deposit that was found under a long-extinct lunar volcano.

On the surface of the moon, a formation known as Compton-Belkovich that is thought to be volcanic was where the granite was discovered. This feature probably developed as a result of the cooling of the magma that powered the lunar volcanoes' explosive eruptions about 3.5 billion years ago.

This area of the moon has long been thought to be an old complex of volcanoes, thus the discovery of volcanic activity remnants there is not entirely surprising. But the size of this region of cooled magma, which has an estimated diameter of about 31 miles (50 kilometers), has surprised the crew. The finding of this substantial mass of granite under the Compton-Belkovich volcanic complex may aid in the explanation of the origin of the lunar crust.

Researchers from the Planetary Science Institute lead by Matthew Siegler used information gathered by NASA's Lunar Reconnaissance Orbiter to make the discovery of the granite body. The crew was able to measure the temperatures beneath Compton-Belkovich's surface thanks to the orbiter's data. The results revealed heat production that could only originate from radioactive elements contained in granite, an igneous rock found in the "plumbing" of volcanoes called "batholith," subterranean rock formations produced when magma cools without erupting, which is only present on the moon.

The Cascade volcanoes in the Pacific Northwest are now being fed by a vast system, according to Siegler. "Any big body of granite that we find on Earth used to feed a big bunch of volcanoes," Siegler said in a statement. "Batholiths are considerably larger than the surface-feeding volcanoes. For instance, the Sierra Nevada mountains are a batholith that was left behind by a long-ago volcanic sequence in the western United States.

On Earth, granite often forms as a result of water and plate tectonics melting vast regions of rock known as melt bodies beneath the surface of our planet. Granites are abundant on Earth but far more rare on the moon because to the lack of plate tectonics and water. This means that this discovery may shed light on the local or global circumstances that existed on the moon at the time that it was the site of volcanic activity.

Granite can only be created under severe conditions if there is no water, according to Siegler. "So, this system has granite despite lacking water and plate tectonics. At least in this one location, did the moon formerly have water? Or was it just very warm?

Between July 9 and July 14, Siegler will present the team's findings at the Goldschmidt Conference in Lyon, France. The team's research is also covered in a publication that appeared in the journal Nature on July 5.