A huge block of ancient granite has been found under the lunar surface, evidence of a type of volcanism we haven’t seen there before. The analysis shows that it is a giant, deeply buried mass of solidified magma, or batholith, that was deposited about 3.5 billion years ago. We see this quite often on Earth, but planetary scientists are excited to see it on the Moon.

“Any large granitic body we find on Earth fed a large group of volcanoes, just as a large system today feeds the Cascade Volcanoes in the Pacific Northwest,” says planetary scientist Matthew Ziegler of Southern Methodist University and Planetary Science.

“Batoliths are much larger than the volcanoes they feed on the surface. For example, the Sierra Nevada Mountains are a remnant of a long-existing volcanic chain in the western United States.”

Granite is found in large quantities on Earth, but is extremely rare in other parts of the solar system, as certain conditions are required for its formation. These conditions include abundant liquid water and plate tectonics that help melt and recycle material in the planet’s crust. Granite production requires multistage remelting of basalt rock or crystal fractionation in liquid basalt.

There is neither liquid water nor plate tectonics on the Moon.

However, near the north pole on the far side of the Moon, under a volcanic region known as Compton-Belkovich, microwave instruments in China’s Chang’e 1 and Chang’e 2 orbiters have detected something strange. They found an anomalous temperature about 20 times higher than the average for the lunar highlands. The researchers were able to analyze publicly available data from the China National Space Administration, and the results surprised them.

“We found that one of these suspected volcanoes, known as Compton-Belkovich, definitely glows in microwave radiation,” says Ziegler. “This means it’s hot below the surface, not on the surface like you see in infrared.

“The only way to explain this is additional heat from somewhere under the deeper lunar crust. Considered a volcano for this reason, Compton-Belkovich also hides a large source of heat underneath.

Compton-Belkovich notes that the region contains large amounts of thorium, a product of radioactive decay. Analyzes by Siegler and colleagues suggest that radioactive elements in the granite matrix are likely the source of heat beneath it. This granite matrix is ​​much larger than they expected, at around 50 kilometers (31 miles) in diameter.

Researchers say this is evidence of an advanced magmatic plumbing system that is much larger than expected for the Moon.

A system this large would require one of three things: a large mantle plume fed by magma from the Moon; an abnormally wet pocket in the middle of the Moon at this location; or a piece of element capable of providing sufficient radiogenic material to generate sufficient heat for successive remelting. All three imply large-scale lunar composition inconsistencies that require explanation.

“If you don’t have water, making granite takes extreme situations. Here is this system without water and without plate tectonics, but you have granite,” says Ziegler.

“Was there water on the Moon – at least at this point? Or was it particularly warm?”