Until now, scientists had believed the moon was made of two basic types of rock: dark basalt and light, calcium-rich feldspar. Both would have come from volcanoes spewing relatively runny basaltic lava.

But the new volcano type oozed thicker lava rich in silica over a light, arrowhead-shaped patch of the moon roughly 18 miles (30 kilometers) across, called Hansteen Alpha, the scientists say.

The new type of moon volcano is now extinct—the last time it oozed any lava was two billion years ago at best, said Timothy Glotch, assistant professor of geosciences at Stony Brook University and co-author of a new paper describing the find.

(Related: "Volcanoes Rocked Dark Side