From Apollo samples and crater counts, we know that the bulk of volcanism on the moon occurred from 3.9 to 3.1 billion years ago, and we’ve long believed that all volcanic activity shut off around a billion years ago. But now, dozens of newly detected topographic anomalies reveal that volcanic activity didn’t stop on the moon abruptly when we thought it did -- rather, it’s slowed gradually, and it may not even be done yet.

The discovery of these rock deposits suggests that volcanoes were erupting on the moon within the last 100 million years -- during the Cretaceous when dinosaurs roamed the Earth -- making the moon much, much warmer than we thought. It may be time to rewrite the textbooks. The findings were published in Nature Geoscience this week.

These rock deposits, called "irregular mare patches” (IMPs), are characterized by a distinct combination of textures: smooth, rounded, shallow mounds next to patches of rough, blocky terrain. They’re the remnants of small basaltic eruptions, and they’re scattered all across volcanic plains. Until recently, these features were considered very rare: Only one, named Ina, was been spotted by Apollo 15 in the 1970s.

Turns out, Ina isn’t a one-off oddity. Using NASA’s Lunar Reconnaissance Orbiter Camera (LROC), researchers from Arizona State University and Westfälische Wilhelms-Universität Münster spotted 70 irregular mare patches on the near side of the moon. These are too small to see from Earth, and in fact, most of the lava flows that make up the dark plains visible to us (including the face of the “man on the moon”) erupted between 3.5 and 1 billion years ago.

Their wide distribution strongly suggest that late-stage volcanic activity was not an anomaly. And their sharp nature and the absence of impact craters greater than 20 meters in diameter indicate that at least three of these IMPs formed in the last 100 million years, according to an LROC release.

Ina may be less than 50 million years old, they found, and volcanic activity at another IMP called Sosigenes only ended about 18 million years ago. Sosigenes (pictured, right) is only about 300 meters deep, 3 kilometers wide, and 7 kilometers long. You can see how sparse the number of craters on the lava flows are.

Because these IMPs formed way after the well-established volcanic shutdown a billion years ago, the findings suggest that the interior of the moon is hotter than previously thought. Our cold, dead moon’s still got some heat in it. “The existence and age of the irregular mare patches tell us that the lunar mantle had to remain hot enough to provide magma for the small-volume eruptions that created these unusual young features,” Sarah Braden of ASU says in a NASA release. “Young volcanism indicates possibly more magma, or magma at higher temperatures, or magma at shallower depths, or all of the above,” she tells New Scientist.

“The big story is that the moon is warmer than we thought,” ASU’s Mark Robinson tells Science. And with lava flows that young, eruptions are apt to happen again.

Images: NASA/GSFC/Arizona State University