Dozens of exceptionally active volcanic sites dot the earth. But geologists have debated the cause of these so-called hotspots for decades. Do they originate in mantle plumes—vast upwellings of superhot rock from the earth's core— or in shallower reservoirs of heat in the upper mantle? Seismologists at the University of California, Berkeley, and Lawrence Berkeley National Laboratory recently took an unprecedented look at what lies below the earth's surface to depths of thousands of kilometers. After performing MRI-like tomographies of the planet's innards, they found more than two dozen mantle plumes rising continuously from the core to the surface—many of them directly feeding hotspots. The plumes, reported in Nature, provide the first direct evidence that these columns of heat generate volcanic hotspots, such as Iceland and the island chain of Hawaii.

By the Numbers

28

Number of mantle plumes rising continuously from the earth's core.

600–800 kilometers

Average width of plumes, three to five times larger than computer models predicted.

44 terawatts (44 trillion joules per second)

Internal heat the earth sheds by way of mantle plumes.

SOURCES: “BROAD PLUMES ROOTED AT THE BASE OF THE EARTH'S MANTLE BENEATH MAJOR HOTSPOTS,” BY SCOTT W. FRENCH AND BARBARA ROMANOWICZ, INNATURE, VOL. 525; SEPTEMBER 3, 2015 (first item); “MANTLE PLUMES SEEN RISING FROM EARTH'S CORE,” BY ERIC HAND, in SCIENCE, VOL. 349; SEPTEMBER 4, 2015 (second and third items)