The theory that volcanoes are driven by mantle plumes—narrow jets of magma originating within the Earth’s core—is a staple of geology. Aspiring geologists, like everyone else, learn in high school that volcanoes far from plate boundaries lie above such plumes. Subsequent education builds on this like layers of strata. So when the theory is challenged, a field of science quakes.
"Mantle plumes have never had a sound physical or logical basis," says Caltech's Professor Don Anderson. "They are akin to Rudyard Kipling's 'Just So Stories' about how giraffes got their long necks."
While no one is sure exactly how mantle plumes form, the theory stuck as a way to explain chains of current and former volcanoes.
Los688. Standard view of how volcano chains form as tectonic plates drift over mantle plumes.
"Much of solid-Earth science for the past 20 years—and large amounts of money—have been spent looking for elusive narrow mantle plumes that wind their way upward through the mantle," says Anderson. While these efforts were unsuccessful, their failures were explained as a result of the jets being too thin to show up on the devices trying to detect them.
However, Anderson deals what he considers a death blow to the mantle plume theory in the Proceedings of the National Academy of Sciences. He explored the Yellowstone, Hawaii and Samoa volcanic provinces with numerous closely packed measuring devices and still found no evidence for narrow jets in the mantle.
Instead, Anderson concludes, “Convection in an isolated planet is characterized by narrow downwellings and broad updrafts—consequences of Archimedes’ principle, the cooling required by the second law of thermodynamics, and the effect of compression on material properties.”
What causes volcanoes then? Anderson says they are “the result of normal broad-scale convection and plate tectonics.”
Lava lamps operate a lot like the traditional view of the Earth's magma. The bottom gets hot enough to drive jets of warm material to the top, where things slowly cool and fall in broad downdrafts. However, lamps have an external energy source. Earth's core warms via radioactive decay, but Anderson notes that—from a geological point of view—the planet is slowly cooling.
Surface cooling, along with plate tectonic movement, produces narrow channels of sinking material called slabs. These slabs displace the lighter magma as they fall, causing broad plumes thousands of kilometres across to rise until they hit the plate bottoms.
Where a plate is cracked, the magma squeezes up through the fissure to surface as a volcano. In Anderson's view, the narrow vent the volcano sits upon is a few hundred kilometres deep, not several thousand as imagined in textbooks.
The model is similar to that proposed by Lord Kelvin in the 19th century. However, while Kelvin is revered for his work on thermodynamics, his vision of a cooling planet ignored radioactivity and operated on a scale orders of magnitude faster than what we now know to be true, causing it to be disgarded.