Forget Yellowstone, Iceland and all those other wannabe volcanic hotspots. There are only two hotspots powered by true deep upwelling of heat and rock in the Earth’s mantle: The central Pacific Ocean and Africa.
A new study published today in Nature today argues that large-scale upwelling happens only in those two places, and also suggests that the two plumes from Earth’s depths have been very stable and have not moved much over the history of our planet. That’s despite the fact that the continents have formed and reformed several times and tectonic plates have been banging around continuously.
“For example, the Pangaea supercontinent formed and broke apart at the surface, but we think that the upwelling locations in the mantle have remained relatively constant despite this activity,” said Clinton Conrad, associate professor of geology at the University of Hawaii–Manoa’s School of Ocean and Earth Science and Technology (SOEST).
Conrad had long noted that tectonic plates tend to move northward. With that in mind, he and his colleagues started looking to see if there was some point in the Northern Hemisphere where the plates tended to be heading. They found it in eastern Asia.
Conrad then started looking for other important points on the planet for plate tectonics motions. With a bit of math, he discovered a plate tectonic “quadrupole” with two points of “net convergence” and two points of “net divergence” of tectonic plates. Overlaid on today’s planet, the researchers found the quadrupole locations of net divergence matched African and central Pacific locations where it’s generally thought mantle upwellings are now underway.
“This observation was interesting and important, and it made sense,” said Conrad.
Next, his team applied the same formula to the history of plate motions and plotted the points. They were astonished to see that the quadrupole points have not moved over geologic time. Why? Nobody knows. But the discovery provides a framework for begining to understand how mantle happenings can be linked to surface geology over the span of Earth’s history. In other words, this could be an underlying effect that could explain a lot of changes that have taken place on the surface of the Earth over the last few billion years.
As for Yellowstone and those others, they are increasingly thought to be shallow upwelling of material subducted by colliding plates. Nothing deep. Just transient stuff compared to the biggies under Africa and the Pacific.