A "Four-Piston Engine" Drives Earth From The Inside, New Study Shows

In a paper appearing in the April 26 issue of Nature, lead author Alessandro Forte of The University of Western Ontario and co-author Jerry Mitrovica of the University of Toronto provide an answer to the longstanding mystery of how the entire Earth, from the deep interior to the surface, is changing over time.

Their work, which draws together results from many disciplines in the earth sciences, shows a heat engine is slowly churning deep within the planet's interior and provides the most comprehensive model to date that explains physical phenomena occurring at the surface.

"In effect, we have found that the solid Earth is being churned by a four-piston heat engine with two immense sinking cold slabs and two equally large rising hot plumes," says Forte. "It really ignites the imagination to realize how things are changing hundreds of kilometres beneath your feet and how this change connects to majestic features on Earth's surface."

"This find allows us to move well beyond the drift of continents described by plate tectonics," says Mitrovica, the University of Toronto's J. Tuzo Wilson Professor in Geophysics. The plate tectonics theory, proposed in the 1960s, suggests Earth's crust is split up into a few immense plates that constantly shift and produce earthquakes, but it does not explain how this movement is linked to processes occurring deep inside the planet, says Forte.

"The first clues to finding that link appeared in the 1980s when earth scientists obtained images of Earth's internal structure using earthquake waves that travel deep inside the planet," says Mitrovica. "This method is similar to medical CAT-scanning used to image the human body. The remarkable images of the mantle, a region below the crust that extends down 3,000 kilometres to the top of Earth's molten core, turned earth science on its head."

The images showed that deep below the margins of the Pacific Ocean are two vast arc-shaped regions where earthquake waves travel faster, while deep below the central Pacific and below Africa are two equally enormous plume-shaped regions where earthquake waves are slowed down. Because the edge of the Pacific is ringed by zones where cold, dense portions of tectonic plates descend into Earth, the "faster" regions were clearly marking areas where slabs of heavy material were sinking into Earth toward the iron core.

A popular view held by earth scientists is that the "slow" regions are simply immense stagnant blobs of material that have remained essentially unaltered since the formation of Earth. Now Forte and Mitrovica have proven that these towering features are actually floating up toward the surface like hot air balloons. Their proof is based on a diverse array of observations ranging from incredibly small variations in Earth's rotation and gravity field to dramatic deflections of continental regions such as southern Africa, which now sits 1,000 metres higher than North Africa.

The pair's multidisciplinary approach is being hailed as the most unified model to date for Earth dynamics and one which provides a framework for modeling of long-term changes in sea-level, topography and climate. The model may also be used to further understanding of other planets in our solar system, such as Venus, Mars and Mercury, says Forte. "We have discovered something grandiose in size and yet remarkably simple and symmetric."

"It's a road map for resolving a contentious debate that has hampered global earth science since the plate tectonics revolution," says Mitrovica.

Forte and Mitrovica's research is funded by the Canada Foundation for Innovation, the Ontario Innovation Trust, the Natural Sciences and Engineering Research Council of Canada and the Canadian Institute for Advanced Research Earth Systems Evolution Program.