There is a lot that scientists don’t know about the mantle, the partly molten, partly solid layer of Earth that makes up 84 percent of the planet’s volume. We know that it's composed of vast cycling masses of material moving around in gigantic loops called “convection currents,” and in-between these loops, superheated plumes rise up through to the surface of Earth, creating near-perpetual sources of volcanism.
Without these currents, plate tectonics wouldn’t have happened. There would be no crust, no continents, no volcanoes, no earthquakes, almost no atmosphere, and almost certainly no life – so, understanding them is of paramount importance. A new study, published in the journal Nature Geoscience, peels back another layer of this mysterious section of the planet, revealing that these convection currents are moving 10 times faster than most estimates.
“Although we're talking about timescales that seem incredibly long to you or me, in geological terms, the Earth's surface bobs up and down like a yo-yo,” Dr. Mark Hoggard, a postdoctoral research fellow at Cambridge's Department of Earth Sciences, and the paper's lead author, said in a statement. “Over a period of a million years, which is our standard unit of measurement, the movement of the mantle can cause the surface to move up and down by hundreds of meters.”
One of the common models of mantle convection. Surachit/Wikimedia Commons; CC BY-SA 3.0
The team of researchers from the University of Cambridge used 2,120 seismic surveys to build up a detailed picture of the mantle. Just like ice sitting atop buried bedrock, the topography of the crust can reveal what is happening beneath it. These mantle convection currents are incredibly powerful, and their upwelling can cause the Earth’s crust to move further skywards, just as their downwelling can cause it to sink.
By determining global changes in oceanic crustal thickness, the team was able to gain an understanding of the types of mantle currents active below it all. Geophysicists already have a fairly in-depth picture of what the mantle is doing beneath the crust, but this study builds on this knowledge by providing more precise measurements of the speed and size of the convective cycles.
This new global map of the mantle, the first of its kind, revealed that these supposedly gargantuan cycles are actually fairly small. Instead of being 10,000 kilometers (6,200 miles) in length, as many predicted, they are more often on the order of 1,000 kilometers (621 miles). If this is true, then it seems unusual that, at these scales, such convective cycles could cause such major changes in ocean crust height.
The dynamic topography of the world. Red indicates rises caused by upwelling mantle currents; blue indicates the opposite. The initial model is depicted in (a), where it is overlaid atop additional geophysical observations in (b). Hoggard et al./Nature Geoscience
However, the researchers realized that this discrepancy could be resolved if old ideas about the speed of mantle convection were thrown out the window. Ultimately, they reasoned that the global variation in oceanic crust height can be explained if these shorter convective cycles are actually moving around 10 times faster than previously thought.
“These results will have wider reaching implications, such as how we map the circulation of the world's oceans in the past, which are affected by how quickly the sea floor is moving up and down and blocking the path of water currents,” added Hoggard. “Considering that the surface is moving much faster than we had previously thought, it could also affect things like the stability of the ice caps and help us to understand past climate change.”