In other words, you’re closer to the edge of space right now than you are to the bottom of a continental plate.
The finding will help scientists improve our understanding of the mechanics of the plates—not just their depth or composition, but how they move around Earth. The Earth is sliced up into many different kinds of layers. Elementary schoolers are familiar with the chemical bands: the crust, which terminates three to 40 miles below the surface; the silicate mantle, which descends 1,800 miles below the surface; and the inner and outer core, a white-hot sphere of iron and nickel that meet 3,959 miles from the surface.
But these familiar terms only describe what makes up the Earth’s layers, its chemistry. They don’t describe how the planet moves or how it reacts to heat. For that, geologists turn more specific words that describe Earth’s mechanical layers—lithosphere and asthenosphere.
“Imagine that you had a hot fudge brownie you had poked. It’s all liquid at first,” says Cin-Ty Lee, a geologist at Rice University who was not affiliated with the study. “Then it will cool from the top and it will generate that crusty hard layer. That top layer, the cold part, that’s the lithosphere. It’s the uppermost parts of the earth.”
The lithosphere, in other words, is the layer of the planet that makes up the tectonic plates. The warmer parts below constitute the asthenosphere. To a human observer, the asthenosphere would also appear as a kind of rock, but over millions of years it functions like a slow and ponderous liquid: upwelling, sliding around, mixing with itself.
In fact, both of these spheres would appear chemically similar, especially at their boundary. But the heat difference between them is stark. The interior of Earth is about 1,400 degrees Celsius (or 2,552 degrees Fahrenheit), while the surface is, on average, a balmy 25 degrees Celsius (77 degrees Fahrenheit).
“Where it transitions from 1,400 degrees to 25 degrees, that’s the lithosphere,” says Lee.
Geologists long ago identified the chemical layers of Earth—they can be predicted from surface research or directly observed via seismic stations or gravity-sensing satellites—but they have struggled to identify the boundary of the lithosphere. What’s more, scientists often arrived at conflicting results: While some estimated the lithosphere’s depth from diamonds found near volcanoes, other tried to sense it by measuring how seismic waves move through the inner Earth. The diamond-informed estimates said the lithosphere ended much closer to the surface, about 100 miles down, than the seismic estimates did.
The Science paper reconciles the two findings. It draws on more data, at high resolution, from every continent to estimate the edge of the lithosphere. “We used seismic waves generated by earthquakes with magnitude 5.5 or greater and recorded at stations all over the world from 1990 to 2015,” said Saikiran Tharimena, a research fellow at the University of Southampton. They also looked specifically at the middle of plates—the center of the Canadian shield, for instance—and not at their rocky and more fragmented coasts.