The Earth's mantle, which lies between the crust and core, is divided into layers of its own. Scientists have now discovered that the boundary between these layers can be very rough, resembling the mountain ranges and smooth plateaus of the Earth's surface. The discovery could reveal lost continents' final resting places.
The deeper layers of the mantle are far beyond humanity's capacity to study directly. However, we can understand the composition of the Earth's inner and outer core in indirect ways, such as from seismic waves triggered by large earthquakes that bounce off the planet's internal boundaries, just as light waves can be partially reflected by transitions between different densities of glass. Dr Jessica Irving of Princeton University applied the same idea to look at the more subtle differences in composition within the mantle.
"You want a big, deep earthquake to get the whole planet to shake," Irving said in a statement. Smaller ones lose too much energy within the crust to be useful.
In 1994, a magnitude 8.2 earthquake beneath Bolivia overthrew ideas about how plate boundaries interact at great depths. At 647 kilometers (402 miles) beneath the Earth's surface, it was at the time the largest recorded quake more than 300 kilometers (180 miles) below. The combination of being so far from the surface and occurring beneath a sparsely populated area meant the death toll was unusually low for such a powerful quake. For geologists, the event was a rich source of information.
Twenty-five years later, Irving has analyzed data from seismographs worldwide and found a stark contrast in the way the earthquake's waves scattered off two mantle transitions. At the 410-kilometer-deep (255-mile) transition, between where the quake occurred and the surface, the waves' response indicated a smooth boundary.
However, Irving reports in Science that things were different at another transition approximately 660 kilometers (410 miles) below sea level. In some parts of the planet, the meeting of the upper and lower mantle were fairly smooth. In others, the transition was rougher than the one between the Earth's surface and its atmosphere. In other words, great mountain ranges like the Andes, under which the quake occurred, have counterparts hundreds of kilometers down where the lower mantle sticks into the upper mantle.
Geologists have disagreed over whether the upper and lower mantle have different compositions, and how easily heat flows between them. Irving and co-authors believe the two layers mix well where the boundary is smooth and are highly distinct where it is rough. They think the roughness is caused by giant crustal rocks of very different chemical composition to the rest of the mantle being subducted in and changing the upper mantle's average make up. The work may reveal the resting places of ancient plates swallowed by the mantle.