Hot and Cold Spots Below Us

Friday, November 01, 1996
To map Earth’s interior, geologists use a worldwide network of seismometers that chart the movement of seismic waves generated by earthquakes. These waves, originating in Earth’s crust or upper mantle, ricochet around the interior, traveling most rapidly through cold, dense regions, and more slowly through hotter rocks. The travel time of the waves reveals the extent and density of Earth’s deepest regions. Until now, though, geologists have lacked a detailed global picture of the narrow boundary between Earth’s molten iron core and the solid rocky mantle, because most seismic waves typically spend only seconds there before passing through or reflecting off it--not enough time to provide geologists with much information about its structure.

Geophysicist Michael Wysession at Washington University in St. Louis has recently created the first high-resolution global model of this elusive region. To construct the map, he selected two types of seismic waves: those that pass straight through the boundary and core, and those that hit the boundary at very low angles. These latter waves are diffracted at the boundary and travel along it for thousands of miles before surfacing.

It’s the same way that sound waves travel around a corner, says Wysession. If you’re standing around a corner and somebody talks, you can hear them. That’s because the sound is diffracting right around the corner. This is the same process.

Since the two types of waves travel similar paths until they reach the boundary, Wysession could isolate variations in the diffracted waves’ speed caused by their journey along the boundary by subtracting one wave type from the other. The map shows hotter regions in red. Cold regions (blue) underlie the areas where oceanic plates have sunk into Earth’s interior in the past, supporting the idea that dense slabs of oceanic crust may penetrate to the lower mantle.
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