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Seismic tomography is the geophysical analogue of a medical CAT scan.
Seismologists measure the speed of seismic waves generated by large earthquakes (white circles) that propagate through the mantle. They can then map temperature variations in the mantle, because seismic waves travel more slowly though hotter regions (yellow and red) and faster through colder regions (green).
The green region here marks the slab of old oceanic plate plunging back into the mantle at the Tonga Trench in the South Pacific Ocean.[back]
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