This is a three-dimensional structure of the Earth's mantle under western North America, down to a depth of 1500 kilometers.
Seismic tomography uses waves generated by naturally occurring, strong earthquakes to probe and "illuminate" the planet's deep interior. Hundreds of thousands of measurements were combined to create this 3-D reconstruction, using algorithms similar to those that deliver 3-D images of the human body in medical tomography.
The Pacific coastlines and mountainous western states are plotted above an expansive, seismically fast structure, colored in purple, known as the Farallon plate. It is a vast piece of ancient ocean floor that has been slowly sinking back into the mantle over the past 150 million years. The tectonic stresses caused by the Farallon's movements deep underground have thrust up the mountain peaks and plateaus of the West. They continue to drive its volcanoes and earthquakes, thus shaping the surface appearance of an entire continent.