Upwellings of light material at oceanic ridges compress plates in the direction of the heavier subducting side, and gravity continues to pull the heavy plate edge downward into the mantle. Scientists including Stanford geophysics Professor Norman Sleep study such recycling of continents. Like the legendary blind men who gave different descriptions of an elephant because one had felt the trunk, another the ears, another the tail, geologists have to look at complex data from many locations worldwide to get a true picture of how plates move. They travel through the planet in different ways, as compressional or shearing waves, and those differences reveal important information. Squeezed pulses alternate with expanded pulses.
Compressional waves can travel through anything.
Seismometers, motion sensors that detect earthquake waves, can distinguish compressional waves from shearing waves. Seismic data are collected at thousands of instrument stations worldwide and arrival time measurements are sent to repositories such as the International Data Center in England and the National Earthquake Information Center in Colorado. Arrival times are triangulated to pinpoint earthquake locations. By characterizing seismic waves according to their origins and time delays, scientists have been able to create a sort of X-ray image of the planet. Earth has a solid inner core of iron with a little nickel surrounded by a liquid outer core of mostly iron.