The ground may feel solid, but far beneath our feet, extremely hot rock at extremely high pressure is flowing like slow, underground rivers. No one has ever seen that far into the earth, but we see the results of that movement in every earthquake and volcano. When an earthquake occurs, it sends what are called “seismic waves” in all directions. Because seismic waves travel faster through some kinds of rock than through others, geologists can guess what sort of rock the waves are traveling through based on how fast the waves travel through that part of the earth. But the speed of the seismic waves can also tell in us which direction, if any, the rock is moving. When soft, hot rock flows under high pressure, the crystals in the rock all line up like tiny toothpicks in the direction the rock is flowing.