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.
Speed and Direction
But the speed of the seismic waves can also tell in us which direction, if any, the rock is moving.Â 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.
And the direction the crystals are pointing makes a difference to the speed seismic waves travel through the rock:Â a wave traveling parallel to the crystals--in other words, along the length of the toothpick--moves faster than a wave traveling in the other direction, across the width of the crystals.
So if seismic waves going through one part of the earth always travel faster in one direction than they do in another, there's a good chance that the rock there is moving in that direction.
Based on this kind of information geologists can construct models of what's going on in parts of the earth no one has ever seen--models that show us the slow movement of molten rock, welling up from the earth's core, escaping here and there through volcanoes, and slowly pushing the continents around on the earth's surface.
Read More: Almanac of Science & Technology: What's New and What's Known (Amazon)