5: Tectonic Plates, Geologic Time, and Earthquakes
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No one doubts that the Earth is the most hospitable planet in the solar system. We have a breathable atmosphere, and the temperature, as Goldilocks said about the porridge, is “just right.” Venus is too hot, Mars is too cold, and the Moon and Mercury have no atmosphere at all to speak of.
But in terms of earthquakes, the other planets could be considered safer places to live than the Earth. That’s because the Earth’s outer shell is broken up into great slabs called plates that jostle and grind against one another like huge ice floes. In the process, all that crunching between plates forces parts of the crust up to create mountains, causing earthquakes in the process. In contrast, the crust of the other inner planets consists entirely of massive rock that experienced most of its mountain-building activity billions of years ago, soon after the planets were formed. Now the crustal movements on these planets have been stilled. There is no grinding of plates against one another to cause them to shake.
But the Earth has active volcanoes and earthquakes, which are geologic phenomena, and to understand them we need a brief introduction to their geologic setting. This requires us to stretch our minds to think about moving masses of rock that are extremely large, many tens of miles thick and hundreds of miles wide. We also must think of great lengths of time. Just as an astronomer asks us to think of great distances of hundreds of billions of miles, a geologist asks us to think about thousands, even millions of years. An earthquake may happen in less than thirty seconds, but it is a response to the slow-motion of massive tectonic plates on the surface of the Earth, building up strain over many thousands of years.
How do we study earthquakes? We can see the effects of past earthquakes in fault ruptures on the Earth’s surface. We can learn about earthquakes as they happen by the squiggles they make on a seismograph record. We can think about future earthquakes by measuring the slow buildup of tectonic strain in the Earth, using orbiting satellites and the Global Positioning System (GPS).