What Makes the Earth Quake?

Much about the forces at work within the Earth remains unknown. But since the 1960s, geologists have increasingly explained seismic events through “plate tectonics.” At the core of this theory is a simple premise – that Earth’s seemingly solid surface is actually fractured and adrift.

Stop the World, I Want to Get Off

The Earth’s surface consists of a hard shell called the crust, which is broken into large chunks called plates. Plates can hold entire continents (as the North American Plate does) or entire oceans (as the Pacific Plate does). Often, they hold bits of both.

The crust is actually a hard exterior on top of a much deeper layer, the mantle. Most of the mantle is made of liquid rock called magma, melted by heat from an even deeper layer – the Earth’s core. The plates that make up the crust basically “float” on the mantle’s magma.

That means the plates and everything on them, including whole continents and oceans, are in continual motion. Their movement is slow, ranging from less than an inch a year to about 6 inches (15 cm) a year, but it’s real.

Setting Boundaries

At certain places – called “divergent plate boundaries” – two plates move away from each other, and magma bubbles up from below the surface and hardens into new crust. When such divergence occurs under an ocean (as it usually does), it’s called “seafloor spreading.” When it occurs on land, it produces a “rift valley.”

At other places – called “convergent plate boundaries” – two plates crash into each other. When two continental plates collide, the crunch they produce can slowly build entire mountain ranges. The highest mountains in the world, the Himalayas, just keep on rising, as the Indian and Eurasian plates continually push against each other.

When two oceanic plates converge, one generally dives deep beneath the other, creating an underwater trench. The deepest part of the Earth, the Marianas Trench, sinks almost 7 miles (11 km) below the ocean surface where the Pacific and Philippine plates meet.

Finally, seismologists are interested in “transform plate boundaries,” places on the surface where two plates slide alongside each other. The most widely known – the San Andreas Fault – runs through California, where the Pacific and North American plates have been sliding past each other for 10 million years.

Ready to Rumble

In some places, plates creep beneath, above, or beside each other in a slow and steady fashion. Such movement creates continual seismic activity, but it’s often so faint that we can’t even feel it. Yet in other areas, friction between two plates can cause them to lock together. When that happens, pressure builds until the friction is eventually overcome. In a sudden lurch, the two plates break free, releasing vast amounts of energy and causing a major earthquake.

The magnitude of an earthquake depends mainly on the amount of energy released, which is partly an effect of how long the two plates have been locked together. Durations can vary from mere minutes to centuries. The buildup helps seismologists predict the timing and intensity of earthquakes, occasionally leading to warnings that a “big one” is due. For now, that’s about as accurate as earthquake prediction gets.

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