Picture yourself enjoying a warm, sunny day in Los Angeles, when the tranquility is suddenly disrupted by a fearsome destructive force. A resonant rumble slowly starts to build up below your feet, and suddenly the ground itself seems to be trying to rip itself apart. What is an earthquake, really? And what forces of nature cause these powerful phenomena?

To explain how earthquakes are caused, one must first study a few things about the make-up of the planet Earth itself. The outer layer of the earth’s surface, known as the lithosphere, is made up of many large plates which float upon the layer below, which is mostly made up of molten rock.

These tectonic plates are either oceanic or continental plates, meaning they either support on their surface an expanse of ocean or a large landmass. Due to the molten, unstable nature of the layer these plates rest upon, they are constantly moving and shifting around, and often times they bump into one another. Depending on the nature of contact and the types of plates doing it, these collisions can be quite catastrophic.

Sometimes, magma from below the lithosphere will manage to make its way up, usually between two oceanic plates. The magma then rapidly cools and hardens, which forces the two plates apart this is generally called an oceanic divergent boundary and is responsible for the massive rift in the middle of the Atlantic Ocean.

These plates that have been moved by a divergent force have nowhere to go but towards other plates; when two plates run into each other, it is called a convergent boundary. If the two plates are both oceanic, then one of the plates is forced underneath the other, or “subducted”; the plate which is subducting eventually heats up and forms magma, which rises to the surface, often creating chains of islands or even volcanoes near where these boundaries occur.

However, the other two situations that occur in plate tectonics are ones that are of great interest to those living near the boundaries. The first is when an oceanic plate converges with a continental plate; the oceanic plate is subducted, but during the process, it often gets caught along the edges of the plate.

Intense amounts of pressure and force drive these two plates into each other, which must be released onto the surface above in the form of earthquakes. These are some of the most powerful earthquakes, and one example is the plate boundary which runs along the western border of Canada and surrounding areas.

The other important creators of earthquakes, albeit slightly less forceful ones, are what are known as transform faults. These occur where two plates run alongside each other, never running head-on into one another, but still causing a significant amount of friction along their border.

The earthquakes that are formed as a result of this rubbing together are usually closer to the surface than those caused by subduction; they also tend to create linear patterns that are much straighter. The most famous example of a transform fault is the San Andreas Fault, which runs along the coast of California, and has caused much grief to the residents therein.

These powerful forces of nature are scary due both to their destructive capabilities and their inability to be controlled or predicted with perfect accuracy, but we can at least be thankful that they are usually limited to specific areas of the globe.

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