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Subduction: the plates collide and one plate is forced under the other. Often, the plate that is forced under will melt when it gets deep and hot enough and the resulting magma forces its way back to the surface as a volcano.
Collision: The plates collide and buckle and fold. Mountains may result. Take India, for example.
Remember this? That’s India. Note that it’s on its own tectonic plate Millions of years ago, the thing rampaged north and crashed into Asia. It created the Himalaya Mountains, including Mt. Everest. Even today, the mountains are still growing by a few millimeters per year. They’ve also found ancient ocean fossils in these, the highest points on Earth.
The fault is the line at which the plates rub past each other.
Normally (hopefully), the plates slide past each other steadily and gradually and we don’t notice it.
Sometimes, though, the crust at the fault sticks, but the plates keep moving. This builds up tremendous pressure on the stuck crusts until they finally unstick and they suddenly move a lot at once. This is when the earthquake happens.
All that kinetic energy is released at once instead of gradually.
The Richter Scale determines the relative strength of an earthquake.
To give you an idea of the energy released by an earthquake, the largest thermonuclear device ever detonated measured 7.0 on the Richter and released energy equivalent to 50 megatons (50 million tons) of TNT.
The 2004 earthquake in the Indian Ocean that caused the tsunami measured 9.0 and released energy equivalent to 30 gigatons (30 BILLION tons) of TNT.
You should be familiar with this. It’s what happened on 12/26/04.
An earthquake occurs on the ocean floor or along the coast. The energy travels as waves through the water and across the ocean until it gets to a coast. The shallow region causes the wave to rush over land and flood the place.
Some are worried that a massive landslide (some 500 billion tons of rock slipping into the ocean) from the Cumbre Vieja volcano off the west coast of Africa could cause a massive tsunami that will devastate the East Coast of the U.S. Others think the concern is overblown and unwarranted.