Plate Tectonics - What Causes Earthquakes
Geologists who study the Earth tell us that the continents and oceans are always moving. Sometimes this movement is quick and sudden and is what causes earthquakes. The Earth's surface is not as smooth as it may look from far away. Continents float on the Earth's outer skin, the crust. When hot, melted rock comes up from the inside of the Earth new crust is created. The old crust is pushed away and moves deeper down into the Earth where it is melted again. This movement of crust and continents is called plate tectonics.
The Earth's crust is not in one big piece. It consists of up to 20 plates. These plates sometimes crash into each other, at other times they move away from each other. When plates move, the continents move with them. Where these plates come together earthquakes occur.
Modern instruments show us that 90% of all earthquakes happen where these plates come together. Other earthquakes take place near the coasts of continents.
California, for example, is geologically not a part of North America. It lies on the Pacific plate which is moving northwest. The North American plate, on the other hand, is moving in the opposite direction. The line on which this happens is called the San Andreas Fault. Los Angeles is about 50 km away from this fault. Many smaller faults are in and around the city.
The Earth's continents have not always been in the place they are today. Hundreds of millions of years ago there was only one continent, called Pangea. This huge continent broke apart about 200 million years ago and started drifting into all directions.
Scientists found out that some of the rocks found in Brazil were the same as the rocks found in Western Africa. They also discovered that the ocean floor and the continents are not made up of the same material. Very often hot, melted rock comes up in valleys in the middle of oceans. It becomes hard and presses older rock away. The new ocean floor of the Atlantic is pushing Europe and America apart from each other. Not fast, but slowly, at the speed of 2 cm a year.
When the ocean floor presses against a continent it is forced down and melts again. That is why the Earth does not become bigger. New ocean floor is created, older one is destroyed.
Volcanoes also occur where plates come together. Scientists call these places "hot spots", areas where melted rock comes out of the inner part of the Earth. The Hawaiian Islands, for example, are in the center of one of these hot spots.
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- area = place, region
- cause = lead to
- coast = where the ocean meets land
- consist of = to be made up of
- create = make, produce
- crust = the hard outer layer of the Earth
- destroy = to damage something so badly that it does not exist any more
- discover = to find out for the first time
- drift = move slowly
- earthquake = when the Earth suddenly shakes; it destroys buildings and sometimes people are killed
- fault =a large crack in the Earth’s outer layer
- float = here: to move on
- force = here: to make something move
- huge = very big
- melt = if something turns into liquid
- movement = to change your position and go from one place to another
- occur = happen
- opposite = other
- plate = one of the large sheets of rock that form the surface of the Earth
- quick = fast
- scientist = someone who works in a laboratory or is trained in science
- smooth = flat, even
- speed = how fast something is
- sudden = unexpected
- surface = the top part of the Earth
- valley = an area of lower land between two lines of mountains