Overview: Tectonic Plates
The earth’s crust and upper mantle are not an even surface, but are made up of plates with irregular boundaries, called tectonic plates, that move and shift underneath the ocean and along the continents. Most of the time those plates move smoothly past each other and do not cause a problem. When irregularities along fault lines lock the plates, pressure builds up until energy is released in the form of seismic waves. Those sudden seismic waves result in an earthquake.
Measurement of Earthquakes
Earthquakes are measured by the Richter scale, as well as the moment-magnitude scale. It is a logarithmic base-10 scale of the ratio of waves measured by a seismometer. This means that every level is 10 times greater than the one before. For example, people usually do not feel earthquakes until they are at a level 3, but at Level 4, people notice shaking objects inside. The 1906 San Francisco earthquake was probably at a Level 8. If earthquakes are large enough, they can be recorded all over the world, because seismic waves travel through the earth.
Volcanoes and Earthquakes
The movement of magma in volcanoes also can cause earthquakes, especially just before an eruption. For example, hundreds of earthquakes were measured before the Mt. St. Helens eruption in Washington State in 1980. Clusters of earthquakes around a volcano alert geologists that an eruption might be imminent.
The Role of Water in Earthquakes
One of the largest fault zones in the world is the San Andreas Fault, located in California. It is famous for high geologic instability. In 2011, geologists measured how electricity was conducted by the types of rock found there. They were able to conclude that rocks found deep within the earth had high water content. The fluids trapped within rocks underneath the San Andreas Fault allow the plates to move more easily. When they get locked and stuck, then earthquakes happen.
The Role of Magma in Earthquakes
Geologists formed an hypothesis that the tectonic plates slid over one another, but they didn’t have direct evidence until very recently. In March, 2013, geologists actually found a layer of magma by measurements taken from a deep trench in the seafloor. They were able to discover an area of molten rock when they were mapping the crust and mantle. The electromagnetic measurements suggested molten rock rather than dissolved water causing the liquid movement.
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