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An earthquake is a sudden motion or trembling in the earth caused by the abrupt release of slowly accumulated energy. All earthquakes occur along a fault, which is a fracture in the earth's crust where tectonic movement occurs. Where the actual break occurred along the fault is called the focus (also called the hypocenter) and the epicenter is the point on the Earth's surface that lies directly above the focus and is where the strongest shock wave is normally felt. Click here to watch a brief video on earthquakes.
Recall that all around the planet, tectonic plates are moving because of convection in the mantle. Tectonic plates are also composed of two types of crust, oceanic and continental. The oceanic crust, which is made mostly of basalt is more dense than continental crust that is made of granite. When these tectonic plates come in contact, the denser oceanic crust subducts below the continental crust. Now sometimes when two tectonic plate come in contact they become stuck. As the rocks begin to bend or strain under tectonic forces, large amounts of energy, called strain, builds. When the stress becomes too great for the rocks to hold, segments may suddenly snap, releasing large amounts of energy, called the elastic rebound theory.
There are several types of faults that earthquakes occur on, which are dependent on whether the fault is occurring because of convergent, divergent, or transform tectonic plate forcing. Geologists use old mining terms to distinguish between different types of faults. Think of a minor walking down into the earth along a fault line. The ground the miner is walking on is called the foot-wall. If the minor needs to hand their lantern, the ceiling is called the hanging-wall.
Strike-slip faults (A) occur along transform boundaries where tectonic plates are moving horizontal or parallel to each other. Deformation of rivers, roads, fences, etc. can occur if they cross over these fault lines. Examples of strike-slip faults are the San Andreas Fault in the United States and the North Anatolian Fault in Turkey.
Normal faults (B) are common along divergent plate boundaries. As extensional forces occur, the foot-wall is forced upward, while the hanging wall slides downward. This can create a series of valleys (called a graben) and mountains (called a horst). Examples of mountain ranges and valleys created by normal faulting are theGrand Tetons, the Basin and Range in the western United States, and the Wasatch Front in Utah.
Reverse faults (C) are caused by compressional forces as tectonic plates collide together forcing one plate to rise above another. Using the mining terminology, movement along a reverse fault would cause the hanging-wall to rise up and the foot-wall to drop lower. The angle of a reverse fault is about 45 degrees, but if the angle of the fault is steeper than 45 degrees it is called a thrust fault. When two plates collide, intense folding and faulting can occur. Examples of where reverse and thrust faults occur are where convergent boundaries are common such as: the Northern Rocky Mountains, the Alps, Himalayas, and the Appalachian mountains.
Earth is made up of a series of tectonic plates, each consisting of oceanic basalt and continental granite. Now because of convection within the mantle, new oceanic crust is created at divergent boundaries creating oceanic ridges. Where tectonic plates converge, the denser oceanic basalt subducts below the lighter continental granite. As the oceanic crust subducts deep enough it begins to melt under great heat and pressure to form molten rock called magma. The molten rock is less dense than the surrounding rock and thus rises to the surface to create volcanoes. Magma cools into rock much slower than on the surface because the heat gets trapped. Because of this, when magma reaches the surface, geologists call it lava.
Shield volcanoes tend to be the tallest volcanoes - and even the tallest mountains - in the world. These volcanoes tend to have gentle slopes with an arc in the shape of a Roman shield. It is their low viscosity lava flows that produces the gentle slopes. Eruptions tend to be mild in comparison to other volcanoes, but lava flows can destroy property and vegetation. The low viscosity magma can flow not only on the surface as lava, but also underground in lava tubes. The most well known shield volcano is Hawaii. There are two types of lava flows, pahoehoe which is a ropy type of lava that flows easily (low viscosity). The other type is called aa and is a blocky type of lava and has a higher viscosity and does not like to flow well.
Cinder cone volcanoes are the smallest type of volcanoes ranging from 300 to 650 feet high. The volcano is built up by eruptions of solid pyroclastic material, specifically tephra, along the volcanic neck. Many people live near cinder volcanoes because the weathered pyroclastic material becomes fertile soil for agriculture. These volcanoes kill few people, but can destroy property.
Composite volcanoes are some of the most dangerous volcanoes on the planet. They tend to occur along oceanic-to-continental and oceanic-to-oceanic convergent boundaries which produces highly viscous pyroclastic material that erupts violently when it reaches the surface. They are also called stratovolcanoes or andesite volcanoes because they erupt volcanic rock - called andesite - which builds up the volcano followed by lava which holds the material in place. This creates stratified layers within the volcano. Examples of composite volcanoes include Mount St. Helens, Mount Rainer, and Mount Pinatubo. Here's a great time-lapse of Mount St. Helens from NASA's Earth Observatory from 1979 to 2013.
Composite volcanoes and other violent volcanos can erupt so violently that they sometimes collapse in on themselves or actually blow themselves up to produce calderas. One of the most powerful volcanoes in the world - Yellowstone - is a massive caldera that has collapsed several times. Sometimes these calderas can fill up with water to produce beautiful lakes such as Mount Mazama (Crater Lake), in Oregon.
The theory of plate tectonics could never explain why some volcanoes form away from any tectonic plate boundaries. These anomaly volcanoes are called hot spots. Instead, they tend to form within tectonic plates in areas where the lithosphere is weak, which allows magma to rise up to the surface to create volcanoes. Though convection within the mantle causes tectonic plates to move, the hot spot does not. The hot spot stays stationary while the tectonic plate moves across it. Examples of hot spots include Hawaii and Yellowstone.
Hawaii is a shield volcano on top of a hot spot. It is a series of volcanic islands that have been created as the Pacific Plate has moved across the hot spot. Hawaii has gentle slopes and is the most active volcano in the world.
Open Geography Education by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License.
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