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A Volcano is a Landform Created by Magma From the Earth

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  A volcano is a landform created by magma from the earth's interior which penetrates through weaknesses in the Earths surface. Most volcanoes are formed at plate boundaries.  A volcano is a mountain that opens downward to a reservoir of molten rock below the surface of the earth. Unlike most mountains, which are pushed up from below, volcanoes are vents through which molten rock escapes to the earth’s surface. When pressure from gases within the molten rock becomes too great, an eruption occurs. Eruptions can be quiet or explosive. There may be lava flows, flattened landscapes, poisonous gases, and flying rock and ash that can sometimes travel hundreds of miles downwind. Magma contains a mixture of silicates and dissolved gases. As magma rises closer to the Earths surface the pressure decreases and the dissolved gases come out of solution to form bubbles. As the bubbles expand the magma is pushed into the volcanic vent until it breaks through weaknesses in the Earth's crust. The explosive potential of the volcano is dependant on the effervescence of the gases and the viscosity of the magma. High gas effervescence and low viscosity cause the most explosive eruptions. As previously mentioned volcanoes occur at weaknesses in the Earths surface, these are in one of three tectonic settings: subduction volcanoes, rift volcanoes and hot spot volcanoes. Subduction volcanoes occur where one tectonic plate is thrust and consumed by another. This type is the most explosive and comprises approximately 80% of the worlds active volcanoes. Rift volcanoes occur where tectonic plates diverge, often on the ocean floor. These are generally less explosive. Hot spot volcanoes occur in the middle of plate boundaries where magma exits from weaknesses in the earth's surface. The Hawaiian Islands are an example of hot spot volcanoes. The most active area of volcanic activity occurs along the pacific plate boundary which is often referred to as the 'Pacific ring of fire'. There are about 500 active volcanoes thoughout the world. In an average year approximately 50 of these erupt. Volcanic hazards create fewer disasters and deaths compared with earthquakes and severe storms. Many of the deaths associated with volcanoes are indirect hazards such as famine due to crop damage or from secondary hazards such as lahars. Volcanoes are composite hazards. There are both primary and secondary hazards which can be caused by volcanic eruptions. The primary hazards include pyroclastic flows, air-fall tephra, lava flows and volcanic gases. The secondary hazards include ground deformation, lahars (mudflows), landslides and possibly tsunamis in ocean floor volcanic eruptions.   A volcano is a mountain that opens downward to a reservoir of molten rock below the surface of the earth. Unlike most mountains, which are pushed up from below, volcanoes are vents through which molten rock escapes to the earth’s surface. When pressure from gases within the molten rock becomes too great, an eruption occurs. Eruptions can be quiet or explosive. There may be lava flows, flattened landscapes, poisonous gases, and flying rock and ash that can sometimes travel hundreds of miles downwind. Because of their intense heat, lava flows are great fire hazards. Lava flows destroy everything in their path, but most move slowly enough that people can move out of the way. Fresh volcanic ash, made of pulverized rock, can be abrasive, acidic, gritty, gassy and odorous. While not immediately dangerous to most adults, the acidic gas and ash can cause lung damage to small infants, to older adults and to those suffering from severe respiratory illnesses. Volcanic ash also can damage machinery, including engines and electrical equipment. Ash accumulations mixed with water become heavy and can collapse roofs. Volcanic ash can affect people hundreds of miles away from the cone of a volcano.    An earthquake is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called the fault or fault plane. The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter. Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens. The largest, main earthquake is called the mainshock. Mainshocks always have aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock! What causes earthquakes and where do they happen? The earth has four major layers: the inner core, outer core, mantle and crust. (figure 2) The crust and the top of the mantle make up a thin skin on the surface of our planet. But this skin is not all in one piece  –  it is made up of many pieces like a puzzle covering the surface of the earth.  (figure 3) Not only that, but these puzzle pieces keep slowly moving around, sliding past one another and bumping into each other. We call these puzzle pieces tectonic plates, and the edges of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake. Why does the earth shake when there is an earthquake? While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us! (see P&S Wave inset) How are earthquakes recorded?
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