Eruption of Volcanic Magnitude VII (Lake Nyos, Cameroon, 1978), was a massive poisonous gas release from the Lake Nyos crater. A second release in 1986 killed approximately 1,800 people in nearby villages. Volcanism on Iceland is thought to be related to the Iceland hot spot and its location underneath a weak patch of the North American plate. Iceland also contains a substantial amount of volcanism related to ice melting as Greenland glaciers flow into the sea, with both the Hekla and Eyjafjallajökull erupting since 1821. The largest volcano in the world is Mauna Loa, on Hawaii. Although several eruptions of explosive violence have occurred within historic times, most volcanoes tend to be effusive in character and thus are at least twice as large as they were during historical time. If Mount Erebus of Antarctica were to erupt again, it would be larger than all other volcanoes. The lava of Mount Etna of Sicily has built up an altitude of in only the last century, whereas other volcanoes show only moderate increases in elevation (like Pinatubo in the Philippines). The active volcanoes on earth account for less than a thousandth of the crust's surface area, but are responsible for almost all of its geologic activity. This figure is probably less than three percent of the total amount of magma produced by all volcanoes since their origin. List of historical eruptions Modern volcanism Volcanology Modern volcanism has generally been observed and studied using the techniques of seismology and geomagnetism. The physical nature of volcanism has been studied for over a century, and volcanologists use different techniques to study lava flows and volcanic gases in their research. The classification of volcanic eruptions is a common method used in volcanology. In classifying an eruption, scientists consider the number, size, and explosiveness of the erupted rocks, the volume of erupted material, and any magma interactions with the Earth's surface and water. The word volcano comes from Vulcan or Vulcano, the Roman name for the Greek god of fire, and is derived from a Latin word meaning forge. Epicentre is a term used in volcanology. An earthquake is said to have an epicenter, which is the point of a quake that makes an oscillation, or oscillation is a small movement. The place where the maximum shaking occurs at the time of the quake is called the epicentre. Tectonic evolution Volcanic activity is thought to be one of the most important processes that shape the geology and topography of the Earth. The continental crust can be divided into terranes which are fragments of continent that have been accreted to a different continent through plate tectonics. Volcanic activity causes earthquakes, uplift, and subsidence. Magma rises to the surface from deep in the earth in zones that are called the "magma chambers". The magma chambers are connected to the crust in various ways including sills, dikes, conduits, and fissures. Magma is often less dense than the rock of the overlying crust, so a chamber forms in the crust. On continents, volcanic eruptions often occur in craters which are formed by the collapse of the magma chamber. Stratigraphy In general, volcanic rocks are defined by the absence of sedimentary rock. One indication is the presence of volcanic ash or pumice. Another is the distinctive crystalline texture that differentiates volcanic rocks from sedimentary rocks. This distinction is based on the fact that volcanic rocks are formed by crystallization from magma, whereas sedimentary rocks are formed from chemical precipitation or mineral crystallization in an aqueous solution. It is likely that all basaltic lava had been subducted prior to erupting, thus becoming sedimentary rock. However, in some cases, volcanic rocks can be classified as sedimentary, and vice versa. Volcanic landforms are classified as volcanic cones, calderas, craters, shield volcanoes, stratovolcanoes, cinder cones, and composite cones. Formed by volcanic ash eruptions, ash cones are the product of hot gases and superheated ash. They are the source for volcanic ash, pumice, and lapilli. Cinder cones consist of a heap of cinders (small angular pieces of material) that were swept up by hot gas and pyroclastic flows. Cinder cones are often found near calderas. Rivers of flowing lava In the tropics, rivers of lava emerge from fissures and then follow the paths of drainage, flowing downhill to the oceans. These volcanic rivers of molten rock appear as flowing lines of hardened lava. The channels between the lava are usually filled with fine sediments washed into the rivers by rain or snow. Lava also occurs as small, isolated vents. Lava lake An example of a lava lake is the caldera of the Taupo volcano, located in the North Island of New Zealand. The lake was created by the eruption of volcanic gases that filled the enormous crater. This lake is the largest known lake of molten rock, and is 4,000 meters deep. Scientists consider this phenomenon an important indicator of an ongoing eruption because no known lava lake has ever disappeared or decreased in size, and a decrease in size would signal a possible eruption. The Taupo Volcanic Zone includes at least 30 active volcanoes. Plinian eruption A Plinian eruption is a massive explosion that hurls large amounts of volcanic ash, and rock into the atmosphere, leaving an ash-laden atmosphere. It can start with magma fragmentation or phreatic explosion, with the latter being more common. Phreatic explosions are air explosions of steam generated by heating rock which can precede an eruption. This is called a phreatic eruption. The Plinian eruptions of Mount Vesuvius show pyroclastic flows and ash clouds. One of the best preserved Plinian eruptions, and probably the largest, is the one that occurred at the Taupo Volcanic Zone in the North Island of New Zealand. During the 1855 eruption of Taupo, at least 69 billion cubic metres of lava was erupted from vents less than a mile from the summit. It is estimated that the total amount of pyroclastic flow material was about 27 cubic kilometres. The Yellowstone Caldera is a supervolcano located in the western United States and northwest Canada. The last Plinian eruption occurred in the year 640,000 B.C. During the time of peak volcanic activity, the volcano would have erupted more than 10 times per year. Volcanic activity and nuclear accidents Numerous volcanoes and volcano-related sites are linked to human activity. Since they were known in historical times, most anthropogenic vents have been active within historic times. For example, the large volcano-and-caldera chain in the Andes was formed by humans through volcanism. There are no known Holocene instances of natural volcanism in the volcano-and-caldera chain, but there is active volcanism in the area today. Some volcanic sites were linked with nuclear accidents and radioactive contamination. For example, the Teapot Dome oil fields are situated on the Mid-Cretaceous Columbia River Basalt Group in the U.S., and the Cinder Cone Geysers is also a location associated with nuclear accidents. On 3 January 1983, the Union Carbide Corporation, along with the Department of Energy, opened an underground nuclear test site in the NTS of southern New Mexico with explosives. The explosion was followed by a fire which spewed black soot and ash over the area. Ashes reached all the way to the Texas-Mexico border. The largest volcanic eruption during human history occurred in the Yellowstone Caldera at an event known as the Yellowstone eruption. The eruption is well known to have spewed ash and destroyed parts of the forest in both Canada and the United States. Causes of volcanism The interior of Earth is composed of three main layers which make up the lithosphere: upper mantle, lower mantle, and crust. These three layers contain three major tectonic processes which cause volcanism: convection, plate tectonics, and mantle plumes. Convection Convection currents in the mantle are responsible for the formation of new land in a process known as continental drift. It forms plates, mantle plumes, and new seafloor. As the plates move apart, they sink beneath the seafloor and new areas of land are formed. Plate tectonics The earth's lithosphere is broken into about 30 or so major and minor plates. Each of these plates move about the earth's mantle and crust. Plate tectonics are responsible for most volcanic activity because they add new crust and form volcanic islands. Plate movements are caused by shifting, or subduction, of plates. Subduction occurs when one plate (known as a descending plate) moves underneath another plate (the overriding plate) to form new oceanic crust in a process known as seafloor spreading. The downward-moving oceanic plate is said to be subducted beneath the overlying continental plate, and the downward movement of the plate causes oceanic crust to melt and be drawn into the subduction zone. Mantle plumes The Earth's mantle is made up of several sections called tectonic plates that float over the Earth's surface. Mantle plumes are columns of hot rock that rise from the mantle to the surface and cause