A volcano is a mountain or hill formed by the accumulation of materials erupted through one or more openings in the earth's surface. Most volcanoes have steep sides but sometimes they can slope down or even be flat. The volcanoes above sea level are the best known, but the most volcanoes lie beneath the sea, formed along the global oceanic ridge. Volcanic eruptions in populated regions are a significant threat to people, property, and agriculture.

The danger is mostly from fast-moving, hot flows of explosively erupted materials, falling ash, and highly destructive lava flows and volcanic debris. In addition, explosive eruptions, even from volcanoes in unpopulated regions, can eject ash high into the atmosphere, creating drifting volcanic ash clouds that pose a serious hazard to airplanes.Lava is magma that breaks the surface and erupts from a volcano. If the magma is very fluid, it flows rapidly down the volcano's slopes.

Lava that is more sticky and less fluid moves slower. Lava flows that have a continuous, smooth, ropy, or billowy surface are called pahoehoe flows. But aa flows have a jagged surface composed of loose, irregularly shaped lava chunks. Once cooled, pahoehoe forms smooth rocks, while aa forms jagged rocks.

The words pahoehoe and aa are Hawaiian terms that describe the texture of the lava
All volcanoes are formed by the accumulation of. Magma can erupt through one or more volcanic vents, which can be a single opening, a cluster of openings, or a long crack, called a fissure vent. It forms deep within the earth, generally within the upper part of the mantle one of the layers of the earth's crust. Or less commonly within the base of the earths crust. High temperatures and pressures are needed to form magma. Volcanic activity ranges from emission of gases, non-explosive lava emissions to extremely violent explosive bursts that may last many hours.

The types of eruptions determine the relative volumes and types of volcaniclastic material and lava flows, consequently the shapes and sizes of volcanoes. A volcanic event occurs when there is a sudden or continuing release of energy caused by near-surface or surface magma movement. The energy can be in the form of earthquakes, gas-emission at the surface, release of heat, explosive release of gases and the non-explosive extrusion or intrusion of magma. An event could be non-destructive without release of solids or magmatic liquid, or if there is anything to destroy, could be destructive with voluminous lava flows or explosive activity. A volcanic event can include an eruptive pulse usually an explosion with an eruption plume, but also non-explosive surges of lava.

A pulse may last a few seconds to minutes. Then an eruptive phase that may last a few hours to days and consist of numerous eruptive pulses that may alternate between explosions and lava surges. Or a single eruption or eruptive episode, composed of several phases that may last a few days or even months. Some volcanoes may form completely within a few weeks or months. Others, such as shield volcanoes and composite volcanoes may show high order discontinuities such as major chemical changes, volcano-tectonic events like caldera collapse, or long erosional intervals.During a single eruption, styles of activity and types of products may change within minutes or hours, depending upon changes in magma composition, volatiles, or other magma chamber and vent conditions.

Volcanic eruptions and eruptive phases are traditionally classified according to a wide range of criteria. Many have been given names from volcanoes where a certain type of behavior. Common eruptions types are Plinian, Hawaiian, Strombolian, and Vulcanian. Volcanoes occur when magma makes its way to the surface of the earth. One type of volcanic eruption is a rift eruption. A rift eruption occurs at a diverging boundary.

They are non-violent eruptions, and mafic magma is usually involved. Subduction eruptions occur at a subduction boundary. They are usually very violent, and involve felsic magma. Hot Spot eruptions occur in the middle of a plate, not at a boundary. They are non-violent. Magma is molten rock found in the mantle.

Mafic magma is low in silica, dark colored magma and does not trap gases, forms basalt. Felsic magma is rich in silica, light colored magma, traps gases, forms granite.Volcanoes erupt differently depending on the composition of the magma beneath the surface, the amount of gas in the magma, and the type of vent from which it erupts. In general, the more viscous, or stiffer, the lava, the more explosive the eruptive activity. During explosive eruptions, the lava erupted is torn into shreds, forming a variety of fragmental or pyroclastic materials depending on the physical state of the lava and on the force of the explosions. Explosive eruptions can eject a large amount of material into the air.

Nonexplosive eruptions produce lava flows and eject very little pyroclastic material into the air.Gases, primarily in the form of steam, are released from volcanoes during eruptions. All eruptions, explosive or nonexplosive, are accompanied by the release of volcanic gas. The sudden escape of high-pressure volcanic gas from magma is the driving force for eruptions.

Gases come from the magma itself or from the hot magma coming into contact with water in the ground
Columnar jointing forms in lava flows, sills, dikes, ignimbrites and shallow intrusions of all compositions. Most columns are straight with parallel sides and diameters from a few centimeters to 3 m. Some columns are curved ; vary in width. Most columns tend to have 5 or 6 sides but have as few as 3 and as many as 7 sides.

The columns form due to stress as the lava. The lava contracts as it cools, forming cracks. Once the crack develops it continues to grow. The growth is perpendicular to the surface of the flow
Plinian Eruptions are widely dispersed sheets of pumice and ash are derived from high eruption columns that result from high-velocity voluminous gas-rich eruptions, commonly lasting for several hours to about four days. Plinian eruptions commonly produce high eruption columns. The energy and characteristics of a Plinian eruption depends on gas content of the magma, exit pressure, viscosity, vent radius and shape, and volume of magma erupted.

Most Plinian eruptions result from explosions of highly evolved rhyolitic to dacitic, trachytic and phonolitic magmas with temperatures from about 750 to 1000 Celsius. Volcanoes come in different shapes and sizes, depending on the makeup of the magma, the style of the eruption, and how often they erupt. The major types of volcanoes, roughly in order of increasing size, are cinder cones, composite volcanoes, shield volcanoes, calderas, and plateaus.
Cinder cones and composite volcanoes have the familiar cone like shape that people most often associate with volcanoes. Although both cinder cones and composite volcanoes are mostly the results of explosive eruptions, cinder cones consist exclusively of fragmental lava. This fragmental lava is erupted explosively and made up of cinders.

Cinder cones are typically much smaller than composite volcanoes for two reasons. They involve only weakly explosive, small volume eruptions of basaltic cinder that does not travel far from the vent; and they usually have a short life often only a single eruptive burst before becoming extinct. Composite volcanoes can grow much larger because they represent the accumulated products of repeated eruptions from the same vents over a long time.Composite volcanoes are composed of explosively erupted pyroclastic materials layered with non-explosively erupted lava flows and deposits of volcanic debris. They are mostly built from materials that come from andesitic or dacitic lava. In some composite volcanoes that undergo a major explosive eruption non-explosive extrusions of lava within the summit crater can later construct a bulbous mound of accumulated lava.

This mound is called a lava dome or a volcanic dome.Shield volcanoes get their name from their distinctive, gently sloping mound-like. Their shapes reflect the fact that they are constructed mainly of countless fluid basaltic lava flows that erupted non-explosively. Such flows can easily spread great distances from the feeding volcanic vents, similar to the spreading out of hot syrup poured onto a plate.

Volcanic shields may be either small or large, and the largest shield volcanoes are many times larger than the largest composite volcanoes. A caldera is a round or oval-shaped low-lying area that forms when the ground collapses because of explosive eruptions. An explosive eruption can explode the top off of the mountain or eject all of the magma that is inside the volcano. Either of these actions may cause the volcano to collapse. Calderas can be bigger than the largest shield volcanoes in diameter.

Such volcanic features they are often outlined by an irregular, steep-walled boundary, which reflects the original ring, like zone, or fault, along which the ground collapse occurred. Some calderas have hills and mountains rising within them, called resurgent domes that reflect volcanic activity after the initial collapse.Some of the largest volcanic features on earth do not actually look like volcanoes. Instead, they form extensive, nearly flat-topped accumulations of erupted materials.

These materials form volcanic plateaus or plains covering many thousands of square kilometers. The volcanic materials can be either very fluid basaltic lava flows or far-traveled pyroclastic flows. The basaltic lava flows are called flood or plateau basalts and are erupted from many fissure vents.The magma-forming regions of the earth and the volcanoes built above them and are confined to several zones and special places.

Eruptions pose direct and indirect volcano hazards to people and property, both on the ground and in the air. Direct hazards are pyroclastic flows, lava flows, falling ash, and debris flows. Pyroclastic flows are mixtures of hot ash, rock fragments, and gas. They are especially deadly because of their high temperatures and fast speeds. Lava flows, which move much more slowly than pyroclastic flows, are rarely life threatening but can produce massive property damage and economic loss. Heavy accumulations of volcanic ash, especially if they become wet from rainfall, can collapse roofs and damage crops.

Debris flows called lahars are composed of wet concrete like mixtures of volcanic debris and water from melted snow or ice or heavy rainfall. Lahars can travel quickly through valleys, destroying everything in their paths.
If the eruption is nonexplosive, as is typical for Hawaiian volcanoes, lava flows are produced. The lava comes out of rifts in the sides of the volcano, or vents in a rift.

Tephra is rarely ejected during a nonexplosive eruption. Nonexplosive eruptions are characterized by basaltic lava and by the type of volcanoes they form, called shield volcanoes. Hawaiian eruptions consist of basaltic, highly fluid lavas of low gas content that produce effusive lava flows and some pyroclastic debris. Thin, fluid lava flows can gradually build up large broad shield volcanoes.

Most Hawaiian eruptions start from fissures, commonly beginning as a line of lava fountains that eventually concentrate at one or more central vents. Most of the vesiculating lava falls back in a still molten condition, coalesces and moves away as lava flows. If fountains are weak, most lava will quietly well out of the ground and move away from a vent as a lava flow. Much lava in shield volcanoes is transmitted through tubes enclosed within lava flows. Strombolian eruptions, named after Stromboli Volcano, Italy, are discrete explosions separated by periods of less than a second to several hours.

They give rise to ash columns and abundant ballistic debris.Volcanic eruptions can obviously cause serious human, economic, and environmental impacts, but volcanoes can also be rich in natural resources. Perhaps the greatest resource from volcanoes is the land formed by the materials they erupt. Volcanic activity has created some of most scenic and fertile regions on earth.

1- "Volcano," Microsoft Encarta Encyclopedia 2000
2- http://volcano.und.nodak.edu/vwdocs/vw_exchange.html
3- http://library.

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