Volcano Eruption

 A volcano is a mountain or hills formed by the accumulation of materials erupted through one or more openings (called volcanic vents) in the earth's surface. The term volcano can also refer to the vents themselves. Most volcanoes have steep sides, but some can be gently sloping mountains or ever flat tablelands, plateaus, or plains. The volcanoes above sea level are the best known, but the vast majority of the world's volcanoes lie beneath the sea, formed along the global oceanic ridge systems that crisscross the deep oceans floor. According to the Smithsonian Institution, 1511 above-sea volcanoes have been active during the past 10,000 years, 539 of them erupting one or more times during written history. On average, 50 to 60 above-sea volcanoes worldwide are active in any given year; about half of these are continuations of eruptions from previous years, and the rest are new.             
Volcanic eruptions in populated regions are a significant thread 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 flows. In addition, explosive eruptions, even from volcanoes in unpopulated reions, can eject ash high into the atmosphere, creating drifting volcanic ash clouds that pose a serious hazard to airplanes           

All volcanoes are formed by the accumulation of magma (moltem rock that forms below the earth's surface). 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 mentle (one of the layers of the earth's crust), or less commonly, within the base of the earth's crust. High temperatures and pressures are needed to form magma. The solid mantle or crustal rock must be melted under conditions typically reached at depths of 80 to 100 km (50 to 60 miles) below the earth's surface.            
Once tiny droplets of magma are formed, they begin to rise because the magma is less dense than the solid rock surrounding it. The processes that cause the magma to rise are poorly understood, but it generally moves upward toward lower pressure regions, squeezing into spaces between minerals within the solid rock. As the individual magma droplets rise, they join to form ever-larger blobs and move towards the surface. The larger the rising blob of magma, the easier it moves. Rising magma does not reach the surface in a steady manner but tends to accumulate in one or more underground storage regions, called magma reservoirs, before it eruption, whether explosion or nonexplosive, the material erupted adds another layer to the growing volcano. After many eruptions, the volcanic materials pile up around the vent or vents. These piles form a topographic feature, such as a hill, mountain, plateau, or crater that we recognize as a volcano. Most of earth's volcanoes are formed beneath the oceans, and their locations have been documented in recent decades by mapping of the oceans floor.
         Three different types of materials may erupt from an active volcano. These materials are lava, tephra (rock fragments), and gases. The type and amount of the material that erupts from an active volcano depends on the composition of the magma inside the volcano.
           
  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 moreexplosive 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 materials inot the air. Nonexplosive eruptions produce lava flows and eject very little pyroclastic material into the air.                
Volcanoes come in different shapes and sizes, depending on the makeup of then 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 (also called strato volcanoes), shield volcanoes, calderas, plateaus. Calderas and plateaus are shaped differently than traditional volcanoes neither has a mountain.