VOLCANO:

a Volcano is a mountain or hill formed by the accumulation of materials erupted through one or more openings called volcanic vents in the earth's surface. volcano can also refer to the vents themselves. Most volcanoes have steep sides, but some can be gently sloping mountains or even flat tablelands, plateaus, or even plains. volcanoes above sea level are the best known, but the vast majority of the world's volcanoes exist beneath the sea. According to the Smithsonian Institution, 1,511 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 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 flows (see Volcano Hazards below). 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.

HOW

A VOLCANO IS FORMED

All volcanoes are formed by the accumulation of magma which is molten 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 mantle, 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 mi) 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 toward 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 erupts onto the surface. With each eruption, whether explosive 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 the earth's volcanoes are formed beneath the oceans, and their locations have been documented in recent decades by mapping of the ocean floor. 

DANGERS AND THREATS POSED BY VOLCANOES TO MAN

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 of 850° C (1600° F) or higher and fast speeds of 250 km/h (160 mph) or greater. 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 concretelike 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. Pyroclastic and volcanic debris flows have caused the most eruption-related deaths in the 20th century. 

Eruption in Colombia

In November 1985 the Nevado del Ruiz volcano in the Cordillera Central region of the Andes Mountains in Colombia erupted after lying dormant since 1845. The eruption caused massive mud flows that swept away neighboring towns and villages, leaving some 23,000 people either dead or missing

Indirect hazards are usually nonvolcanic effects that accompany or follow eruptions. Examples are earthquakes, tsunamis, rainfall-caused debris flow, and posteruption disease and famine. Tsunamis are large seismic sea waves generated by sudden movement of the seafloor. This sudden seafloor movement can be caused by a large earthquake or by the collapse of an island volcano during or after an eruption. Tsunamis can devastate low-lying coastal areas and can be deadly if people living in such areas are not evacuated. Indirect hazards also include volcanic deposits from large eruptions. These deposits can blanket farm fields and grazing lands, leading to the loss of crops and livestock and ultimately to the starvation of people dependent on them for life. During the period from the 17th century to the 19th century, tsunamis and posteruption starvation and disease caused most eruption-related deaths

Starting in the early 1980s, another indirect volcanic hazard began to attract increasing attention: jet aircraft encounters with airborne volcanic ash. More than 60 airplanes, mostly commercial jetliners, have been damaged by such encounters.