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Types of Volcano eruptions

A volcano is a mountain that opens downward to a pool of molten rock below the surface of the earth. A volcano is a vent in the Earth from which molten rock (magma) and gas erupt. The molten rock that erupts from the volcano (lava) forms a hill or mountain around the vent.

All volcanic eruptions are not alike. Some eruptions, are quiet, with lava slowly oozing from a vent. Other eruptions are very violent, with lava and other materials being hurled hundreds of miles into the air. Gases from within the earth’s interior mix with huge quantities of dust and ash and rise into the air as great dark clouds that can be seen from many kilometers away.

Some dark-colored lava is thin and runny, and tends to flow. Explosive eruptions are caused when lava in the vents hardens into rock. Steam and lava build up under the rocks. When the pressure of the steam and new lava becomes great, a violent explosion occurs. When pressure builds up, eruptions occur. Gases and rock shoot up through the opening and spill over or fill the air with lava fragments. Volcano eruptions have been known to knock down entire forests. An erupting volcano can trigger tsunamis, flashfloods, earthquakes, mudflows and rockfalls.

Fresh volcanic ash, made of pulverized rock, can be harsh, acidic, gritty, glassy and smelly.

Types of Volcanoes

The form of a volcano is determined by the ingredients of the erupting magma. Their shapes are determined by the explosivity of the eruptions and to the amount of water in the magma.

Shield Volcanoes – are large volcanic forms with broad summit areas and low, sloping sides. This type of volcano is caused by slow, basaltic lava flows. A good example of a shield volcano is the island of Hawaii (the “Big Island”).

Cinder Cones

Cinder cones are mounds that are formed by streaming gases that carry lava blobs and ribbons into the atmosphere to form lava fountains. The lava blobs commonly harden during flight through the air before landing on the ground. If gas pressure drops, the final stage of building a cinder cone may be a lava flow that breaks through the base of the cone. The longer the eruption, the higher the cone. Some are no larger than a few meters and others rise to as high as 610 meters or more, such as Paricutin volcano, Mexico, that was a nearly continuous eruption from 1943 to 1952 and eventually destroyed the village.

Composite Volcanoes (Stratovolcanoes)

Composite volcanoes are constructed from multiple eruptions, sometimes recurring over hundreds of thousands of years, sometimes over a few hundred. Most of the tallest volcanoes are composite volcanoes. These form from a cycle of quiet eruptions of fluid lava followed by explosive eruptions of viscous lava. The fluid lava creates an erosion-resistant shell over the explosive debris, forming strong, steep-sided volcanic cones

Volcanic Activity

Volcanoes are rather unpredictable phenomena. Some volcanoes erupt fairly regularly; others have not erupted within modern history. In order to indicate the relative activity of volcanoes, scientist classify them as active, dormant, or extinct. An active volcano is one that erupts either continually or periodically.

Volcanic Eruptions

Some mild eruptions merely discharge steam and other gases, whereas other eruptions quietly extrude quantities of lava. The most spectacular eruptions consist of violent explosions that blast great clouds of gas-laden debris into the atmosphere.

The type of volcanic eruption Is often labeled with the name of a well-known volcano where characteristic behavior is similar–hence the use of such terms as “Strombolian,” “Vulcanian,” “Vesuvian,” “Pelean,” “Hawaiian,” and others. Some volcanoes may exhibit only one characteristic type of eruption during an interval of activity–others may display an entire sequence of types.

The most powerful eruptions are called “plinian” and involve the explosive ejection of relatively viscous lava. Large plinian eruptions–such as during 18 May 1980 at Mount St. Helens -can send ash and volcanic gas tens of miles into the air. The resulting ash fallout can affect large areas hundreds of miles downwind. Fast-moving deadly pyroclastic flows (“nuées ardentes”) are also commonly associated with plinian eruptions.

”  Preatic” ( or steam-blast) eruptions are driven by explosive expanding steam resulting from cold ground or surface water coming into contact with hot rock or magma. The distinguishing feature of phreatic explosions is that they only blast out fragments of preexisting solid rock from the volcanic conduit; no new magma is erupted.

In a Strombolian-type eruption observed during the 1965 activity of Irazú Volcano in Costa Rica, huge clots of molten lava burst from the summit crater to form luminous arcs through the sky. Collecting on the flanks of the cone, lava clots combined to stream down the slopes in fiery rivulets.

Hawaiian” eruptions may occur along fissures or fractures that serve as linear vents, such as during the eruption of Mauna Loa Volcano in Hawaii in 1950; or they may occur at a central vent such as during the 1959 eruption in Kilauea Iki Crater of Kilauea Volcano, Hawaii. In fissure-type eruptions, molten, incandescent lava spurts from a fissure on the volcano’s rift zone and feeds lava streams that flow downslope.

In contrast, the eruptive activity of Parícutin Volcano in 1947 demonstrated a “Vulcanian”-type eruption, in which a dense cloud of ash-laden gas explodes from the crater and rises high above the peak. Steaming ash forms a whitish cloud near the upper level of the cone.

In a “Vesuvian” eruption, as typified by the eruption of Mount Vesuvius in Italy in A.D. 79, great quantities of ash-laden gas are violently discharged to form cauliflower-shaped cloud high above the volcano.

In a “Peléan” or “Nuée Ardente (glowing cloud) eruption, such as occurred on the Mayon Volcano in the Philippines in 1968, a large quantity of gas, dust, ash, and incandescent lava fragments are blown out of a central crater, fall back, and form tongue-like, glowing avalanches that move downslope at velocities as great as 100 miles per hour.

Some Volcanoe Facts :

The May 18, 1980 eruption of Mount St. Helens in the Cascade Range of Washington State happened after more than 100 years of dormancy ( a time when the volcano was “asleep.”) When the volcano erupted, it took the lives of 58 people and caused $1.2 billion in damage.

There are more than 500 active volcanoes in the world.

The rock debris carried by a lateral blast of Mt. St. Helens traveled as fast as 250 miles per hour.

Crater Lake in Oregon formed from a high volcano that lost its top after a series of tremendous explosions about 6,600 years ago.

More than 80 percent of the earth’s surface is volcanic in origin. The sea floor and some mountains were formed by countless volcanic eruptions. Gaseous emissions from volcano formed the earth’s atmosphere.

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