Active and Dormant Volcanoe volcano that is erupting or has erupted during historic times is called active. [img=http://2wapworld.com/get106155/c54b88408.jpg] Mt.Etna Italy (active) A Volcano in whthere is no evidence of any activity is consider extinct. Volcanoes that appear recent but have recorded activity other than the quiet escape are called dormant. [img=http://2wapworld.com/get106156/647ec7927.jpg] Some volcanoes are much more active than mt Stromboli, in the Lipari Islands near Sicily, has constantly active since ancient times. Many act volcanoes are found in a belt, called the Ring that circles the Pacific Ocean. Yet other volcano such as Mount Vesuvius, continue in a state of activity for longer or shorter periods and then become dormant. The eruption that follows dormancy is violent. This was shown in the 19 eruption of Mount Saint Helens in the state of Washington. It had been dormant for a period years of being in a dormant state. The danger posed by active volcanoes is not limited to era of molten rock or showers of ash and cinders. Mudflows are also equally troublesome. A mud triggered by an eruption that melted ice and sand on Colombia's Nevado del Ruiz volcano in 198 killed more than 25,000 people.

VOLCANO : DATE OF ERUPTION ACTIVITY Popocatepetl, Mexico: December 6, 1997 Bezymianny, Kamchatka December 5, 1997 Kilauea, Hawaii 1983-continuing Ruapehu, New Zealand October, 1997 Adatara, Honshu, Japan September 15, 1997 Karymsky, Kamchatka, Russia: August 2, 1997 Etna, Sicily, Italy July 31, 1997 Pavlof, Alaska June 3, 1997 Merapi, Indonesia Nov 22, 1994 Fogo, Cape Verde April 2, 1995 Akutan, Alaska: March 10, 1996 Mount St. Helens January 1 - Septembe 1995 Cerro Negro, Nicaragua: November 30, 1995 Hosho, Kyushu, Japan October 12, 1995

Location and Causes of Volcanoes There are about 2,500 known active volcanoes world. Nearly all of them are located in chains stretching across the earth, often for many thousands of miles. Geologists believe that this distribution is related to a theory of the develo of the earths surface called plate tectonics. The plate tectonics theory says that the surface the earth is divided into segments, called plate which are moving relative to one another. Wh two adjacent plates are moving away from eac other material rises from beneath the plates to the gap, which is therefore marked by a line of volcanoes. In the opposite situation, in which two plates a moving together, a string of volcanoes is also developed by the melting of one plate as it des beneath the other. The Pacific Ocean is nearly surrounded by such lines of collision between along which are located two thirds of the world active volcanoes, forming the "ring of fire". A third type of volcanic chain is formed when plate moves over what appears to be a "hot sp deep in the earth. Volcanoes formed when ove hot spot are carried away by the motion of the The Hawaiian Islands are an example of such chain.

Structure and Formation At the top of most volcanic mountains is a crater which marks the upper end of the vent. A large crater, or caldera, may be formed by the collap the mountain top after the withdrawal of mag Crater Lake in Oregon occupies 20 sq mi of a c left by the collapse of Mounth Mazama, an anc extict volcano. Underwater eruptions are come especially in the seas near island arcs. In Japa 1952, a underwater eruption of Myozin-San su ship with all on board. Most new volcanoes start on the flanks of olde ones, forming adventive cones. Paricutin was on Feb. 20 1943 in the state of Michoacan, Mex Gases and hot ashes began to emerge from a fi in a cornfield, and soon later a cone 30 ft high been built around the vent. In a week it had gr to 450 ft and the roar of the eruption was hear mi away. When its activity ceased nine years l the volcano had built a cone 1,300 ft high and lava had buried two towns

products of Volcanoes [img=http://2wapworld.com/get106154/57fd2f330.jpg] Volcanoes can emit gases, molten rock, or solid particles. Volcanic gases are composed mainly water vapor, hydrogen, hydrogen chloride, hyd sulfide, carbon monoxide, and carbon dioxide. The molten rock, or magma, that rises in a volcano is called lava after it flows out at the surface. and hardened lava may have either a smooth rough surface. Smooth lava, called pahoehoe, ropy, folded surface. Rough clinkery lava is cal aa. Both words are of Hawaiian origin. The ga remaining in lava after its eruption forms bub which leave small holes, or vesicles, in the har rock. Scoria is hardened lava that contains magma vesicles. Magma Composition and Origin The kind of material erupted depends on the n of the eruption, which in turn is related to the magma composition. Magmas are classified by amount of silica that they contain. Eruptions o material are usally quiet outporuings of lava t cools to form a dark colored rock called basalt. magmas contain about two thirds silica. They stiff and viscious, and gases are released explo after building up enough pressure to shatter th magma. The earth's interior, below the thin crust, was thought to be entirely molten. It is now known most of the earth is solid rock, and that only p the central core is liquid. Most magma comes f pockets in the upper mantle--the thick, normall solid layer between the crust and the core. The material erupted from Hawaii's volcanoes is fr depth of about 40 mi.

Types of Eruptions Icelandic - Volcanic eruptions may be classifie order of increasing violence. Icelandic eruptio involve the quiet outpouring of lava from long fissures. Most than 2 1/2 cu mi of lava flowed f Iceland's Lake Fissure in 1783, the largest emis lava in historic times. Eruptions of the Icelandi during the Tertiary Period built up vast basalt plateaus in many parts of the World. One of th largest is the Columbia Plateau, more than 20,0 mi in the northwestern United States. Hawaiian - Lava issues from a single vent in eruptions of the Hawaiian type. Each flow adds of rock to a broad, gently sloping mountain cal shielt volcano. In calderas at their summits shi volcanoes may contain lava lakes in which lav continuosly circulates from below. The volcano Niragongo in Africa contains a lava lake, as di Kilauea in Hawaii. The escape of gas at the ve during Hawaiian erputions often produces lava fountains several hundred feet high. Strombolian - Continuos eruptions that emit s of lava at rhythmic intervals are called stromb after the volcano Stromboli, north of Sicily. Mu the erupted material is fragmental, so that the steep sided. A cross section would show altern layers of pyroclastic material and lava flows, f a mountain known as a stratovolcano. Vulcanian - In Vulcanian eruptions, which also produce stratovolcanoes, the magma is more a less fluid than in the preceding types. The ope phase is explosive, blowing away the thick cru the vent and often destroying part of the moun Lava flows may emerge later. Both Vulcano, fr which such eruptions are named, and Italy's Vesuvius illustrate this type of activity. In 79 A eruption of Mount Vesuvius buried the city of under a layer of pumice, and many of its 20,00 inhabitants were killed. A nearby city, Hercula was buried by a lahar, or mudflow, caused by that accompanied the eruption. A single Vulca eruption where none has occurred before prod cinder cone, or pyroclastic cone, surrounded b flows. Paricutin is an example. pelean - The most explosive eruption is the Pel type, named for the 1902 eruption of Mount Pel the island of Martinique in the West Indies. On morning of May 8 a blast of steam and red-hot shot laterally from the volcano and destroyed t of Saint Pierre and all but two of more than 26 inhabitants. This was the greatest loss of life e known to result directly from an eruption, alth 80,000 lives were lost in the famine that was c by the eruption in 1815 of Tambora in the East The blast that destroyed Saint Pierre was a nu ardente, a stream of ash-laden gas that forced passage through the viscous magma that clogg crater. Some months later a volcanic spine, or of stiff lava, arose from the crater. It reached a of 1,020 ft above the crater floor before collapsi

phreatic - Violent phreatic eruptions, similar to Pelean type, are caused by the vaporization of underground water by magma. In the phreatic eruption of Bandai-san, in Japan, 461 persons l their lives in 1888.

predicting and Controlling Eruption There are a few ways to predict future volcanic activity. The first method, which is the most w used, is to study the geographic area where th volcano resides. Usually, volcanoes may produce many small earthquakes preceeding a volcanic eruption, as Mount Saint Helens did in Washinton in 1980. Vulcanologists are continually studyin these and other effects, such as increases in th temperature of the volcano, to find ways of giv warning of an coming eruption.Although there seems to be little man can do to prevent an eruption, some attempts to lessen the damage caused by lava flows have been successful. Stre of lava have been diverted from a course whic threatened to overwhelm a town by bombing t flow nearer the eruption vent and diverting th lava into another area. Slowing part of a flow building walls or by cooling the lava with strea water may be possible in some cases.

Lava Flows [img=http://2wapworld.com/get106153/1bd20760a.gif] Under certain circumstances, instead of being from a central vent, lava pours out along verti fissures that usually stretch for several kilome across the land surface. Flows of this sort have created thick sheets of basalt covering hundre square kilometers.

Theories of Volcanism Geologists have incorporated volcanism into th theory of plate tectonics. Their theory says tha active volcanoes obtain their energy from the movements of crustal plates. It is because of th that volcanoes tend to be associated with majo plate boundaries. Convergent and divergent are two kinds of plat boundaries where volcanoes form. When one p plunges beneath the other, material on the upp surface of the subducted plate is dragged down into the earth's crust until it reaches a depth it becomes molten. It then rises and is ejected surface through a volcanic vent. At divergent p boundaries, such as the Mid-Atlantic Ridge, wh oceanic crust is being stretched and rifted apar linear zone of weakness forms. When the volcano of Paricutn erupted in the of a central Mexican cornfield in 1943, scientist the opportunity to study the birth and the orde emited materials firsthand. The lava flows completely buried the village of Paricutn.

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