A Volcano is a vent in the earth through which hot gases and molten rock rise to the surface. A cone-shaped mountain of erupted material around such a vent is also called a volcano. The name is taken from Vulcano, an island north of Sicily in the Mediterranean Sea. The island was given its name by the ancient Romans because it spewed smokelike vapor, and was thought to be the home of Vulcan, their god of fire.
A Volcano consists of a fissure in the earth’s crust, above which a cone of volcanic material has accumulated. At the top of the funnel is a bowl-shaped vent called a crater. The cone is formed by the deposition of molten or solid matter that flows or is ejected through the vent from the interior of the earth. The study of volcanoes and volcanic phenomena is called volcanology. Most volcanoes are composite landforms built up partly of lava flows and somewhat of fragmental materials. Italy’s Mount Etna, in Sicily is an example of a composite cone.
In successive eruptions, the solid materials fall around the vent on the slopes of the cone, while lava streams issue from the vent and fissures on the flanks of the funnel. Thus, the cone is built up of layers of fragmental materials and flows of lava, all inclined outward away from the vent. Some enormous, craterlike basins, called calderas, at the top of long-dormant or extinct volcanoes, are eventually occupied by deep lakes, such as Crater Lake in Oregon. Some calderas are the result of cataclysmic explosions that destroy the erupting volcano. Others form when the subterranean magma chamber, emptied by repeated eruptions, can no longer support the weight of the volcanic pile above it. Therefore it collapses.
Active and Dormant Volcanoes
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A volcano that is erupting or has erupted during historic times is called active. A Volcano in which there is no evidence of any activity is considered extinct. Volcanoes that appear recent but have no recorded activity other than the quiet escape of gas are called dormant.
Some volcanoes are much more active than others. Stromboli, in the Lipari Islands near Sicily, has continuously been active since ancient times. Many active volcanoes are found in a belt, called the Ring of Fire, that circles the Pacific Ocean. Yet other volcanoes, such as Mount Vesuvius, continue in a state of activity for longer or shorter periods and then become dormant. The eruption that follows long dormancy is violent. This was shown in the 1980 eruption of Mount Saint Helens in the state of Washington. It had been dormant for 123 years of being in a dormant state. The danger to life posed by active volcanoes is not limited to an eruption of molten rock or showers of ash and cinders. Mudflows are also equally troublesome. A mudflow triggered by an explosion that melted ice and snow on Colombia’s Nevado del Ruiz volcano in 1985 killed more than 25,000 people.
Below is a list of some active volcanoes in the world, with a link to more information.
|VOLCANO:||DATE OF ERUPTION OR ACTIVITY:|
|Popocatepetl, Mexico:||December 6, 1997|
|Bezymianny, Kamchatka||December 5, 1997|
|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 – September 30, 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 in the 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 development of the earth’s surface called plate tectonics.
The plate tectonics theory says that the surface of the earth is divided into segments, called plates, which are moving relative to one another. Where two adjacent plates are moving away from each other material rises from beneath the platters to fill the gap, which is therefore marked by a line of volcanoes.
In the opposite situation, in which two plates are moving together, a string of volcanoes is also developed by the melting of one dish as it descends beneath the other. The Pacific Ocean is nearly surrounded by such lines of a collision between plates, along which are located two-thirds of the world’s active volcanoes, forming the “ring of fire.”
The third type of volcanic chain is formed when a plate moves over what appears to be a “hot spot” deep in the earth. Volcanoes formed when over the hot spot are carried away by the motion of the plate. The Hawaiian Islands are an example of such a chain.
Structure and Formation
At the top of most volcanic mountains is a crater, which marks the upper end of the vent. An enormous crater, or caldera, may be formed by the collapse of the mountain top after the withdrawal of magma. Crater Lake in Oregon occupies 20 sq mi of a caldera left by the failure of Mount Mazama, an ancient extinct volcano. Underwater eruptions are frequent, especially in the seas near island arcs. In Japan in 1952, an underwater eruption of Myosin-San sunk a ship with all on board.
Most new volcanoes start on the flanks of older ones, forming adventive cones. Paricutin was born on Feb. 20, 1943, in the state of Michoacan, Mexico. Gases and hot ashes began to emerge from a fissure in a cornfield, and soon later a cone 30 ft high had been built around the vent. In a week it had grown to 450 ft, and the roar of the eruption was heard 200 mi away. When its activity ceased nine years later, the volcano had built a cone 1,300 ft high, and its lava had buried two towns.
Products of Volcanoes
Volcanoes can emit gases, molten rock, or solid particles. Volcanic gases are composed mainly of water vapor, hydrogen, hydrogen chloride, hydrogen 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. Cooled and hardened lava may have either a smooth or a rough surface. Flowing lava, called pahoehoe, has a ropy, folded surface. Rough clinkery lava is called aa. Both words are of Hawaiian origin. The gas is remaining in lava after its eruption forms bubbles, which leave small holes, or vesicles, in the hardened rock. Scoria is hardened lava that contains many vesicles.
Pieces of magma that are hurled into the air are the third product of volcanic eruptions. The largest, which may be several feet in diameter, are called volcanic bombs. Smaller fragments are called lapilli, and volcanic cinders are the size of sand grains. The smallest particles are volcanic ash, which may be carried hundreds of miles through the air. In 1883 volcanic ash from the eruption of Krakatoa in the East Indies fell all over the world. Most of the debris of such violent eruptions are derived from hardened lava that was deposited previously, rather than from magma directly. Fragmental volcanic material is called pyroclastic, from the Greek pyr and Klastos. Light-colored pumice, so filled with small holes that it floats in water, is a pyroclastic rock that is mined for use an abrasive.
Magma Composition and Origin
The kind of material erupted depends on the nature of the eruption, which in turn is related to the magma composition. Magmas are classified by the amount of silica that they contain. Eruptions of this material are usually quiet outpourings of lava that cools to form a dark colored rock called basalt. Acid magmas contain about two-thirds silica. They are stiff and viscous, and gases are released explosively after building up enough pressure to shatter the magma.
The earth’s interior, below the thin crust, was once thought to be entirely molten. It is now known that most of the planet is a solid rock and that only part of the central core is liquid. Most magma comes from pockets in the upper mantle–the thick, usually solid layer between the crust and the core. The material erupted from Hawaii’s volcanoes is from a depth of about 40 mi.
Predicting and Controlling Eruptions
There are a few ways to predict future volcanic activity. The first method, which is the most widely used, is to study the geographic area where the volcano resides. Usually, volcanoes may produce many small earthquakes preceding a volcanic eruption, as Mount Saint Helens did in Washington in 1980. Vulcanologists are continually studying these and other effects, such as increases in the temperature of the volcano, to find ways of giving warning of a coming eruption. Although there seems to be a little man can do to prevent an eruption, some attempts to lessen the damage caused by lava flows have been successful. Streams of lava have been diverted from a course which threatened to overwhelm a town by bombing the flow nearer the eruption vent and diverting the fluid lava into another area. Slowing part of flow by building walls or by cooling the lava with streams of water may be possible in some cases.
After a volcano has finished erupting materials, the eruption will continue to emit acid gases. This is called the fumarolic stage. After this phase, hot springs may arise from the volcano. This type of activity can be seen in the geysers of Yellowstone National Park in Wyoming. Eventually, after a long time, the last traces of volcanic heat will disappear. Springs of cold water may come from the volcano and the ground in its area, signaling the dormancy of the volcano.
After becoming inactive, a volcano undergoes a reduction in size through erosion caused by running water, glaciers, wind, or waves. Finally, the volcano may become obliterated entirely, leaving only a volcanic pipe.
Under certain circumstances, instead of being emitted from a central vent, lava pours out along vertical fissures that usually stretch for several kilometers across the land surface. Flows of this sort have created thick sheets of basalt covering hundreds of square kilometers.
Theories of Volcanism
Geologists have incorporated volcanism into the theory of plate tectonics. Their opinion says that active volcanoes obtain their energy from the movements of crustal plates. It is because of this that volcanoes tend to be associated with significant plate boundaries.
Convergent and divergent are two kinds of plate boundaries where volcanoes form. When one plate plunges beneath the other, the material on the upper surface of the subducted plate is dragged down into the earth’s crust until it reaches a depth where it becomes molten. It then rises and is ejected at the surface through a volcanic vent. At divergent plate boundaries, such as the Mid-Atlantic Ridge, where oceanic crust is being stretched and drifted apart, a linear zone of weakness forms.
When the volcano of Paricutín erupted in the middle of a central Mexican cornfield in 1943, scientists had the opportunity to study the birth and the order of emitted materials firsthand. The lava flows completely buried the village of Paricutín.
- Aa: Hawaiian word used to describe a lava flow whose surface is broken into rough, angular fragments.
- Active volcano: A volcano that is erupting. Also, a volcano that is not presently erupting but that has erupted within historical time and is considered likely to do so in the future.
- Ash: Fine particles of pulverized rock blown from an explosion vent. Ash may be either solid or molten when first erupted.
- Ashfall: Volcanic ash that has fallen through the air from an eruption cloud. A deposit so formed is usually well sorted and layered.
- Ash-flow: A turbulent mixture of gas and rock fragments, most of which are ash-sized particles, ejected violently from a crater or fissure. The mass of pyroclastics is generally of very high temperature and rushes down the slopes.
- Avalanche: A large mass of material or mixtures of material falling or sliding rapidly under the force of gravity.
- Bomb: Fragment of molten or semi-molten rock, 2 1/2 inches to many feet in diameter, which is blown out during an eruption. Because of their plastic condition, bombs are often modified in shape during their flight or upon impact.
- Caldera: The Spanish word for cauldron, a basin-shaped volcanic depression; by definition, at least a mile in diameter. Such large depressions are typically formed by the subsidence of volcanoes. Crater Lake occupies the best-known caldera in the Cascades.
- Central Vent: A central vent is an opening at the Earth’s surface of a volcanic conduit of cylindrical or pipelike form.
- Central Volcano: A volcano constructed by the ejection of debris and lava flows from a central point, forming a more or less symmetrical volcano.
- Cinder cone: A volcanic cone built entirely of loose, fragmented material (pyroclastics.)
- Composite Volcano: A steep volcanic cone formed by both lava flows and pyroclastic eruptions.
- Compound Volcano: A volcano that consists of a complex of two or more vents, or a volcano that has an associated volcanic dome, either in its crater or on its flanks. Examples are Vesuvius and Mont Pelee.
- Conduit: A passage followed by magma in a volcano.
- Crater: A steep-sided, usually circular depression formed by either explosion or collapse at a volcanic vent.
- Debris avalanche: A rapid and unusually sudden sliding or flowage of unsorted masses of rock and other material. As applied to the significant landslide involved in the eruption of Mount St. Helens, a rapid mass movement that included fragmented cold and hot volcanic rock, water, snow, glacier ice, trees, and some hot pyroclastic material.
- Dome: A steep-sided mass of lava extruded from a volcanic vent. Its surface is often rough and blocky as a result of the fragmentation of the colder, outer crust during the growth of the dome.
- Dormant volcano: The term is used to describe a volcano which is presently inactive but which may erupt again.
- Eruption: The process by which solid, liquid, and gaseous materials are ejected into the earth’s atmosphere and onto the earth’s surface by volcanic activity. Eruptions range from the quiet overflow of molten rock to the tremendously violent expulsion of pyroclastics.
- Eruption cloud: The column of gases, ash, and larger rock fragments rising from a crater or other vent. If it is of sufficient volume and velocity, this gaseous column may reach many miles into the stratosphere, where high winds will carry it long distances.
- Eruptive vent: The opening through which volcanic material is emitted.
- Fault: A crack or fracture in the earth’s surface. Movement along the fault can cause earthquakes or, in the process of mountain-building, can release underlying magma and permit it to rise to the surface.
- Fissures: Elongated fractures or cracks on the slopes of a volcano. Fissure eruptions typically produce liquid flows, but pyroclastics may also be ejected.
- Fumarole: A vent or opening through which issue steam, hydrogen sulfide, or other gases. The craters of many dormant volcanoes contain active fumaroles.
- Harmonic tremor: A release of seismic energy associated with the underground movement of magma.
- Hot Spot: A volcanic center, 60 to 120 miles across and persistent for at least a few tens of millions of years that is thought to be the surface expression of a persistent rising plume of hot mantle material. Hot spots are not linked to arcs, and may not be associated with ocean ridges.
- Hyaloclastite: A deposit formed by the flowing or intrusion of lava or magma into the water, ice, or water-saturated sediment, and its consequent granulation or shattering into small angular fragments.
- Intensity: A measure of the effects of an earthquake at a particular place. Power depends not only on the magnitude of the quake but also on the distance from the epicenter and the local geology.
- Intrusion: The process of emplacement of magma in pre-existing rock. Also refers to igneous rock mass so formed within the surrounding rock.
- Joint: A surface of fracture in a rock.
- Lahar: A flow of water-saturated volcanic debris down the slope of a volcano in response to gravity. A type of mudflow.
- Lava: Magma which has reached the surface through a volcanic eruption. The term is applied to streams of liquid rock that flow from a crater or fissure.
- Lava Flow: An outpouring of lava onto the land surface from a vent or fissure. Also, a solidified tongue like or sheet-like body formed by outpouring lava.
- Lava lake: A lake of molten lava, usually basaltic, in a volcanic crater or depression. The term refers to solidified and partially solidified stages as well as to the melted, active lava lake.
- Magma: Molten rock beneath the surface of the earth.
- Magma chamber: The subterranean cavity containing the gas-rich liquid magma which feeds a volcano.
- Mantle: The zone of the earth below the crust and above the core.
- Mudflow: A flowage of water-saturated earth material possessing a high degree of fluidity during movement. A less-saturated flowing mass is often called a debris flow. A mudflow originating on the flank of a volcano is rightly called a lahar.
- Phreatic eruption: An explosive volcanic eruption caused when water and heated volcanic rocks interact to produce a violent expulsion of steam and pulverized rocks. Magma is not involved.
- Plinian eruption: An explosive eruption in which a steady, turbulent stream of fragmented magma and magmatic gases is released at a high velocity from a vent. Large volumes of tephra and tall eruption columns are characteristic.
- Pyroclastic: About fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent.
- Pyroclastic flow: Flowage of a mixture of hot gases and unsorted pyroclastic material that can move at a maximum speed of 100 miles an hour.
- Repose: The interval of time between volcanic eruptions.
- Rift zone: A zone of volcanic features associated with underlying dikes. The location of the rift is marked by cracks, faults, and vents.
- Ring of Fire: The regions of mountain-building earthquakes and volcanoes which surround the Pacific Ocean.
- Seamount: A submarine volcano.
- Stratovolcano: A volcano composed of both lava flows and pyroclastic material.
- Strombolian eruption: A type of volcanic eruption characterized by jetting of clots or fountains of fluid basaltic lava from a central crater.
- Vent: The opening at the earth’s surface through which volcanic materials issue forth — usage of a vent.
- Vesicle: A small air pocket or cavity formed in volcanic rock during solidification.
- Volcano: A vent in the surface of the Earth through which magma and associated gases and ash erupt; also, the form or structure, usually conical, that is produced by the ejected material.
- Volcanic arc: A generally curved linear belt of volcanoes above a subduction zone, and the volcanic and plutonic rocks formed there.
- Vulcan: Roman god of fire and the forge, after whom volcanoes are named.
- Vulcanian: A type of eruption consisting of the explosive ejection of incandescent fragments of new viscous lava, usually on the form of blocks.