Volcano: sides, but some can be gently

Volcano: sides, but some can be gently

Volcano: defined 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. The term 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 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 ocean 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 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 volcanicdebris flows. 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.

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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 which is 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 50 to 60 mi. (80 to 100 km) 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 floorThree different types of materials may erupt from an active volcano. These materials are lava, tephra which are 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.Lava is magma that breaks the surface and erupts from a volcano.

If the magma is very fluid, it flows rapidly down the volcano’s slopes. Lava that is more sticky and less fluid moves slower. Lava flows that have a continuous, smooth, ropy, or billowy surface are called pahoehoe (pronounced pah HOH ee hoh ee) flows, while aa (pronounced ah ah) flows have a jagged surface composed of loose, irregularly shaped lava chunks. Once cooled, pahoehoe forms smooth rocks, while aa forms jagged rocks. The words pahoehoe and aa are Hawaiian terms that describe the texture of the lava. Lava may also be described in terms of its composition and the type of rock it forms. Basalt, andesite, dacite, and rhyolite are all different kinds of rock that form from lava.

Each type of rock, and the lava from which it forms, contains a different amount of the compound silicon dioxide. Basaltic lava has the least amount of silicon dioxide, andesitic and dacitic lava have medium levels of silicon dioxide, while rhyolitic lava has the most.Tephra, or pyroclastic material, is made of rock fragments formed by explosive shattering of sticky magma. The term pyroclastic is of Greek origin and means “fire-broken” (pyro, “fire”; klastos, “broken”). Tephra refers to any airborne pyroclastic material regardless of size or shape. The best-known tephra materials include pumice, cinders, and volcanic ash. These fragments are exploded when gases build up inside a volcano and produce an explosion.

The pieces of magma are shot into the air during the explosion. Ash refers to fragments smaller than 2 mm (0.08 in) in diameter. The finest ash is called volcanic dust and is made up of particles that are less than 0.06 mm (0.002 in) in diameter.

Volcanic blocks, or bombs, are the largest fragments of tephra, more than 64 mm (2.5 in) in diameter which is baseball size or larger. Some bombs can be the size of a small car.Gases, primarily in the form of steam, are released from volcanoes during eruptions. All eruptions, explosive or nonexplosive, are accompanied by the release of volcanic gas. The sudden escape of high-pressure volcanic gas from magma is the driving force for eruptions.

Gases come from the magma itself or from the hot magma coming into contact with water in the ground. Volcanic plumes can appear dark during an eruption because the gases are mixed with dark-colored materials such as tephra. Most volcanic gases predominantly consist of water vapor which is steam, with carbon dioxide (CO2) and sulfur dioxide (SO2) being the next two most common compounds along with smaller amounts of chlorine and fluorine gases.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 more explosive 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 material into the air. Nonexplosive eruptions produce lava flows and eject very little pyroclastic material into the air.Explosive eruptions can eject liquid and semisolid lava as well as solid fragments of volcanic or nonvolcanic rock that have been carried along by the rising magma before eruption. Very violent explosive eruptions are called Plinian eruptions, after Roman naturalist Pliny the Elder. These eruptions can last for several hours to days and eject a large amount of pyroclastic material. Some volcanoes can produce much more energetic eruptions that eject materials farther from the vents because of their andesitic and dacitic composition.

Andesitic and dacitic lava is generally thicker than basaltic lava. Stiff lava generally produces more-explosive eruptions.If the eruption is nonexplosive, as is typical for Hawaiian volcanoes, lava flows are produced.

The lava comes out of rifts in the sides of the volcano, or vents in a rift. Tephra is rarely ejected during a nonexplosive eruption. Nonexplosive eruptions are characterized by basaltic lava and by the type of volcanoes they form, called shield Aerial view of Mount St. Helen in WashingtonVolcanic eruptions can hurl hot rocks for at least 20 miles.

Floods,airborne ash, or noxious fumes can spread 100 miles or more. If you livenear a known volcano, active or dormant, be ready to evacuate at aLearn about your community warning systems.Be prepared for these disasters that can be spawned by volcanoes.Landslides and mudflows You want to get to high ground away from the eruption. Plan a route outand have a backup route in mind.Develop an emergency communication plan.In case family members are separated from one another during avolcanic eruption (a real possibility during the day when adults are atwork and children are at school), have a plan for getting back together.

Ask an out-of-state relative or friend to serve as the “family contact.”After a disaster, it’s often easier to call long distance. Make sureeveryone knows the name, address, and phone number of the contactHave disaster supplies on hand.

Flashlight and extra batteries Portable, battery-operated radio and extra batteries First aid kit and manual Emergency food and water Nonelectric can opener Essential medicinesCash and credit cards Get a pair of goggles and a throw-away breathing mask for eachmember of the household.Contact your local emergency management office or American RedCross chapter for more information on volcanoes.Although it may seem safe to stay at home and wait out an eruption,doing so could be very dangerous. The rock debris from a volcano canbreak windows and set buildings on fire.

Stay safe. Follow authorities’instructions and leave the area before the disaster begins.Follow the evacuation order issued by authorities.Avoid areas downwind of the volcano.

Close all windows, doors, and dampers. Put all machinery inside a garage or barn. Bring animals and livestock into closed shelters.

Seek shelter indoors. If caught in a rockfall, roll into a ball to protect head. Avoid low-lying area where poisonous gases can collect and flashfloods can be most dangerous. If caught near a stream, beware of mudflows. Wear long sleeved shirts and pants. Use goggles to protect eyes. Use a dust-mask or hold a damp cloth over face to helpKeep car or truck engines off.

A lateral blast of a volcano can travel many miles from the mountain.Trying to watch an erupting volcano is a deadly idea.Mudflows are powerful “rivers” of mud that can move faster than peoplecan walk or run. Mudflows occur when rain falls through ash-carryingclouds or when rivers are damed during an eruption. They are mostdangerous close to stream channels. When you approach a bridge, firstlook upstream. If a mudflow is approaching or moving beneath thebridge, do not cross the bridge.

The power of the mudflow can destroyListen to a battery-powered radio or television for the latest emergencyStay away from volcanic ashfall.Cover your mouth and nose. A number of victims of the MountSt. Helens volcano died from inhaling ash. Wear goggles to protect your eyes. Keep skin covered to avoid irritation or burns. If you have a respiratory ailment, avoid contact with any amount of ash.

Stay indoors until local health officials advise it is safe to go outside.Avoid driving in heavy ashfall.Driving will stir up more ash that can clog engines and stall vehicles.Ashfall is very heavy and can cause buildings to collapse.Remember to help your neighbors who may require specialassistance–infants, elderly people, and people with disabilities.

Is Chemistry Involved in Studying Volcanoes?Chemistry is involved for studying volcanoes in a number of ways. For one thing,volcanologists (scientist who study volcanoes) are often interested in the processes that go on down deep within a volcano such as within a magma chamber. By studying in detail the chemical characteristics of some of the minerals in a volcanic rock, they can start to get a handle on these processes. They can also start to understand what goes on when the first magma is produced. For example, when the upper mantle starts to partially melt to produce magma, not all the minerals in the mantle melt at the same rate.

Some start melting first so their chemical constituents will be concentrated in the first blobs of melt. As melting continues other minerals start to melt as well, and the chemistry of the magma changes. From studies like this geologists have determined that the early stages of a Hawaiian volcano’s life is generated by magmas derived from only small degrees of partial melting (such as only 5% of the source melting and the rest staying behind). On the other hand, when the volcano really gets going (such as at Kilauea and Mauna Loa), the percentage ofsource rock that melts to produce the magma is perhaps as high as 20%. There are many other ways in which chemistry is used in volcanology, including the studies of volcanic gases, crater lakes, and trying to determine the temperatures that lava had at the time they were erupted even long after the eruption has ended.Bibliography:Bibliography1999 Microsoft Encarta; Volcanoes1980 USGS; www.vulcan.wr.usgs.gov2000 Volcano World; http://volcano.und.nodak.edu/vwdocs/frequent-questions/grp13/question1544.html1980-2000 USGS; www.vulcan.wr.usgs.gov/photo/volcanoes/MSH/framework.html1993-2000 Microsoft Corporation; various volcano media2000 FEMA; www.fema.gov/library/volcanof.htm

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