By Tricia Severson2nd hour Science4/30/98A volcano is a vent, or opening, in the surface of the Earth through which magma andassociated gases and ash erupt. The word also refers to the form or structure, usuallyconical, produced by accumulations of erupted material. Volcanoes occur mainly nearplate tectonic boundaries and are especially common around the Pacific basin, called thePacific Ring of Fire (see Plate Tectonics).
Humanity has long been awed by this powerful force of nature. The Romans attributedvolcanic events to Vulcan, the god of fire and metalworking. In AD 79 the eruption ofMount Vesuvius destroyed the Roman cities of Pompeii and Herculaneum. Polynesiansbelieve volcanoes to be ruled by the fire goddess Pele.
One of the most spectacularvolcanic eruptions in recorded history occurred in 1883 with the explosion of Krakatoa,an island in the Sunda Strait near Java (see Krakatoa). A more recent example is thedramatic 1980 eruption of Mount St. Helens in the Cascade Range in Washington State. Volcano Formation and EruptionsVolcanic eruptions may be violent, even catastrophic, or relatively mild. The mostexplosive eruptions are essentially blasts of steam that create spectacular displays.
Quieter fissure eruptions occur when molten rock pushes through long cracks in theEarth’s crust and floods the surrounding landscape. Such repeated outpourings of lava canfill surrounding valleys and bury low hills, creating thick lava sequences that eventuallybecome plateaus (see Plateau). The origin of molten rock, referred to by geologists as magma, is not clearlyunderstood. About 80 percent of all magma is composed of basalt rock. Geophysicalresearch suggests that volcanic magma forms near the base of the Earth’s crust and movesupward to a shallow magma chamber before erupting at the surface.
Magmas risebecause they are less dense than the rocks at lower depths, and their heat probablyweakens surrounding rocks. The upward movement of magma may also be due toexpanding gases within the molten rock or to chemical reactions that dissolve rocksabove the magma. Volcanic material moves toward the surface through channelways, orvolcanic conduits, and is extruded through vents at the Earth’s surface. (See also Lavaand Magma. ) Eruptions take different forms depending on the composition of the magma when itreaches the surface. Sudden eruptions are often associated with low-viscosity (morefluid) magma where the expanding gases form a froth that becomes a light, glassy rockcalled pumice.
In eruptions of high-viscosity (thicker) magmas, the gas pressure shattersthe rock into fragments. Pyroclastic rocks, formed by volcanic explosion, are namedaccording to size: volcanic ash if sand-sized or smaller, volcanic bombs if larger. Consolidated ash is called tuff. Quieter, more passive eruptions release fluid basalt lavafrom dikes or dike swarms (magma intrusions that cut across layers of rock). Theseeruptions cover large areas and often produce ropy, or pahoehoe, lava flows.
Thickerbasalt lava breaks into chunks or blocks, forming blocky lava flows, called aa. The products of volcanism may be classified into two groups: lava and pyroclastics. Lava is the fluid phase of volcanic activity. Pyroclastics (also called tephra) arevarious-sized particles of hot debris thrown out of a volcano. Whether lava orpyroclastics are being ejected, the eruption is normally accompanied by the expulsion ofwater and gases, many of which are poisonous.
Lava usually forms long, narrow rivers ofmolten rock that flow down the slopes of a volcano. Explosive eruptions tend to be spectacular events best observed from a safe distance. Earthquakes, high columns of vapors, lightning, and strong whirlwinds often accompanythe explosions. The eruption of Krakatoa unleashed a tsunami, a large seismic sea wave,that swept the coasts of Java and Sumatra and drowned more than 36,000 people. Avolcano can grow with frightening speed and often affects territory far beyond the areaon which the cone forms. When volcanoes are born in the sea, the eruptions may be moreviolent than those on land because the contact between molten rock and seawaterproduces steam.
Volcanoes also create craters and calderas. Craters are formed either by the massivecollapse of material during volcanic activity, by unusually violent explosions, or later byerosion during dormancy. Calderas are large, basin-shaped depressions. Most of them areformed after a magma chamber drains and no longer supports the overlying cone, whichthen collapses inward to create the basin.
One of the most famous examples is thestill-active Kilauea caldera in Hawaii. Types of VolcanoesVolcanoes are usually classified by shape and size. These are determined by such factorsas the volume and type of volcanic material ejected, the sequence and variety oferuptions, and the environment. Among the most common types are shield volcanoes,stratovolcanoes, and cinder cones.
Shield volcanoes have a low, broad profile created by highly fluid basalt flows thatspread over wide areas. The fluid basalt cannot build up a cone with sides much steeperthan 7 degrees. Over thousands of years, however, these cones can reach massive size. The Hawaiian Islands are composed of shield volcanoes that have built up from the seafloor to the surface some 3 miles (5 kilometers) above.
Peaks such as Mauna Loa andMauna Kea rise to more than 13,600 feet (4,145 meters) above sea level. Hawaii is thelargest lava structure in the world, while Mauna Loa, if measured from the sea floor, isthe world’s largest mountain in terms of both height and volume. Stratovolcanoes are the most common volcanic form. They are composed of alternatinglayers of lava and pyroclastic material. When a quiet lava flow ends, it creates a seal ofsolidified lava within the conduit of the volcano.
Pressure gradually builds up below,setting the stage for a violent blast of pyroclastic material. These alternating cycles repeatthemselves, giving stratovolcanoes a violent reputation. A cinder cone is a conical hill of mostly cinder-sized pyroclastics. The profile of thecone is determined by the angle of repose, that is, the steepest angle at which debrisremains stable and does not slide downhill.
Larger cinder fragments, which fall near thesummit, can form slopes exceeding 30 degrees. Finer particles are carried farther fromthe vent and form gentle slopes of about 10 degrees at the base of the cone. Thesevolcanoes tend to be explosive but may also extrude some lava. Cinder cones arenumerous, occur in all sizes, and tend to rise steeply above the surrounding area.
Thoseoccurring on the flanks of larger volcanoes are called parasitic cones. Volcanic activity typically alternates between short active periods and much longerdormant periods. An extinct volcano is one that is not erupting and is not likely to eruptin the future. A dormant volcano, while currently inactive, has erupted within historictimes and is likely to do so in the future.
An inactive volcano is one that has not beenknown to erupt within historic times. Such classification is arbitrary, however, sincealmost any volcano is capable of erupting again. In the late stages of volcanic activity, magma can heat circulating groundwater,producing hot springs and geysers (see Geyser and Fumarole). A geyser is a hot-waterfountain that spouts intermittently with great force. One of the best-known examples isOld Faithful in Yellowstone National Park.
Fumaroles are vents that emit gas fumes orsteam. Volcanoes occur along belts of tension, where continental plates diverge, and alongbelts of compression, where the plates converge. Styles of eruption and types of lava areassociated with different kinds of plate boundaries. Most lavas that issue from vents inoceanic divergence zones and from midoceanic volcanoes are basaltic. Where oceanplates collide, the rock types basalt and andesite predominate.
Near the zone where anocean plate and continental margin converge, consolidated ash flows are found. Nearly 1,900 volcanoes are active today or known to have been active in historicaltimes. Of these, almost 90 percent are situated in the Pacific Ring of Fire. This belt partlycoincides with the young mountain ranges of western North and South America, and thevolcanic island arcs fringing the north and western sides of the Pacific basin. TheMediterranean-Asian orogenic belt has few volcanoes, except for Indonesia and theMediterranean where they are more numerous.
Oceanic volcanoes are strung along theworld’s oceanic ridges, while the remaining active volcanoes are associated with theAfrican rift valleys. Study of Volcanic Eruptions Volcanology, a branch of geology, is the study of volcanoes and volcanic activity. Although volcanoes are difficult to study because of the hazards involved, volcanoobservatories have existed for decades. Scientists observe active volcanoes to obtain information that might help predict thetiming and intensity of eruptions. Sensitive instruments detect changes in temperature,chemical composition of emissions, Earth movements, magnetic fields, gravity, and otherphysical properties of the volcano.
Modern networks of seismographs provideinformation on the internal structure and activity of volcanoes (see Earthquake). Theintensity, frequency, and location of earthquakes provide important clues to volcanicactivity, particularly impending eruptions. Movements of magma typically producenumerous tremors, sometimes exceeding 1,000 per day. An almost continuous tremorgenerally accompanies a lava outpouring. Tiltmeters (instruments that measure tilting ofthe ground) help detect swelling and deflation of the volcano caused by the accumulationand movement of magma.
Researchers also monitor variations in the chemistry andpetrology of the lavas and the chemistry of emitted gases. Volcanoes erupt in a wide variety of ways. Even a single volcano may go throughseveral eruption phases in one active period. Eruptions are classified according to thegeochemical composition and viscosity of the lavas, nature of the flows or ash release,and associated phenomena.
Magmatic eruptions are the most common, but the mostviolent arise from steam explosions when the fiery magma reaches surface water, ice, orgroundwater. Pelean eruptions, named after the 1902 eruption of Mount Pele on the Caribbeanisland of Martinique, are characterized by incandescent flows of rock and pumicefragments. The entrapment of high-temperature gases in these “glowing avalanches,”known by the French term nue ardente, is associated with a particularly violent phase oferuption. Eruptions of intermediate force are typified by Plinian eruptions, named after Pliny theElder, who witnessed the volcanic destruction of Pompeii and Herculaneum.
Plinianeruptions are characterized by both the extrusion of high-viscosity lava flows and theviolent explosion of released gases that blast huge quantities of ash, cinders, bombs, andblocks skyward. Volcanic mudflows, landslides, and lahars (flows of volcanic debris)may also follow, particularly if the eruptions are accompanied by rainstorms. Less violent Hawaiian and Strombolian-type eruptions are associated with fissures thatoften produce a line of fire fountains. These geyserlike fountains of lava may shootseveral hundred feet into the air and form a nearly continuous curtain of fire. The basaltlava is extremely fluid and flows down the mountain sides in torrents.
When thesestreams reach the sea, they form pillow lavas, lobes of stacked lava that resemble a pileof pillows. Volcanoes provide a wealth of natural resources. Emissions of volcanic rock, gas, andsteam are sources of important industrial materials and chemicals, such as pumice, boricacid, ammonia, and carbon dioxide. In Iceland most of the homes in Reykjavk areheated by hot water tapped from volcanic springs. Greenhouses heated in the same waycan provide fresh vegetables and tropical fruits to this subarctic island. Geothermal steamis exploited as a source of energy for the production of electricity in Italy, New Zealand,the United States, Mexico, Japan, and Russia.
The scientific study of volcanoes providesuseful information on Earth processes.