Volcanoes Essay Research Paper By Tricia Severson2nd

Volcanoes Essay, Research Paper

By Tricia Severson2nd hour Science4/30/98

A 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 Eruptions

Volcanic 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 Volcanoes

Volcanoes 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