By Tricia Severson
2nd hour Science

A volcano is a vent, or opening, in the surface of the Earth through which magma and
associated gases and ash erupt. The word also refers to the form or structure, usually
conical, produced by accumulations of erupted material. Volcanoes occur mainly near
plate tectonic boundaries and are especially common around the Pacific basin, called the
Pacific Ring of Fire (see Plate Tectonics).

   Humanity has long been awed by this powerful force of nature. The Romans attributed
volcanic events to Vulcan, the god of fire and metalworking. In AD 79 the eruption of
Mount Vesuvius destroyed the Roman cities of Pompeii and Herculaneum. Polynesians
believe volcanoes to be ruled by the fire goddess Pele. One of the most spectacular
volcanic 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 the
dramatic 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 most
explosive eruptions are essentially blasts of steam that create spectacular displays.
Quieter fissure eruptions occur when molten rock pushes through long cracks in the
Earth's crust and floods the surrounding landscape. Such repeated outpourings of lava can
fill surrounding valleys and bury low hills, creating thick lava sequences that eventually
become plateaus (see Plateau).

   The origin of molten rock, referred to by geologists as magma, is not clearly
understood. About 80 percent of all magma is composed of basalt rock. Geophysical
research suggests that volcanic magma forms near the base of the Earth's crust and moves
upward to a shallow magma chamber before erupting at the surface. Magmas rise
because they are less dense than the rocks at lower depths, and their heat probably
weakens surrounding rocks. The upward movement of magma may also be due to
expanding gases within the molten rock or to chemical reactions that dissolve rocks
above the magma. Volcanic material moves toward the surface through channelways, or
volcanic conduits, and is extruded through vents at the Earth's surface. (See also Lava
and Magma.)

   Eruptions take different forms depending on the composition of the magma when it
reaches the surface. Sudden eruptions are often associated with low-viscosity (more
fluid) magma where the expanding gases form a froth that becomes a light, glassy rock
called pumice. In eruptions of high-viscosity (thicker) magmas, the gas pressure shatters
the rock into fragments. Pyroclastic rocks, formed by volcanic explosion, are named
according 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 lava
from dikes or dike swarms (magma intrusions that cut across layers of rock). These
eruptions cover large areas and often produce ropy, or pahoehoe, lava flows. Thicker
basalt 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) are
various-sized particles of hot debris thrown out of a volcano. Whether lava or
pyroclastics are being ejected, the eruption is normally accompanied by the expulsion of
water and gases, many of which are poisonous. Lava usually forms long, narrow rivers of
molten 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 accompany
the 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. A
volcano can grow with frightening speed and often affects territory far beyond the area
on which the cone forms. When volcanoes are born in the sea, the eruptions may be more
violent than those on land because the contact between molten rock and seawater
produces steam.

   Volcanoes also create craters and calderas. Craters are formed either by the massive
collapse of material during volcanic activity, by unusually violent explosions, or later by
erosion during dormancy. Calderas are large, basin-shaped depressions. Most of them are
formed after a magma chamber drains and no longer supports the overlying cone, which
then collapses inward to create the basin. One of the most famous examples is the
still-active Kilauea caldera in Hawaii.

Types of Volcanoes

Volcanoes are usually classified by shape and size. These are determined by such factors
as the volume and type of volcanic material ejected, the sequence and