Questions: 1.) What determines the degree of violence associated with volcanic activity?

2.) Why can we state confidently that active volcanism in Hawaii poses only slight danger to humans when violent explosions, such as Mt. St. Helens, occur in the Cascade Mountains?

Whether eruptions are very explosive or relatively "quiet" is largely determined by two factors:

-- the amount of gas in the lava or magma (the gases include mainly water vapor and carbon dioxide)

-- the ease or difficulty with which the gas escapes to the atmosphere.

Viscosity - resistance to flow of a lava determines how easily the gas escapes. The more viscous the lava and the greater the volume of gas trying to escape, the more violent the eruption will be. Viscosity is most affected by temperature and silica content.

There are three common types of magma which are as follows from high temperature to low temperature: basaltic (50% SiO2); andesitic (60% SiO2); and rhyolitic (70% SiO2). Basaltic magma has a low viscosity whereas rhyolitic magma has a high viscosity.

Volcano - a hill or mountain formed by the extrusion of lava or ejection of rock fragments from a vent.

Shield volcanoes - broad, gently sloping cones constructed of solidified lava flows. The eruption from a shield volcano is relatively nonviolent because the lavas are fairly fluid (less viscous). The slope of a shield volcano is less than 10°. One characteristic surface of a basalt flow is called pahoehoe which has a ropy or billowy surface. Basaltic eruptions are non-explosive. During the 1908's, Kilauea volcano, Hawaii, was especially active. Over 800 million cubic meters of lava erupted - enough to cover a four-lane highway extending from the east coast to the west coast of North America with a 10-meter-thick layer of lava.

Cinder cones - a volcano constructed of loose rock fragments ejected from a central vent. Most of the ejecta lands near the vent to form a cone with a slope of up to 30°.

Composite volcano - one constructed of alternating layers of pyroclastics and rock solidified from lava flow with slopes between those of cinder cones and shield volcanoes. Aconcagua, a composite volcano in the Andes, is 6,960 meters above sea level and the highest peak in the western hemisphere.

Volcanic dome - steep-sided, dome, or spine-shaped masses of volcanic rock formed from viscous lava that solidifies in or immediately above a volcanic vent. The most viscous lavas form volcanic domes which are rich in silica. They solidify as rhyolite or, less commonly, andesite if minerals crystallize, or as obsidian if no minerals crystallize. Rhyolitic eruptions are most likely to be explosive. The material blown out of a volcano is called pyroclastic debris, or tephra. Bubbles that form quickly in a huge mass of sticky rhyolitic magma can shatter into a froth of innumerable tiny glass-walled bubbles, producing a rock called pumice.

Vent - opening through which an eruption takes place.

Crater - a basinlike depression over a vent at the summit of the cone of a volcano.

Flank Eruption - lava pours from a vent on the side of a volcano.

Caldera - a volcanic depression much larger than the original crater.

Nuées ardentes - clouds of red-hot ash and dust blown out by gas explosions. In 1902, such a gas cloud descended lake an avalanche down the mountainside, engulfing the port town of St. Pierre and incinerated everything in its path including 30,000 people.

Welded tuff - the material made of particles from the nuées ardentes which becomes welded upon cooling.

Plateau basalts - produced during the geologic past by bast outpourings of lava. The Columbia plateau area of Washington, Idaho, and Oregon is constructed of layer upon layer of basalt, in places as thick as 3,000 meters.

Pillow basalts - elongate blobs of lava break out of a thin skin of solid basalt over the top of a flow that is submerged in water.

Maria - on the Moon are basalt flows. Volcanoes are also known on Mercury, Venus, Mars, and Io.

Distribution of composite volcanoes:

Circum-Pacific belt - Krakatoa (Indonesia), Fujiyama (Japan), Cascade volcanoes (U.S.), Aconcagua (Chile), Erebus (Antarctica).

Mediterranean belt - Vesuvius (Italy), Etna (Sicily)