vooes

1. The primary factors that determine the nature of volcanic eruptions include the magma’s composition, its temperature, and the amount of dissolved gases it contains. As lava cools, it begins to congeal and, as viscosity increases, its mobility decreases. The viscosity of magma is also directly related to its silica content. Rhyolitic (felsic) lava, with its high silica content (over 70%), is very viscous and forms short, thick flows. Basaltic (mafic) lava, with a lower silica content (~50%), is more fluid and may travel a long distance before congealing. Dissolved gases tend to make magma more fluid, and as they expand, provide the force that propels molten rock from the volcano.

2. The materials associated with a volcanic eruption include (1) lava flows (pahoehoe flows, which resemble twisted braids; and aa flows, consisting of rough, jagged blocks; both form from basaltic lavas); (2) gases (primarily water vapor); and (3) pyroclastic material (pulverized rock and lava fragments blown from the volcano’s vent, which include ash, pumice, lapilli, cinders, blocks, and bombs.

3. Successive eruptions of lava from a central vent result in a mountainous accumulation of material known as a volcano. Located at the summit of many volcanoes is a steep-walled depression called a crater. Shield cones are broad, slightly domed volcanoes built primarily of fluid, basaltic lava. Cinder cones have steep slopes composed of pyroclastic material. Composite cones, or stratovolcanoes, are large, nearly symmetrical structures built of interbedded lavas and pyroclastic deposits. Composite cones produce some of the most violent volcani activity. Often associated with a violent eruption is a nuee ardente, a fiery coloud of hot gases infused with incandescent ash that races down steep volcanic slopes. Large composite cones may also generate a type of mudflow known as a lahar.

4. Most volcanoes are fed by conduits or pipes. As erosion progresses, the rock occupying the pipe, which is often more resistant, may remain standing above the surrounding terrain as a volcanic neck. The summits of some volcanoes have large, nearly