COMPOSITE VOLCANOES & STRATO-VOLCANOES

A composite cone or strato-volcano is a large, nearly symmetrical structure composed of interbedded lavas and pyroclastic deposits, emitted mainly from a central vent. Conposite cones are produced when relatively viscous lavas of andesitic composition are extruded. A composite cone may extrude viscous lava for a long period of time. Then, suddenly, the eruptive style changes and the volcano violently ejects pyroclastic material. Most of the ejected pyroclastic material falls near the summit, building a steep-sided mound of cinders. In time this debris becomes covered by lava. Most composite volcanoes have a crater at the summit which contains these central vents.

Composite cone volcanoes are very large, between one and ten kilometers across. Their eruptions are the most violent and deadly of all the types of volcanoes. The essential feature of a composite volcano is a conduit system through which magma from a reservoir in the earth's crust rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, and ash are added to its slopes.

When a composite volcano becomes dormant, erosion begins to destroy the cone. After long periods of erosion the volcano appears to vanish. Depressions left after an exploded, eroded composite cone volcano are known as Calderas. They are usually large, steep-walled, basin-shaped depressions formed by the collapse of the volcanic vent or vents. An example of this is Crater Lake in Oregon. Geologists state that Crater Lake was once a giant volcano called Mt. Mazama. After a series of tremendous explosions the volcano lost its top and became the lake it is today. Some of the most beautiful mountains in the world are composite volcanoes. Some examples of these are:

• Mt. Saint Helens

• Mt. Vesuvius, Pavlof, and Mt. Popacatepetl

• Calderas

• Stromboli

Created by: Lenny Lehrer, Amber Reed, and Shannon Jones.