The volcano, a geological landform, consisting of a fissure in
the earth's crust, above which a cone of volcanic material has
accumulated. At the top of the cone is a bowl-shaped vent called
a crater. The cone is formed by molten or solid matter that
flows through the vent from the interior of the earth. The study
of volcanoes is called volcanology. Most volcanoes are composite
landforms built up partly of lava flows and partly of fragmental
materials. Italy's Etna, in Sicily, and Vesuvius, near Naples,
are examples of composite cones. In successive eruptions, the
solid materials fall around the vent on the slopes of the cone,
while lava streams come up from the vent on to the flanks of the
cone. Thus, the cone is built up of layers of fragmental
materials and flows of lava, all inclined outward away from the
vent. Some enormous, craterlike basins, called calderas, at the
top of long-dormant or extinct volcanoes, are eventually
occupied by deep lakes, such as Crater Lake in Oregon. Some
calderas are the resul