The Opening
of Earth

A volcano is a place on the Earth's surface where molten rock, gases and pyroclastic debris erupt through the earth's crust.

Composite

recorded history, explosive eruptions at subduction zone (convergent-boundary) volcanoes have posed the greatest hazard to civilizations.Subduction-zone stratovolcanoes, like Mount St. Helens and Mount Pinatubo, typically erupt with explosive force: the magma is too stiff to allow easy escape of volcanic gases. As a consequence the tremendous internal pressures of the trapped volcanic gases remain in the pasty magma. Following the breaching of the magma chamber, the magma degasses explosively. Such an explosive process can be likened to shaking a bottle of carbonated water vigorously, and then quickly removing the cap. The shaking action nucleates the dissolution of CO2 from the liquid as bubbles, increasing the internal volume.

Cinder

The rock fragments, often called cinders or scoria, are glassy and contain numerous gas bubbles "frozen" into place as magma exploded into the air and then cooled quickly. Cinder cones range in size from tens to hundreds of meters tall. Cinder cones are made of pyroclastic material. Many cinder cones have a bowl-shaped crater at the summit. During the waning stage of a cinder-cone eruption, the magma has lost most of its gas content. This gas-depleted magma does not fountain but oozes quietly into the crater or beneath the base of the cone as lava.

Shield

Shield volcanoes are one of the four major types of volcanoes, distinguished from the three other major volcanic types, stratovolcanoes, lava domes, and cinder cones, by distinct differences in structure and composition. Stratovolcanoes are built up by the accumulation of thick, viscous lavas, cinder cones are constructed of tephra ejected in explosive eruptions, and lava domes are built from extremely thick lava which can't flow far from the vent. In comparison, shield volcanoes are usually built of low viscosity basaltic lavas that erupt in longer cycles than that of a stratovolcano.