According to plate tectonics, now accepted by essentially all geologists, the earth is covered by plates, huge tiles of crust shifting on the subsurface. The ocean plates are mostly basalt, and heavier than the continental plates. The continental plates are mostly granitic rock. The plates move against each other, one sometimes going beneath another, as on the coast of California, in a subduction zone. In other places, the plates grate along side one another, or even diverge.
Most volcanoes occur at plate boundaries, ones where the plates are divergent as well as near the subduction zones where one tectonic plate goes under another and helps create a supply of melted rock. Some volcanic activity does occur in the middle of plates, such as in Hawaii and at Yellowstone, but this is believed to be due to the plates passing over hotspots in the mantle beneath them.
The tall stratovolcanoes volcanoes that circle the Pacific Ocean, the Ring of Fire, are made mostly of andesite rock. They are found near subduction zones, and their andesite lava contains water absorbed beneath the ocean. When an oceanic plate slides beneath a continental plate in subduction water is released. This water helps melt the mantle rocks above and produces magma. Magma is melted rock, underground. Lava is melted rock that surfaces.
Another kind of magma is mafic, or basaltic. It occurs at places where the tectonic plates are diverging or where the crust is stretched. It rises to the surface more quickly than andesite, and retains more of its original composition.
When these two kinds of lava make their way to the surface, they produce two different kinds of volcanoes. One is gently sloped, the shield volcano. The other is the tall classic cone we associate with such volcanoes as Mount Fuji.
Of course escaping lava may produce other features as well, such as volcanic plateaus, formed when lava oozes from a fissure; cinder cones piled up of fragments ejected from a vent; and spatter cones formed when globs of lava fall in heaps. Obsidian domes are formed when high silicate lava congeals. The shield volcanoes and the stratovolcanoes are, however, the most common types.
Shield volcanoes spread. Their lava is less explosive, and forms the kind of slow flaming lava flows that tourists thrill to watch. The volcanoes this lava forms are broad and low.
Stratovolcanoes are steep, and their eruptions may be extremely dangerous. Deadly volcanic gases are released. The explosion produces pyroclastic material, fragmented debris produced by fire. In Pompeii, deadly gases killed the inhabitants, and they were buried in pyroclastic materials and ash.
Hot lava from the stratovolcano's vent is instantly chilled as it hits the air and expands. It may form volcanic glass that is thrown high into the atmosphere. Hot lava runs down the sides of the volcanoes, often releasing more deadly gas. It glows with heat, and may travel at 300 kilometers per hour. Snow capped volcanoes may send sudden avalanches of debris down their flanks, which can destroy mountain towns at what seems a safe distance. The eruption of Mt. Pinatubo did this kind of damage.
After an eruption subsides, the molten lava in the vent of the volcano solidifies. This hardened lava forms a plug, and new magma making its way up from the mantle may be trapped beneath it. A volcanic dome may form. Eventually the cone may be destroyed, when the magma held within the dome explodes its way free.
This happened at Mount St. Helens, where a new dome is now forming, and the same process created beautiful Crater Lake in Oregon. The lake is in a caldera, a depression formed when the volcano was blasted away.
Volcanoes erupt when heat and pressure below force molten rock to the surface. The eruption may be slow and stately, as volcanoes go, or it may be a deadly blast of heat, fumes, and flying rock.