Photograph courtesy USGS

Ancient Greeks and Romans believed that volcanoes were under the control of gods. How else to explain the fiery spectacles that occurred without warning and with such destruction?

Since that time, scientists have learned much about volcanoes—vents in the Earth's surface from which molten rock, debris, and steam issue.

About 1,900 volcanoes are active today or known to have been active in historical times. Almost 90 percent are in the Ring of Fire, a band of volcanoes circling the edges of the Pacific Ocean.

If a volcano erupts suddenly in a heavily populated area or in a remote region, the result can be a staggering death toll and the decimation of towns, industries, and vegetation.

But volcanoes bring benefits as well. They provide valuable mineral deposits, fertile soils, and geothermal energy. Lava flows can build new land, as they have in Hawaii.

	Photograph by R.W. Decker

Two important traits that characterize volcanoes are explosiveness and viscosity.

An eruption begins when magma, the molten rock from deep in the Earth's crust, rises toward the surface.

Dissolved gases in the magma determine whether the eruption will be explosive or nonexplosive. Lower amounts of dissolved gases lead to effusive, nonexplosive eruptions. Higher amounts of dissolved gases lead to explosive eruptions.

Anyone who has shaken a soda bottle and then opened it has seen a similar phenomenon—the dissolved gases come out of solution and the liquid erupts violently from the bottle.

The magma's silica content determines its viscosity. Viscosity is the measure of a substance's ability to resist flow. The lower the viscosity, the more fluid the lava is.

Low amounts of silica lead to less viscous magma. Often, low-viscosity magma lets dissolving gases escape to the surface, and the erupting lava is more fluid, or runny.

High amounts of silica, however, lead to high-viscosity magma, which often traps the dissolved gases. Pressure builds in the magma until the gases explode violently from the volcano's vents. The magma erupts as hard fragments of rock (pyroclasts) and ash.

Volcanoes can erupt in many different ways: explosively with primarily hard pyroclastic material; explosively with primarily fluid lava (lava fountains); nonexplosively, with thicker, more solid flows; and effusively, with highly fluid lava.

Volcanoes typically alternate between short active periods and much longer dormant periods. Although some volcanoes are considered extinct, almost any volcano is capable of rumbling to life again.