Volcanic eruptions are spectacular, sometimes deadly displays of nature's force. The power of volcanoes can make human endeavors appear insignificant and transient by comparison.

Volcanoes form when the planet needs to let heat escape from its interior. Rocks that melt deep beneath the planet's surface become liquid magma. The magma rises from Earth's interior to spew forth from a volcano at Earth's surface.

In this guide, volcanologist Haraldur Sigurdsson gets to the bottom of volcanic activity.

Volcanoes and history
The human species has grown up with volcanoes. Our earliest ancestors originated millions of years ago among the volcanoes of East Africa. Although we do not know what these ancestors thought about volcanic activity, we know that they made use of volcanic rocks, especially obsidian (volcanic glass formed as magma cools). Obsidian, with its sharp edges, provided a raw material for arrowheads, spear tips, and other tools.

When early scholars began to think about Earth, they pondered the origin of volcanoes. More than 2,000 years ago ancient Greek philosophers proposed that volcanic eruptions resulted from wind within Earth. They regarded volcanoes as safety valves or vents that released these winds.

About AD 100 the ancient Romans proposed that volcanoes were like furnaces. They believed that underground fires of sulfur, bitumen, coal, and other flammable substances caused volcanic eruptions. Vulcan, the Roman god of fire, gave his name to volcanoes. Fires cannot burn inside Earth, however, because no free oxygen is available to fuel them.

Today we know that magma, or molten rock, forms volcanoes. Heat released by enormous pressure in Earth's interior melts the rock. The pressure is caused by the movement of tectonic plates, which make up Earth's crust, or outer rigid shell.

Why volcanoes form
Volcanoes form where magma comes to Earth's surface. This melted rock, liquefied deep in Earth, is at a very high temperature when it reaches the surface, ranging from 1000° to 1250°C (1800° to 2300°F). Magma that erupts at Earth's surface is called lava. Knowing how magma forms is essential to a basic understanding of volcanic activity.

Most magma is formed by the melting of peridotite, a rock that makes up Earth's mantle--the part between the crust and the core. Peridotite is solid throughout most of the mantle, but it begins to melt in areas where the pressure on it decreases. The melting point of most materials changes with pressure--the lower the pressure, the lower the melting point. Where Earth's crustal plates pull apart, pressure on the mantle rock beneath decreases; peridotite rises as a result and spontaneously melts to form magma. Peridotite melts only partially, however, becoming a spongy rock. About 10 percent of this spongy substance is liquid magma, which separates out of the rock and rises to the surface to erupt.

Magma also forms in regions called hot spots, which include many volcanic islands--Hawaii, the Galapagos, and the Azores, for example. Hot-spot magma forms because of deep upwellings within the mantle, rather than as a direct result of plate motion. These upwellings may originate as deep as the boundary between the mantle and the core, some 2,900 km (1,800 mi) below Earth's crust.

Visiting volcanoes
Active volcanoes occur in many parts of the world, as shown on this Smithsonian Institution map. They are not randomly scattered but form in well-defined volcanic belts or arcs. This systematic arrangement results from the fact that volcanoes appear mainly at the boundaries of the tectonic plates that make up Earth's crust.

Most volcanic activity on Earth occurs above plate boundaries on the ocean floor, where crustal plates are pulling apart. In only a few places do these so-called mid-ocean ridges rise above the surface of the ocean, where we can observe them easily. One such place is Iceland, where scientists can study the process of plate separation and the resulting volcanic activity.

Volcanoes also form where crustal plates converge. These volcanoes are the easiest to visit, but they can also be dangerous. An example is Mount Saint Helens in Washington State, which erupted in 1980, killing 57 people.

Hot-spot volcanism is easily studied and observed on the big island of Hawaii, where eruptions of Kilauea volcano are frequent.

Mars also has a number of volcanoes, including the largest volcano in the solar system: Olympus Mons. This monster volcano is more than 26 km (16 mi) high and more than 500 km (300 mi) wide. Scientists do not know if any of Mars's volcanoes are active.

Pancake-like lava domes are the most striking of many volcano types on Venus. Scientists discovered these domes through radar beams that penetrated the planet's thick atmosphere and made it possible to map surface features.