Volcanism is the eruption of molten rock (magma) onto the surface of a planet. A volcano is the vent through which magma and gases are discharged. Magma that reaches the surface is called "lava." Volcanoes are named for Vulcan -- the Roman god of fire!
Why and where do volcanoes form?
Volcanism is the result of a planet losing its internal heat. Volcanoes can form where rock near the surface becomes hot enough to melt. On Earth, this often happens in association with plate boundaries. Where two plates move apart, such as at mid-ocean volcanic ridges, material from Earth's interior slowly rises up, melts when it reaches lower pressures, and fills in the gap. Where one plate is being subducted under another, chambers of magma may form. These magma bodies feed the volcanic islands and continental volcanoes that mark subduction zones; the magma is silica-rich, forming explosive composite volcanoes.
Although most volcanic activity takes place at plate boundaries, volcanism also can occur within the plate interiors at hotspots. Hotspots are thought to be from large "plumes" of extremely hot material rising from deep in Earth's interior. The hot material rises slowly, eventually melting as it reaches lower pressures near Earth's surface. When the material erupts it forms massive lava flows of fine-grained dark volcanic rock -- basalt, which has less silica. The broad, gentle shield volcanoes of Hawai'i come from a hotspot.
What do Earth's volcanoes tell us?
The fact that Earth has volcanoes tells us that Earth's interior is circulating and is hot -- hot enough to melt. Earth is cooling; volcanoes are one way to lose heat. The pattern of distribution of volcanoes on Earth gives us a clue that Earth's outer surface is divided into plates; the chains of volcanoes associated with mid-ocean ridges and subduction zones mark the plate edges. Other planets have volcanic features -- some recently active -- telling geologists that they, too, are losing heat from their interiors and that there is circulation. However, these planets do not display the pattern that Earth's volcanoes do.
What evidence is there of volcanism on other planets?
Moon: Our closest neighbor has small volcanoes, fissures (breaks in the crust) and extensive flows of basalt, a fine-grained dark volcanic rock. The large dark basins that you can see on the Moon are the maria -- areas of these lava flows. However, all these volcanic features are old. There are no active volcanic features on the Moon. Most of the volcanic activity took place early in the Moon's history, before about 3 billion years ago. The most recent lava flow occurred about 1 billion years ago.
Mercury: The MESSENGER mission has photographed much of Mercury's surface and found evidence of volcanic activity shaping its surface. Some of the lava flows are between one billion and two billion years old. Although that is still relatively old by terrestrial standards, it tells planetary scientists that volcanic activity continued well after Mercury formed about 4.5 billion years ago.
Mars: Mars has volcanic features that are similar in shape to those on Earth, although much larger. There are large shield volcanoes -- like those in Hawai'i -- that contain 100 times more mass that those on Earth. Olympus Mons is the tallest volcano in our solar system. It is 22 km (14 miles) tall, compared to Mauna Loa's 9 km (almost 6 miles). It is 600 km across (375 miles), which is large enough to cover the state of Arizona! Several of the volcanoes on Mars, including Olympus Mons, occur in the Tharsis region; the magma for the volcanoes may come from hot material welling up in plumes from deep in Mars' interior. Many scientists consider Mars to be volcanically active, even if we have not observed an eruption. Basalt meteorites from Mars indicate that volcanism has occurred in the last 180 million years. Very few impact craters occur on the lava flows of Olympus Mons, suggesting that this volcano has probably erupted in the last few million years.
Venus: Venus has more than 1700 volcanic features and many of these look fresh -- unweathered. Much of the surface of Venus has been covered by huge flows of basalt lava, probably in the last few hundred million years. This blanket of lava completely covered the surface features, such as impact craters. The fact that only a few craters dot the surface provides evidence of the recent nature of this resurfacing.
Io: Jupiter's innermost moon, Io, is the most volcanically active body in our entire solar system! NASA missions imaged massive plumes shooting hundreds of kilometers above the surface, active lava flows and walls of fire associated with magma flowing from fissures. The entire surface of Io is covered with volcanic centers and lava flows, which have covered all of its impact craters.
Why don't we find active volcanoes on all planets and moons?
Active volcanoes occur on planets that are still hot. In general, the larger the planet, the slower it cools. Small planets or moons, like Mercury and our Moon, have cooled to the point that they are no longer hot enough to melt rock. Larger planets, like Earth and Venus, are still hot and still have active volcanism. Some small moons, like Io, have active volcanism because of tidal heating.
Earth Volcanoes: Types and Locations
Volcanoes on the Moon, Venus and Mars
Volcanoes on the Moons of Jupiter and Saturn
Education and Public Outreach for Volcanism in the Solar System
This revised and updated site has been built by volcanologists; it has a variety of information on volcanoes, a detailed glossary, and data about volcanic eruptions on Earth.
USGS Volcano Hazards Program
This site has maps, reports and images of recent and ongoing volcanic activity on Earth.
PBS' Savage Earth, Out of the Inferno: Volcanoes
This is a great resource for middle school students and older -- and the public. Includes an article about volcanoes on other planets.