PART V
Extraterrestrial Volcanism


Extraterrestrial VolcanismIn the Space Age we have discovered that volcanoes occur on most of the rocky bodies of our Solar System and future explorations of other worlds will without doubt reveal volcanism as an important feature of planetary evolution in the universe as a whole. The evidence for volcanism on other bodies of the Solar System is reviewed in the five following chapters in Part V of the Encyclopedia of Volcanoes, Extraterrestrial Volcanism.

Although just beginning, the exploration of extraterrestrial volcanoes has revealed some amazing surprises, as described in this part of the volume. We begin with the chapter Volcanism on the Moon, studying the volcanism of our nearest neighbor. When Apollo 11 landed on the Moon in 1969 it touched down on the surface of a basaltic lava flow. Ever since Galileo aimed his telescope to the Moon in 1610, we have wondered about the origin of its numerous and large craters. The lunar craters, however, turned out to be of impact origin, whereas the dark plains, which are clearly visible with the naked eye from Earth and were termed maria by Galileo, have turned out to be made up of great floods of basaltic lavas. Studies of the rocks returned back to Earth laboratories have shown that the Moon has been extinct for over one billion years—a consequence of the small diameter of this rapidly cooling planetary body.

In contrast to our volcanically "dead" Moon, the Jovian satellite Io has a continuous display of spectacular eruptions, as described in the chapter Volcanism on Io. It is in fact the only body outside the Earth known to have large-scale active volcanism today. Io is indeed a strange and colorful world, with a surface decorated by a wonderful mosaic of yellows, oranges and reds, and sulfurous volcanic plumes that rise over 300 km above its surface. We are accustomed to thinking of volcanic energy as largely derived from primordial heat in the planet, but Io's energy source is different. The intense and continuous volcanism of Io is due to heat from great tidal stresses generated by its giant neighbor Jupiter.

Venus—our nearest planetary neighbor—has volcanic features that are in several respects similar to the activity on Earth, as described in the chapter Volcanism on Venus. While there is no evidence of Earth-style plate tectonics on the Moon, Io or Mars, there is indication that such global-scale processes may have been operating on Venus, and this may be reflected in its volcanism also. Here both lava flood plains and large central volcanoes occur. But Venus also possesses enigmatic coronae, which are enormous circular structures that are several hundred km in diameter and found nowhere else in our Solar System.

The chapter Volcanism on Mars answers the question: how did a planet one-half the size of the Earth generate volcanoes that are several times larger than the largest volcanoes on Earth? Mars has widespread volcanic plains, like the Moon, but the characteristic Martian volcanic features are huge central volcanoes that dwarf any terrestrial volcanoes. These immense lava shields are the result of hot spot activity and the largest is Olympus Mons, 25 km high, 600 km diameter. Is the size of these volcanoes a reflection of a much thicker lithosphere and the absence of plate tectonics?

In the farthest reaches of our Solar System, on the moons of Uranus, Saturn and Jupiter, a unique form of volcanism occurs, where geyser-like plumes of nitrogen and water-rich "magmas" are ejected from volcano-like structures. The final chapter of this section, Cryovolcanism in the Outer Solar System, discusses this distinctive form of volcanism. Most of the moons around Jupiter, Uranus and Neptune are made up of water ice and other volatiles. From time to time, heat generated within these satellites churns up a slush of ice and volatiles that are erupted at the surface, forming volcano-like manifestations of "dirty ices" mixed with ammonia, silicates and other impurities. The discovery of nitrogen-rich and geyser-like plumes on Triton, Neptune's distant moon, by Voyager 2 in 1989 shows that this type of volcanism occurs today.

Haraldur Sigurdsson
University of Rhode Island

[Return to Contents]