Io is the only planetary body in the solar system other than Earth known to exhibit active volcanism. Eight volcanic plumes were discovered by Voyager 1, six of which were still active when Voyager 2 sped by four months later.
Two blue volcanic eruption plumes extend 100 kilometers (62 miles) above the surface of Io. The plumes were first observed by Voyager 1 in March 1979. This image was taken by Voyager 2 on July 9, 1979.
Unlike Earth, the lava flows and eruptions on Io are composed primarily of sulfur. The difference in color of the flows may be related to impurities in the sulfur and cooling of the flows as they move away from the volcanic vent.
This enhanced false color image from Voyager 1 shows a symmetrical volcanic plume from the volcano Loki. Volcanoes on Io eject gases and particles at 3,300 kilometers per hour (2,000 miles per hour), more than 50 times faster than material ejected from Mount Etna, one of the Earth's most explosive volcanoes.
No impact craters have been observed on Io. The surface of the moon is continually covered by deposits from the active volcanoes. Scientists believe that the planet is subject to extreme stresses as it is gravitationally attracted inwards by Jupiter, and outwards by the Galilean satellites Europa and Ganymede. These tidal forces heat the crust of the moon creating fractures and the numerous volcanoes.