The forces of volcanism are visible in this artwork.  Evidence of volcanism appears throughout the Solar System.  Remnants of extinct volcanoes can be found on all the rocky inner planets and the larger rocky moons.  There are still active volcanoes on Venus, Earth, and Jupiter’s moon Io.  

Photo Credit: NASA Radar-mapped Image of the Surface of Venus

Photo Credit: NASA False-color Image of Jupiter's Moon, Io

The heat that drives volcanoes on these planetary bodies is a by-product of their formation.  Planets and moons were formed when many smaller bodies (called planetesimals), which dominated the Solar System during its early formation, crashed together.  As the bodies collided with one another at incredibly high speeds, a great deal of heat was released.  They became molten and to this day remain molten under a thin crust. 

The decay of radioactive elements in the interior of a planetary body is another source of the heat that forms volcanoes.  In the case of Jupiter’s moon Io, the heat source is something else.  There, tidal forces caused by Jupiter’s strong gravity and the perturbing influence of the nearby moon Europa keep Io from coming to rest.  This stress constantly flexes the interior of Io, producing intense heat.  Io’s surface experiences more volcanic eruptions than any other body in the Solar System.

Internal planetary heat rises slowly through the process called convection, clearly visible in ”Convection Cells.”  It must either seep through the rocky crust to be radiated away at the surface or travel more directly through cracks in the crust.  When the latter happens, a volcano erupts.  As it works its way up through cracks, hot rock melts pathways, which are constantly changing.  The density of the hot rock is less than that of solid rock, so it rises.  As the solid rock is eroded by the dynamic flow of the lava, chunks break off and sink into the flow, where they dissolve.  As the magma chamber of molten rock nears the surface it can form a bulge.  Once the magma begins to flow out above the surface it creates a volcanic peak.  If the flow is later blocked by changes below, the magma chamber may collapse, causing the surface to slump.  Collapsing magma chambers form a geological feature called a caldera.  This phenomenon is represented vividly in both “ Rift Zone” and “Caldera.”