Calderas: A planetary perspective

Abstract

Caldera‐bearing volcanoes are the largest volcanic edifices on a planet and are expressions of long‐lived, near‐surface magma chambers. On earth only three morphological classes of calderas are common: shield calderas (e.g., Mauna Loa, Hawaii), stratocone calderas (e.g., Crater Lake, Oregon), and ash flow calderas (e.g., Valles Caldera, New Mexico). Each of these classes has a characteristic caldera size, magma type, and tectonic location. No calderas have been recognized on the moon or Mercury, but Mars has large shield calderas as well as the unique highland patera calderas. The latter may represent an explosive, ultramafic style of volcanism, unlike any volcanic activity in modern earth history. Radar images suggest that Venus has at least one large caldera‐bearing shield volcano and possible calderas of other types. Jupiter's satellite Io has abundant, large shieldlike calderas but no shield massifs. The number and variety of calderas on a planet may be proportional to planet mass, consistent with a previous suggestion that volcanic complexity increases with planet size. Io is an exception, but heating in Io is generated tidally rather than by the decay of radioactive elements, so that its calderas are not related to its size. The occurrence of shield volcanoes on four solar system bodies suggests that thermal and tectonic mechanisms (i.e. hot spots) giving rise to construction of shield mountains and associated collapsed summits are basic processes in planetary geology. Hot spot volcanism is expected to be most important on planets lacking plate tectonics. On earth, large shields have lifetimes of only a few million years, but on Mars, large shields may have been active for 1.5×109 years. Shield calderas on earth are smaller than 20 km in diameter, but known or suspected shield calderas on Mars, Venus, and Io have considerably larger diameters. Assuming that caldera diameters are related to magma chamber diameters and depths, magma chambers on other planets are significantly larger and/or deeper than their counterparts on earth. Terrestrial magma chambers are small because terrestrial shield volcanoes are small in comparison to shields on Mars and Venus, and terrestrial shields are small because the earth's mobile plates move growing shields away from their deep sources, thus distributing magma along a narrow chain rather than piling it up around a central vent. Finally, the long lava flows of Mars are also explained if the volume of erupted magma has any simple relation to magma chamber volume.