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., Vailes 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, ultramarie style of volcanism, unlike any volcanic activity in modem 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 x 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. TERRESTRIAL CALDERAS As a guide to interpreting possible volcanic depressions on other planets and satellites, I first summarize information on the types, geometries, numbers, distribution, and formation processes of terrestrial calderas. Investigations of caldera origins were widespread during the first third of this century, but little new work appeared for 2:5 years following the publication of Williams'[ 1941] classic memoir "Calderas and their origins." This paper is not subject to U.S. copyright. Published i...

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“…This would mean that, except in regions where magma plumes continue to rise to the surface, continental uplift could slow down or stop, allowing the weathering rate to exceed the mountain-building rate, potentially resulting in less high altitude land area (Meadows, 2007). However, the lack of plate movements could allow volcanoes surrounding magma plumes (hot spot volcanoes) to grow taller than those on the present Earth as material is able to accumulate in one location for a longer period of time -the driver behind very tall martian volcanoes, such as Olympus Mons, which stands at a height of 21 km (Wood, 1984).…”

Flank vents and graben as indicators of Late Amazonian volcanotectonic activity on Olympus Mons

“…This is slightly east of the geographic center of the volcano as a function of Olympus Mons's asymmetry relative to its caldera complex. The center of the nested caldera complex (approximately 18.33°N, 226.83°E) was chosen as the center here, because direct pathways to the magma chamber below are assumed [ Wood , ; Zuber and Mouginis‐Mark , ; Thomas et al , ]. As ~2 km of lava from Tharsis and the Tharsis Montes have embayed Olympus Mons to the east, all vent elevation values are referenced to the elevation of the plains west of Olympus Mons that sit at an elevation of −2.5 km.…”

“…The shield hosts a variety of complex and enormous geologic features (Figure ). A nested caldera complex approximately 90 × 60 km in size is situated at the summit [ Carr , ; Wood , ; Mouginis‐Mark and Rowland , ; Plescia , ]. Most of the lava flows observed to have originated from Olympus Mons are embayed by flows from Tharsis, either from the Tharsis Montes, the smaller shields, or from sources that are no longer visible [ Carr et al, ; Isherwood et al, ; Chadwick et al, ].…”

“…Olympus Mons displays a flat summit plateau, steeper upper flanks (~5–9°), and gentler sloping lower flanks (~4°), with the steepest slopes (~25–40°) restricted to the basal escarpment [ King and Riehle , ; Blasius and Cutts , ; Plescia , ; Byrne et al, ]. The physical volcanological landforms on the flanks of Olympus Mons render it morphologically similar to the Hawaiian volcanic shields, and so a basaltic composition has been inferred for its erupted lavas [ Carr , ; Wood , ; Hulme , ; Zuber and Mouginis‐Mark , ; Bleacher et al, ]. Although development of the Olympus Mons edifice probably started during the early Hesperian (~3.6 Ga), most of the shield's surface has been dated to the Late Amazonian (<700 Ma) [ Neukum et al, ; Werner , ; Isherwood et al, ; Tanaka et al, ].…”

Omori law for eruption foreshocks and aftershocks

“…The 20 km size of the seismic gap is characteristical of the size of the volcanic edifices worldwide. Moreover, one possible mechanism to reproduce the larger gap zone around non subduction volcanoes (Figure 12) is the larger size of shield hot spot volcanoes [ Wood , 1984]. It possibly drives a regional crust damage at larger distances from eruption than for the other eruptive styles, accounting for the larger seismic shadow zone (Figure 12).…”