Calderas are volcanic depressions, roughly circular in surface plan, with a diameter greater than depth, and representing roof collapse into shallow underlying magma reservoirs.
The term “caldera” comes from the Latin word “caldaria” meaning “boiling pot,” and was originally used in the Canary Islands for any large “bowl-shaped” depression. Only in the last 50 years has their origin and potential hazards been fully appreciated. Calderas may occur in volcanoes of all compositions, in all tectonic environments, and show a wide range of forms. Consequently, it is difficult to classify calderas, although common collapse processes, provide “end-member” possibilities (Figure 1). The simplest form is “piston” or “plate” collapse within a cylindrical (ring) fault. This occurs within many smaller (typically basaltic) calderas, but is rare in larger (typically rhyolitic) structures. The latter are more likely to show either “piecemeal” collapse, around a number of centers in the caldera, or “downsag,” where parts of the structure dip towards the center of the caldera. Regional faults can be an important boundary influence, and collapse may preferentially occur along one of these faults, with the opposite side showing “downsag,” to produce a “trapdoor” caldera.
Many larger calderas have experienced multiple collapse events, often separated by tens of thousands of years. Such calderas should more correctly be called “caldera complexes”. Each collapse is likely to be accompanied by explosive eruptions, usually producing pyroclastic flows, which deposit widespread ignimbrites (ash flow tuffs). Some of the ignimbrite will pond in the caldera (intra-caldera ignimbrite), whereas the remainder will be distributed radially around the caldera (outflow sheets).
While collapse is the key to caldera formation and is rapid (hours to days), it is only one phase in a process that may take tens to hundreds of thousand years. Pre-collapse volcanism is common, sometimes accompanied by uplift (“tumescence”), and most rhyolitic calderas are followed by post-collapse volcanism (usually forming lava domes and airfall tephra), often accompanied by uplift (“resurgence”). A cross section of a generalized “piston” caldera is shown in Figure 2. Hydrothermal activity and mineralization is likely to occur throughout the life of a caldera volcano, but is particularly important in the post-collapse stages. Once volcanism ceases, erosion will progressively remove much of the surface volcanism (over millions of years). This structure is called a “cauldron,” when caldera-floor rocks become exposed. Once a substantial amount of the underlying magma reservoir is exposed, the term “ring-structure” is commonly used.
Calderas are a major natural hazard. The accompanying pyroclastic flows can cause total devastation for hundreds to thousands of square kilometers around the volcano. The largest of these, the “Supervolcano” eruptions, can produce >1,000 km3 of ignimbrite and can influence climate with fine ash remaining in the atmosphere to cause a “global winter” for many years! During pre-collapse tumescence and post-collapse resurgence, ground movement is likely, which will affect structures built in the area. While there are likely to be precursor events to caldera formation (e.g., earthquake swarms, gas discharge, etc.), such events do not always culminate in an eruption. Such “false alarms” are a major problem for effective prediction.
Volcanoes and Volcanic Eruptions