Calderas

A caldera is a large depression, more or less circular in form, caused by the collapse of a volcano during or after eruption. With the exception of impacts by asteroid-sized meteorites, the largest caldera-forming eruptions represent the most catastrophic geologic events known. Ancient calderas are sites of many of the earth's ore deposits, and recently formed calderas are important sources of geothermal energy.

Calderas and Craters

The term caldera (Spanish for “kettle” or “cauldron”) was used by inhabitants of the Canary Islands to refer to all natural depressions, including the island's volcanic craters and calderas. The term was introduced into the geologic literature in the nineteenth century to describe volcanic depressions. There remains, however, a debate over the difference between craters and calderas.

In general, craters are caused by the explosive removal of material, while calderas form by the subsidence of the surface during or immediately after explosive volcanism. Because subsidence structures are usually larger than craters, many geologists consider all volcanic depressions larger than one mile (or one kilometer) to be calderas. Other geologists prefer to emphasize origin; they use “craters” for depressions produced by explosions and “calderas” for all collapse depressions. It is only when explosion structures are extremely large or calderas are small that a problem exists. Geologists, for example, are about evenly divided on whether the large depression produced by the 1980 eruption of Mount St. Helens is, by origin, a crater or, by size, a caldera. One of the small ironies of science is that all volcanologists agree that one of the world's best examples of a caldera is Oregon's Crater Lake.

In general, then, calderas form when the support provided by the underlying molten rock or magma is removed, either by eruption or by withdrawal to a lower level. Three classes of calderas are common: those located at the summits of basaltic shield volcanoes, such as Hawaii's Mauna Loa; those that “behead” andesitic stratovolcanoes, such as Crater Lake; and those that contain the source vents for widespread layers of rhyolitic ash, such as Wyoming's Yellowstone caldera. In addition, planetary geologists have discovered shieldlike volcanoes with summit calderas on Mars, Venus, and Jupiter's satellite Io.

Basaltic Shield Volcanoes

The smallest terrestrial calderas are associated with basaltic shield volcanoes, such as those in the Hawaiian Islands. Shield volcanoes are composed mostly of layers of basalt, a dark-colored volcanic rock rich in magnesium and iron but relatively poor in silica. The Hawaiian Islands are the exposed southeastern end of a largely submarine mountain range of volcanic origin. Volcanic islands and submerged seamounts can be traced for nearly 6,000 kilometers to where they disappear into a deep oceanic trench off Alaska's Aleutian Peninsula. Plate tectonics theory and age determinations performed on volcanic rock suggest that the entire chain took nearly eighty million years to be produced as the Pacific Ocean floor moved at an average rate of about 8.6 centimeters per year over a subcrustal magma source, or hot spot. Hawaii is now located over the hot spot and contains two active shield volcanoes, Mauna Loa and Kilauea. Each shield volcano has a summit caldera from which emanates radial rift zones marked by recent lava flows, minor vents, and lines of craters. A magma chamber is located at a relatively high level within each volcano.

Although basaltic shield volcanoes erupt frequently, their eruption style is the mildest known. A typical eruptive sequence begins with the magma reservoir within the volcano gradually filling and producing a measurable inflation of the volcano's summit. Swarms of small earthquakes caused by magma movement occur below the impending vent site. The eruption commonly begins with lava fountaining at the summit. As magma works its way along the rift zones to erupt at lower elevations, activity ceases at the summit. The continued removal of magmatic support causes a part of the summit area to subside along arcuate faults, forming a caldera.

Summit calderas are slightly elliptical in outline, with flat floors and steep walls. Because of the frequency with which basaltic shield volcanoes erupt, summit calderas have a complex history of collapse, uplift, and infilling by later lava eruptions. Kilauea's summit contains several collapse features, including Kilauea caldera, the major structure. Its approximate dimensions are four by three kilometers, and its average depth is about 100 meters. A second, smaller caldera, Kilauea Iki, is within Kilauea caldera, and both structures are surrounded by arcuate faults along which minor collapse has occurred. Mauna Loa's summit caldera is similar in size and also consists of multiple collapse zones.

Shield volcanoes with summit calderas are by no means restricted to the Hawaiian Islands. They are commonly found where large outflows of fluid basalt occur, and prominent examples are found in Iceland and the Galápagos Islands. Summit shield calderas on what appear to be basaltic volcanoes are also found on Mars. The most impressive example is Olympus Mons, probably the largest volcano in the solar system. It is more than 600 kilometers across and 23 kilometers high, and its summit contains an eighty-kilometer-wide caldera complex. Scientists speculate that Mars has hot spots but does not have independently moving plates. They believe Olympus Mons may have been volcanically active for 1.5 billion years as basaltic magma was fed upward from its mantle source.

Stratovolcanoes

Calderas are also associated with stratovolcanoes. Stratovolcanoes, with slopes of every grade, most closely resemble the stereotype of the volcano. Lava and pyroclastic material accumulate around a central vent to produce mountains rising as much as five kilometers above their bases. The rapid erosion of material from these lofty peaks, sometimes as disastrous mudflows, produces aprons of sediment around the volcanoes' flanks. They are the most abundant type of large volcano on the earth's surface and the characteristic volcanic landform found on the island arcs and continental margins fringing the Pacific Ocean. Although andesite, a dark- to medium-colored volcanic rock with an intermediate silica content, is the most common type of rock erupted, stratovolcanic eruptions produce a wide range of magma types.

Stratovolcanoes show prolonged periods of dormancy broken by eruptive phases that range from mild degassing to catastrophic eruptions that greatly alter or destroy the volcano's shape. Large eruptions from stratovolcanoes are commonly associated with the emplacement of rocks called ignimbrites. During an ignimbrite eruption, parts of the cone may be blasted away, or the volcano may founder into an immense caldera.

Historic caldera-forming eruptions have been impressive. The eruption of Krakatau in modern-day Indonesia in 1883 took place on a deserted volcanic island, yet a giant wave produced by the volcano's collapse killed more than 30,000 people on neighboring shores. The great Tambora eruption of 1815 caused the deaths of more than 90,000 people, either directly by eruption or by the ensuing famine. Prehistoric eruptions must have been even more spectacular. The Bronze Age eruption of Santorini in the Mediterranean Sea, for example, has been linked to the decline of the Minoan civilization and, therefore, may have changed the course of Western history.

Andesitic and Rhyolitic Stratovolcanoes

Crater Lake, Oregon, has contributed much to the understanding of calderas and serves as a good example of caldera formation on andesitic stratovolcanoes. Crater Lake is a circular caldera approximately ten kilometers in diameter. The average depth of the lake is about 600 meters, and the surrounding cliffs rise from 150 to 600 meters. Wizard Island, a small cinder cone, rises 225 meters above the level of the lake. The eruption that formed Crater Lake occurred approximately 6,845 years ago, following thousands of years of intermittent activity that built a large stratovolcano that geologists call Mount Mazama. It is estimated that the cone was approximately 3,500 meters high and was capped by glacial ice.

Detailed field studies around Crater Lake have shown that the initial eruption was from a single vent, which fed ash and pumice into an eruption column that reached into the stratosphere and drifted with the prevailing wind. As the eruption intensified, so much material was emplaced into the cloud that, despite its heat, the cloud's density exceeded that of the surrounding air; it gravitationally collapsed to feed ground-hugging clouds of incandescent ash and pumice. These ash flows had great mobility and moved at hurricane speed to deposit ignimbrite around Mount Mazama. When about thirty cubic kilometers of magma had been expelled, the roof of the magma chamber collapsed to form a caldera. Venting, however, continued to eject another twenty cubic kilometers of magma from multiple vents located along the ring-fracture system bounding the caldera. The caldera continued to subside as venting progressed. Much of the ash fell back into the depression and mixed with rock that was sliding from the oversteepened walls to pile up on the caldera floor. A small cinder cone, Wizard Island, subsequently formed on the caldera floor, and Oregon's abundant rainfall produced the caldera lake.

Stratovolcanoes grade with increasing silica content to volcanoes composed mostly of rhyolite, a silica-rich volcanic rock usually light in color. Although the eruption frequency of the more silicic volcanoes tends to decrease, their eruption volume and caldera size increase. Rhyolitic volcanoes are dominated by ignimbrite eruptions, and they tend to look very unlike volcanoes as most people picture them. A rhyolitic volcanic field consists of a rhyolitic ignimbrite plateau, punctuated here and there by large calderas. In most of these structures, the caldera floor has resurged or been uplifted and arched to form what is known as a resurgent dome. Resurgence, combined with the effects of sedimentation, continued volcanism, and erosion, may even make the caldera difficult to detect. Hundreds of such calderas are known or await discovery, hidden among the rhyolitic ignimbrites of western North America. Many of these ancient calderas are associated with that region's important ore deposits. Most calderas of this type tend to be circular, but the largest examples are elongated. Their irregular shape may be caused by their piecemeal collapse into larger, more complex magma chambers or may reflect stresses in the crust. The largest calderas are more properly called volcano-tectonic depressions. The Lake Toba caldera, a volcano-tectonic depression on the island of Sumatra, is 100 kilometers long by thirty-five kilometers wide, the largest caldera yet recognized. Dormant periods between minor eruptions at rhyolitic volcanoes may be measured in thousands of years. Repeated large eruptions may be separated by one million years and can form compound structures such as the Yellowstone caldera, a good example of a recently active, rhyolitic volcano.

Yellowstone National Park

Yellowstone National Park, famous for its hot springs and geysers, takes its name from volcanic rocks altered to bright colors by hot water and steam. The park's volcanic rocks belong to an igneous province composed mostly of basalt and rhyolite that extends southwest into Idaho's Snake River plain. Like Hawaii's, these volcanic rocks appear to be related to a hot spot. The Yellowstone hot spot, however, underlies the thicker and more silica-rich crust of the North American continent, and it is this continental crust that is believed to be the source for the enormous amounts of rhyolite.

Geologists of the US Geological Survey have shown that the Yellowstone area has been the site of three large caldera-forming eruptions and numerous minor eruptions during the past two million years. The ash emplaced within several tens of kilometers of the calderas was hot enough to anneal, or weld, into hard, rhyolite-capped plateaus. Remnants of the more widely dispersed ash have been found as far away as Texas. The last caldera-forming eruption occurred 600,000 years ago, when 1,000 cubic kilometers of magma was expelled, and a caldera forty-five kilometers wide and seventy-five kilometers long was formed over the partly drained magma chamber. Within a few thousand years, magma elevated the caldera floor and arched it into the two resurgent domes in this complex structure. Over 600,000 years, much of the caldera has been filled with lava and sediment. Yellowstone Lake now covers part of it but remains thermally and seismically active.

Principal Terms

hot spot: a volcanic center that has persisted for tens of millions of years and that is thought to be the surface expression of a rising plume of mantle material

ignimbrite: an igneous rock deposited from a hot, mobile, ground-hugging cloud of ash and pumice

plate tectonics: a theory that describes the earth's outer layer as consisting of large, independently moving fragments

pyroclastic: pertaining to volcanic material formed by explosion

shield volcano: a volcano in the shape of a flattened dome, broad and low, built by flows of very fluid basaltic lava

stratovolcano: a volcano constructed of layers of lava and pyroclastic rock; also called a composite volcano

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