Calderas

In the evolution of the planet, major land forms have often been dependent on igneous activity. Features ranging from the mighty Andes, to the smallest hot spring are all dependent on the same processes of fire and earth that form and fuel the heat engines of the planet. One of the land forms that is common in these igneous environments is the feature known as a caldera. Calderas are important land forms, not only due to their local control of the surface features, but also due to the importance of their ash flows in the geologic chronology and climate of the region.They also have fascinating life cycles which are widely varied dependent on location, rock composition and tectonics. Therefore, lets now examine calderas:their formation, differences and locations on the planets surface.

Before the processes of caldera formation are described, the term caldera must first be defined. A caldera is a more or less circular volcanic depression, which is presumably formed by the collapse of an underlying magma chamber. (Hyndman p. 265) The size of a caldera is variable, but it is generally larger than a volcanic crater, but smaller than a volcano-tectonic depression (Steinberg p 267) Calderas are further subdivided into two types, explosive and subsidive. (Summerfield p. 118) Explosive calderas are generally formed in association with large volcanic blasts and are essentially, larger versions of volcanic craters. Subsidence calderas are features that form subsequent to large eruptions that lower the level of magma in a chamber to the point that the chamber's roof can no longer support itself and collapses. Another structure, closely related to calderas, is a calderon. This is a caldera like structure, with rectilinear of graben-like bounding fractures that are controlled by the regional tectonic framework. Finally, calderas form in two environments. In epicontinental sialic rocks, and in basaltic shield volcanoes.

First are explosive calderas. These are large blast craters formed from the violent explosions of volcanoes, usually andesitic in composition. They form from the disintegration of a volcanic summit following the collapse of the magma chamber after an explosive eruption. (Summerfield p 118). The size of explosive calderas varies but is limited to less than 10 km. (Steinberg p 29-30) The source of this explosion is the differentiation of the magma chambers lava, over time. As the lava column becomes vertically stratified, the more viscous, gas rich, magma rises to the top. As pressure is released by initial eruptions, and the level of the magma chamber falls, the pressure is lowered on the magma causing the underlying layers to degas violently. (Summerfield p 118-119) This degassing is the cause of the violent caldera forming explosion and it irresistibly obliterates all overlying structure into the caldera form.

A second type of caldera is the type that is found on the summits of basalt shield volcanoes. These occur at the summit of the shield complex and are associated with the rise and fall of magma in the magma chamber. As an eruption occurs, the magma level in the chamber falls. With the loss of buoyant support, the ceiling of the chamber collapses, forming the caldera. At the onset of a new eruption cycle, the floor of the caldera often rises . This lifting of the down faulted block is a result the refilling of the magma chamber. Eventually the chamber is again drained through eruptions and the cycle repeats itself forming a complex record of ring dikes, faults and flows.

The final, most complex, type of caldera is the subsidence calderas that form in the marginal and epicontinental zones of an orogenetic belt. These generally form in pyroclastic parent rocks and produce sialic ignibrites and lava flows during their eruptions (Hyndman p. 275). The origin of the intrusions that form these calderas can be extrapolated by the examination of their extrusive products. Epicontinental andesites are derived from the partial melting of a subducting oceanic plate under the continent. The shallower the dip of the subducting plate, the further inland plutons of this composition can be formed. Dacites and rhyolotes are derived from partial melting of the continental crust, generally through the contact melting of a basaltic intrusion rising in a zone of extension. Differentiation in an andesitic or intermediate composition pluton may also be the source of these acidic rocks (Hyndman p. 286). The composition of the pluton is important, not so much in the formation of the caldera but in the type of pyroclastic and ignibrite flows that will be produced during the caldera eruptions.

The size of the caldera is generally proportional to the size of the magma chamber (Hyndman p. 269). Like a basaltic shield caldera, subsidence calderas form by the collapse of the roof of a magma chamber after it partially drains. The chamber generally collapses before more than one tenth of its magma is drained. This removal is usually by large eruptions of ignibrites which extend over large areas. These flows are useful in the determination of the calderas relative age.

There are several stages that occur in the development of a subsidence caldera. The first of these stages is the regional swelling that occur with the emplacement of a pluton. Minor eruptions occur along extensional ring fractures in the periphery of the dome.

The second phase of caldera formation is the eruption of major ash flows from extensional ring fractures. This may or may not be concurrent with phase three, the collapse of the magma chamber roof to form the caldera. The collapse occurs along down faulting blocks and may be from multiple eruptive events.

The determination of the concurrence of phase two and three, is dependent on the thickness of ash fall deposits associated with the eruption. If the ash fall deposits are uniformly thick, both inside and outside the caldera, phase 2 preceded phase 3. If the ash flow deposits are thicker inside the caldera, then the eruption and down faulting of the central block were concurrent. At the end of the eruption cycle there are often felsic lava flows extruded to the caldera floor. These occur when the upper, gas rich, members of the differentiated magma chamber are exhausted allowing the more viscous lower magmas to reach to the surface.

Phase four of the growth of the caldera is an intermediate phase of erosion and deposition. The caldera walls erode in talus slides, and alluvial fans fill the caldera rim. Smaller eruptions and lava flows add to the filling in of the depression. Lakes often fill the caldera (As seen in the famous Crater lake of Oregon.) and if this occurs, lacuestrine deposits are often found. Eventually the lake will overflow and breach the caldera walls allowing erosional processes to take over again. Denudation of the caldera rim ultimately buries the ring fractures and back cuts the caldera walls through slope retreat. As a result, the ring fractures are often found well within the topographic expression of the caldera rim.

Phase five of caldera evolution is the resurgence doming that occurs in large calderas. This process occurs as the magma chamber refills and buoyantly uplifts the down faulted, caldera floor. Some ring fracture and dome fracture volcanism often occur, continuing the filling of the caldera depression.

Phase six of the caldera's development consists of major ring fracture volcanism which exhibits rhyolitic lava flows and the formation of volcanic domes and pyroclastic cones. At this point three things can occur: phase five, resurgent doming, can continue, phase two can initiate as a younger caldera is formed or the caldera can advance to phase seven, cooling of the pluton. This phase is exhibited with fumeroles, hydrothermal activity, continued erosion and filling of the caldera depression. Ore deposits are emplaced and the pluton finally crystallizes. The end of phase seven, represents the end of the caldera cycle for a igneous pluton.

So as one compares the formation of the multiple types of calderas, several similarities can be found. All calderas are the result of the collapse of a magma chamber, either through the failure of the chamber's ceiling, or through its violent disintegration through an explosion. Magma composition plays a major part in the formation of calderas. Mafic composition magmas cause shield collapse calderas, while felsic compositions cause explosion or subsidence calderas. Finally the tectonic environment plays a major role in caldera types: mafic calderas in ocean ocean trenches, mid ocean ridges and hot spots, and felsic calderas in extensional or ocean continent trenches. All these environments form plutons which eventually result in a magma chamber that fills an empties forming collapses or pressure change explosions that give birth to calderas.

Bibliography

Hildreth, Wes, Mahood, Gail, Nestled Calderas and Trapdoor Uplift at Pantelleria, Straight of Sicily, Geology, v. 11, pp 722-726, December 1983.

Hyndman, Donald, W., Petrology of Igneous and Metamorphic Rocks,McGraw Hill Pub. Co., 1985

Steinberg, G. S., On Explosive Caldera Formation, Modern Geology, v. 5, pp 27-30, 1974.

Summerfield, Michael, A., Global Geomorphology, John Wiley and Sons, New York, 1991.

Read but Not Referenced:

Brow, Everett, Foster, McGarvie, Rhymer, New light on caldera evolution-Askja, Iceland, Geology, v. 19, pp. 352-355, April 1991.

Christiansen, Robert, L., Timber Mountian-Oasis Valley caldera complex of southern Nevada, Geological Society of America Bulletin, v. 88, pp 943-949, July 1977.

Dewitt, Ed, Fridrich, C.J., McKee, E.H., Smith, R. P., Structural, eruptive, and intrusive evolution of the Grizzly Peak caldera, Sawatch, Range, Colorado, Geology, v. 11, pp 722-726, December, 1983.