Caldera Formation

A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is gone. The ground surface then collapses into the emptied or partially emptied magma chamber, leaving a large depression at the surface (from one to dozens of kilometers in diameter). Although sometimes described as a crater, the feature is actually a type of sinkhole, as it is formed through subsidence and collapse rather than an explosion or impact. Compared to the thousands of volcanic eruptions that occur each century, the formation of a caldera is a rare event, occurring only a few times per century. Only seven caldera-forming collapses are known to have occurred between 1911 and 2016. More recently, a caldera collapse occurred at Kīlauea, Hawaii in 2018.

Etymology

The term ''caldera'' comes from Spanish ', and Latin ', meaning "cooking pot". In some texts the English term ''cauldron'' is also used, though in more recent work the term ''cauldron'' refers to a caldera that has been deeply eroded to expose the beds under the caldera floor. The term ''caldera'' was introduced into the geological vocabulary by the German geologist Leopold von Buch when he published his memoirs of his 1815 visit to the Canary Islands, where he first saw the Las Cañadas caldera on Tenerife, with Mount Teide dominating the landscape, and then the Caldera de Taburiente on La Palma.

Caldera formation

A collapse is triggered by the emptying of the magma chamber beneath the volcano, sometimes as the result of a large explosive volcanic eruption (see Tambora in 1815), but also during effusive eruptions on the flanks of a volcano (see Piton de la Fournaise in 2007) or in a connected fissure system (see Bárðarbunga in 2014–2015). If enough magma is ejected, the emptied chamber is unable to support the weight of the volcanic edifice above it. A roughly circular fracture, the "ring fault", develops around the edge of the chamber. Ring fractures serve as feeders for fault intrusions which are also known as ring dikes. Secondary volcanic vents may form above the ring fracture. As the magma chamber empties, the center of the volcano within the ring fracture begins to collapse. The collapse may occur as the result of a single cataclysmic eruption, or it may occur in stages as the result of a series of eruptions. The total area that collapses may be hundreds of square kilometers.

Mineralization in calderas

Some calderas are known to host rich ore deposits. Metal-rich fluids can circulate through the caldera, forming hydrothermal ore deposits of metals such as lead, silver, gold, mercury, lithium, and uranium. One of the world's best-preserved mineralized calderas is the Sturgeon Lake Caldera in northwestern Ontario, Canada, which formed during the Neoarchean era about 2.7 billion years ago. In the San Juan volcanic field, ore veins were emplaced in fractures associated with several calderas, with the greatest mineralization taking place near the youngest and most silicic intrusions associated with each caldera.

Types of caldera

Explosive caldera eruptions

Explosive caldera eruptions are produced by a magma chamber whose magma is rich in silica. Silica-rich magma has a high viscosity, and therefore does not flow easily like basalt. The magma typically also contains a large amount of dissolved gases, up to 7 wt% for the most silica-rich magmas. When the magma approaches the surface of the Earth, the drop in confining pressure causes the trapped gases to rapidly bubble out of the magma, fragmenting the magma to produce a mixture of volcanic ash and other tephra with the very hot gases.

The mixture of ash and volcanic gases initially rises into the atmosphere as an eruption column. However, as the volume of erupted material increases, the eruption column is unable to entrain enough air to remain buoyant, and the eruption column collapses into a tephra fountain that falls back to the surface to form pyroclastic flows. Eruptions of this type can spread ash over vast areas, so that ash flow tuffs emplaced by silicic caldera eruptions are the only volcanic product with volumes rivaling those of flood basalts. For example, when Yellowstone Caldera last erupted some 650,000 years ago, it released about 1,000 km³ of material (as measured in dense rock equivalent (DRE)), covering a substantial part of North America in up to two metres of debris.

Eruptions forming even larger calderas are known, such as the La Garita Caldera in the San Juan Mountains of Colorado, where the Fish Canyon Tuff was blasted out in eruptions about 27.8 million years ago.

The caldera produced by such eruptions is typically filled in with tuff, rhyolite, and other igneous rocks. The caldera is surrounded by an outflow sheet of ash flow tuff (also called an ash flow sheet).

If magma continues to be injected into the collapsed magma chamber, the center of the caldera may be uplifted in the form of a ''resurgent dome'' such as is seen at the Valles Caldera, Lake Toba, the San Juan volcanic field, Cerro Galán, Yellowstone, and many other calderas.

Because a silicic caldera may erupt hundreds or even thousands of cubic kilometers of material in a single event, it can cause catastrophic environmental effects. Even small caldera-forming eruptions, such as Krakatoa in 1883 or Mount Pinatubo in 1991, may result in significant local destruction and a noticeable drop in temperature around the world. Large calderas may have even greater effects. The ecological effects of the eruption of a large caldera can be seen in the record of the Lake Toba eruption in Indonesia.

At some points in geological time, rhyolitic calderas have appeared in distinct clusters. The remnants of such clusters may be found in places such as the Eocene Rum Complex of Scotland, the San Juan Mountains of Colorado (formed during the Oligocene, Miocene, and Pliocene