CALDERA
A caldera is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a
volcanic eruption. When large volumes of magma are erupted over a short time, structural support for
the rock above the magma chamber is lost. The ground surface then collapses downward into the
emptied or partially emptied magma chamber, leaving a massive depression at the surface (from one to
dozens of kilometers in diameter).[1] 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.
Only seven caldera-forming collapses are known to have occurred since 1900, most recently at
Bárðarbunga volcano, Iceland in 2014.[2]
Etymology
The term caldera comes from Spanish caldera, and Latin caldaria, meaning "cooking pot".[3] In some
texts the English term cauldron is also used,[4] 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.[3] 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,[note 1] 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.[5][3]
Caldera formation
Animation of analogue experiment showing the origin of the volcanic caldera in box filled with flour.
Landsat image of Lake Toba, on the island of Sumatra, Indonesia (100 km/62 mi long and 30 km/19 mi
wide, one of the world's largest calderas). A resurgent dome formed the island of Samosir.
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[6] in 1815), but also during effusive eruptions
on the flanks of a volcano (see Piton de la Fournaise in 2007)[7] 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.[8]:86–89 Secondary volcanic vents may form above the ring fracture.[9] 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.[3]
Mineralization in calderas
Caldera formation under water.
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.[10] One of the world's best-preserved mineralized calderas is the Sturgeon Lake Caldera in
northwestern Ontario, Canada, which formed during the Neoarchean era[11] about 2.7 billion years
ago.[12] 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.[13]
Types of caldera
Explosive caldera eruptions
Further information: Explosive eruption
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.[8]:23–26 The magma typically
also contains a large amount of dissolved gases, up to 7 wt% for the most silica-rich magmas.[14] 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.[15]
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.[16] 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.[8]:77 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.[17]
Eruptions forming even larger calderas are known, such as the La Garita Caldera in the San Juan
Mountains of Colorado, where the 5,000 cubic kilometres (1,200 cu mi) Fish Canyon Tuff was blasted out
in eruptions about 27.8 million years ago.[18][19]
The caldera produced by such eruptions is typically filled in with tuff, rhyolite, and other igneous
rocks.[20] The caldera is surrounded by an outflow sheet of ash flow tuff (also called an ash flow
sheet).[21][22]
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,[4] Cerro Galán,[23] Yellowstone,[24] and many other calderas.[4]
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[25] or Mount Pinatubo in 1991,[26] 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,[20] the San Juan
Mountains of Colorado (formed during the Oligocene, Miocene, and Pliocene epochs) or the Saint
Francois Mountain Range of Missouri (erupted during the Proterozoic eon).[27]