The empty chamber of a volcano formed after the completion of the eruption is known as the Caldera. As the eruption of the volcano comes to an end, the vent and crater remain as a hollow shaped cavity. According to H.J de Blij,
“A volcano‘s lava and pyroclastics come from a subterranean magma chamber, a reservoir of active molten rock material that forces its way upward through the volcanic vent. When the magma reservoir ceases to support the volcano, the chamber may empty out, and the interior of the mountain may literally become hollow. Left unsupported by magma the wall of the vent may collapse creating a caldera. Such an event can happen after the completion of the eruption when the weakened structure of the volcano is shaken by an earthquake. A caldera can also result from a violent eruption, which destroys the peak and crater of the volcano.”
Crater Lake Oregon is a classic example of a caldera, which was produced in prehistoric period. In 1883, Krakatoa, a volcanic island of Indonesia explode very violently, thus creating a huge caldera. Similarly, in 1992, a great explosion took place at Alsaka’s region of Katmai, a caldera of more than 2 miles in width was produced.
The word “Caldera” was derived from the Spanish “Caldera”, and Latin “Caldaria”, which means a cooking pot. As in English sometimes, the caldera is assumed to be derived from Cauldron. The first introduction of the term “Caldera” had been done 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 Montana Teide dominating the landscape, and then the Caldera de Taburiente on La Palma.
Minerals in Calderas:
Some calderas are known to be enriched with ore minerals. Sturgeon Lake Caldera is famous for best-preserved mineralization. The movement of rock-material in the caldera makes circulate the mineral ores, like gold, iron, silver, uranium, Lead and Lithium, thus concentrated each individual mineral separately. The Sturgeon Lake Caldera is located in Northwest of Ontario, Canada, which was formed during Neoarchean era (2.7 billion year 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:
There are two types of calderas:
(1). Explosive Calderas:
The explosive caldera is a caldera formed purely due to the violent eruption and explosion of the volcano. The high-pressure up-thrust of the magma makes its way out, thus forming a huge caldera. As the magma of rhyolitic volcanoes is enriched with silica, which gives it high viscosity. Due to the high viscosity and adhesive quality, the magma is not drained by the lava fluid, but rather, it is moved by the pressure exerted by the eruption. The magma of rhyolitic/silica-rich volcanoes also contains a reasonable amount of gases. 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 caldera is generally filled with tuff, igneous rocks, and rhyolite.
(2). Non-explosive Calderas:
Very large volcanoes like “Shield volcanoes of Kilauea” and Muana Lao” (Hawaii island) form calderas in quite a different way. A major part of the magma of these volcanoes comprises Basalt, which has a very little ratio of Silica. The low quantity of silica reduces the viscosity of the magma, as compared to rhyolitic volcanoes. Due to less viscosity, the magma chamber is drained up by the fast flow of lava fluid. A caldera formed due to lava flow, rather than by volcanic explosion, is known as a Non-explosive caldera. . For instance, the caldera atop Fernandina Island collapsed in 1968 when parts of the caldera floor dropped 350 meters (1,150 ft).