Caldera

Caldera, originally uploaded by Wolfgang Staudt.

A caldera is a volcanic feature formed by the collapse of land following a volcanic eruption. They are often confused with volcanic craters. The word 'caldera' comes from the Spanish language, meaning "cauldron".

Caldera formation

A collapse is triggered by the emptying of the magma chamber beneath the volcano, usually as the result of a large volcanic eruption. If enough magma is erupted, the emptied chamber will not be able to support the weight of the volcanic edifice (the mountain) above. Fractures will form around the edge of the chamber, usually in a roughly circular shape. These ring fractures may in fact serve as volcanic vents. As the magma chamber empties, the center of the volcano within the ring fractures begins to collapse. The collapse may occur as the result of a single massive eruption, or it may occur in stages as the result of a series of eruptions. The total area that collapses may be hundreds or thousands of square kilometers.

Explosive calderas

If the magma is rich in silica, the caldera is often filled in with ignimbrite, tuff, rhyolite, and other igneous rocks. Silica-rich magma is very viscous. As a result, gases tend to become trapped at high pressure within the magma. When the magma gets near the surface of the Earth, the gas expands quickly, causing explosions and spreading volcanic ash over wide areas. Further lava flows may be erupted, and the center of the caldera is often uplifted in the form of a resurgent dome by subsequent intrusion of magma. A silicic or rhyolitic caldera may erupt hundreds or even thousands of cubic kilometers of material in a single event. 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.

When Yellowstone Caldera (last) erupted 640,000 years ago it released 1,000 cubic kilometers of material, covering all of North America in up to two meters of debris. By comparison, when Mount St. Helens erupted in 1980, it released 1.2 cubic kilometers of ejecta. The ecological effects of the eruption of a large caldera can be seen in the record of the Lake Toba eruption in Indonesia. About 75,000 years ago, this volcano released 2,800 cubic kilometers of ejecta, the largest known eruption within the Quaternary Period (last 1.8 million years). In the late 1990s, archeologist Stanley Ambrose proposed that a volcanic winter induced by this eruption reduced the human population to a few thousand individuals, resulting in a population bottleneck (see Toba catastrophe theory). Even larger caldera-forming eruptions are known, especially La Garita Caldera in the San Juan Mountains of Colorado, where the 5,000 cubic kilometer Fish Canyon Tuff was blasted out in a truly major single eruption 27.8 million years ago.

At some points in geologic time, rhyolitic calderas have appeared in distinct clusters. The remnants of such clusters may be found in places such as the San Juan Mountains of Colorado (erupted during the Tertiary Period) or the Saint Francois Mountain Range of Missouri (erupted during the Proterozoic).

Non-explosive calderas

Some volcanoes, such as Kīlauea on the island of Hawaii, form calderas in a different fashion. In the case of Kilauea, the magma feeding the volcano is relatively silica poor. As a result, the magma is much less viscous than the magma of a rhyolitic volcano, and the magma chamber is drained by large lava flows rather than by explosive events. The resulting calderas are also known as subsidence calderas, and can form more gradually than explosive calderas. For instance, the caldera atop Fernandina Island underwent a collapse in 1968, when parts of the caldera floor dropped 350 meters. Kilauea Caldera has an inner crater known as Halema‘uma‘u, which has often been filled by a lava lake. The largest volcano on Earth, Mauna Loa is also capped by a subsidence caldera called Moku‘āweoweo Caldera.


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Mount Etna, Sicily - Italy

Mount Etna Eruption, originally uploaded by P•A•U•L | Photography.

Mount Etna (Αἴτνη (Aítnē) in Classical Greek, Aetna in Latin, also known as Muncibeddu (beautiful mountain) in Sicilian and Mongibello in Italian (from the Latin mons and the Arabic gibel, both meaning mountain) is an active stratovolcano on the east coast of Sicily, close to Messina and Catania. Its Arabic name was Jebel Utlamat (the Mountain of Fire). According to Adrian Room’s book Placenames of the World, the name Etna is said to have originated from a Phoenician word attuna meaning “furnace.” He dismisses the theory that Etna is from a Greek source meaning “I burn.” It is the largest active volcano in Europe, currently standing 3,329 metres (10,922 ft) high, though this varies with summit eruptions; the mountain is 21 m (69 ft) lower now than it was in 1981. It is the highest mountain in Italy south of the Alps. Etna covers an area of 1,190 km² (460 sq mi) with a basal circumference of 140 km. This makes it by far the largest of the three active volcanoes in Italy, being about two and a half times the height of the next largest, Mount Vesuvius. Only Mount Teide in Tenerife surpasses it in the whole of the European region (though geographically Tenerife is an island of Africa). In Greek Mythology, the deadly monster Typhon was trapped under this mountain by Zeus, the god of the sky.
Mount Etna is one of the most active volcanoes in the world and is in an almost constant state of activity. The fertile volcanic soils support extensive agriculture, with vineyards and orchards spread across the lower slopes of the mountain and the broad Plain of Catania to the south. Due to its history of recent activity and nearby population, Mount Etna has been designated a Decade Volcano by the United Nations.

"A journey of a thousand miles must begin with a single step." -- Lao Tzu Copyright © Demetrios the Traveler

Santorini Caldera View, Greece

Caldera View, originally uploaded by Klearchos Kapoutsis.

Santorini (Greek: Σαντορίνη, pronounced [ˌsa(n)do̞ˈrini]) is a small, circular archipelago of volcanic islands located in the southern Aegean Sea, about 200 km (120 mi) southeast from Greece's mainland. The largest island is known as Thēra (or Thira, Greek Θήρα [ˈθira]), forming the southernmost member of the Cyclades group of islands, with an area of approximately 73 km2 (28 sq mi) and a 2001 census population of 13,670. It is composed of the Municipality of Thira (pop. 12,440) and the Community of Oía (Οία, pop. 1,230, which includes 268 inhabitants resident on the offshore island of Therasia, lying to the west). These have a total land area of 90.623 km2 (34.990 sq mi), which also includes the uninhabited islands of Nea Kameni, Palaia Kameni, Aspronisi, and Christiana (all part of the Municipality of Thira).
Santorini is essentially what remains of an enormous volcanic explosion, destroying the earliest settlements on what was formerly a single island, and leading to the creation of the current geological caldera.
A giant central lagoon, more or less rectangular, and measuring about 12 by 7 km (7.5 by 4.3 mi), is surrounded by 300 m (980 ft) high steep cliffs on three sides. The island slopes downward from the cliffs to the surrounding Aegean Sea. On the fourth side, the lagoon is separated from the sea by another much smaller island called Therasia; the lagoon merges with the sea in two places, in the northwest and southwest. The water in the centre of the lagoon is nearly 400 m (1,300 ft) deep, thus making it a safe harbour for all kinds of shipping. The island's harbours all lie in the lagoon and there are no ports on the outer perimeter of the island; the capital, Fira, clings to the top of the cliff looking down on the lagoon. The volcanic rocks present from the prior eruptions feature olivine and have a notably small presence of hornblende.[1]
It is the most active volcanic centre in the South Aegean Volcanic Arc, though what remains today is chiefly a water-filled caldera. The volcanic arc is approximately 500 km long and 20-40 km wide. The region first became volcanically active around 3-4 million years ago though volcanism on Thera began around 2 million years ago with the extrusion of dacitic lavas from vents around the region of Akrotiri.
The island is the site of one of the largest volcanic eruptions in recorded history: the Minoan eruption (sometimes called the Thera eruption), which occurred some 3,600 years ago at the height of the Minoan civilization. The eruption left a large caldera surrounded by volcanic ash deposits hundreds of feet deep and may have led indirectly to the collapse of the Minoan civilization on the island of Crete, 110 km (68 mi) to the south, through the creation of a gigantic tsunami. Another popular theory holds that the Thera eruption is the source of the legend of Atlantis.
The name Santorini was given to it by the Latin empire in the thirteenth century, and is a reference to Saint Irene. Before then it was known as Kallístē (Καλλίστη, "the most beautiful one"), Strongýlē[citation needed] (Στρογγύλη, "the circular one"), or Thēra.

"A journey of a thousand miles must begin with a single step." -- Lao Tzu
Copyright © Demetrios the Traveler