Phreatic Eruption

A phreatic eruption is a type of volcanic eruption which occurs when rising magma makes contact surface water. The extreme temperature of the magma (anywhere from 600 to 1,170 °C) causes near-instantaneous evaporation to steam resulting in an explosion of steam, water, ash, rock, and volcanic bombs. At Mount St. Helens hundreds of steam explosions preceded a 1980 plinian eruption of the volcano. A less intense geothermal event may result in a mud volcano. In 1949, Thomas Jaggar described this type of activity as a steam-blast eruption.

Phreatic eruptions typically include steam and rock fragments; the inclusion of lava is unusual. The temperature of the fragments can range from cold to incandescent. If molten material is included, the term phreato-magmatic may be used. These eruptions occasionally create broad, low-relief craters called maars. Phreatic explosions can be accompanied by carbon dioxide or hydrogen sulfide gas emissions. The former can asphyxiate at sufficient concentration; the latter is a broad spectrum poison. A 1979 phreatic eruption on the island of Java killed 149 people, most of whom were overcome by poisonous gases.

It is believed the 1883 eruption of Krakatoa, which obliterated most of the volcanic island and created the loudest sound in recorded history, was a phreatic event. Kilauea, in Hawaii, has a long record of phreatic explosions; a 1924 phreatic eruption hurled rocks estimated at eight tons up to a distance of one kilometer. Additional examples are the 1963–65 eruption of Surtsey, the 1965 eruption of Taal Volcano, and the 1982 Mount Tarumae eruption.

Hydrothermal Circulation

Hydrothermal circulation is the circulation of hot water underneath the earth. Hydrothermal circulation occurs most often in the vicinity of sources of heat within the Earth's crust. It usually occurs near volcanic activity, but can occur in the deep crust related to the intrusion of granite, or as the result of orogeny or metamorphism. The word "hydrothermal" derives from Greek: "Hydros" means water and "thermos" means heat.

Hydrothermal circulation in the oceans is the passage of the water through mid-oceanic ridge systems. The term includes both the circulation of the well known, high temperature vent waters near the ridge crests, and the much lower temperature, diffuse flow of water through sediments and buried basalts further from the ridge crests.

Hydrothermal circulation is not limited to ocean ridge environments. The source water for geysers and hot springs is heated groundwater convecting below and lateral to the hot water vent. Hydrothermal circulating convection cells exist any place an anomalous source of heat, such as an intruding magma or volcanic vent, comes into contact with the groundwater system.

Skjaldbreidur Shield Volcano

Skjaldbreidur is an Icelandic shield volcano which was formed in a protracted eruption that occurred roughly 9,000 years ago. The extensive lava fields which were produced by this eruption, flowed southwards, and formed the basin of pingvallavatn, Iceland's largest lake, and pingvellir, the "Parliament Plains" where the Icelandic national assembly, the Alping was founded in the year 930.

The Skjaldbreidur volcano rises up to 1,060 meters, and its crater measures roughly 300 meters in diameter. Straddling the Mid-Atlantic ridge, the lava fields from Skjaldbreidur have been torn and twisted over the millennia, forming a multitude of fissures and rifts inside the pingvellir National Park, the best known of which are Almannagja, Hrafnagja and Flosagja.

Mayon Volcano

Mayon volcano, also known as Mount Mayon, is an active stratovolcano in the province of Albay, on the island of Luzon, in the Philippines. Rising to 2,462 m above the Albay Gulf, it is one of the most beautifully symmetrical volcano in the world. The current cone was formed through pyroclastic and lava flows from past eruptions. Mayon volcano is the most active volcano in the Philippines, having erupted over 49 times in the past 400 years.

The Mayon volcano historical eruptions date back to 1616, ranging from strombolian to basaltic plinian, with cyclical activity beginning with basaltic eruptions, followed by longer term andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic flows and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often devastated populated lowland areas.

Nevado del Ruiz

Nevado del Ruiz is an active stratovolcano which is found the Colombian Andean Volcanic Belt. It lies about 129 kilometers west of Bogota in the Tolima Department of Colombia. It is made up of many layers of lava alternating hardened volcanic ash and other pyroclastic rocks. Nevado del Ruiz has been active for about two million years, since the early Pleistocene or late Pliocene epoch, with three major eruptive periods. The current volcanic cone formed during the "present" eruptive period, which began 150 thousand years ago.

Nevado del Ruiz generates Plinian eruptions, which produce swift-moving currents of hot gas and rock called pyroclastic flows. These eruptions often cause massive lahars (mud and debris flows), which pose a threat to human life and the environment. In 1595, the Nevado del Ruiz erupted and a lahar swept down the valleys of the River Guali and the River Lagunillas, killing 636 people. In 1845, an immense lahar flooded the upper valley of the River Lagunillas, killing over 1000 people. It continued for 70 kilometers downstream before spreading across a plain in the lower valley floor. The young village of Armero was built directly on top of the 1845 mudflow deposit. Over the ensuing years, Armero grew into a vibrant town with over 27,000 residents.

On November 13, 1985, a small eruption produced an enormous lahar that buried and desolated the town of Armero in Tolima Department, causing an estimated 23,000 deaths. This event later became known as the Armero tragedy—the deadliest lahar in recorded history. Similar but less deadly incidents occurred in 1595 and 1845, consisting of a small explosive eruption followed by a large lahar.

The Nevado del Ruiz is part of the Ruiz–Tolima volcanic massif (or Cordillera Central), a group of five ice-capped volcanoes which includes the Tolima, Santa Isabel, Quindio and Machin. The volcano is part of Los Nevados National Park, which also contains several other volcanoes. The summit of Nevado del Ruiz is covered by large glaciers, although these have retreated significantly since 1985 because of atmospheric warming. The volcano continues to pose a threat to the nearby towns and villages, and it is estimated that up to 500,000 people could be at risk from lahars.

Satellite photograph of the Nevado del Ruiz

1985 Eruption of Nevado del Ruiz (Video)

Mount Pelee

Mount Pelee is an active stratovolcano located on the northern tip of the French island of Martinique in the Lesser Antilles island arc of the Caribbean. It is among the deadliest stratovolcanoes on Earth. Its volcanic cone is composed of layers of volcanic ash and hardened lava.

Mount Pelee is the result of a subduction zone which formed the Lesser Antilles island arc, a curved chain of volcanoes approximately 850 kilometers in length, between Puerto Rico and Venezuela, where the Caribbean Plate meets Atlantic Oceanic crust belonging to the South American Plate. Other volcanoes in the island arc are also active, including Saint Vincent's La Soufrière, Guadeloupe's Soufriere volcano, Montserrat's Soufrière Hills, and the submarine volcano Kick-'em-Jenny.

Geological History of Mount Pelee

In the geological evolution of Mount Pelee volcano, three different phases have been identified: initial, intermediate, and modern. In an initial phase, called the Paleo-Pelee stage, Mount Pelee was a common stratovolcano. The cone of Paleo-Pelee was composed of many layers of lava flows and fragmented volcanic debris. Remains of the Paleo-Pelee cone are still visible at the northern view at the volcano today.

A second stage, now called the intermediate phase, started around 100,000 years ago, after a long period of quiescence. This stage is grouped by the formation of the Morne Macouba lava dome, then later on, the Morne Macouba caldera. During the intermediate phase, there were several eruptions which produced pyroclastic flows like those that destroyed Saint-Pierre in the 1902 eruption. Around 25,000 years ago, a large Southwest sector collapse occurred, forming a landslide. This event is similar to the eruption of Mount Saint Helens in 1980.

The modern stage of the evolution of Mount Pelée has created most of the current cone, with deposits of pumice and the results of past pyroclastic flows. More than 30 eruptions have been identified during the last 5,000 years of the volcano's activity.

Eruption of Mount Pelee

The Mount Pelee is famous for its eruption in 1902 and the destruction that resulted, now dubbed the worst volcanic disaster of the 20th century. The eruption killed about 30,121 people. Most deaths came from the city of Saint-Pierre, at that time the largest city in Martinique, due to its pyroclastic flows. Before the tragic 1902 eruption, as early as the summer of 1900, signs of increased fumarole activity had been present in the Étang Sec crater near the summit.

Relatively minor phreatic eruptions which had occurred in 1792 and 1851 were evidence that the volcano was active and potentially dangerous. Local natives, the Carib people, knew it as "fire mountain" from previous eruptions in ancient times.

Mount Pelee began its eruptions on April 23, 1902. In early April, excursionists noted the appearance of sulfurous vapors emitting from fumaroles near the mountaintop. This was not regarded as important, as fumaroles had been appearing and disappearing in the past. Nevertheless, on April 23, the mountain caused a light rain of cinders on its southern and western side, together with sharp underground shocks. At 11:30 p.m. on May 2, the mountain produced loud explosions, earthquakes, and a massive pillar of dense black smoke. On Saturday, May 3, the wind blew the ash cloud northwards, alleviating the situation in Saint-Pierre.

On Monday, May 5, the mountain apparently calmed down somewhat; however, at about 1 PM, the sea suddenly receded about 100 metres (330 ft) and then rushed back, flooding parts of the city, and a large cloud of smoke appeared westwards of the mountain. One wall of the Étang Sec crater collapsed and propelled a mass of boiling water and mud, or lahar, into Blanche River, flooding the Guérin sugar works and burying about 150 victims under 60 metres (200 ft) to 90 metres (300 ft) of mud.

On Wednesday, May 7, at around 4 AM, the mountain stepped up its activity; the clouds of ash caused numerous bolts of volcanic lightning around the mountaintop, and both the craters glowed reddish orange into the night. Through the day, people were leaving the city, but more people from the countryside were attempting to find refuge in the city, which increased its population by several thousand.

The main volcanic eruption of Mount Pelee occurred on May 8, 1902, Ascension Day. The horizontal pyroclastic cloud hugged the ground and rushed down towards the city of Saint-Pierre, appearing black and heavy, glowing hot from the inside. It consisted of superheated steam and volcanic gases and dust, with temperatures exceeding 1,075 °C (1,967 °F). In under a minute it reached and covered the entire city, instantly igniting everything flammable with which it came in contact.

Pyroclastic flows completely destroyed St. Pierre, Martinique, a town of 30,000 people, following the eruption of Mont Pelee in 1902. The eruption left only two survivors in the direct path of the volcano: Louis-Auguste Cyparis survived because he was in a poorly ventilated, dungeon-like jail cell; Léon Compère-Léandre, living on the edge of the city, escaped with severe burns. The event marked the only major volcanic disaster in the history of France and its overseas territories.

Jorullo

El Jorullo is a cinder cone volcano in Michoacán, central Mexico, on the southwest slope of the central plateau, 33 miles (53 kilometers) southeast of Uruapan. Jorullo has four smaller cinder cones which have grown from it. The vents of Jorullo are aligned in a northeast to southwest direction. Lava from these vents cover nine square km around the volcano.

El Jorullo is one of two known volcanoes to have developed in Mexico in recent history. The second, born about 183 years later, was named Parícutin after a nearby village that it eventually destroyed. Parícutin is about 50 miles (80 km) northwest of this volcano. El Jorullo originated on September 29, 1759. Earthquakes occurred prior to this first day of eruption. Once the volcano started erupting, it continued for 15 years until in 1774. El Jorullo didn’t develop on a corn field like Parícutin did, but it did destroy what had been a rich agricultural area. It grew approximately 820 feet from the ground in the first six weeks.

The eruptions of El Jorullo were phreatic and phreatomagmatic. They covered the area with sticky mud flows, water flows and ash falls. All but the youngest lava flows were covered by this ash fall. Later eruptions were magmatic with neither mud nor water flows. This 15 year eruption was the only one Jorullo ever had, and was the longest cinder cone eruption known.

Lakagigar Fissure Vent

Lakagígar, or Laki, is a volcanic fissure vent located in the south of Iceland, not far from the canyon of Eldgjá and the small town Kirkjubæjarklaustur, in Skaftafell National Park. Lakagigar is part of a volcanic system, centering on the Grímsvötn volcano and including the Thórdarhyrna volcano. It lies between the glaciers of Mýrdalsjökull and Vatnajökull, in an area of fissures which run in a south-west to north-east direction. The 25 km long crater row called Lakagigar was created during a relatively short, intensive, and catastrophic eruption between the 8th of June 1783 and February 1874. It was among the biggest and poisonous lava eruptions of the earth during historical times.

The Lakagigar system erupted over an 8 month period during 1783-1784 from the Laki fissure and the adjoining Grímsvötn volcano, pouring out an estimated 14 km3 (3.4 cu mi) of basalt lava and clouds of poisonous hydrofluoric acid/sulfur-dioxide compounds that killed over 50% of Iceland's livestock population, leading to famine which killed approximately 25% of the population. Laki created two vast lava fields with a total area of 565 km², and the total volume of tephra emitted was estimated to have been 12,3 km³. The consequences were enormous.

The Lakagigar eruption has been estimated to have killed over two million people globally, making it the deadliest volcanic eruption in the history of mankind. The drop in temperatures, due to the sulfuric dioxide gases spewed into the northern hemisphere, caused crop failures in Europe, droughts in India, and Japan experienced its worst famine.

On 8 June 1783, a fissure with 130 craters opened with phreatomagmatic explosions because of the underground water interacting with the rising basalt magma. Over a few days the eruptions became less explosive, Strombolian, and later Hawaiian in character, with high rates of lava effusion. This event is rated as VEI 6 on the Volcanic Explosivity Index, but the eight month emission of sulfuric aerosols resulted in one of the most important climatic and socially repercussive events of the last millennium. The eruption continued until 7 February 1784, but most of the lava was erupted in the first five months. Grímsvötn volcano, from which the Laki fissure extends, was also erupting at the time from 1783 until 1785.

Artistic portrayal of Lakagigar eruption of 1783

Lakagigar today

Redoubt Volcano

Redoubt Volcano is an active stratovolcano which is situated in the largely volcanic Aleutian Range of the State of Alaska, in Lake Clark National Park. It is just west of Cook Inlet, in the Kenai Peninsula Borough about 110 mi southwest of Anchorage. Redoubt Volcano, or Mount Redoubt, is 9,000 feet high, towering above the surrounding valleys to the north, south, and southeast. It is also the third highest within the range, with nearby Mount Torbert, at 11,413 feet, being the highest and Mount Spurr at 11,070 feet being the second highest.

Redoubt has been the most active Holocene volcano in the upper Cook Inlet. The volcano began to build up about 890,000 years ago over Mesozoic granitic rocks of the Alaska-Aleutian Range batholith. Collapse of the summit of Redoubt 10,500-13,000 years ago produced a major debris avalanche that reached Cook Inlet. Holocene activity has included the emplacement of a large debris avalanche and clay-rich lahars that dammed Lake Crescent on the south side and reached Cook Inlet about 3500 years ago. Eruptions during the past few centuries have affected only the Drift River drainage on the north. Historical eruptions have originated from a vent at the north end of the 1.8-km-wide breached summit crater. The 1989-90 eruption of Redoubt had severe economic impact on the Cook Inlet region and affected air traffic far beyond the volcano.

Phreatomagmatic Eruption

Phreatomagmatic eruptions are volcanic eruptions in which large amounts of steam and magmatic gases are emitted. They are juvenile forming eruptions as a result of interaction between water and magma. They are different from magmatic and phreatic eruptions. Unlike magmatic eruptions, the products of phreatomagmatic eruptions contain juvenile clasts and are the result of interaction between magma and water. It is very common for a large explosive eruption to have magmatic and phreatomagmatic components.

A phreatomagmatic eruption ends when the water supply is exhausted, and not, as in most other eruptions when the magma stops rising. The development of an explosive surge is dependent on the magma coming into contact with water and forming a phreatomagmatic eruption. Water for a phreatomagmatic eruption could come from the harbors, rivers, streams, subsurface aquifers or from sediments at the surface. Several theories have been written as to the exact mechanism of its formation. The most common is the theory of explosive thermal contraction of particles under rapid cooling from contact with water. In many cases the water is supplied by the sea, for example Surtsey. In other cases the water may be present in a lake or Caldera-lake, for example Santorini where the phreatomagmatic component of the Minoan eruption was a result of both a lake and later the sea.

There have also been examples of interaction between magma and water in an aquifer, many of the cinder cones on Tenerife are believed to be phreatomagmatic because of these circumstances. The other competing theory is based on fuel-coolant reactions, which have been modeled for the nuclear industry. Under this theory the magma (in this case the fuel) fragments upon contact with a coolant (the sea, a lake or aquifer), the propagating stress waves and thermal contraction widening cracks and increasing the interaction surface area leading to explosively rapid cooling rates. The two mechanisms proposed are very similar and the reality is most likely a combination of both.

Sarychev Peak

Sarychev Peak is a young stratovolcano which covers almost the entirety of Matua Island in the Kuril Islands, Russia. A highly symmetrical stratovolcanic cone, Sarychev Peak is one of the most active volcanoes of the Kuril Islands. It lies in the NW corner of Matua Island in the central Kuriles. The andesitic central cone was constructed within a 3-3.5 km wide caldera, whose rim is exposed only on the SW side. A dramatic 250-m-wide, very steep-walled crater with a jagged rim caps the volcano.

The substantially higher SE rim of the Sarychev forms the 1496 m high point of the island. Fresh-looking lava flows descend all sides of Sarychev Peak and often form capes along the coast. Much of the lower-angle outer flanks of the volcano are overlain by pyroclastic-flow deposits. Sarychev Peak eruptions have been recorded since the 1760's and include both quiet lava effusion and violent explosions. One of the largest historical eruptions of Sarychev Peak in 1946 produced pyroclastic flows that reached the sea.

On 12 June 2009, the Sarychev volcano erupted again, spewing out ash plumes high into the sky. As the volcano is near some of the main air routes between East Asia and North America, there was some disruption to air traffic. During the eruption, the International Space Station passed overhead and astronauts were able to photograph the event. A hole in the overhead clouds, possibly caused by the shock wave from the explosion, allowed a clear view of the plume and pyroclastic flow down the sides of the mountain. A cap-like pileus cloud is visible atop the rising column. Sarychev Peak had previously erupted in 1760, 1805, 1879, 1923, 1927, 1928, 1930, 1932, 1946, 1954, 1960, 1965, 1976, 1986 and 1989.

Sarychev Peak

Sarychev Volcano Eruption Seen From The International Space Station

Fissure Vent

A fissure vent, or volcanic fissure, is a linear volcanic vent through which lava gushes out onto the earth surface. A fissure vent eruption usually occurs without any explosive activity. It is a few meters wide and may be many kilometers long, causing large flood basalts and lava channels. This type of volcano is usually hard to recognize from the ground and from outer space because it has no central caldera and the surface is mostly flat. A fissure vent can usually be seen as a crack in the ground or on the ocean floor. Narrow fissures can be filled in with lava that hardens. As erosion removes its surroundings, the lava mass could stand above the surface as a dyke. The dykes that feed fissures reach the surface from depths of a few kilometers. Fissures are usually found in or along rifts and rift zones, such as Iceland and the Great Rift Valley in Africa. Fissure vents are often found in shield volcanoes.

Fissure vents in Iceland are often long fissures parallel to the rift zone where lithospheric plates are diverging. Renewed eruptions generally occur from new parallel fractures offset by a few hundred to thousands of metres from the earlier fissures. This distribution of vents and voluminous eruptions of fluid basaltic lava usually build up a thick lava plateau rather than a single volcanic edifice. The Laki fissure system produced the biggest eruption on earth in historical times, in the form of a flood basalt, during the Eldgjá eruption A.D. 934, which released 19.6 km³ (4.7 mi³) of lava.

The radial fissure vents of Hawaiian volcanoes produce “curtains of fire” as lava fountains erupt along a portion of a fissure. These vents produce low ramparts of basaltic spatter on both sides of the fissure. More isolated lava fountains along the fissure produce crater rows of small spatter and cinder cones. The fragments that form a spatter cone are hot and plastic enough to weld together, while the fragments that form a cinder cone remain separate because of their lower temperature.

Plinian Eruption

Plinian eruption is a volcanic eruption which is marked by columns of gas and volcanic ash extending high into the stratosphere, a high layer of the atmosphere. The key characteristics are ejection of large amount of pumice and very powerful continuous gas blast eruptions. Plinian eruptions include the most violent forms of all volcanic activity. These outbursts expel fine ash and pumice, composed of rhyolite, dacite, phonolite, trachyte or andesite often at supersonic speeds.

Short Plinian eruptions can end in less than a day, but longer events can take several days to months. The longer eruptions begin with production of clouds of volcanic ash, sometimes with pyroclastic flows. The amount of magma erupted can be so large that the top of the volcano may collapse, resulting in a caldera. Fine ash can deposit over large areas. Plinian eruptions are often accompanied by loud noises, such as those generated by Krakatoa.

Plinian eruptions, also known as 'Vesuvian eruptions', are volcanic eruptions marked by their similarity to the eruption of Mount Vesuvius in AD 79 and is named after Pliny the Elder who who got killed in that cataclysmic event.

Plinian eruption of Mt Redoubt

Pyroclastic Rocks

Pyroclastic rocks are clastic rocks, which are fragments of pre-existing rock, composed primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic. Commonly associated with explosive volcanic activity–such as Plinian or krakatoan eruption styles, or phreatomagmatic eruptions–pyroclastic deposits are commonly formed from airborne ash, lapilli and bombs or blocks ejected from the volcano itself, mixed in with shattered country rock.

Pyroclastic rocks are rocks of extrusive (volcanic) origin, composed of rock fragments produced directly by explosive eruptions. Pyroclastic fragments may represent shattered and comminuted older rocks (volcanic, plutonic, sedimentary, or metamorphic) or solidified lava droplets formed by violent explosion. A pyroclastic rock is produced from the consolidation of pyroclastic accumulations into a coherent rock type.

Pyroclastic rocks may be composed of a large range of clast sizes; from the largest agglomerates, to very fine ashes and tuffs. Pyroclasts of different sizes are classified as volcanic bombs, lapilli and volcanic ash. Ash is considered to be pyroclastic because it is a fine dust made up of volcanic rock. One of the most spectacular forms of pyroclastic deposit are the ignimbrites, deposits formed by the high-temperature gas and ash mix of a pyroclastic flow event.

There are three modes through which pyroclastic rocks can be transported: pyroclastic flow, pyroclastic surge, and pyroclastic fall. During Plinian eruptions, pumice and ash are formed when silicic magma is fragmented in the volcanic conduit, because of decompression and the growth of bubbles. Pyroclasts are then entrained in a buoyant eruption plume which can rise several kilometers into the air and cause aviation hazards. Particles falling from the eruption clouds form layers on the ground (this is pyroclastic fall or tephra). Pyroclastic density currents, which are referred to as 'flows' or 'surges' depending on particle concentration and the level turbulence, are sometimes called glowing avalanches. The deposits of pumice-rich pyroclastic flows can be called ignimbrites.

Pyroclastic flow (video)

Chaiten Volcano

Chaiten volcano is a volcanic caldera 2 miles in diameter, located 11 miles west of the elongated, ice-capped Michinmahuida volcano and 6 miles northeast of the town of Chaiten, near the Gulf of Corcovado in southern Chile. The most recent eruptive phase of the volcano began on May 2, 2008. According to the Global Volcanism Program, radiocarbon dating of older tephra from the volcano suggests that its last previous eruption had been in 7420 BC ± 75 years. The caldera rim reaches 3,681 ft above sea level. Prior to the current eruption, it was mostly filled by a rhyolite obsidian lava dome that reached a height of 3,156 ft, partly devoid of vegetation. Two small lakes occupied the caldera floor on the west and north sides of the lava dome.

Chaiten Volcano Eruption (Video)

Volcanic Caldera

A volcanic caldera is a large bowl-shaped volcanic depression, which is formed as a result of subsidence. As the magma founders, so the center of the volcano collapses. The Plateau of Giant Craters in Tanzania contains many impressive calderas, including Ngorongoro, which attains a diameter of 22 km. A volcanic caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption such as the ones at Yellowstone National Park in the US and Glen Coe in Scotland. They are sometimes confused with volcanic craters. The word comes from Spanish caldera, and this from Latin CALDARIA, meaning "cooking pot". In some texts the English term cauldron is also used.

Calderas occur primarily in three different volcanic settings, each of which affects their shape and evolution: basaltic shield cones, stratovolcanoes, and volcanic centers consisting of preexisting clusters of volcanoes. These last calderas, associated with broad, large-volume andesitic to rhyolitic ignimbrite sheets, are generally the largest and most impressive, and are those generally denoted by the term.

A collapse is triggered by withdrawal of magma from an underlying chamber some 2.4 – 3.6 miles beneath the surface, resulting in foundering of the roof into the chamber. 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. These ring fractures serve as feeders for fault intrusions which are also known as ring dykes. 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 or thousands of square kilometers.

In 1815, the German geologist Leopold von Buch visited the Las Cañadas Caldera Teide, Tenerife, and the Caldera de Taburiente, La Palma, both in the Canary Islands. When he published his memoirs he introduced the term "caldera" into the geological vocabulary.

Formation of a volcanic caldera

Volcanic Caldera on Fernandina Island in the Galapagos

Cerro Negro (Volcano)

Cerro Negro is a cinder cone volcano which is situated in the Maribios mountain range in Nicaragua, about 10 km from the village of Malpaisillo. It is 675-meter high and is the youngest volcano in Central America, having first appeared in April 1850. Cerro Negro is composed of a gravelly basaltic cinder cone, which contrasts greatly with the surrounding verdant hillsides, and gives rise to its name, which means Black Hill. Cerro Negro has erupted frequently since its first eruption. One unusual aspect of several eruptions has been the emission of ash from the top of the cone, while lava erupts from fractures at the base.

Cerro Negro is a polygenetic cinder cone that is part of the Central American Volcanic Arc, which formed as a result of the Cocos plate subducting under the Caribbean plate, at a rate of 9 cm/year. It has erupted more than 20 times since its birth in 1850. Explosive eruptions from the central crater are often accompanied by lava flows from the base of the cone. It is the youngest of four cinder cones scattered along a 20 km line east-southeast of Telica. It is the largest and southernmost of four cinder cones that have formed along a NW-SE trend line in the Cordillera de los Maribios mountain range. Despite its youth, Cerro Negro has been one of the most active volcanoes in Nicaragua, with its latest eruption occurring in 1999. Since its birth in 1850, it has erupted approximately 23 times, making Cerro Negro the most active cinder cone volcano in the history of our planet.

Paricutin Volcano

Paricutin is a cinder cone volcano in the Mexican state of Michoacan. It lies close to a lava-covered village of the same name. Paricutin is part of the Michoacan-Guanajuato Volcanic Field, which covers much of west central Mexico. The volcano began as a fissure in a cornfield owned by a P'urhépecha farmer, Dionisio Pulido on February 20, 1943. Pulido, his wife, and their son all witnessed the initial eruption of ash and stones first-hand as they plowed the field.

The Paricutin cinder cone volcano grew quickly, reaching five stories tall in just a week, and it could be seen from afar in a month. Much of the volcano's growth occurred during its first year, while it was still in the explosive pyroclastic phase. Nearby villages Paricutín (after which the volcano was named) and San Juan Parangaricutiro were both buried in lava and ash; the residents relocated to vacant land nearby. At the end of this phase, after roughly one year, the volcano had grown 336 meters (1,102.36 ft) tall. For the next eight years the volcano would continue to erupt, although this was dominated by relatively quiet eruptions of lava that would scorch the surrounding 25 km² (9.65 mi²) of land.

Paricutin's activity would slowly decline during this period until the last six months of the eruption, during which violent and explosive activity was frequent. In 1952 the eruption ended and Parícutin went quiet, attaining a final height of 424 meters (1,391.08 ft) above the cornfield from which it was born. The volcano has been quiet since. Like most cinder cones, Parícutin is a monogenetic volcano, which means that it will never erupt again. Any new eruptions in a monogenetic volcanic field erupt in a new random location.

Mount Pinatubo

Mount Pinatubo is an active stratovolcano which is situated on the island of Luzon, in the Philippines. It is located in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is 42 km (26 mi) west of the dormant and more prominent Mount Arayat, occasionally mistaken for Pinatubo. Before 1991, the Mount Pinatubo was inconspicuous and heavily eroded and was covered in dense forest which supported a population of several thousand indigenous people, the Aeta.

The Pinatubo eruption in June 1991 produced the second largest terrestrial eruption of the 20th century (after the 1912 eruption of Novarupta) and the largest eruption in living memory. The colossal 1991 eruption had a Volcanic Explosivity Index (VEI) of 6, and came some 450–500 years after the volcano's last known eruptive activity (estimated as VEI 5, the level of the 1980 eruption of Mount St. Helens), and some 1000 years after previous VEI 6 eruptive activity. Successful predictions of the onset of the climactic eruption led to the evacuation of tens of thousands of people from the surrounding areas, saving many lives, but surrounding areas were severely damaged by pyroclastic flows, ash deposits, and later by lahars caused by rainwater remobilizing earlier volcanic deposits: thousands of houses and other buildings were destroyed.

The effects of Mt. Pinatubo eruption were felt worldwide. It ejected roughly 10 billion metric tonnes of magma, and 20 million tons of SO2, bringing vast quantities of minerals and metals to the surface environment. It injected large amounts of aerosols into the stratosphere—more than any eruption since that of Krakatoa in 1883. Over the following months, the aerosols formed a global layer of sulfuric acid haze. Global temperatures dropped by about 0.5 °C (0.9 °F), and ozone depletion temporarily increased substantially.

Eruption of Mount Pinatubo June 1991 (video)

Shield Volcano

A shield volcano is a tall, broad volcano with gently sloping sides. Shield volcanoes are usually built up over time by flow after flow of fluid basaltic lava that comes out from vents or fissures on the surface of the volcano, resulting in their relatively flat, broad profile. In contrast, steeply sloped stratovolcanoes better match the popular stereotype of a volcano. Some of the largest volcanoes on Earth are shield volcanoes. The name derives from a translation of "Skjaldbreiður", an Icelandic shield volcano whose name means "broad shield", from its resemblance to a warrior's shield. Shield volcanoes are characteristic of the Hawaiian chain; Manua Kea is an example, with a basal diameter of around 200?km, 4000?m beneath the sea.

Shield volcanoes can be so large that they are sometimes considered to be a mountain range, such as the Ilgachuz Range and the Rainbow Range, both of which are located in Canada. These shield volcanoes formed when the North American Plate moved over a hotspot similar to the one feeding the Hawaiian Islands, called the Anahim hotspot. Shield volcanoes can be found in many places around the world, including Australia, Ethiopia, and the Galapagos Islands. The Piton de la Fournaise, on Reunion Island, is one of the most active shield volcanoes on earth, with one eruption per year on average.

Mount Okmok, Aleutian Islands (A Shield Volcano)

A Shield Volcano Kilauea, Hawaii (Video)