Abu is the name of a group of shield volcanoes located on the coast of Japan on the southwest end of the island of Honshū. It is primarily based in the city of Hagi, Yamaguchi Prefecture. The group dates from 800.000 years ago and was active into the Holocene era. The last eruption occurred around 9000 years ago. The group of volcanoes consists of basalt and dacitic lava flows, small shield volcanoes, cinder cones, and lava domes. Altogether, there are about 40 volcanoes, with the highest peak being Irao-yama. Volcanic activity in the region i related to subduction of the Philippine Sea Pl ...
Mount Awu is the largest volcano in the Sangihe chain, located in North Sulawesi, Indonesia. Powerful eruptions occurred in 1711, 1812, 1856, 1892 and 1966 with devastating pyroclastic flows and lahars that have resulted in more than 8.000 fatalities altogether. A 4.5 km wide crater is found at the summit and a deep valley forms a passageways for lahars, splitting the flanks from the crater. This is a volcano in the Ring of Fire.
Aguilera is a stratovolcano in southern Chile, which rises above the edge of the Southern Patagonian Ice Field. It is a remote volcano that was identified as such in 1985, but the first ascent only occurred in 2014, making it the last unclimbed major Andean volcano. It is located west of Lake Argentino and northeast of Peel Fjord in the southern Andes and erupted mainly dacites and pyroclastic tephra. It has erupted several times in the Holocene, with a major eruption taking place 3.000 ± 1.000 years before present. Its eruptions have spread ashfalls over Patagonia.
Volcan de Agua is a stratovolcano located in the departments of Sacatepequez and Escuintla in Guatemala. At 3.760 m, Agua Volcano towers more than 3.500 m above the Pacific coastal plain to the south and 2.000 m above the Guatemalan Highlands to the north. It dominates the local landscape except when hidden by cloud cover. The volcano is within 5 to 10 km of the city of Antigua Guatemala and several other large towns situated on its northern apron. These towns have a combined population of nearly 100.000. It is within about 20 km of Escuintla to the south. Coffee is grown on the volcanos l ...
Agua de Pau Massif is a stratovolcanic complex, located in central part of the island of São Miguel, in the Portuguese archipelago of the Azores. More recognizable for the Lagoa do Fogo at its centre, the volcanic complex includes centuries of geomorphological structures that include lava domes, cones and encrusted lava flows that have marked its history from, the last, 45.000 years BC.
Mount Adams, known by some Native American tribes as Pahto or Klickitat, is a potentially active stratovolcano in the Cascade Range. Although Adams has not erupted in more than 1.000 years, it is not considered extinct. It is the second-highest mountain in the U.S. state of Washington, after Mount Rainier. Adams, named for President John Adams, is a member of the Cascade Volcanic Arc, and is one of the arcs largest volcanoes, located in a remote wilderness approximately 34 miles 55 km east of Mount St. Helens. The Mount Adams Wilderness consists of the upper and western part of the volcano ...
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
Earths volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in its mantle. Therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging, and most are found underwater. For example, a mid-oceanic ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by divergent tectonic plates whereas the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates. Volcanoes can also form where there is stretching and thinning of the crusts plates, e.g., in the East African Rift and the Wells Gray-Clearwater volcanic field and Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "plate hypothesis" volcanism. Volcanism away from plate boundaries has also been explained as mantle plumes. These so-called "hotspots", for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core–mantle boundary, 3.000 km deep in the Earth. Volcanoes are usually not created where two tectonic plates slide past one another.
Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. One such hazard is that volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature; the melted particles then adhere to the turbine blades and alter their shape, disrupting the operation of the turbine. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earths lower atmosphere or troposphere; however, they also absorb heat radiated from the Earth, thereby warming the upper atmosphere or stratosphere. Historically, volcanic winters have caused catastrophic famines.
The word volcano is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn comes from Vulcan, the god of fire in Roman mythology. The study of volcanoes is called volcanology, sometimes spelled vulcanology.
2.1. Plate tectonics Divergent plate boundaries
At the mid-oceanic ridges, two tectonic plates diverge from one another as new oceanic crust is formed by the cooling and solidifying of hot molten rock. Because the crust is very thin at these ridges due to the pull of the tectonic plates, the release of pressure leads to adiabatic expansion without transfer of heat or matter and the partial melting of the mantle, causing volcanism and creating new oceanic crust. Most divergent plate boundaries are at the bottom of the oceans; therefore, most volcanic activity on the Earth is submarine, forming new seafloor. Black smokers also known as deep sea vents are evidence of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed; for example, Iceland.
2.2. Plate tectonics Convergent plate boundaries
Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges, under the continental plate, forming a deep ocean trench just offshore. In a process called flux melting, water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, thus creating magma. This magma tends to be extremely viscous because of its high silica content, so it often does not attain the surface but cools and solidifies at depth. When it does reach the surface, however, a volcano is formed. Typical examples are Mount Etna and the volcanoes in the Pacific Ring of Fire.
2.3. Plate tectonics Hotspots
Hotspots are volcanic areas believed to be formed by mantle plumes, which are hypothesized to be columns of hot material rising from the core-mantle boundary in a fixed space that causes large-volume melting. Because tectonic plates move across them, each volcano becomes dormant and is eventually re-formed as the plate advances over the postulated plume. The Hawaiian Islands are said to have been formed in such a manner; so has the Snake River Plain, with the Yellowstone Caldera being the part of the North American plate above the hot spot. This theory, however, has been doubted.
3. Volcanic features
The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit; however, this describes just one of the many types of volcano. The features of volcanoes are much more complicated and their structure and behavior depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have landscape features such as massive plateaus. Vents that issue volcanic material including lava and ash and gases mainly steam and magmatic gases can develop anywhere on the landform and may give rise to smaller cones such as Puu Ōō on a flank of Hawaiis Kīlauea. Other types of volcano include cryovolcanoes or ice volcanoes, particularly on some moons of Jupiter, Saturn, and Neptune; and mud volcanoes, which are formations often not associated with known magmatic activity. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes except when the mud volcano is actually a vent of an igneous volcano.
3.1. Volcanic features Fissure vents
Volcanic fissure vents are flat, linear fractures through which lava emerges.
3.2. Volcanic features Shield volcanoes
Shield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent. They generally do not explode catastrophically. Since low-viscosity magma is typically low in silica, shield volcanoes are more common in oceanic than continental settings. The Hawaiian volcanic chain is a series of shield cones, and they are common in Iceland, as well.
3.3. Volcanic features Cryptodomes
Cryptodomes are formed when viscous lava is forced upward causing the surface to bulge. The 1980 eruption of Mount St. Helens was an example; lava beneath the surface of the mountain created an upward bulge which slid down the north side of the mountain.
3.4. Volcanic features Volcanic cones cinder cones
Volcanic cones or cinder cones result from eruptions of mostly small pieces of scoria and pyroclastics both resemble cinders, hence the name of this volcano type that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 400 meters high. Most cinder cones erupt only once. Cinder cones may form as flank vents on larger volcanoes, or occur on their own. Paricutin in Mexico and Sunset Crater in Arizona are examples of cinder cones. In New Mexico, Caja del Rio is a volcanic field of over 60 cinder cones.
Based on satellite images it was suggested that cinder cones might occur on other terrestrial bodies in the Solar system too; on the surface of Mars and the Moon.
3.5. Volcanic features Stratovolcanoes composite volcanoes
Stratovolcanoes or composite volcanoes are tall conical mountains composed of lava flows and other ejecta in alternate layers, the strata that gives rise to the name. Stratovolcanoes are also known as composite volcanoes because they are created from multiple structures during different kinds of eruptions. Strato/composite volcanoes are made of cinders, ash, and lava. Cinders and ash pile on top of each other, lava flows on top of the ash, where it cools and hardens, and then the process repeats. Classic examples include Mount Fuji in Japan, Mayon Volcano in the Philippines, and Mount Vesuvius and Stromboli in Italy.
Throughout recorded history, ash produced by the explosive eruption of stratovolcanoes has posed the greatest volcanic hazard to civilizations. Not only do stratovolcanoes have greater pressure buildup from the underlying lava flow than shield volcanoes, but their fissure vents and monogenetic volcanic fields volcanic cones also have more powerful eruptions because they are often under extension. They are also steeper than shield volcanoes, with slopes of 30–35° compared to slopes of generally 5–10°, and their loose tephra are material for dangerous lahars. Large pieces of tephra are called volcanic bombs. Big bombs can measure more than 4 feet1.2 meters across and weigh several tons.
3.6. Volcanic features Supervolcanoes
A supervolcano usually has a large caldera and can produce devastation on an enormous, sometimes continental, scale. Such volcanoes are able to severely cool global temperatures for many years after the eruption due to the huge volumes of sulfur and ash released into the atmosphere. They are the most dangerous type of volcano. Examples include Yellowstone Caldera in Yellowstone National Park and Valles Caldera in New Mexico both western United States; Lake Taupo in New Zealand; Lake Toba in Sumatra, Indonesia; and Ngorongoro Crater in Tanzania. Because of the enormous area they may cover, supervolcanoes are hard to identify centuries after an eruption. Similarly, large igneous provinces are also considered supervolcanoes because of the vast amount of basalt lava erupted even though the lava flow is non-explosive.
3.7. Volcanic features Underwater volcanoes
Submarine volcanoes are common features of the ocean floor. In shallow water, active volcanoes disclose their presence by blasting steam and rocky debris high above the oceans surface. In the oceans deep, the tremendous weight of the water above prevents the explosive release of steam and gases; however, they can be detected by hydrophones and discoloration of water because of volcanic gases. Pillow lava is a common eruptive product of submarine volcanoes and is characterized by thick sequences of discontinuous pillow-shaped masses which form under water. Even large submarine eruptions may not disturb the ocean surface due to the rapid cooling effect and increased buoyancy of water as compared to air which often causes volcanic vents to form steep pillars on the ocean floor. Hydrothermal vents are common near these volcanoes, and some support peculiar ecosystems based on dissolved minerals. Over time, the formations created by submarine volcanoes may become so large that they break the ocean surface as new islands or floating pumice rafts.
In 2018, a multitude of seismic signals were detected by earthquake monitoring agencies all over the world in May and June. They created a weird humming sound and some of the signals detected in November of that year had a duration of up to 20 minutes. An oceanographic campaign in May 2019 showed that the previously mysterious humming noises were caused by the formation of an underwater volcano off the coast of Mayotte.
3.8. Volcanic features Subglacial volcanoes
Subglacial volcanoes develop underneath icecaps. They are made up of flat lava which flows at the top of extensive pillow lavas and palagonite. When the icecap melts, the lava on top collapses, leaving a flat-topped mountain. These volcanoes are also called table mountains, tuyas, or uncommonly mobergs. Very good examples of this type of volcano can be seen in Iceland, however, there are also tuyas in British Columbia. The origin of the term comes from Tuya Butte, which is one of the several tuyas in the area of the Tuya River and Tuya Range in northern British Columbia. Tuya Butte was the first such landform analyzed and so its name has entered the geological literature for this kind of volcanic formation. The Tuya Mountains Provincial Park was recently established to protect this unusual landscape, which lies north of Tuya Lake and south of the Jennings River near the boundary with the Yukon Territory.
3.9. Volcanic features Mud volcanoes
Mud volcanoes or mud domes are formations created by geo-excreted liquids and gases, although there are several processes which may cause such activity. The largest structures are 10 kilometers in diameter and reach 700 meters high.
4.1. Erupted material Lava composition
Another way of classifying volcanoes is by the composition of material erupted lava, since this affects the shape of the volcano. Lava can be broadly classified into four different compositions:
- If the erupted magma contains a high percentage > 63% of silica, the lava is called felsic.
- Felsic lavas dacites or rhyolites tend to be highly viscous not very fluid and are erupted as domes or short, stubby flows. Viscous lavas tend to form stratovolcanoes or lava domes. Lassen Peak in California is an example of a volcano formed from felsic lava and is actually a large lava dome.
- Because siliceous magmas are so viscous, they tend to trap volatiles gases that are present, which cause the magma to erupt catastrophically, eventually forming stratovolcanoes. Pyroclastic flows ignimbrites are highly hazardous products of such volcanoes, since they are composed of molten volcanic ash too heavy to go up into the atmosphere, so they hug the volcanos slopes and travel far from their vents during large eruptions. Temperatures as high as 1.200 °C are known to occur in pyroclastic flows, which will incinerate everything flammable in their path and thick layers of hot pyroclastic flow deposits can be laid down, often up to many meters thick. Alaskas Valley of Ten Thousand Smokes, formed by the eruption of Novarupta near Katmai in 1912, is an example of a thick pyroclastic flow or ignimbrite deposit. Volcanic ash that is light enough to be erupted high into the Earths atmosphere may travel many kilometres before it falls back to ground as a tuff.
- If the erupted magma contains 52–63% silica, the lava is of intermediate composition.
- These "andesitic" volcanoes generally only occur above subduction zones e.g. Mount Merapi in Indonesia.
- Melting of subducted slab containing sediments
- Hydration melting of peridotite and fractional crystallization
- Magma mixing between felsic rhyolitic and mafic basaltic magmas in an intermediate reservoir prior to emplacement or lava flow.
- Andesitic lava is typically formed at convergent boundary margins of tectonic plates, by several processes
- At mid-ocean ridges, where two oceanic plates are pulling apart, basaltic lava erupts as pillows to fill the gap;
- If the erupted magma contains 45% silica, the lava is called mafic because it contains higher percentages of magnesium Mg and iron Fe) or basaltic. These lavas are usually much less viscous than rhyolitic lavas, depending on their eruption temperature; they also tend to be hotter than felsic lavas. Mafic lavas occur in a wide range of settings
- Shield volcanoes e.g. the Hawaiian Islands, including Mauna Loa and Kilauea, on both oceanic and continental crust;
- As continental flood basalts.
- Some erupted magmas contain