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INTRODUCTION TO VOLCANOES. What is a volcano?. A volcano is a place on the Earth's surface (or any other planet's or moon's surface) where molten rock, gases and pyroclastic debris erupt through the earth's crust.
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What is a volcano? • A volcano is a place on the Earth's surface (or any other planet's or moon's surface) where molten rock, gases and pyroclastic debris erupt through the earth's crust. • Volcanoes vary quite a bit in their structure - some are cracks in the earth's crust where lava erupts, and some are domes, shields, or mountain-like structures with a crater at the summit.
Terminology • A volcano constitutes a vent, a pipe, a crater, and a cone. • The vent is an opening at the Earth's surface. • The pipe/ conduit is a passageway in the volcano in which the magma rises through to the surface during an eruption. • The crater is a bowl-shaped depression at the top of the volcano where volcanic materials like, ash, lava, and other pyroclastic materials are released. • Solidified lava, ashes, and cinder form the cone. Layers of lava, alternate with layers of ash to build the steep sided cone higher and higher.
Magma is molten rock within the Earth's crust. An eruption begins when pressure on a magma chamber forces magma up through the conduit (The passage followed by magma in a volcano) and out the volcano's vents. When the magma chamber has been completely filled, the type of eruption partly depends on the amount of gases and silica in the magma. The amount of silica determines how sticky (level of viscosity) the magma is and water provides the explosive potential of steam. When magma erupts through the earth's surface it is called lava. Lava can be thick and slow-moving (andesitic) or thin and fast-moving (basaltic). How do volcanoes erupt?
Types of Volcanic Eruptions In ascending order of magnitude ( gentlest to most explosive ). • Icelandic • Hawaiian • Strombolian • Vulcanian • Vesuvian • Krakatoan • Pelean • Plinian
Distribution of volcanoes • The Pacific Ring of Fire is an area where large numbers of earthquakes and volcanic eruptions occur in the basin of the Pacific Ocean. • In a 40,000 km horseshoe shape, it is associated with a nearly continuous series of oceanic trenches, volcanic arcs, and volcanic belts and/or plate movements. • It is sometimes called the circum-Pacific belt or the circum-Pacific seismic belt
Types of Volcanoes • The shape of a volcano depends on the proportions of fluid lava and pyroclastic debris it contains. Volcanoes eject fragmented (pyroclastic) material of various sizes. These include : • Tephra (volcanic glass shards) • Ash • Lapilli (small stones) • Bombs (larger material) • Pumice (solidified foam /scum on the top of the lava flow • These may then form pyroclastic flows, which may take the form of either a fast moving cloud of very hot toxic gases known as nuee ardente (e.g. Mont Pelee Martinique , 8 May 1902).
Types of Volcanoes The most common types of volcanoes are: • Composite Volcanoes- also called strato volcanoes are formed by alternating layers of lava and rock fragments. Composite volcanoes usually erupt in an explosive way. This is usually formed by viscous magma. • Shield Volcanoes-Shield Volcanoes are huge in size. They may be produced by hotspots which lay far away from the edges of tectonic plates, shields also occur along the mid-oceanic ridge, where sea-floor spreading is in progress and along subduction related volcanic arcs. • Cinder Cones- A cinder cone is a conical hill formed above a vent. Cinder cones are among the most common volcanic landforms found in the world. They are not famous as their eruptions don’t usually don’t cause any loss of life. Cinder cones are chiefly formed by Strombolian eruptions. • Complex Volcanoes: These are usually volcanoes that consists of two or more vents.
Types of Volcanoes COMPOSITE VOLCANOES • Composite volcanoes usually erupt in an explosive way. This is usually caused by viscous magma. • Composite volcanoes are composed of alternating layers of ash and lava usually resulting from alternating types of erupting ,where first ash (from initial violent eruption ) and then lava ( usually acidic) are ejected. • When very viscous magma rises to the surface, it usually clogs the craterpipe, and gas in the craterpipe gets locked up. • Therefore, the pressure will increase resulting in an explosive eruption.
Types of Volcanoes SHIELD VOLCANOES ( Central Vent) • Shield volcanoes are huge in size. They are built by many layers of runny lava flows. • Lava spills out of a central vent or group of vents. • A broad shaped, gently sloping cone is formed. • This is caused by the very fluid, basaltic lava which can't be piled up into steep mounds. Famous shield volcanoes can be found for example in Hawaii
Types of Volcanoes FISSURE ERUPTIONS • At constructive margins , lava may erupt through fissures, rather than through a central vent. Heimaey, Iceland (1973) , began an fissure eruption, 2km in length, but Laki, Iceland (1783)was 30 km in length. • As the lava is basic, it flows great distances and may form large plateaux , for example those can be seen as basaltic columns in Iceland, Greenland .
Types of Volcanoes LAVA CONES • The slope of the cone depends on whether the molten lava was fluid or viscous ( basic or acidic) and cones are built up from repeated lava flows. • Fluid basic lava give rise to more gently sloping cones (e.g. Mauna Loa , Hawaii, -400km in diameter at the sea floor and 112 km at the sea level). • Viscous acidic lavas give rise to more steeply sloping cones, which may be convex in appearance as the lava tends to solidify close to the equator. • In the case of Mont Pelee, Martinique, the lava was so viscous and solidified so quickly as it was coming up the vent that it was extruded as a spine or plug.
Types of Volcanoes ASH & CINDER CONES • A cinder cone is a steep conical hill formed above a vent. Cinder cones are among the most common volcanic landforms found in the world. • They aren't famous as their eruptions usually don't cause any loss of life. • The cones usually grow up in groups and they often occur on the flanks of shield volcanoes. A great example of a cinder cone is Paricutín in Mexico.
Types of volcanoes SPATTER CONES • When hot erupting lava contains just enough explosive gas to prevent the formation of a lava flow, but not enough to shatter it into small fragments the lava is torn by expanding gases into fluid hot clots, ranging in size from 1cm to 50cm across, called spatter. • When the spatter falls back to Earth the clots weld themselves together and solidify forming steep-sided accumulations. • These accumulations focused on an individual vent are called spatter cones.
Types of volcanoes CALDERAS • Occasionally, volcanic eruptions are so violent that they clear the magma chamber beneath the volcano. • The summit of the cone may then sink into the empty magma chamber beneath the vent, creating a huge crater that may be several kilometres in diameter. • These calderas may later become flooded (e.g. Lake Toba, northern Sumatra), and later eruptions may create conelets in these lagoons.
Types of Volcanoes VOLCANIC DOMES • Rhyolitic and Andesitic Lavas tend to be more viscous and flow less readily . • They ooze out at the surface like thick toothpaste from a tube , piling up close to the volcanic vent, rather than spreading freely . • The resulting structure is a more compact and steep sided volcanic dome. • An example of this is Mount Helens which comprises of this stiff, viscous lava .
Types of Volcanoes UNDERWATER VOLCANOES • If volcanoes erupt under the sea (e.g. at plate margins), the lava cools very quickly due to the cool temperatures of surrounding water. • This may give rise to interesting forms such as pillow lava, so called as it appears to billow outwards.
Types of Volcanoes HOTSPOTS • Not all volcanoes are found above plate margins. Hotspots are areas of the Earth’s crust where there is an unusually high flow of heat , marked by volcanic activity . • They are usually found on oceanic crust , although they may also occur on continental crust .For example, it is thought that there is a ‘superplume’ underlying Yellowstone Park, Wyoming , USA. • Of the 125 hotspots that are thought to have been active over the past ten million years, most are located away from the boundaries of tectonic plates, and it is generally thought that they result from hot plumes of molten rock in the mantle that rise to the surface. • Should the hotspot be stationary, chains of volcanoes may be develop on the overlying crust as it is moved by tectonic activity .E.g. Hawaiian Islands , Pacific Ocean , Lesser Antilles Chain of Islands.
Volcanic features - Igneous Structures Intrusive Batholith Stock Lopolith Laccolith Volcanic neck Sill Dike Extrusive Lava flow or plateau Volcano (many types) Crater Caldera Fissure 29
Intrusive Features /Landforms Form within the earths crust may become visible at the surface. Dykes- tabular igneous intrusions that cut across the country rock. They may be less or more resistant than the country rock. Sills- tabular ,sheet like igneous intrusions that lie between parallel layers of bedrock. Laccoliths- lens shaped intrusion of magma along bedding planes. The overlying strata is impacted. Batholiths – The lens shape intrusion becomes exposed as a result of weathering. As a result of heat that has been released the surrounding rock may become metamorphosed or altered.
Extrusive Features/ Landforms These are formed above the earth’s surface from the extrusion of lava. Fissure eruptions and Landforms produced. Fissure eruptions- Lava may erupt through fissures rather than through a central vent .(Heimaey,Iceland 1973).This fissure was about 2km in length. Flood Basalts/plateaus –Basic lavas flow great distances forming large plains or accumulate as a plateau.(Columbian Plateau 200,000km2 ) Plateaus formed by flood basalts are found on every continent. Ash –Flow deposits- Fissure eruptions of pyroclastic material have produced extensive sheets of hard volcanic tuffs called ash-flow deposits. Humans have never experienced this. The early Tertiary ash flow deposits of Great Basin In Nevada.
