Volcanoes
Volcanoes are places where magma emerges through the crust and onto the surface. The word‘volcano’ comes from Vulcano Island in the Mediterranean. Vulcan, the ancient Roman god of fire and blacksmith to the gods, was supposed to have forged his weapons in the fire beneath the mountain. |
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Types of volcano
There are many types of volcano . The most distinctive are the cone-shaped composite volcanoes, which build up from alternating layers of ash and lava in successive eruptions.Megma chamber
Beneath a composite volcano there is typically a large reservoir of magma called a magma chamber. Magma collects in the chamber before an eruption.Chimney From the magma chamber
A narrow chimney, or vent, leads up to the surface. It passes through the cone of debris from previous eruptions. Eruption of megma When a volcano erupts, the magma is driven up the vent by the gases within it. As the magma nears the surface, the pressure drops, allowing the gases dissolved in the magma to boil out. The expanding gases - mostly carbon dioxide and steam - push the molten rock upwards and out of the vent.At Urgup, Turkey, volcanic ash has been blown into tall cones by gas fumes bubbling up. The cones have hardened like huge salt cellars. People have dug them out to make homes. Before each eruption, the vent is clogged by old volcanic material from previous eruptions.
The explosion blows the plug into tiny pieces of ash and cinder, and blasts them high into the air Central vent If the level of magma in the magma chamber drops, the top of the volcano’s cone may collapse into it, forming a giant crater called a caldera. Caldera is Spanish for ‘boiling pot’ Cones When a volcano with a caldera subsides, the whole cone may collapse into the old magma chamber.
The caldera may fill with water to form a crater lake, such as Crater Lake in Oregon, USA. All the magma does not gush up the central vent. Some exits through branching side vents, often forming their own small ‘parasitic’ cones on the side of the main one. the level of magma in the magma chamber drops, the top of the volcano’s cone may collapse into it, forming a giant crater called a caldera.
When a volcano with a caldera subsides, the whole cone may collapse into the old magma chamber. The caldera may fill with water to form a crater lake, such as Crater Lake in Oregon, USA. All the magma does not gush up the central vent. Some exits through branching side vents, often forming their own small‘parasitic’ cones on the side of the main one.
When a volcano erupts it sends out a variety of hot materials, including lava, tephra, ash and gases.
Lava
Lava is hot molten rock from the Earths interior. It is called magma while it is still underground.Tephra
Tephra is material blasted into the air by an eruption. It includes pyroclasts (solid lava) and volcanic bombs.Pyroclasts
Pyroclasts are big chunks of volcanic rock that are thrown out by explosive volcanoes when the plug in the volcano’s vent shatters. ‘Pyroclast’ means fire broken. Pyroclasts are usually 0.3 -1 m across.Big eruptions can blast pyroclasts weighing 1 tonne or more up into the air at the speed of a jet plane.Ash
The ash hurled out from a volcano can settle and form a layer many metres deep, completely covering roads.Cinders and lapilli
Cinders and lapilli are small pyroclasts. Cinders are 6.4-30 cm in diameter; lapilli are 0.1-6.4 cm.Volcanic blobs
Volcanic bombs are blobs of molten magma that cool and harden in flight.Breadcrust blobs
Breadcrust bombs are bombs that stretch into loaf shapes in flight gases inside them create a ‘crust’. Around 90 percent of the material ejected by explosive volcanoes is not lava, but tephra and ash. Pumice rock is made from hardened lava froth - it is so full of air bubbles that it floats. Kinds of volcano Each volcano and each eruption are slightly different.e.gShield volcanoes
Shield volcanoes are shaped like upturned shields. They form where lava is runny and spreads over a wide area.Fissure Volcanoes
Fissure volcanoes are found where floods of lava pour out of a long crack in the ground. Composite volcanoes are cone- shaped. They build up in layers from a succession of explosive eruptions. Cinder cones are built up from ash, with little lava.Strombolian eruptions
Strombolian eruptions are eruptions from sticky magma. They spit out sizzling clots of red-hot lava.Vulcanian eruptions
Vulcanian eruptions are explosive eruptions from sticky magma. The magma clogs the volcano’s vent between cannon-like blasts of ash. These volcanoes are a shield volcano (top), clouds and thick lava flows. a crater volcano (middle) and a cone-shaped volcano (hotttom).Plinian eruptions
Plinian eruptions are the most explosive kind of eruption. They are named after Pliny who witnessed the eruption of Vesuvius in AD79 .In Plinian eruptions boiling gases blast clouds of ash and volcanic fragments up into the stratosphere. A Fissure volcanoes shoot lava fountains in the air. This happens when gases in the lava boil suddenly as they reach the surface. The above picture shows the eruption of Mount St Helens, USA. There are about 60 major volcanic eruptions each year around the world, including two or three huge, violent eruptions.Megma
Volcanic eruptions are produced by magma, the hot liquid rock under the Earth’s crust. Magma is less dense than the rock above, and so it tries to bubble to the surface. When magma is runny, eruptions are ‘effusive’, which means they ooze lava gently all the time.When magma is sticky, eruptions are explosive.The magma clogs the volcano’s vent until so much pressure builds up that the magma bursts out, like a popping champagne cork. The explosion shatters the plug of hard magma that blocks the volcano’s vent, reducing it to ash and cinder.
Volcanoes and earthquakes Explosive eruptions are driven by expanding bubbles of carbon dioxide gas and steam inside the magma. An explosive eruption blasts globs of hot magma, ash, cinder, gas and steam high up into the air. Repose time "It is the interval between erruptions"
Volcanoes usually erupt again and again. The interval between eruptions, called the repose time, varies from a few minutes to thousands of years.
Magma near subduction zones contains 10 times more gas, so the volcanic eruptions here are violent.
The gas inside magma can expand hundreds of times in just a few seconds. Krakatau is a volcano in Indonesia. It erupted in 1883 and produced sea waves almost 40 m high, which drowned about 36,000 people.
Volcano zones
Worldwide there are over 1500 volcanoes; 500 of these are active. A volcano can have a lifespan of a million years and not erupt for several centuries. Volcanoes are said to be active if they have erupted recently. The official Smithsonian Institute list of active volcanoes includes any that have erupted in the past 10,000 years. Extinct volcanoes will never erupt again.Volcanoes occur either along the margins of tectonic plates, or over hot spots in the Earth’s interior.A Most volcanoes are found around the Pacific Ocean. They also occur in Ice-land, Hawaii and southern Europe. Volcanoes and earthquakes Some volcanoes erupt where the plates are pulling apart, such as under the sea along mid-ocean ridges. Some volcanoes lie near subduction zones, forming either an arc of volcanic islands or a line of volcanoes on land, called a volcanic arc.
Subduction zone volcanoes are explosive, because the magma gets contaminated and acidic as it burns up through the overlying plate. Acidic magma is sticky and gassy. It clogs up volcanic vents then blasts its way out. Around the Pacific there is a ring of explosive volcanoes called the Ring of Fire. It includes Mt Pinatubo in the Philippines, and Mt St Helens in Washington State, USA. Away from subduction zones magma is basaltic.
It is runny and low in gas, so the volcanoes here gush lava. Effusive volcanoes pour out lava frequently but gently. 3D radar interferometry from satellites may pick up the minutest swelling on every active volcano in the world. In this way it helps to predict when eruptions may occur. Hot-spot volcanoes About five percent of volcanoes are not near the margins of tectonic plates.
They are over especially hot places in the Earths interior called hot spots. Hot spots are created by mantle plumes - hot currents that rise all the way from the core through the mantle.
When mantle plumes come up under the crust, they burn their way through to become hot-spot volcanoes.- Famous hot-spot volcanoes include the Hawaiian island volcanoes and Reunion Island in the Indian Ocean. Hot-spot volcanoes ooze runny lava that spreads out to create shield volcanoes (see kinds of volcano).
Lava from hot-spot volcanoes also creates plateaux, such as the Massif Central in France.The geysers, hot springs and bubbling mud pots of Yellowstone National Park, USA, indicate a hot spot below. Yellowstone has had three huge eruptions in the past 2 million years.
The first produced over 2000 times as much lava as the 1980 eruption of Mt St Helens. Hot spots stay in the same place while tectonic plates slide over the top of them. Hawaiian hot spot has created a chain of old volcanoes 6000 km long. It starts with the Meiji seamount under the sea north of Japan, and ends with the Hawaiian islands.
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