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Volcanism

Volcanism is a geological term for processes at the Earth's surface caused by the rise of magma in the Earth's crust . The magma rises up from the magma chamber by a volcanic pipe or through cracks in the Earth's crust which are called dykes. Volcanic eruptions are one example of volcanism, but also escaping steam or gases causes by magma in the subsurface is covered by volcanism. When magma reaches the surface, it is called lava.

A volcano is a place where lava , gases or chunks of rock to come to the surface during eruptions. This type of volcanism occurs mainly near boundaries of tectonic plates, but also at hotspots.


Kerið volcano crater, Iceland

Causes

Volcanism occurs mainly along the edges of tectonic plates. The cause of this is the phenomenon tectonics. Because heat is produces in the interior of the Earth, caused by radioactive decay , there are convection currents in the Earth's mantle. These convection currents are the engine of plate tectonics.

When material flows upwards in the mantle and crust, the rock will melt at a given moment due to decrease of pressure on the smaller depth. It forms a magma chamber. The rock may also partially melt, and the unmelted minerals can become separated from themagma. This is called magma differentiation. This process explains the large variety of igneous rocks that occur on Earth.

Because of the movement of the tectonic plates, oceanic crust subducts under continental crust in subduction zones. On the continental crust this yields a row of volcanoes. Hot spots may occur anywhere because they are caused by mantle plumes from the mantle.

Types of volcanoes and eruptions

Because lava composition and viscosity varies, there are different types of volcanoes: stratovolcano, shield volcano and fissure volcano. Tougher lava causes more explosive eruptions, and forms a stratovolcano. Depending on the type of volcanic activity and the possible presence of water, a crater may be formed.

Fissure volcanoes occur mainly on the spreading zones in the oceans where continents are drifting apart. For example, the mid - oceanic ridge in the Atlantic ocean. This fissure volcanoes erupt material for the formation of new oceanic crust, usually Basalt.

Stratovolcanoes are high and have a conical shapeand satisfy the classic image of a volcano. Stratovolcanoes can occur in places where explosive eruptions occur. Because of the composition of the magma, the lava is quite tough and gases may accumulate. Lava flows are not very far and explosive eruptions by the sustained high gas pressure in the volcano ensure ashfall and pyroclastic flows. This creates a cone-shaped volcano which is made up of tephra and lava. Stratovolcanoes typically occur in subduction zones. Usually in volcanic island arcs or lines of volcanoes on land. The best example is the "ring of fire" around the Pacific Ocean.

A shield volcano is recognizable by its very flat slopes. This is in contrast to stratovolcanoes that are high and conical. Because of the composition, the magma is pretty thin and the lava flows over a large area. This creates the flat shape of the shield volcano. Shield volcanoes tend to have relatively peaceful non-explosive eruptions. Shield volcanoes occur for example in Iceland and in oceanic hotspots such as Hawaii.

Volcanic deposits

When lava is released during an eruption, rock like Basalt, Andesite, Dacite, Rhyolite and Obsidian can form depending on the rate of cooling and the mineral composition. During explosive eruptions  volcanic ash and tephra will form, which can cover large areas. Especially with deposition of volcanic ash in water, animal or plant remains are buried quickly and fossils may form. The lava rocks often contain cavities, formed by volcanic gases in the rock. In these cavities dissolved minerals can be deposited, and agates or geodes are formed in the course of geological time.

Ripples in lava at the Askja volcano, Iceland.

During an eruption, pyroclastic flows  can occes, caused by large amounts of cooling ash.  These clouds can travel dowhhill with great speed where everything in its path is destroyed. In Roman times Pompeii and Herculaneum, destroyed were destroyed by a pyroclastic flow in AD 79 from the volcano Vesuvius in Italy.

When large amounts of ash and tephra is erupted, it is likely that landslides occur in the material. When it rains, there is high probability of lahars during and after the eruption, mudflows of volcanic ash. These can cause devastation up to many kilometers from the volcano.

Other phenomena

Volcanism is a collective name for geological processes on the surface, which are the result of the rise of hot material from the interior of a planet. The rising material can consist of magma (molten rock), or volatile substances such as gases or liquids. If such material reaches the surface is called an eruption. When magma flows out over the surface, it is called lava. An eruption may however be accompanied by explosions, which causes rubble and ashes (called tephra or pyroclastic material) to be thrown into the atmosphere. At the location of a volcanic eruption a crater can occur when the eruption has an explosive nature.


The geyser Geysir, Iceland. Photo Herman Zevenberg.

Traces of past volcanic activity are sometimes found in the form of an intrusion. This is magma which intruded into an already present rock and then solidified.

A caldera is a large crater caused by the collapse of the original volcano because of the depletion of the magma chamber by an eruption. Calderas can be many kilometers in diameter, and not readily identifiable as a crater.

When magma is present at shallow depths in the crust, geysers or hot springs can occur at the surface. Fumaroles may also occur, where volcanic gases (mostly SO2 and CO2) escape from the ground.


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