Through fracture, fissures found into the crust magma found inside the crust can sometimes reach the surface of the crust and consolidate from there. In this case the features formed are Extrusive features. In this case the features formed are known as Intrusive features. The causes of volcanicity and volcanic eruption can include: Increased quantity of magma in the mantle leading to increase to pressure pushing this magma out wards. Presence of fissure and cracks allowing magma to move towards the crust. Increase in the temperature of magma inside making the magma very light to move along a crack.

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Created and produced by QA International. All rights reserved. Most of these are associated with the boundaries of the enormous rigid plates that make up the lithosphere —the crust and upper mantle. The majority of active terrestrial volcanoes roughly 80 percent and related phenomena occur where two tectonic plates converge and one overrides the other, forcing it down into the mantle to be reabsorbed.

Long curved chains of islands known as island arcs form at such subduction zones. Volcanoes of the explosive type make up many of the islands of a single arc or the inner row of islands of a double arc. All such islands that border the Pacific basin are built up from the seafloor, usually by the extrusion of basaltic and andesitic magmas. A second major site of active volcanism is along the axis of the oceanic ridge system, where the plates move apart on both sides of the ridge and magma wells up from the mantle, creating new ocean floor along the trailing edges of both plates.

Virtually all of this volcanic activity occurs underwater. In a few places the oceanic ridges are sufficiently elevated above the deep seafloor that they emerge from the ocean, and subaerial volcanism occurs. Iceland is the best-known example. The magmas that are erupted along the oceanic ridges are basaltic in composition. A relatively small number of volcanoes occur within plates far from their margins.

These magmas characteristically generate a chain of progressively older volcanoes that mark the direction of past motion of the plate over a particular hot spot. The active volcanoes of the East African Rift Valley also occur within a plate the African Plate , but they appear to result from a different mechanism—possibly the beginning of a new region of plates moving apart.

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Main article: Stratovolcano 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. 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. Throughout recorded history , ash produced by the explosive eruption of stratovolcanoes has posed the greatest volcanic hazard to civilizations.





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