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write about volcanic eruption its causes.

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.Magma is molten rock in the Earth's crust. When magma erupts through the earth's surface it is called lava.The largest volcano on Earth is Hawaii's Mauna Loa. Mauna Loa is about 6 miles (10 km) tall from the sea floor to its summit (it rises about 4 km above sea level). It also has the greatest volume of any volcano, 10,200 cubic miles (42,500 cubic kilometers).
The largest volcano in our Solar System is perhaps Olympus Mons on the planet Mars. This enormous volcano is 17 miles (27 km) tall and over 320 miles (520 km) across. There is a great range in the explosivity of volcanic eruptions. Many eruptions are relatively quiescent and are characterized by the calm, nonviolent extrusion of lava flows on the earth's surface. Other eruptions, however, are highly explosive and are characterized by the violent ejection of fragmented volcanic debris, called tephra, which can extend tens of kilometers into the atmosphere above the volcano.Whether or not an eruption falls into one of these end-member types depends on a variety of factors, which are ultimately linked to the composition of the magma (molten rock) underlying the volcano. Magma composition is discussed below, followed by a description of the controlling factors on explosivity -- viscosity, temperature, and the amount of dissolved gases in the magma.
Volcanic eruptions, like earthquakes, don't strike randomly; they occur in specific areas. More than half of the world's active volcanoes above sea level encircle the Pacific Ocean to form the famous "Ring of Fire" – the most volcanically (and seismically) active zone on Earth. That's because both are the result of plate tectonics, and the Pacific Ocean is where the Pacific plate meets several others.

According to this now widely-accepted theory, the Earth's surface is made up of about a dozen large plates which collide, spread apart or slide past each other at a rate ranging from less than one centimetre to 10 centimetres a year – about the speed at which fingernails grow. Volcanoes tend to form where plates collide or spread apart, but they can also grow in the middle of a plate.

Subduction zones: where plates collide
When an old, hard edge of an oceanic plate slides into and underneath a more buoyant continental plate, the wet oceanic plate sinks into the mantle, its absorbed water rising into the overlying mantle. This water causes the overlying mantle to melt to form magma. Bubbles of sulphur dioxide, water vapour and carbon dioxide rise with the magma, growing in size as they approach the Earth's surface. Like a can of pop shaken too hard, the gases want out. When that happens, we have a volcanic eruption. The more gas there is, the faster it will force its way out, and the more likely it will explode when it gets to the surface.

Spreading centers: where plates spread apart

While volcanic eruptions at subduction zones have historically posed the greatest hazard to human civilization, most volcanoes erupt under the sea at mid-ocean ridges where tectonic plates pull apart and magma rises to fill the gap. As the magma hardens, it becomes part of the existing plates, extending them. There is debate over whether the magma causes the plates to move apart or if the magma reacts to fill in the space. Regardless of what triggers the event, it is the counterpart to subduction zones, which consume crust. Almost continuous, these spreading centers, as they're called, create a 75,000-kilometre long Mid-Ocean ridge that snakes around the globe like the seam of a baseball.


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