Volcanoes

Volcano

A volcano is an opening (or rupture) in the Earth's surface or crust, which allows hot, molten rock, ash, and gases to escape from deep below the surface.

Volcanic activity involving the extrusion of rock tends to form mountains or features like mountains over a period of time. Volcanoes are generally found where two to three tectonic plates pull apart or come together.

A mid-oceanic ridge, like the Mid-Atlantic Ridge, has examples of volcanoes caused by "divergent tectonic plates" pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by "convergent tectonic plates" coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another (like the San Andreas fault).

Volcanoes can also form where there is stretching of the Earth's crust and where the crust grows thin (called "non-hotspot intraplate volcanism"), such as in the African Rift Valley or the European Rhine Graben with its Eifel volcanoes). Finally, volcanoes can be caused by "mantle plumes," so-called "hotspots;" these hotspots can occur far from plate boundaries, such as the Hawaiian Islands.

Interestingly, hotspot volcanoes are also found elsewhere in the solar system, especially on rocky planets and moons. The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater in its top.

This describes just one of many types of volcano and the features of volcanoes are much more complicated. Supervolcano is the popular term for a large volcano that usually has a large caldera and can potentially produce devastation on an enormous, sometimes continental, scale.

Such eruptions would be able to cause severe cooling of global temperatures for many years afterwards because of the huge volumes of sulfur and ash erupted.

They are the most dangerous type of volcano.
Examples include Yellowstone Caldera in Yellowstone National Park of western USA, Lake Taupo in New Zealand and Lake Toba in Sumatra, Indonesia..
  
Active and non-active volcanoes

There are volcanoes in different phases of activity:

• Active volcanoes: which are likely to erupt at any moment

• Dormant volcanoes: which lie dormant for centuries, but then erupt suddenly and violently

• Extinct volcanoes: ones no longer likely to erupt.

Type of Volcanoes

In the surroundings of boundaries of tectonic plates the following types of volcanoes occur:

• The fissure volcano: Is a long crack in the earth's surface through which magma erupts. These cracks may form as two tectonic plates pull apart. You'll find them mainly near mid-ocean ridges.
  

The ordinary volcanoes can be divided in different types, relating to their forms:

• The shield volcano: This is a broad, shallow volcanic cone, which arises because the running lava, which is fluid and hot, cools slowly. 
• The dome volcano: This one has a steep, convex slope from thick, fast-cooling lava. 
• The ash-cinder volcano: Throws out - besides lava - much ash into the air. Through this the volcanic cone is built up from alternate layers of ash and cinder. 
• The composite volcano: These are also built up from alternate layers of lava and ash but, besides its main crater, it has many little craters on its slope. 
• The caldera volcano: An older volcano with a large crater which can be 62 miles(100km) wide. In this crater many little new craters are formed.
  
  
  
  
  
  
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