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When there is enough pressure, it causes the volcano to erupt. The pressure blows off the top of the volcano. The magma comes out, sometimes quickly and sometimes slowly.
Types of volcanoes
These volcanoes are formed by low-silica (i.e. basic) lava.
Shield volcanoes are built out of layers of lava from continual eruptions (without explosions). Because the lava is so fluid, it spreads out, often over a wide area. Shield volcanoes do not grow to a great height, and the layers of lava spread out to give the volcano gently sloping sides. Shield volcanoes can produce huge areas of basalt, which is usually what lava is when cooled.
The base of the volcano increases in size over successive eruptions where solidified lava spreads out and accumulates. Some of the world's largest volcanoes are shield volcanoes.
Unlike shield volcanoes, stratovolcanoes have a steep profile and periodic eruptions. The lava that flows from stratovolcanoes cools and hardens before spreading far. It is sticky, that is, it has high viscosity. The magma forming this lava is often felsic, with high-to-intermediate levels of silica, and less mafic magma. Big felsic lava flows are uncommon, but have travelled as far as 15 km (9.3 mi).
Two famous stratovolcanoes are Japan's Mount Fuji, and Vesuvius. Both have big bases and steep sides that get steeper and steeper as it goes near the top. Vesuvius is famous for its destruction of the towns Pompeii and Herculaneum in 79 AD, killing thousands.
A caldera is a basin-like feature formed by collapse of land after a volcanic eruption. This happens after a huge stratovolcano blows its top off. The base of the crater then sinks, leaving a caldera where the top of the volcano was before. Krakatoa, best known for its catastrophic eruption in 1883, is much smaller now.
How volcanoes are formed
There are two main processes.
Volcanoes are made when two tectonic plates come together. When these two plates meet, one of them (usually the oceanic plate) goes under the continental plate. This is the process of subduction. Afterwards, it melts and makes magma (inside the magma chamber), and the pressure builds up until the magma bursts through the Earth's crust.
The second way is when a tectonic plate moves over a hot spot in the Earth's crust. The hot spot works its way through the crust until it breaks through. The caldera of Yellowstone Park was formed in that way; so were the Hawaiian Islands.
A traditional way to classify or identify volcanoes is by its pattern of eruptions. Those volcanoes which may erupt again at any time are called active. Those that are now quiet called dormant (inactive). Those volcanos which have not erupted in historical times are called extinct.
- Kilauea (Hawaii, USA)
- Krakatoa (Rakata, Indonesia)
- Mauna Loa (Hawaii, USA)
- Mauna Kea (Hawaii, USA)
- Mount Ashitaka (Japan)
- Mount Baker (Washington, USA)
- Mount Edziza (British Columbia, Canada)
- Mount Etna (Sicily, Italy)
- Mount Erebus (Ross Island, Antarctica)
- Mount Hood (Oregon, USA)
- Mount Fuji (Honshu, Japan)
- Mount Rainier (Washington, USA)
- Mount Ruapehu (North Island, New Zealand)
- Mount Shasta (California, USA)
- Mount St. Helens (Washington, USA)
- Novarupta (Alaska, USA)
- Olympus Mons (Mars (planet))
- Popocatépetl (Mexico-Puebla state line, Mexico)
- Surtsey (Surtsey island, Iceland)
- Santorini (Santorini island, Greece)
- Tambora (Sumbawa, Indonesia)
- Teide (Tenerife, Canary Islands, Spain)
- Vesuvius (Gulf of Naples, Italy)
- Yellowstone Caldera (Wyoming, USA)
Largest volcano on Earth
The Earth's largest volcano has been discovered. It is 2km below the sea on an underwater plateau known as the Shatsky Rise. This is about 1,600km east of Japan. The previous record-holder, Mauna Loa in Hawaii, is still the largest volcano on land.
The 310,000 sq km (119,000 sq mi) volcano, Tamu Massif, is comparable in size to Mars' vast Olympus Mons volcano, which is the largest in the Solar System. It was formed about 145 million years ago when massive lava flows erupted from the centre of the volcano to form a broad, shield-like feature. That suggests the volcano produced a flood basalt eruption.
The Tamu Massif extends some 30 km (18 miles) into the Earth's crust. The researchers doubted the submerged volcano's peak ever rose above sea level during its lifetime and say it is unlikely to erupt again.
- "The bottom line is that we think that Tamu Massif was built in a short (geologically speaking) time of one to several million years and it has been extinct since," co-author William Sager, of the University of Houston told the AFP news agency.
- "There were lots of oceanic plateaus (that) erupted during the Cretaceous period (145-65 million years ago) but we don't see them since. Scientists would like to know why... The biggest oceanic plateau is Ontong Java plateau, near the equator in the Pacific, east of the Solomon Islands. It is much bigger than Tamu – it's the size of France".
- The plural of volcano can be either volcanos or volcanoes. Both are equally correct, and it is not a matter of British vs US spelling. Oxford English Dictionary. Spelling in any particular Simple page tries to be consistent.
- Earth Science. Austin, Texas: Holt, Rinehart and Winston. 2001. .
- Principal types of volcanoes. USGS. 
- "Garibaldi volcanic belt: Garibaldi Lake volcanic field". Catalogue of Canadian volcanoes. Geological Survey of Canada. 2009-04-01. http://wayback.archive.org/web/20090626082839/http://gsc.nrcan.gc.ca/volcanoes/cat/feature_garibaldi_e.php. Retrieved 2010-06-27.
- Ball, Jessica (September 8, 2010). "Voices: Dead or alive ... or neither? Why a dormant volcano is not a dead one". Earth Magazine. http://www.earthmagazine.org/article/voices-dead-or-alive-or-neither-why-dormant-volcano-not-dead-one. Retrieved 14 August 2012.
- University of Edinburgh, "Holyrood Park Geology"; retrieved 2012-8-2.
- World's largest volcano discovered beneath Pacific. BBC Science & Environment. 
- Witze, Alexandra 2013. Underwater volcano is Earth's biggest: Tamu Massif rivals the size of Olympus Mons on Mars.