When there is enough pressure, the volcano erupts. Some volcanic eruptions blow off the top of the volcano. The magma comes out, sometimes quickly and sometimes slowly. Some eruptions come out at a side instead of the top.
Volcanologists are scientists who study volcanoes using methods from geology, chemistry, geography, mineralogy, physics and sociology.
Types of volcanoes
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.
An extinct volcano has not erupted in the past 10,000 years. Edinburgh Castle in Scotland is located on top of an extinct volcano.
- 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".
A 2007 eruptive column at Mount Etna producing volcanic ash, pumice and lava bombs
Aerial view of the Barren Island, Andaman Islands, India, during an eruption in 1995. It is the only active volcano in South Asia.
Santa Ana Volcano, El Salvador. A close-up aerial view of the nested summit calderas and craters, along with the crater lake.
Lakagigar fissure vent in Iceland, source of the major world climate alteration of 1783–84
Skjaldbreiður, a shield volcano whose name means "broad shield"
Izalco (volcano), located in the Cordillera de Apaneca volcanic range complex in El Salvador. Only a few generations old, Izalco is the youngest and best known cone volcano. Izalco erupted almost continuously from 1770 (when it formed) to 1958, earning it the nickname of "Lighthouse of the Pacific".
Pāhoehoe lava flow on Hawaii. The picture shows overflows of a main lava channel.
San Miguel (volcano), El Salvador. On December 29, 2013, San Miguel volcano, also known as "Chaparrastique", erupted at 10:30 local time, spewing a large column of ash and smoke into the sky; the eruption, the first in 11 years, was seen from space and prompted the evacuation of thousands of people living in a 3 km radius around the volcano.
Ash plume from San Miguel (volcano) "Chaparrastique", seen from a satellite, as it heads towards the Pacific Ocean from the El Salvador Central America coast, December 29, 2013
Fourpeaked volcano, Alaska, in September 2006 after being thought extinct for over 10,000 years
Lava flows at Holuhraun, Iceland, September 2014
Ash plume rising from Eyjafjallajökull on April 17, 2010
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