Volcano

erupting on May 18, 1980]]
 peninsula, eastern Russia]]

A **volcano** is a mountain that has lava (hot, liquid rock) coming out from a magma chamber under the  or did have in the past. Volcanoes are formed by the movement of tectonic plates.

Volcanoes can also form where there is stretching and thinning of the crust's plates, e.g., in the East African Rift. Volcanoes are not typically found where two tectonic plates slide past one another.

Volcanism away from plate boundaries is caused by mantle plumes. These so-called "hotspots", for example Hawaii, are thought to arise from upwelling magma from the core–mantle boundary, 3,000&nbsp;km deep in the Earth.  

Most volcanoes have a volcanic crater at the top. When a volcano is active, materials come out of it. The materials include lava, steam, gaseous sulfur compounds, ash and broken rock pieces.

When there is enough pressure, the volcano erupts. Some volcanic eruptions blow off the top of the volcano. Sometimes, the magma comes out quickly or slowly depending on the eruption. Some eruptions come out at a side instead of the top.

Volcanoes are found on planets other than Earth. An example is Olympus Mons on Mars.

Volcanologists are scientists who study volcanoes using methods from geology, chemistry, geography, mineralogy, physics and sociology.

The world's biggest volcano is named Mauna Loa in Hawaii. Mauna Loa is part of the five volcanoes on Hawaii's 'Big Island'. The most recent time this volcano erupted was in 1984. It erupted 33 times in the last 170 years. Like all the other Hawaiian volcanoes, Mauna Loa was created by the movement of the Pacific tectonic plate which moved over the Hawaii hotspot in the Earth's mantle. Mauna Loa is 4,196&nbsp;meters tall. It is a shield volcano. The largest recent eruption from Mauna Loa left a lava trail  long.

## Types of volcanoes 
The lava and pyroclastic material (clouds of ash, lava fragments and vapor) that comes out from volcanoes can make many different kinds of land shapes. There are two basic kinds of volcanoes.

### Shield volcanoes 
These volcanoes are formed by fluid low-silica mafic 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 . 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.

Even though their sides are not very *steep*, shield volcanoes can be huge. Mauna Kea in Hawaii is the biggest mountain on Earth if it is measured from its base on the floor of the sea.

### Stratovolcanoes 
, an active
 stratovolcano in Japan that last erupted in 1707&ndash;08]]
, an active stratovolcano near Rabaul in Papua New Guinea]]

A stratovolcano, also known as a **composite volcano**, is a tall, conical volcano. It is built up of many layers of hardened lava, tephra, pumice, and volcanic ash.

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 .

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&nbsp;AD, killing thousands.

#### Caldera 
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.

### Dormant (inactive) 
A dormant volcano is "sleeping," but it could awaken in the future. Mount Rainier in the United States is considered dormant.

## Largest volcano on Earth 
The Earth's largest volcano has been discovered. It is 2&nbsp;km below the sea on an underwater plateau known as the Shatsky Rise. This is about 1,600&nbsp;km east of Japan. The previous record-holder, Mauna Loa in Hawaii, is still the largest volcano on land.

The 310,000&nbsp;km2 (119,000&nbsp;sq mi) volcano, Tamu Massif, is comparable in size to Mars' vast Olympus Mons volcano, which is the largest known volcano 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&nbsp;km (18&nbsp;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".

## Related pages 
* List of volcanoes

## References 
## Other websites 
* The United States Geological Service (USGS) Volcano page 
* Glossary of Volcanic Terms from USGS 
* Volcanic and Geologic Terms  from Volcano World  - University of North Dakota (UND)
* Television program (BBC) on the prediction of Popocatepetl's 2000 eruption