page_title: Volcano
hierarchical_section_title: Volcano
caption_attribution_description: English: Mount St. Helens eruptingFrançais&#160;: Éruption du mont Saint Helens en 1980.
context_page_description: A volcano is a mountain that has lava coming out from a magma chamber under the ground, or did have in the past. Volcanoes are formed by the movement of tectonic plates.
  The Earth's crust is broken into 17 major, rigid tectonic plates. These float on a hotter, softer layer in its mantle. Volcanoes are often found where tectonic plates are moving apart or coming together.
  Volcanoes can also form where there is stretching and thinning of the crust's plates, e.g., in the East African Rift. Volcanoes are usually not 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 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 and sometimes it comes slowly.
context_section_description: A volcano is a mountain that has lava (hot, liquid rock) coming out from a magma chamber under the ground, or did have in the past. Volcanoes are formed by the movement of tectonic plates.
  The Earth's crust is broken into 17 major, rigid tectonic plates. These float on a hotter, softer layer in its mantle. Volcanoes are often found where tectonic plates are moving apart or coming together.   
  Volcanoes can also form where there is stretching and thinning of the crust's plates, e.g., in the East African Rift. Volcanoes are usually not 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 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 and sometimes it comes slowly. 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 meters tall. It is a shield volcano. The largest recent eruption from Mauna Loa left a lava trail 51 kilometres (32 miles) long.
caption: Mount St. Helens erupting on May 18, 1980
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