What causes a volcano erupt?

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A volcano may refer to

  • a vent or opening in the surface of the Earth through which lava, hot gases, ash and rock fragments erupt OR

  • the usually conical mountain formed by the deposition and accumulation of erupted material around the opening.
A volcano begins as melted rock (magma) which rises from deep inside the earth toward the surface. As the gas-filled magma rises, it melts gaps in the surrounding rock and forms a large chamber. Pressure from the solid rock around it forces the magma up to the surface through the conduit (channel) in a weakened part of the rock. The gas and melted rock blast out an opening called the central vent.

The erupted materials gradually pile up around the vent, forming a volcano. A bowl-like crater generally forms at the top of the volcano. The vent lies at the bottom of the crater.

Cross-section of A Volcano (modified from U.S. Geological Survey)

Volcanic eruptions are spectacular sights ... chunks of hot rock, red-hot ash and cinders shoot out and huge fiery clouds rise over the mountain. Most volcanic materials erupt through the central vent. However, some magma may break through the main conduit wall and branch out into smaller channels and escape through vents formed in the sides of the volcano.

Some eruptions occur on volcanic islands. Such islands are the tops of volcanic mountains that have been built up from the ocean floor by repeated eruptions.

A volcano is classified as

  • "active" when it is erupting or has erupted recently
  • "dormant" when it has not erupted recently but is likely to erupt in the future
  • "extinct" when it has not erupted for a long time and is not expected to erupt in the future

Flash movies:   1 How a volcano forms   2 BrainPOP Movie   3 CBBC Animation
Types of Volcanoes: 4 Explanation | 5 Animation 6 How volcanoes work

"George Files" by Parenting the Next Generation