A volcano is an opening in the Earths crust through which lava, volcanic ash, and gases escape. Volcanic eruptions are partly driven by pressure from dissolved gas, much as escaping gases force the cork out of a bottle of champagne. Beneath a volcano, liquid magma containing dissolved gases rises through cracks in the Earth’s crust. As the magma rises, pressure decreases, allowing the gases to form bubbles. How the magma (lava) behaves when it reaches the surface depends on both its gas content and chemical composition. Lavas with low silica contents have low viscosities and flow freely, allowing any gas bubbles to escape readily, while lavas with high silica contents are more viscous (resistant to flow), so that any trapped gases cannot escape gradually.
Volcanoes can be found on Earth and other planets in our solar system. The word "volcano" comes from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn comes from Vulcan, the god of fire in Roman mythology. The study of volcanoes is called volcanology, sometimes spelled vulcanology.
Some key features of volcanoes include:
- Types: There are many types of volcanoes, including shield volcanoes, cinder cone volcanoes, and composite volcanoes.
- Structure: The features of volcanoes are much more complicated than just a conical mountain with a summit crater. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have landscape features such as massive plateaus.
- Activity: Volcanoes can be active, dormant, or extinct. Active volcanoes are volcanoes that have had recent eruptions or are expected to have eruptions in the near future. Dormant volcanoes no longer produce eruptions, but might again sometime in the future. Extinct volcanoes will likely never erupt again.
Volcanoes can cause a variety of hazards, including lava flows, ash clouds, mudslides, avalanches, and floods. The United States contains several active volcanoes, mainly in Hawaii, Alaska, California, Oregon, and Washington.
