A volcano is an opening in Earth’s crust. When a volcano erupts, hot gases and melted rock from deep within Earth find their way up to the surface. This material may flow slowly out of a fissure, or crack, in the ground, or it may explode suddenly into the air. Volcanic eruptions may be very destructive. But they also create new landforms. There are more than 1,500 potentially active volcanoes in the world today.
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"Volcano." Britannica School. Encyclopædia Britannica, Inc., 2015. Web. 2 Sep. 2015. <http://school.eb.com/levels/middle/article/353902>.
Yellowstone National Park in the United States sits on top of what scientists call a super volcano. The image below shows how super volcanos are formed. For more information on the super volcano in Yellowstone, go to http://volcanoes.usgs.gov/volcanoes/yellowstone/yellowstone_sub_page_49.html