Volcanoes are generally cone shaped landforms of once-molten magma scattered all over the surface of Earth. They are holes on the crust which allow molten rock, hot gases, steam and ash to be expelled from below. In a way, they are like gigantic rock pimples. As volcanic stuff settles around the hole it builds up forming mounds and mountains. Volcanoes can form islands when they erupt under the sea.
Hundreds of volcanoes are active today. The only time we hear about them is when they result in death or destruction. To geologists they are interesting all the time, for they are the only way scientists are able to study firsthand what rock is below the crust: the mantle.
Volcanoes form in many ways, but they all result from magma pushing up and breaking through the crust. For a long time scientists did not know why volcanoes formed. It was not until the development of plate tectonics that they understood how they formed and the reason for their locations. Crustal plate interactions drive volcano activity. Because of this most volcanoes are located near and around crust boundaries.
Convergent boundaries are most commonly associated with volcanoes. As plates collide, heavier plates slide under lighter plates and melt as they move into the mantle. Mount St. Helens in Oregon is the most famous volcano in the United States. It is the result of the Pacific plate sliding under the North American continent. The Pacific plate melted and rose towards the surface, pushing its way through the rock.
Pressure formed by rising melted rock increases as it pushes through the crust. When the magma plume reaches a point in the crust where the overlying pressure is no longer enough to contain the pressure, it explodes or erupts.
Eruptions can take place in one of three locations: divergent boundaries (spreading zones), convergent boundaries ('squishing' zones) and in the middle of plates. A large popular spreading zone located in the Atlantic Ocean is known as the Mid-Atlantic ridge. Convergent zones are along the Pacific coast, of North and South America.
Volcanoes form three main shapes: shield volcanoes, cinder cones and composite volcanoes (stratovolcanoes).
Shield volcanoes are formed from runny mafic lava that spreads out forming a large, broad, slightly convex shape. These volcanoes rarely explode but ooze lava from the vent. Large shield volcanoes are located at Hawai'i Volcanoes National Park.
Cinder cones are small volcanoes that expel ash and cinders forming steep sides. These volcanoes are explosive (pyroclastic) and do not have lava flowing from the opening. Material from these volcanoes is thick and not runny at all. Because of its thickness, it traps gases inside the rock, sometimes causing it to explode into the air.
Composite volcanoes (stratovolcanoes) are a mixture of shield volcanoes and cinder cones. This volcano type emits both lava and pyroclastic material that over time forms layers. They are steeper than a shield volcano and broader than a cinder cone. These are the most common volcanoes on continents. Mount Shasta and Mount St. Helens are examples of composite volcanoes.
Other volcanic landforms are:
Craters are deep circular holes that form at the tops of volcanoes and are where volcanic stuff comes out.
Calderas are the blasted out craters of volcanoes. This occurs when gases trapped below a plug of rock or hardened lava build up and blow out the stopper. A common example of the same reaction is a wine bottle and the cork. Another way calderas form is when an empty lava chamber can no longer support itself and caves in, i.e., Crater Lake in Oregon's Crater Lake National Park or Fish Lake in southern Utah. Most of Yellowstone National Park is the smoking remnant of an enormous caldera that blew its top 600,000 years ago.
More information about igneous rocks can be found in the Igneous, In or Out activity.
Included National Parks and other sites:
Utah Science Core:
4th Grade Standard 4 Objective 1,2
5th Grade Standard 2 Objective 1,2,3