Resource: Volcanism

Print Background Essay

The image most people have of volcanoes is that of a steep-sided, barren cone that spews molten lava down its slopes. This image is depicted in movies, art, and toys, and is often reflected in the volcano models students create for school projects. In reality, while many volcanoes fit this quintessential image, there are countless exceptions to the rule. Volcano slopes may be gentle or their profiles may be jagged and asymmetrical. Dormant volcanoes are often covered with snow and ice, or draped in dense vegetation. In fact, many volcanoes are easily mistaken for non-volcanic mountains that have been formed by the folding and buckling of Earth's crust.

What sets volcanoes apart from other mountains is the manner in which they form. Volcanoes result when magma — a mixture of molten rock, suspended mineral grains, and dissolved gases formed in Earth's interior — is forced to the surface. This occurs most often at the boundaries of tectonic plates, but also at "hot spots" in the interior of these plates.

Scientists recognize four different types of volcanoes, which they categorize according to height, shape, and where and how they formed. The type that most closely resembles the typical volcano image described above is the cinder cone. These relatively small, symmetrical, steep-sided volcanoes usually form on or near other volcanoes of the same or different types. Their shape and vertical structure results from volcanic material being launched nearly straight up into the air during eruptions and then falling back down and settling near the volcano's peak. High concentrations of gas dissolved in the magma of these volcanoes cause these explosive eruptions.

Other types of volcanoes are the lava dome, the composite volcano, and the shield volcano. The lava dome, like the cinder cone, is a relatively small volcano — often called a secondary volcano — that forms on or near another larger volcano. What distinguishes lava domes from cinder cones are their rounded, lumpy shapes that result from the very thick, or viscous, magma that feeds them. Composite volcanoes are typically large and asymmetrical. They can erupt explosively because their magma is both viscous and contains high concentrations of dissolved gas. Shield volcanoes erupt less violently, because their magma is less viscous. Flowing more fluidly, lava erupting from a shield volcano's vent creates gently sloping sides and a more symmetrical shape.

To learn more about volcanoes and how they form, check out Volcanic Features, Plate Tectonics: The Hawaiian Archipelago, and Tectonic Plates, Earthquakes, and Volcanoes.

To learn about one of the largest and deadliest volcanic eruptions in recent history, check out Mount Pinatubo: Predicting a Volcanic Eruption and Mount Pinatubo: The Aftermath of a Volcanic Eruption.

To learn about the role of magma in the rock cycle, check out Rock Cycle Animation.

Print Questions for Discussion

• What is a volcano?
• Volcanoes occur all over the world; however, they occur more often in some locations. Where are volcanoes more likely to occur and what is the explanation for these patterns?
• Compare the volcanoes on the Big Island of Hawaiʻi and Mount Rainier. For each one, identify its type, how it was formed, and its location in terms of plate boundaries.
• Are there any volcanoes in your state? If so, are they active, dormant, or extinct? What volcano erupted most recently in the U.S.?
• The name of any rock is determined by what it is made of and how it is formed. Igneous rocks always start as magma. What is the difference between the two major types of igneous rocks? Give the name of the two major types and explain how each is formed into a solid rock.