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Lesson Plans

Volcanoes: Can we predict volcanic eruptions? Dynamic Earth
Earth's Plates: Use clues about volcanoes
        to find out where they're located. Why do volcanoes erupt in different ways?

Most volcanoes occur on plate boundaries. Plate boundaries are areas where Earth's shifting plates meet or split apart, usually with violent results.

California's Mt. Shasta
is a stratovolcano.

Plate margins that are coming together are called convergent margins, while those that are splitting apart are called divergent. A third type, transform-fault margins, are sliding against each other, going in opposite directions (like those of the San Andreas Fault). Volcanoes can occur on convergent or divergent plate margins or over a hotspot, a spot inside the mantle that heats an area of the plate above it.

Colliding Plates
Along convergent margins, when two plates meet, sometimes one descends, usually of oceanic composition, beneath the other, usually of continental composition, in a process called subduction. As the descending plate is forced deeper into the mantle, parts of it begin to melt and form magma that rises to the surface, often in explosive eruptions. Subduction zones tend to create large, classic, cone-shaped volcanoes called stratovolcanoes, such as Mt. St. Helens in Washington State, or Mt. Shasta in California.

Separating Plates
At divergent margins, plates are coming apart and hot rock forces its way to the surface. Many divergent plate margins are under the oceans, creating long undersea rift zones that fill with lava. In some eruptions at divergent margins, the relatively calm, smooth flow of lava creates volcanoes with gently sloping sides, called shield volcanoes.

Hotspots can also cause shield volcanoes to form. As plates move over hotspots, volcanoes spring up and die down in turn, often creating an island chain. The Hawaiian Islands are the result of a hotspot.

Hot Spots video clip (Quicktime, 34 seconds, 3,479K)
Take a look at how hot spots form.

[Read More About DYNAMIC EARTH]

IntroductionMelting RocksDynamic EarthJudging HazardsForecastingCoping with RiskRelated Resources

"Volcanoes" is inspired by programs from Earth Revealed.


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