Annenberg Learner Interactives -- Volcanoes

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Eruptions at Separating Plates
Basaltic eruptions are the rule at oceanic ridges (undersea areas where plates are separating) and at hotspots where the plate above the hotspot is composed of oceanic crust.

Belknap Volcano in Oregon
is a shield volcano.

Although spectacular lava fountains and substantial cinder cones are common products of basaltic eruptions, these eruptions are normally dominated by lava flows. As is the case in Hawaii, the large volume of lava that accumulates forms wide volcanoes with gently sloping sides, known as shield volcanoes.

As basaltic magmas are normally low in volatiles and are less viscous than magmas of other compositions, volcanic eruptions that occur at these locations tend to be less dangerous than those occurring at convergent margins.

Eruptions at Colliding Plates
At subduction zones (where a plate plunges beneath another), eruptions can be very explosive. The viscous lava released at convergent margins, combined with the high volatile content, can cause pressure to build in the volcano. This causes explosive eruptions.

Eruptions tend to consist of large amounts of fragmented material released into the atmosphere. Most of the fragments fall back to Earth in the region of the eruption. Layers of erupted material build up over time, forming the magnificent, steep-sided volcanoes called stratovolcanoes (or composite volcanoes). In many regions of the world, you can be sure of the presence of a convergent margin by the line of stratovolcanoes that marks it.

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"Volcanoes" is inspired by programs from Earth Revealed.