Lesson Objectives

• Describe how the locations of volcanoes are related to plate tectonics.
• Suggest why volcanoes are found at convergent and divergent plate boundaries.
• Describe how intraplate volcanoes can form.

Vocabulary

• fissure

Introduction

Volcanoes are a vibrant manifestation of plate tectonics processes. Volcanoes are common along convergent and divergent plate boundaries. Volcanoes are also found within lithospheric plates away from plate boundaries. Wherever mantle is able to melt, volcanoes may be the result.

See if you can give a geological explanation for the locations of all the volcanoes in Figure above. What is the Pacific Ring of Fire? Why are the Hawaiian volcanoes located away from any plate boundaries? What is the cause of the volcanoes along the mid-Atlantic ridge?

Volcanoes erupt because mantle rock melts. This is the first stage in creating a volcano. Remember from the chapter “Rocks” that mantle may melt if temperature rises, pressure lowers, or water is added. Be sure to think about how melting occurs in each of the following volcanic settings.

Convergent Plate Boundaries

Why does melting occur at convergent plate boundaries? The subducting plate heats up as it sinks into the mantle. Also, water is mixed in with the sediments lying on top of the subducting plate. This water lowers the melting point of the mantle material, which increases melting. Volcanoes at convergent plate boundaries are found all along the Pacific Ocean basin, primarily at the edges of the Pacific, Cocos, and Nazca plates. Trenches mark subduction zones, although only the Aleutian Trench and the Java Trench appear on the map in Figure above.

Remember your plate tectonics knowledge. Large earthquakes are extremely common along convergent plate boundaries. Since the Pacific Ocean is rimmed by convergent and transform boundaries, about 80% of all earthquakes strike around the Pacific Ocean basin (Figure below). Why are 75% of the world’s volcanoes found around the Pacific basin? Of course, these volcanoes are caused by the abundance of convergent plate boundaries around the Pacific.

A description of the Pacific Ring of Fire along western North America is a description of the plate boundaries.

• Subduction at the Middle American Trench creates volcanoes in Central America.
• The San Andreas Fault is a transform boundary.
• Subduction of the Juan de Fuca plate beneath the North American plate creates the Cascade volcanoes.
• Subduction of the Pacific plate beneath the North American plate in the north creates the Aleutian Islands volcanoes.

The Cascades are shown on this interactive map with photos and descriptions of each of the volcanoes: http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/interactive/6.Volcanoes4Rollover.swf.

This incredible explosive eruption of Mount Vesuvius in Italy in A.D. 79 is an example of a composite volcano that forms as the result of a convergent plate boundary (3f): http://www.youtube.com/watch?v=1u1Ys4m5zY4 (1:53).

Divergent plate boundaries

Why does melting occur at divergent plate boundaries? Hot mantle rock rises where the plates are moving apart. This releases pressure on the mantle, which lowers its melting temperature. Lava erupts through long cracks in the ground, or fissures.

Footage of Undersea Volcanic Eruptions is seen in National Geographic Videos, Environment Video, Habitat, Ocean section: http://video.nationalgeographic.com/video/player/environment/.

• Fantastic footage of undersea volcanic eruption is in the “Deepest Ocean Eruption Ever Filmed.”
• “Giant Undersea Volcano Revealed” explores a volcano and its life off of Indonesia.

Volcanoes erupt at mid-ocean ridges, such as the Mid-Atlantic ridge, where seafloor spreading creates new seafloor in the rift valleys. Where a hotspot is located along the ridge, such as at Iceland, volcanoes grow high enough to create islands (Figure below).

Eruptions are found at divergent plate boundaries as continents break apart. The volcanoes in Figure below are in the East African Rift between the African and Arabian plates.

Volcanic Hotspots

Although most volcanoes are found at convergent or divergent plate boundaries, intraplate volcanoes are found in the middle of a tectonic plate. Why is there melting at these locations? The Hawaiian Islands are the exposed peaks of a great chain of volcanoes that lie on the Pacific plate. These islands are in the middle of the Pacific plate. The youngest island sits directly above a column of hot rock called a mantle plume. As the plume rises through the mantle, pressure is released and mantle melts to create a hotspot (Figure below).

Earth is home to about 50 known hot spots. Most of these are in the oceans because they are better able to penetrate oceanic lithosphere to create volcanoes. The hotspots that are known beneath continents are extremely large, such as Yellowstone (Figure below).

A hot spot beneath Hawaii, the origin of the voluminous lava produced by the shield volcano Kilauea can be viewed here(3f): http://www.youtube.com/watch?v=byJp5o49IF4 (2:06).

How would you be able to tell hotspot volcanoes from island arc volcanoes? At island arcs, the volcanoes are all about the same age. By contrast, at hotspots the volcanoes are youngest at one end of the chain and oldest at the other.

Lesson Summary

• Most volcanoes are found along convergent or divergent plate boundaries.
• The Pacific Ring of Fire is the most geologically active region in the world.
• Volcanoes such as those that form the islands of Hawaii form over hotspots, which are melting zones above mantle plumes.

Review Questions

1. Why are there volcanoes along the west coast of the United States?
2. Why does melting occur at divergent plate boundaries?
3. In Figure above, explain the geologic reason for every group of volcanoes in the diagram.
4. How did the Pacific Ring of Fire get its name? Does it deserve it?
5. What is a mantle plume?
6. Suppose a new volcano suddenly formed in the middle of the United States. How might you explain what caused this volcano?

Points to Consider

• Some volcanoes are no longer active. What could cause a volcano to become extinct?
• Hot spots are still poorly understood by Earth scientists. Why do you think it’s hard to understand hotspots? What clues are there regarding these geological phenomena?
• Volcanoes have been found on Venus, Mars, and even Jupiter’s moon Io. What do you think this indicates to planetary geologists?