Plate Tectonics and Volcanic Activity

The global distribution of volcanism is not random. Most active volcanoes are located along the margins of the ocean basins— notably within the circum-Pacific belt known as the Ring of Fire. These volcanoes consist mainly of composite cones that emit volatile-rich magma having an intermediate (andesitic) composition and that occasionally produce awe-inspiring eruptions.

A second group includes the basaltic shields that emit very fluid lavas. These volcanic structures comprise most of the islands of the deep ocean basins, including the Hawaiian Islands, the Galapagos Islands, and Easter Island. In addition, this group includes many active submarine volcanoes that dot the ocean floor.

A third group includes volcanic structures that appear to be somewhat randomly distributed in the interiors of the continents. None are found in Australia nor in the eastern two-thirds of North and South America. Africa is notable because it has many potentially active volcanoes including Mount Kilimanjaro, the highest point on the continent (5895 m).

We will examine three zones of igneous activity and their relationship to plate boundaries. These active areas are located (1) along convergent plate boundaries where plates move toward each other and one sinks beneath the other; (2) along divergent plate boundaries, where plates move away from each other and new seafloor is created; and (3) areas within the plates proper that are not associated with any plate boundary.

Volcanism at a convergent plate margin results in the development of a slightly curved chain of volcanoes called a volcanic arc. These volcanic chains develop roughly parallel to the associated trench—at distances of 200 to 300 km. Volcanic arcs can be constructed on oceanic, or continental, lithosphere. Those that develop within the ocean and grow large enough for their tops to rise above the surface are labeled island archipelagos in most atlases. Geologists prefer the more descriptive term volcanic island arcs, or simply island arcs. Several young volcanic island arcs border the western Pacific basin, including the Aleutians, the Tongas, and the Marianas. Volcanism associated with convergent plate boundaries may also develop where slabs of oceanic lithosphere are subducted under continental lithosphere to produce a continental volcanic arc. The mechanisms that generate these mantle derived magmas are essentially the same as those operating at island arcs.

As for volcanism at divergent plate boundaries, one should remember that the greatest volume of magma (perhaps 60% of Earth’s total yearly output) is produced along the oceanic ridge system in association with seafloor spreading. Below the ridge axis where lithospheric plates are continually being pulled apart, the solid yet mobile mantle responds to the decrease in overburden and rises to fill the rift. Аs rock rises, it experiences a decrease in confining pressure and undergoes melting without the addition of heat. This process, called decompression melting, is the most common process by which mantle rocks melt.

Partial melting of mantle rock at spreading centers produces basaltic magma. Because this newly formed magma is less dense than the mantle rock from which it was derived, it rises and collects in reservoirs located just beneath the ridge crest. About 10% of this melt eventually migrates upward along fissures to erupt on the ocean floor. This activity continuously adds new basaltic rock to plate margins, temporarily welding them together, only to break again as spreading continues. Along some ridges, outpourings of bulbous pillow lavas build numerous small seamounts.

Task 3. Copy out all the proper nouns (geographical names) from the text in Task 2. Try to define the geographical position of all the mentioned objects on the map. Transcribe the proper nouns.

Task 4. Look at Figure 8.2, A and B. Describe and discuss:

• the three ways of the global distribution of volcanism;

• convergent plate volcanism (island arc);

• divergent plate volcanism (oceanic ridge).