A Little Background

If you were to cut away a section of the Earth, you would see layers. The uppermost layer is known as the lithosphere, and includes the rocky parts of Earth’s surface. This layer is up to 100 km (60 mi) deep and is divided into the crust and upper mantle. The upper part of the crust is made of igneous, sedimentary and metamorphic rocks; this makes up the continents.The ocean floors are primarily made of igneous rocks covered with sediments. Everything we are able to see directly from the Earth’s surface is really the lithosphere: all rocks, soil, formations—even deep canyons like the Grand Canyon or the Marianas Trench only cut a few kilometers into the lithosphere.

The remaining layers are known as the mantle and the core. Rock material in the mantle exists in a state which acts much like a solid but also flows like plastic putty under pressure. The mantle helps to convect heat from the molten outer core which is composed primarily of iron. In the inner core, the iron has been compressed under intense gravity and is so dense it is solid.

Plates and hot spots

Most volcanoes exist at the boundaries of the enormous, rigid tectonic “plates” that make up the Earth's lithosphere. These plates move very, very slowly (a few centimeters a year) in random directions over Earth's surface, “floating” on the semi-liquid mantle. Plate movement is in response to the changing conditions in the mantle, where the semi-liquid rock material is convecting heat away from the outer core. Plate movement is also responsible for causing earthquakes and forming new mountain ridges.

In some places the plates "crash" into each other (converging), in other places they are drifting apart (diverging), and in still other places they grind and slide past each other like two cars sideswiping each other. (See Illustration below)

Close to eighty percent of the volcanoes on large bodies of land are found where two tectonic plates converge and one of the plates is forced downward into the hot mantle. When this happens, the temperature and pressure increases, eventually causing some of the rocks in the plate to melt and form magma. The magma formed in these regions, called subduction zones, makes its way to the surface and forms volcanoes.

Where subduction occurs under a continent, a string of volcanoes may form inland over the top of the subduction zone. The Cascade volcanoes in the American northwest are an example. Where subduction occurs in oceanic regions, a volcanic island arc forms. The islands of Japan and the Aleutian Islands are examples. A vast circle of volcanic activity, nicknamed the "ring of fire," surrounds the Pacific plate where it subducts under Asia, Australia, and the Americas. A subduction zone at the eastern end of the Caribbean Sea gave rise to the volcanoes that formed the Lesser Antilles, including the island of Martinique. (See the “Puerto Rico Trench in the illustration below.) You may recall the earlier illustration which showed the volcanic ridges in the Caribbean formed by the North American plate converging on the Caribbean plate and sliding underneath to create a subduction zone.

A few volcanoes occur on top of "hot spots" far from the edges of plates. In these spots, columns of hot material from the crust and mantle convect upwards and "burn through" the surface to form volcanoes. As continental plates move over hot spots, the hot material periodically burns through the plate to form a new volcano. The result is a chain of progressively older volcanoes marking the direction of the movement of the plate over the hot spot. The Hawaiian Islands and the geyser activity of Yellowstone National Park were formed in this way.

Of gas and viscosity

When a once sleeping volcano returns to life, the first signs of awakening are often clusters of small earthquakes. These tremors are caused by the magma forcing its way upwards into crevices, cracking the brittle surface rocks, and shoving them aside. Whether there are explosions or slow releases of lava over time depends mostly on two factors: the magma's "viscosity," and the amount of dissolved gases in it. If the viscosity is low, or syrupy, due to lower amounts of silica, the magma flows quickly downhill and forms runny lava flows. If the viscosity is high like toothpaste, due to higher amounts of silica, the magma oozes slowly onto the surface and forms a thick, stubby dome. If the magma contains large amounts of dissolved gases, like steam, the volcano erupts like a bottle of carbonated soda that has been shaken and opened quickly. As the gas-charged magma nears the surface, the pressure drops, and the gas comes out of the super-heated liquid in the form of bubbles that expand so rapidly they create explosions.

Scientists use the word "effusive" to describe volcanoes with flowing magma. The gassy, explosive volcanoes are described as, you guessed it, "explosive."

Review Questions

1. How would a scientist explain the presence of sedimentary rock in the Earth's mantle? What does this have to do with subduction?
2. Tectonic plates move very, very slowly. Explain why they move.
3. Explain how the eruption of molten rock onto Earth's surface is part of the rock cycle.