Chapter 9 Causes of Volcanic Eruptions What You

Chapter 9: Causes of Volcanic Eruptions

What You Will Learn • Describe the formation and movement of magma. • Explain the relationship between volcanoes and plate tectonics. • Summarize the methods scientists use to predict volcanic eruptions.

• Today, even more people are living on and near active volcanoes. Scientists closely monitor volcanoes to avoid this type of disaster. • They study the gases coming from active volcanoes and look for slight changes in the volcano’s shape that could indicate that an eruption is near • Scientists know much more about the causes of eruptions than the ancient Pompeiians did, but there is much more to be discovered.

The Formation of Magma • Understanding how magma forms helps explain why volcanoes erupt. • Magma forms in the deeper regions of the Earth’s crust and in the uppermost layers of the mantle where the temperature and pressure are very high. • Changes in pressure and temperature cause magma to form.

Pressure and Temperature • Part of the upper mantle is made of very hot, puttylike rock that flows slowly. • The rock of the mantle is hot enough to melt at Earth’s surface, but it remains a puttylike solid because of pressure. • This pressure is caused by the weight of the rock above the mantle. • In other words, the rock above the mantle presses the atoms of the mantle so close together that the rock cannot melt. • Rock melts when its temperature increases or when the pressure on the rock decreases.

Magma Formation in the Mantle • Because the temperature of the mantle is fairly constant, a decrease in pressure is the most common cause of magma formation. • Magma often forms at the boundary between separating tectonic plates, where pressure is decreased. • Once formed, the magma is less dense than the surrounding rock, so the magma slowly rises toward the surface like an air bubble in a jar of honey.

Where Volcanoes Form • The locations of volcanoes give clues about how volcanoes form. • A large number of volcanoes lie directly on tectonic plate boundaries. • In fact, the plate boundaries surrounding the Pacific Ocean have so many volcanoes that the area is called the Ring of Fire.

Plate Boundaries • Tectonic plate boundaries areas where tectonic plates either collide, separate, or slide past one another. • At these boundaries, it is possible for magma to form and travel to the surface. • About 80% of active volcanoes on land form where plates collide, and about 15% form where plates separate. • The remaining few occur far from tectonic plate boundaries.

When Tectonic Plates Separate • At a divergent boundary, tectonic plates move away from each other. • As tectonic plates separate, a set of deep cracks called a rift zone forms between the plates. • Mantle rock then rises to fill in the gap. When mantle rock gets closer to the surface, the pressure decreases. • The pressure decrease causes the mantle rock to melt and form magma. • Because magma is less dense than the surrounding rock, it rises through the rifts. • When the magma reaches the surface, it spills out and hardens, creating new crust

When Tectonic Plates Collide • If you slide two pieces of notebook paper into one another on a flat desktop, the papers will either buckle upward or one piece of paper will move under the other. • This is similar to what happens at a convergent boundary. A convergent boundary is a place where tectonic plates collide. • When an oceanic plate collides with a continental plate, the oceanic plate usually slides underneath the continental plate. • The process of subduction, the movement of one tectonic plate underneath another, is shown in Figure 4. • Oceanic crust is subducted because it is denser and thinner than continental crust.

Hot Spots • Not all magma develops along tectonic plate boundaries. • For example, the Hawaiian Islands, some of the most well-known volcanoes on Earth, are nowhere near a plate boundary. • The volcanoes of Hawaii and several other places on Earth are known as hot spots. • Hot spots are volcanically active places on the Earth’s surface that are far from plate boundaries.

Measuring Small Quakes and Volcanic Gases • Most active volcanoes produce small earthquakes as the magma within them moves upward and causes the surrounding rock to shift. • Monitoring these quakes is one of the best ways to predict an eruption. • The ratio of certain gases, especially that of sulfur dioxide, SO 2, to carbon dioxide, CO 2, may be important in predicting eruptions.

Measuring Slope and Temperature • As magma moves upward prior to an eruption, it can cause the Earth’s surface to swell. • The side of a volcano may even bulge as the magma moves upward • An instrument called a tiltmeter helps scientists detect small changes in the angle of a volcano’s slope.