1. Chapter 9: Causes of Volcanic Eruptions

2. 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.

3. Today, even more people are living on and near active volcanoes
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.

4. 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.

5. 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.

7. 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.

8. The locations of volcanoes give clues about how volcanoes form.
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.

10. Plate Boundaries
Tectonic plate boundaries are 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.

11. 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

13. 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.

15. 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.

17. 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, SO2, to carbon dioxide, CO2, may be important in predicting eruptions.

19. 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.