Vocabulary 1. rift zone 2. hot spot Causes of Volcanic Eruptions Chapter 6 Section 3 p

It all begins with formation of magma and the temperature and pressure. 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. Because people are living on or near active volcanoes, scientists must closely monitor them. They study the gases coming from active volcanoes and look for slight changes in volcano’s shape.

Where Volcanoes Form p. 167 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. About 80% of active volcanoes on land form where plates collide, and about 15% form where plates separate.

When Tectonic Plates Separate p. 168 At a divergent boundary, tectonic plates move away from each other forming a rift zone. Lava that flows from undersea rift zones creates mid-ocean ridges. Most volcanic activity on Earth occurs at mid-ocean ridges.

When Tectonic Plates Collide p. 169 A convergent boundary is a place where tectonic plates collide. The oceanic plate slides under the continental plate creating a subduction zone.

Hot Spots p. 170 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. Some scientists think that hot spots are directly above columns of rising magma called mantle plumes. Other scientists think that hot spots are the result of cracks in the Earth’s crust. According to one theory, a string of volcanic islands forms as a tectonic plate passes over a mantle plume.

Predicting Volcanic Eruptions p. 170 Volcanoes are classified in three categories. Extinct volcanoes have not erupted in recorded history and probably will never erupt again. Dormant volcanoes are currently not erupting, but the record of past eruptions suggests that they may erupt again. Active volcanoes are currently erupting or show signs of erupting in the near future. Active volcano Dormant volcano Extinct volcano

Measuring Small Quakes and Volcanic Gases p. 170 Most active volcanoes produce small earthquakes as the magma within them moves upward and causes the surrounding rock to shift. Just before an eruption, the number and intensity of the earthquakes increase and the occurrence of the quakes may be continuous. Monitoring these quakes is one of the best ways to predict an eruption. Scientists also study the volume and composition of volcanic gases. The ratio of certain gases, especially that of sulfur dioxide, to carbon dioxide may be important in predicting eruptions.

Measuring Slope and Temperature p. 171 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. Scientists use different instruments to detect even small changes in the angle of a volcano’s slope. One of the newest methods for predicting volcanic eruptions includes using satellite images. These images record changes in the surface temperature and gas emissions of a volcano over time. If the site is getting hotter, the magma is probably rising!

The Volcano Song

Quiz Time! 1. Why are most volcanoes on plate boundaries? Magma tends to form at plate boundaries. 2. In a subduction zone, which plate is more dense – oceanic or continental? oceanic 3. What is the Ring of Fire? a group of volcanoes that are located on convergent plate boundaries in the Pacific Ocean

4. Which of the following words is a synonym for dormant? A. predictable B. active C. dead D. sleeping D Science Humor… In California, when a bridge falls down, it must be San Andreas Fault! A volcano is a mountain with hiccups!