Chapter 6 Volcanoes • You know that if you want to open a bottle of soda, you must do so carefully. Otherwise, the soda might spray out of the bottle as soon as you loosen the cap. What causes the soda to rush out with such force? How is this similar to what happens when a volcano erupts? Explain. What causes volcanoes, and how do they change Earth’s surface?

Chapter 6 Volcanoes Unfamiliar words Hot spot is the subject of the sentence. Definition, follows boldface Explanation Example Use Clues to Determine Meaning Some volcanoes result from hot spots in Earthís mantle. A hot spot is an area where material from within the mantle rises and then melts. A volcano forms above a hot spot when the hot material erupts through the crust and reaches the surface. A hot spot in the ocean oor can gradually form a series of volcanic mountains. For example, the Hawaiian Islands formed one by one over millions of years as the Pacific plate drifted over a hot spot.

Chapter 6 Volcanoes Apply It! Review the clues to the meaning of hot spot. Then complete the following. 1. What clue tells you that hot spot might be followed by a definition? The term is in boldface and is the subject of the sentence. 2. What example helps you understand hot spots? The Hawaiian Islands formed one by one over millions of years as the Pacific plate drifted over a hot spot

Chapter 6 Volcanoes End of Chapter Preview

Warm-Up • Describe what happens along a fault beneath Earth’s surface when an earthquake occurs?

Warm-Up • How do volcanoes change the land?

Volcanoes

Chapter 6 Volcanoes Section 1:Volcanoes and Plate Tectonics Where are most of Earth’s volcanoes found? How do hot spot volcanoes form?

A volcano is a weak spot in the crust where molten material, or magma, comes to the surface.

Magma is molten material that has not reached the surface and is still underground.

Lava is molten material, or magma, that has reached the surface.

Ring of Fire is a major volcanic belt that rims or circles the Pacific Ocean.

Island Arc is a string of islands formed by weak spots or hot spots in the crust where magma burns through.

Chapter 6 Volcanoes Volcanoes and Plate Boundaries • Volcanic belts form along the boundaries of Earth’s plates.

Chapter 6 Volcanoes Volcanoes and Plate Boundaries • Volcanoes often form where two oceanic plates collide or where an oceanic plate collides with a continental plate. In both situations, an oceanic plate sinks through a trench. Rock above the plate melts to form magma, which then erupts to the surface as lava.

Chapter 6 Volcanoes Hot Spot Volcanoes • A volcano forms above a hot spot when magma erupts through the crust and reaches the surface.

Chapter 6 Volcanoes More on Volcanoes • Click the PHSchool.com button for an activityabout volcanoes.

Chapter 6 Volcanoes End of Section:Volcanoes and Plate Tectonics

Warm-Up • What causes volcanoes to form at a spreading boundary?

Warm-Up • Where are most of Earth’s volcanoes found? Explain.

Warm-Up • Describe the order of parts through which magma travels as it moves to the surface.

Warm-Up • Which is more likely to be dangerous – a volcano that erupts frequently or a volcano that has been inactive for a hundred years? Why?

Chapter 6 Volcanoes Section 2:Volcanic Eruptions What happens when a volcano erupts? What are the two types of volcanic eruptions? What are a volcano’s stages of activity?

Volcanic Eruptions

Chapter 6 Volcanoes Magma Reaches Earth’s Surface • When a volcano erupts, the force of the expanding gases pushes magma from the magma chamber through the pipe until it flows or explodes out of the vent.

Chapter 6 Volcanoes Magma Reaches Earth’s Surface • Magma chamber is a pocket beneath a volcano where magma is collected or stored.

Chapter 6 Volcanoes Magma Reaches Earth’s Surface • Pipe is a long tube inside a volcano that connects the magma chamber to the Earth’s surface.

Chapter 6 Volcanoes Magma Reaches Earth’s Surface • Vent is an opening through which molten rock and gas leave the volcano.

Chapter 6 Volcanoes Magma Reaches Earth’s Surface • Lava flow is the area covered by lava as it pours out of a vent.

Chapter 6 Volcanoes Magma Reaches Earth’s Surface • Crater is a bowl-shaped area that may form at the top of a volcano.

Chapter 6 Volcanoes Magma Reaches Earth’s Surface • Pahoehoe is a very thin, fast moving lava. Pahoehoe contains low amounts of silica, which gives it it’s thinness. When Pahoehoe hardens, it forms wrinkle and rope-like rock formations. Pahoehoe is hotter than Aa.

Chapter 6 Volcanoes Magma Reaches Earth’s Surface • Aa is a very thick, slow moving lava. Aa contains high amounts of silica, which gives it it’s thickness. When aa hardens, it forms a rough surface consisting of jagged (sharp) lava chunks.

Chapter 6 Volcanoes Magma Reaches Earth’s Surface • Pyroclastic flow is an explosive eruption that includes hot gases, cinders (volcanic ash), and bombs (volcanic rocks).

Chapter 6 Volcanoes Composite Volcano Eruption Activity • Click the Active Art button to open a browser window and access Active Art about composite volcano eruption.

Chapter 6 Volcanoes Magma Composition • Magma varies in composition and is classified according to the amount of silica it contains. The graphs show the average composition of the two types of magma. Use the graphs to answer the questions.

Chapter 6 Volcanoes Silica, other oxides, and other solids. Reading Graphs: Study both graphs. What materials make up both types of magma? Magma Composition

Chapter 6 Volcanoes Rhyolite-forming magma; about 70 percent. Reading Graphs: Which type of magma has more silica? About how much silica does this type of magma contain? Magma Composition

Chapter 6 Volcanoes About 60 percent Estimating: A third type of magma has a silica content that is halfway between that of the other two types. About how much silica does this type of magma contain? Magma Composition

Chapter 6 Volcanoes Rhyolite-forming magma would have higher viscosity because it is higher in silica. Predicting: What type of magma would have a higher viscosity? Explain. Magma Composition

Chapter 6 Volcanoes Kinds of Volcanic Eruptions • Within the last 150 years, major volcanic eruptions have greatly affected the land and people around them.

Deadly Eruptions

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