Transcript File
Section 1 Nonexplosive Eruptions • Slow-Flowing Lava Nonexplosive eruptions are the most common type of eruption. These eruptions produce relatively calm flows of lava. Explosive Eruptions • Thrown Debris During an explosive eruption, clouds of hot debris, ash, and gas rapidly shoot out from a volcano. Instead of producing lava flows, explosive eruptions cause molten rock to be blown into tiny End of Slide particles that harden in the air. Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 1 Volcanic Eruptions What Is Inside a Volcano? • Magma Chamber A magma chamber is a body of molten rock deep underground that feeds a volcano. Magma rises from the magma chamber through cracks in the Earth’s crust to openings called vents. End of Slide Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 1 Volcanic Eruptions What Makes Up Magma? • Water and Magma Are an Explosive Combination If the water content of magma is high, an explosive eruption is more likely. • Silica-Rich Magma Traps Explosive Gases Magma that has a high silica content also tends to cause explosive End of Slide eruptions. Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 1 What Erupts from a Volcano? • Types of Lava The viscosity of lava, or how lava flows, varies greatly. The next slide shows four different types of lava. • Types of Pyroclastic Material Pyroclastic material forms when magma explodes from a volcano and solidifies in the air. The size of pyroclastic material ranges from boulders that are the size of houses to tiny particles. • Pyroclastic Flows Pyroclastic flows are produced when enormous amounts of hot ash, dust, and gases are ejected from a volcano.End of Slide Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 1 Four Types of Lava Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 2 Effects of Volcanic Eruptions Science Journal Entry Look through this section, and write a definition for the following terms: shield volcano, cinder cone volcano, composite volcano, volcanic crater, and caldera. Record your definitions in your science journal. Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 2 Effects of Volcanic Eruptions Volcanic Eruptions and Climate Change • Change in Temperature As volcanic ash and gases spread throughout the atmosphere, they can block enough sunlight to cause global temperatures to drop. Such a change in temperature can disrupt climates all over the world. End of Slide Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 2 Different Types of Volcanoes • Shield Volcanoes Shield volcanoes are built of layers of lava released from repeated nonexplosive eruptions. • Cinder Cone Volcanoes Cinder cone volcanoes are made of pyroclastic material usually produced from moderately explosive eruptions. • Composite Volcanoes Composite volcanoes, sometimes called stratovolcanoes, are one of the most common types of volcanoes. They form from explosive eruptions of pyroclastic material followed End of Slide by quieter flows of lava. Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 2 Effects of Volcanic Eruptions Other Types of Volcanic Landforms • Craters Around the central vent at the top of many volcanoes is a funnel-shaped pit called a crater. • Calderas A caldera is a large, semicircular depression that forms when the chamber that supplies magma to a volcano partially empties and the chamber’s roof collapses. The formation of a caldera is illustrated on the next slide. • Lava Plateaus A landform that results from repeated eruptions of lava spread over a large area is called a lava plateau. End of Slide Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 2 The Formation of a Caldera Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 3 Causes of Volcanic Eruptions Science Journal Entry Imagine you live on a volcanic island. List the signals that would tell you the volcano was about to erupt. Record your responses in your science journal. Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 3 Causes of Volcanic Eruptions The Formation of Magma • Pressure and Temperature 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. End of Slide Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 3 Causes of Volcanic Eruptions Where Volcanoes Form • Location The locations of volcanoes give clues about how volcanoes form. The next slide shows the locations of major volcanoes around the world. • Plate Boundaries About 80% of active volcanoes on land form where plates collide, and about 15% form where plates separate. End of Slide Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 3 The Location of Major Volcanoes Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 3 Causes of Volcanic Eruptions When Tectonic Plates Separate • Divergent Boundaries At a divergent boundary, tectonic plates move away from each other. • Mid-Ocean Ridges Form at Divergent Boundaries Most volcanic activity on Earth occurs at mid-ocean ridges. The next slide shows how magma forms at divergent boundaries such as those found along mid-ocean ridges. End of Slide Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 3 How Magma Forms at a Divergent Boundary Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 3 Causes of Volcanic Eruptions When Tectonic Plates Collide • Convergent Boundary A convergent boundary is a place where tectonic plates collide. • Subduction Produces Magma As the descending oceanic crust scrapes past the continental crust, the temperature and pressure increase. This causes the water contained in the oceanic crust to be released. The water mixes with the mantle rock, which lowers the rock’s melting point, causing it to melt into magma. This process is illustrated on the next slide. End of Slide Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 3 How Magma Forms at a Convergent Boundary Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 3 Causes of Volcanic Eruptions Hot Spots • What Are Hot Spots? Hot spots are volcanically active places on the Earth’s surface that are far from plate boundaries. • What Do Hot Spots Form? A hot spot often produces a long chain of volcanoes. •How do hot spots Form? A hot spot forms from Mantle Plumes, or areas of the mantle that are hotter then others End of Slide Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 3 Causes of Volcanic Eruptions Predicting Volcanic Eruptions • Measuring Small Quakes and Volcanic Gases Most active volcanoes produce small earthquakes as the magma within them moves upward. Also the ratio of certain gases 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. Also, infrared satellite images record changes in the surface temperature. If the site is getting hotter, the magma below is probably rising. End of Slide Copyright © by Holt, Rinehart and Winston. All rights reserved.