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Directed Reading B Section: Causes of Volcanic Eruptions Circle the letter of the best answer for each question. 1. What do scientists study to help them predict a volcanic eruption? ocean currents and temperature old volcanoes and craters vents and calderas gases and changes in a volcano’s shape THE FORMATION OF MAGMA 2. Where does magma form? in the Earth’s crust and mantle in the Earth’s coat and crust in the Earth’s oceans and rivers on the Earth’s surface 3. What causes magma to form? puttylike rock changes in temperature and pressure atoms of the mantle low temperatures Pressure and Temperature 4. What is the most common cause of magma formation? increase in temperature decrease in temperature increase in pressure decrease in pressure Copyright © by Holt, Rinehart and Winston. All rights reserved. Magma Formation in the Mantle Circle the letter of the best answer for each question. 5. Magma is less dense than the surrounding rock, so the magma slowly sinks to the bottom. rises to the surface. melts into liquid rock. moves to the side. WHERE VOLCANOES FORM 6. Why are the plate boundaries surrounding the Pacific Ocean called the Ring of Fire? There is a huge lava plateau there. There are many active volcanoes in the area. There is a large ring-shaped crater there. The shallow water is a golden color. 7. Where do most active volcanoes form? where tectonic plates join where tectonic plates separate where tectonic plates collide where tectonic plates slide past each other WHEN TECTONIC PLATES SEPARATE 8. Where do tectonic plates move away from each other? at a rift zone at a divergent boundary in the mantle rock in the Earth’s crust Copyright © by Holt, Rinehart and Winston. All rights reserved. Circle the letter of the best answer for each question. 9. What is a deep set of cracks between plates called? a divergent boundary a mantle rock a crater column a rift zone 10. Why does magma rise to the Earth’s surface? Magma is heavier than rock. Magma is less dense than the rock around it. Magma is harder than the rock around it. Magma is tricky. Mid-Ocean Ridges Form at Divergent Boundaries 11. What is the name of a mountain chain created by lava from undersea rift zones? a.divergent boundary b.tectonic plate mid-ocean ridge hot springs Copyright © by Holt, Rinehart and Winston. All rights reserved. WHEN TECTONIC PLATES COLLIDE Read the words in the box. Read the sentences. Fill in each blank with the word or phrase that best completes the sentence. mantle plumes subduction hot spot convergent boundary temperature cracks 12. The place where tectonic plates collide is called a(n) . 13. The movement of one tectonic plate under another is called . Subduction Produces Magma 14. When oceanic crust scrapes under continental crust, the and pressure increase. HOT SPOTS 15. A volcanically active place that is far from any plate boundaries is called a(n) . 16. Some scientists believe that hot spots are places above , which are columns of rising magma. 17. Some scientists believe that hot spots form along in the Earth’s crust. Copyright © by Holt, Rinehart and Winston. All rights reserved. PREDICTING VOLCANIC ERUPTIONS Read the description. Then, draw a line from the dot next to each description to the matching word. 18. probably won’t erupt again 19. will probably erupt sometime in the future 20. will probably erupt soon •• • a. active volcano b. dormant volcano c. extinct volcano Measuring Small Quakes and Volcanic Gases Circle the letter of the best answer for each question. 21. What happens just before a volcanic eruption? The number of small earthquakes increases. Lava fountains spew out all around the volcano. Gases turn to poison. There is lots of thunder and lightning. 22. What can changes in volcanic gases indicate? changes in temperature changes in the magma chamber below changes in an earthquake’s intensity changes in weather Measuring Slope and Temperature 23. What is a tiltmeter? an instrument that measures sound an instrument that measures gases an instrument that measures earthquakes an instrument that measures small changes in a volcano’s slope Copyright © by Holt, Rinehart and Winston. All rights reserved.