Study guide chapter 9 1. What are the three ways solid mantle material can change phase to a liquid? Associate a boundary/hot spot to each way mantle material changes phase. 1. A decrease in pressure which lowers the melting point of the asthenosphere; along rift valleys (divergent plates) where lithosphere is thinner 2. An increase in temperature; usually at hotspots 3. An increase in water in the asthenosphere; at subduction zones (convergent boundaries) 2. Be able to define: Aa Caldera Cinder cone Composite volcano Hot spot Lahar Lava Lava plateau Pahoehoe Pillow lava Pyroclastic flow Pyroclastic material Shield volcano Viscosity Volcano
3. Where at subduction boundaries do volcanos form? Explain how and why subduction occurs. Volcanoes form on the overriding plate of the subduction boundary. 4. Explain the difference between the formation of an island arc and coastal volcanic mountains. In an oceanic-continental collision, volcanoes form on the continental plate creating coastal volcanic mountains. o Example: Cascade Range along the Pacific coast In oceanic-oceanic collisions, volcanoes form in a chain of volcanic islands called a volcanic arc. o Example: Mariana Islands in Pacific Ocean
5. Explain what rifts are and at which boundaries they occur. Rifts are cracks or openings in Earth s crust. They may form rift valleys at points where lithospheric plates are moving apart, such as at a mid-ocean ridge. 6. What is a divergent boundary? Divergent boundary a boundary between two lithospheric plates that are moving apart How does a divergent boundary form a mid-ocean ridge? Magma rises from deeper hotter regions; Earth s mantle Rifts cause lower pressure which lowers melting temperatures Large amounts of magma form Less dense magma rises Most magma reaches earth s surface at divergent boundaries Example: mid-ocean ridge that rises above sea level in Iceland What type of volcanic activity is associated with divergent boundaries at mid ocean ridges? Usually submarine volcanoes (deep sea vents are one example) Example: mid-ocean ridge that rises above sea level in Iceland Mafic magma Non-explosive
7. How is a hot spot formed? Hot spots areas of volcanic activity resulting from plumes of hot solid materials that rise from deep within the Earth s mantle; as the material rises it melts at areas of lower pressure How do hot spots create island chains? Magma rises at areas of low pressure Hot spot remains stationary as areas of lithospheric plate moves across it Hawaiian Islands have formed as pacific plate has moved northwest over the hot spot What is an example of a hot spot volcano? Hawaii currently lies over the hot spot New island formation 30 km off the coast of Hawaii, Loihi Seamount in 10,000 years it will reach sea level and become the newest island 8. Explain viscosity. Resistance to flow (or stickiness ) Viscosity increases with silica content of lava Viscosity increases the explosively of volcanoes due to trapped gas build up
9. Be able to list and describe the three types of magma and their characteristics: Type of Magma 1) Basaltic 2) Andesitic 3) Rhyolitic Silica Content 50% 60% 70% Gas Content Least Intermediate Most Viscosity Least Intermediate Most Type of Eruption Rarely explosive; quiet Sometimes explosive Usually explosive Melting Temperature Highest Intermediate Lowest Location (type of boundary) Rifts & Oceanic Hot Spots Subduction boundaries Continental hot spots 10. List and describe the three types of lava flows. 1. Pahoehoe lava flows occur on land and when hardened, have a smooth ropy surface.
2. Aa lava flows occur on land and when hardened, have a rough, jagged surface. 3. Pillow lava forms at underwater volcanic vents and has small, rounded lumps. 11. Describe pyroclastic flows and the materials associated with them. After a violent eruption, a dense, super-heated cloud travels downhill at amazing speed. Pyroclastic flows contain pyroclastic material: Ash: smallest, <2mm Lapilli: 2-64 mm Bombs: >64mm Gases: dense superheated cloud, can travel at 100km/hr
12. Describe the three common types of volcanoes, how their types of magma affect their size and shape. Give examples of each type of volcano: Volcano Type Magma Size & Shape Example Shield (form at hot spots; divergent boundaries) Basaltic (thin); not explosive Broad base with gentle sloping sides Mauna Loa, Hawaii Cinder cone Composite (form at convergent subduction boundaries) Rhyolitic (fragments of hardened lava); explosive Usually andesitic (but not always); quiet and explosive; when explosive they release lots of pyroclastic material Small with steep slopes; cone-shaped; smaller than other volcanoes; form on the sides of larger volcanoes Mountain shaped; Alternating layers of volcanic ash and lava Capulin, NM Paricutin, Mexico Mt. St. Helen, Mt. Shasta, Mt. Fuji; Andes Mountains of South America 13. Explain the formation of a caldera. After a large composite volcano erupts, the underground magma chamber is partially emptied, creating a hollow cavity in the top of the volcano. The volcanic cone collapses inward, forming a basin, or caldera. Over time, rainwater may fill the caldera, creating a lake. Example: Crater Lake, Oregon 14. Explain the formation of a lava plateau. Lava plateaus form when basaltic lava pours out of fissures in Earth s surface. The lava spreads across great distances before cooling, forming a large plateau. Example: Columbia Plateau in NW US
Things to Know: What is special about Iceland s mid-ocean ridge? What type of eruptions release the most pyroclastic flow? The least? Magma vs. Lava Why does magma rise to tue surface? Where does most magma reach earth s surface? Be able to label magma descriptions with magma type (andesitic, basaltic, rhyolitic). Be abe to label volcano descriptions with volcano type (composite, shield, cinder). Completely understand how a chain of islands forms over a volcanic hot spot. Know why (conditions) and where (locations) volcanoes occur.