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Question of the Day Question: How do we find the epicenter of an earthquake? Answer: … … … Lava & Volcanoes Types of Lava Composition • Felsic lava/magma – – – – Quartz -sic = silica = quartz = SiO2 Forms continental crust Lighter colored rocks lower density rocks Granite Feldspar Types of Lava Composition • Mafic lava/magma – – – – -fic = ferric = Fe = iron Forms oceanic crust Darker colored rocks higher density rocks Hornblende Basalt Mafic Lava • Mafic Lava – – – – Magma from the mantle rises to the surface Magma melts through oceanic crust only Very little SiO2 is picked up by the magma Erupting lava is mafic in composition Mafic Lava • At which plate boundaries does mafic lava erupt? A. B. C. D. Continental/Oceanic Convergent Oceanic/ Oceanic Convergent Continental/ Continental Convergent Divergent Mafic Lava • At which plate boundaries does mafic lava form? B. Oceanic/ Oceanic Convergent D. Divergent Felsic Lava • Felsic Lava – Magma from the mantle rises toward the surface – Magma melts through continental crust – Lots of SiO2 is picked up by the magma from the continental crust – Erupting lava is felsic in composition Felsic Lava • At which plate boundaries does felsic lava form? A.Continental/Oceanic Convergent B.Oceanic/ Oceanic Convergent C.Continental/ Continental Convergent D.Transform Felsic Lava • At which plate boundaries does felsic lava form? A.Continental/Oceanic Convergent Why is SiO2 important? • SiO2 thickens lava – Changes appearance of igneous rocks produced – Changes types of volcanoes produced – Changes the style of eruptions from those volcanoes SiO2 & Rocks • SiO2 makes igneous rocks lighter in color • Which of these rocks is felsic? A. Granite B. Basalt SiO2 & Rocks • SiO2 makes igneous rocks lighter in color • Which of these rocks is felsic? A. Granite- FELSIC B. Basalt- MAFIC SiO2 & Rocks • Since felsic lava is formed from continental crust, felsic rocks will be _______________ than mafic rocks A. B. C. D. Denser Less dense Heavier Darker SiO2 & Rocks • Since felsic lava is formed from continental crust, felsic rocks will be _______________ than mafic rocks A. B. C. D. Denser Less dense Heavier Darker Properties of Felsic Lava • • • Very thick, slow-moving (high viscosity) Lava cools before it moves very far Volcanoes are A. Narrow with steep sides B. Wide with shallow sloping sides Properties of Felsic Lava • • • Very thick, slow-moving (high viscosity) Lava cools before it moves very far Volcanoes are A. Narrow with steep sides B. Wide with shallow sloping sides Properties of Felsic Lava • High viscosity makes it difficult for volcanic gases (CO2,SO2, H2O) to escape • Trapped gases cause eruptions to be more explosive Mt. Pinatubo 1991 Composite Cone/Stratovolcano • Up to 22,000 feet tall • steep-sided, symmetrical cones • Eruptions are extremely explosive Mount Fuji , Japan Composite Cone/Stratovolcano • alternating layers of lava flows, volcanic ash, and rocks. • Created from felsic lava Volcanic Dome/ Lava Dome • Rebuilding Stratovolcano • Steep sides because lava cools quickly, before spreading out • Created by felsic lava • Eruptions are extremely explosive Mt. St. Helens, WA (before and after) Properties of Mafic Lava • • • • • Thin, fast- moving lava/magma (low viscosity) Lava spreads out and cools Produces wide volcanoes Volcanic gases escape easily Eruptions are _______________ A. Explosive B. Not very explosive Properties of Mafic Lava • • • • • Thin, fast- moving lava/magma (low viscosity) Lava spreads out and cools Produces wide volcanoes Volcanic gases escape easily Eruptions are _______________ B. Not very explosive Shield Volcano • broad, gentle slopes • Created by fluid mafic lava that spreads out quickly before cooling • largest volcanoes on Earth Kilauea, Hawaii Mauna Loa, Hawaii What about hot spots? • Hawaii – Magma melts through oceanic crust so lava is A. Mafic B. Felsic What about hot spots? • Hawaii – Magma melts through oceanic crust so lava is A. Mafic What about hot spots? • Hawaii – Magma melts through oceanic crust so lava is A. Mafic • Yellowstone – Magma melts through continental crust so lava is A. Mafic B. Felsic What about hot spots? • Hawaii – Magma melts through oceanic crust so lava is A. Mafic • Yellowstone – Magma melts through continental crust so lava is B. Felsic • It all depends on the type of crust it’s under!
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