Mafic magma rising from the subducting oceanic slab heats the felsic continental crust causing the continental crust to melt to form the felsic (rhyolitic) magma.Amount of water (more water=more gas and therefore more violent eruptions)Viscosity of the magma (higher silica and oxygen content = higher viscosity and therefore more violent eruptions)Low water content = low gas contentLow silicon and oxygen content = low viscosityHigh water content = high gas contentHigh silicon and oxygen content = high viscosityThe intense lateral blast at the beginning of the eruption created a pyroclastic flow that blew them all down with their tops pointing away from the site of the blast.Lahars (or mudflows). Lahars are so dangerous because they can occur at any time. They flow down valleys where people live at high speed and are as dense as wet concrete.A shield volcano of mafic lava flowsA stratovolcano of felsic/intermediate ash and lavaThe mafic flows of shield volcanoes have low viscosity so the lava travels great distances forming gentle slopes; the felsic/intermediate magma of stratovolcanoes is much more viscous resulting in violent eruptions of ash which piles up around the central vent followed by viscous lava flows which ‘glue’ the ash together forming a steep sided volcano..Name: _______________________Learning Unit 3: Volcano Review QuestionsThis assignment is designed to assess your understanding of Unit 3 and includes some of the Questions forReview at the end of Chapters 6 and 7 from your text plus a few additional questions. Each question can be answered in one to two sentences. Please limit yourself to a maximum of three sentences. Access the assignment, complete it with ANSWERS IN A DIFFERENT COLOR FONT as a separate file, and send it back for evaluation and grading through the assignment tab by or before the due date. 1. In what tectonic environments do volcanic eruptions typically occur?- Divergent boundaries including mid-ocean ridges and rift valleys.- Convergent boundaries where subduction of oceanic lithosphere is occurring.- Over hot-spot mantle plumes.2. What changes in heat and pressure are necessary to melt rock and create magma?Increasing heat (i.e. at subduction zones) or decreasing pressure (i.e. at divergent boundaries) will cause rock to melt to create magma.3. How does felsic (rhyolite) magma form in the line of arc volcanoes like the Cascades?Mafic magma rising from the subducting oceanic slab heats the felsic continental crust causing the continental crust to melt to form the felsic (rhyolitic) magma.4. How does lava differ from magma?Lava is magma that erupts onto the Earth’s surface.5. What are the two most-abundant gases in magmas? Water vapor and carbon dioxide6. What factors influence the violence or style of an eruption? - Amount of water (more water=more gas and therefore more violent eruptions)- Viscosity of the magma (higher silica and oxygen content = higher viscosity and therefore more violent eruptions)7. What properties of mafic magma control its eruptive behavior?- Low water content = low gas content- Low silicon and oxygen content = low viscosity8. What properties of felsic/intermediate magmas control their eruptive behavior?- High water content = high gas content- High silicon and oxygen content = high viscosity9. What causes differences in viscosity among the major magma types?Different compositions (different percentages of silica). The higher the percentage of silica, the more viscous the magma10. If you visit Mount St. Helens, Washington, you will see thousands of trees lying on the ground, all parallel to one another. Explain how they got that way.The intense lateral blast at the beginning of the eruption created a pyroclastic flow that blew them all down with their tops pointing away from the site of the blast.11. Which of the hazards of volcanoes kill more people than anything else? Why are they so dangerous?Lahars (or mudflows). Lahars are so dangerous because they can occur at any time. They flow down valleys where people live at high speed and are as dense as wet concrete.12. Why do landslides sometimes trigger explosive eruptions?Removal of load by landsliding decreases the pressure on the gases in the magma and causes them to expand rapidly and explosively.13. Which gas is heavier than air, collects in low areas, and can asphyxiate people or animals if it is in high concentrations?Carbon Dioxide14. An extremely large volcano with very gentle slopes is likely to be what kind of volcano and is likely to have originated from what composition magma (felsic, intermediate, mafic)? A shield volcano of mafic lava flows15. A large, steep-sided volcano is likely to be what kind of volcano and is likely to have originated from what composition magma (felsic, intermediate, mafic)? A stratovolcano of felsic/intermediate ash and lava16. Why do stratovolcanoes have steeply sloping sides but shield volcanoes have very gently sloping sides?The mafic flows of shield volcanoes have low viscosity so the lava travels great distances forming gentle slopes; the felsic/intermediate magma of stratovolcanoes is much more viscous resulting in violent eruptions of ash which piles up around the central vent followed by viscous lava flows which ‘glue’ the ash together forming a steep sided volcano..17. What is the driving force behind the explosive activity of a cinder cone? Water in the ground boils to steam. Expanding steam blows basalt cinders out of the vent.18. Yellowstone Park has two huge calderas, each more than 20 km across. How do such calderas form?Explosive supervolcanic eruptions. The caldera is formed by the collapse of land following the volcanic eruption. Supervolcanic eruptions sometimes empty their storesof magma so swiftly that they cause the overlying land to collapse into the emptied magma chamber, forming a geographic depression called a caldera.19. What signs (precursors) suggest that a volcano may be preparing to erupt?Some precursors to an impending eruption include numerous small earthquakes (harmonic tremors), steam blasts, small ash eruptions, opening fractures, increased groundwater temperatures, growth of a bulge, increased gas emissions.20. Harmonic tremors recorded on seismographs near volcanoes are generally interpreted as:Moving
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