Name Learning Unit 3 Volcano Review Questions This assignment is designed to assess your understanding of Unit 3 and includes some of the Questions for Review 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 dioxide 6 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 viscosity 8 What properties of felsic intermediate magmas control their eruptive behavior High water content high gas content High silicon and oxygen content high viscosity 9 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 magma 10 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 Dioxide 14 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 flows 15 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 lava 16 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 stores of 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 magma
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