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Magma formation • Magma formation is influenced by depth, pressure, temperature, and the presence or absence of water (which influences melting point). • Three major igneous rock types, basalt, andesite, and rhyolite, form three corresponding types of magma. • Different magmas have different viscosities, which influences their behavior
Basaltic magma • Forms when rocks in the upper mantle melt • Low viscosity • Rises quickly to surface and reacts little with crustal rocks • Contains small amounts of dissolved gases and silica, so erupts quietly
Andesitic magma • Found along continental margins, where oceanic crust is subducted into Earth’s mantle • Source for magma can be either oceanic crust or oceanic sediments • Contains about 60% silica (vs. 50% for basaltic magma), so is more explosive and more viscous
Rhyolitic magma • Forms when molten material rises and mixes with overlying silica- and water-rich continental crust • High viscosity inhibits its movement • Resistance to flow + lots of gas trapped inside it make it very explosive
Magma that cools within the Earth is called a PLUTON • Batholiths: the largest plutons, cover ≥ 100km2. Found in the interiors of major mountain chains. Mostly made of granite. • Stocks: irregularly-sized plutons that are smaller than batholiths • Laccoliths: mushroom-shaped plutons (rock bows upward as a result of intense heat and pressure of magma body) • Sills: plutons that form when magma intrudes parallel to layers of rock • Dikes: plutons that cut across preexisting rock
Types of Volcanoes • A shield volcano is circular, with gently sloping sides. Layers of lava accumulate during non-explosive eruptions. Thus, formed of basaltic lava • A cinder cone volcano is made of more viscous magma. Explosions rocket material high into the air. It piles up around the vent. • A composite volcano forms when layers of volcanic pieces alternate with lava. Larger than cinder cone volcanoes; potentially dangerous.
Where do volcanoes occur? • Divergent boundaries
Where do volcanoes occur? • Hot spots, either on land or at sea (more common)
What about Yellowstone? • http://www.dailymail.co.uk/sciencetech/article-1350123/Worlds-largest-volcano-Yellowstone-National-Park-wipe-thirds-US.html • Is it likely to erupt? • What kind of volcano is it? • Why would there be a volcanic eruption in the continental interior?