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Chapter 8 Section 2 Types of Volcanoes Key Concept: Tectonic plate motions can result in volcanic activity at plate boundaries. Volcanoes How do volcanoes work? • Heat and pressure cause rocks to melt and form magma. • Magma needs to get out, too much pressure!!!!!! • Rise in temperature or drop in pressure causes magma to form faster. How do volcanoes work? • Magma is forced onto Earth’s surface. • It dries and hardens, this happens many times over thousands of years. • Eventually a mountain called a volcano is formed. Parts of a Volcano • Most volcanoes share a specific set of features. • The magma that feeds the eruptions pools deep underground in a structure called a magma chamber. • At Earth’s surface, lava is released through openings called vents. • Flowing lava in the interior travels through long, pipelike structures known as lava tubes. Where do most volcanoes occur? • Volcanoes occur at both divergent and convergent boundaries and also at hot spots. Why do we have different types of Volcanoes? • The process of magma formation is different at each type of plate boundary. • Therefore, the composition of magma differs in each tectonic setting. • Tectonic settings determine the types of volcanoes that form and the types of eruptions that take place. Volcanoes at Divergent Boundaries • At a divergent boundary, the lithosphere becomes thinner as two plates pull away from each other. • A set of deep cracks form in an area called a rift zone. • Hot mantle rock rises to fill these cracks. • As the rock rises, a decrease in pressure causes hot mantle rock to melt and form magma. • The magma that reaches Earth’s surface is called lava. Basaltic magma rises to Earth’s surface through these fissures and erupts nonexplosively Types of volcanoes Types of volcanoes • Shield volcano: usually form at hot spots, from non explosive eruptions • Cinder cone volcano: form from explosive eruptions, very steep. • Composite volcano: form from both explosive and non explosive eruptions. Shield volcano • • • • Form from many layers of “runny” lava. Very wide, not to steep. Biggest type of volcanoes Tallest mountain in the world is Mauna Kea (measures from sea floor to top) • non explosive eruptions Mauna Kea, Hawaii Cinder cone volcano • Smallest type of volcano • Most common • Made from pyroclastic material (material shot out of a volcano) • Form a large crater • Explosive! Paricutin, Mexico Composite volcano: • Eruptions alternate between explosive and non-explosive. • Sometimes they have runny lava layers, other times the have pyroclastic materials form layers. • Have a wide base and steep sides. • Have a crater • Mount Fuji Mount Fuji 2 Types of eruptions • Explosive: volcanoes that build enough pressure to blow its top, sending pyroclastic material into the air. • Non explosive: Build only enough pressure to allow lava to run down its sides. Non explosive eruption • Mafic: refers to rocks and magma rich in iron and magnesium. • This type of lava that is very runny. • As magma nears the surface there is little pressure, causing gasses escape easily. • Magma low in Silica have quiet eruptions Explosive eruptions • Felsic: means magma with high silica and feldspar content. • Felsic magma traps water and gas bubbles, which leads to lots of pressure. • Silica acts like a cork • Explosive eruptions are caused by a build up of high pressure. • Convergent zones contain lots of water, therefore have explosive eruptions. Pyroclastic materials • Material that is thrown into the air during an explosion. • Volcanic bombs: large blobs of magma that harden in the air. • Lapilli: pebble size rocks • Volcanic ash: tiny powder like material Four types of lava • Aa: lava that is thick and sharp • Pahoehoe: lava that forms thin crust and wrinkles • Pillow lava: lava that erupts under water, has a round shape • Blocky lava: cooler, lava that does not travel far from eruption, jagged when it dries. Aa • lava that is thick and sharp Pahoehoe: • lava that forms thin crust and wrinkles Pillow lava • lava that erupts under water, has a round shape Blocky lava • cooler, lava that does not travel far from eruption, jagged when it dries.