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Volcanoes (part 2) Earth Science - 6th Grade; Week 16; Mrs. Rubright Review of Volcano Formation • 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. 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. Types of volcanoes Types of volcanoes • Shield volcano: usually form at hot spots, from non explosive eruptions • Cinder cone volcano: a small, cone-shaped volcano built of ash and cinders; form from explosive eruptions, very steep. • Composite volcano: a deep-sided volcano of lava, ash, and cinders; 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. • Eruption Recipe Thin magma, little gas → quiet eruption Thin magma, much gas → big eruption Thick magma, little gas → quiet eruption Thick magma, much gas → BOOM! 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 Pyroclastic materials • Solid volcanic material such as ash and rocks that is thrown into the air during an explosion. • Volcanic bombs: large blobs of magma that are shot into the air; harden in the air. • Lapilli: pebble size rocks • Volcanic ash: tiny powder like material Four types of lava • Aa: lava that has a rough surface • Pahoehoe: lava that has a smooth or billowy surface; 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: • forms thin crust and wrinkles Pillow lava • lava that erupts under water, has a round shape Stages of a Volcano • There are 3 stages to a volcano's life – Active: a volcano that has erupted recently or will erupt soon – Dormant: a volcano that has not erupted for a long time by may erupt again – Extinct: a volcano that can no longer erupt Questions • What is the difference between magma and lava? • What do you think determines how easily lava flows? • Relate the shapes of volcanoes to their explosivity. • Why do you think that many people believe all volcanoes are dangerous and explosive?