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VOLCANISM: All processes associated with the discharge of magma, hot fluids, and gasses Volcano- a mountain that forms when layers of lava and volcanic ash erupt and build up. * most volcanoes on Earth are dormant (not active) *There are about 600 active volcanoes on Earth. Magma- molten (melted) rock that is located beneath the Earth’s surface. The more silica magma contains the thicker it is. Lava- magma that flows to the Earth’s surface. lava magma 1) Divergent boundaries * When 2 plates are moving apart, it creates an opening or ridge. * Magma flows into the ridge and is cooled by sea water. * pillow lava * non-explosive 2) Convergent boundaries* Magma created in the subduction zone is forced upward because the HOT magma is less dense than cooled rock of the surrounding crust. * example: Mount St. Helens in Washington State Formation of a volcano at subduction zone TWO major belts (volcanoes at convergent boundaries): 1) Circum-Pacific (Ring of Fire) * Pacific plate 2) Mediterranean Belt: * Eurasian, African, & Arabian plates 3) Hot Spots* Some areas of the mantle are hotter than others. These areas are called hot spots. * These hot spots melt rock which is forced upward to the Earth’s crust. Hot Spots- OCEANIC * As the plates move over the hot spot, many volcanic islands may form. * Example- Hawaiian Islands A chain of island volcanoes can form at a hot spot. Hot Spots- CONTINENTAL * Flood basalts form when lava flows out of long cracks in the Earth’s crust called fissures. * This lava flow results in flat plains called plateaus. * Ex: Columbia Plateau in Washington & Oregon 1)Shield volcano- * quiet eruptions * slow-moving, non-viscous lava flows out in flat layers (low silica content) * creates a broad volcano with gently sloping sides 2) Cinder Cone Volcano- * explosive eruptions that shoot lava high into the air (thick, viscous lava that has high silica content) * lava then falls back down and cools * this creates a small, steep-sided volcanic mountain (cone shape) 3) Composite volcano (stratovolcano)- * both explosive and quiet eruptions * First, an explosive period shooting up lava. Then, a quiet period of slow moving lava flows. * large, steep-sided and symmetrical built of layers of ash and lava A Size Comparison of the Three Types of Volcanoes Figure 4.14 The type of eruption is determined by the composition of magma. • • • • Temperature Pressure Presence of water Amount of silica AMOUNT OF SILICA high silica= high viscosity traps gasses EXPLOSIVE!! low silica= not viscous easy flow QUIET & NON-EXPLOSIVE!! 1) Basaltic 2) Andesitic 3) Rhyolitic The Nature of Volcanic Eruptions • Factors affecting viscosity – Lower silica content = lower viscosity or more fluid-like behavior (e.g., mafic lava such as basalt) • Dissolved gases – Gas content affects magma mobility – Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure – The violence of an eruption is related to how easily gases escape from magma The Nature of Volcanic Eruptions • Factors affecting viscosity In summary • Fluid basaltic lavas generally produce quiet eruptions • Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions Ash cloud Crater Vent Conduit Magma Chamber CRATER Bowl-shaped depression formed around the vent, Less than 1 km in diameter VS. CALDERA Larger depression, can be up to 50 km in diameter, often forms when top or sides of the volcano collapses into the magma chamber Terms to know: Tephra Pyroclastic flow Lahar Pyroclastic Materials • “Fire fragments”; Tephra • • • • • • Ash and dust – fine, glassy fragments Pumice – porous rock from “frothy” lava Lapilli – walnut-sized material Cinders – pea-sized material Blocks – hardened or cooled lava Bombs – ejected as hot lava A Volcanic Bomb Figure 4.9 left A Pyroclastic Flow Moving Rapidly Down the Forested Slopes of Mt. Unzen Figure 4.21 B Pompeii Aa: forms from cooler, thicker lava. Hardens into a rock with a rough, jagged surface A Typical Aa Flow Figure 4.6 A Pahoehoe: formed from thin, runny lava. Hardens into a rock with a smooth, rope-like surface. A Typical Pahoehoe Flow Figure 4.6 B Pillow lava: lava formed from eruptions under water Pillow Lava Figure 16.18 Active: a volcano that is erupting or shows signs that it will erupt in the near future Dormant: volcano that scientist expect to awake in the future and become active Extinct: volcano that is unlikely to erupt again