Scarica Vulcani ed eruzioni vulcaniche e più Appunti in PDF di Scienze della Terra solo su Docsity! VOLCANOES AND VOLCANIC ERUPTIONS Magmas and lava ● magma is a mixture of liquid rock, crystals and dissolved gas ○ basaltic: SiO2 45-55%, temp 1200° ○ intermediate: SiO2 55-65%, temp 800-1000° ○ rhyolitic: SiO2 65-75%, temp 650-800° ● gas gives magmas their explosive character → H2O, Sulfur, Cl, F ● viscosity is the resistance to flow ○ magmas with more SiO2 have higher viscosity ○ magmas with lower temperature have higher viscosity ○ basaltic magmas have lower viscosity, rhyolitic magmas have higher viscosity Lava Flows 1. Pahoehoe flows ○ high temperature, low viscosity, basaltic, low content of gas ○ starts to cool when exposed to low temperature of the atmosphere ○ forms a surface skin → expands and forms pahoehoe toes ○ can travel long distances 2. A’A’ flows ○ high viscosity, rough surface, usually andesitic ○ develop a smooth surface skin, but breaks easily 3. Pillow Lavas ○ lava erupts in a body of water, usually basaltic ○ forms surface skin and inflates ○ inflated balloons of magma drop off and stack up like a pile of pillows 4. Lava Domes ○ high viscosity, andesitic/rhyolitic, low in gas ○ lava does not flow away from the vent and piles up over the vent Volcanic landforms ● volcanoes form after an eruption and depend on the type of lava 1. Shield volcano ○ characterized by gentle upper slopes ○ composed by thin lava flows (low viscosity, basaltic) ○ low viscosity allows the lava to travel gently 2. Composite volcano ○ have steeper slopes ○ show inter-layering of lava flow and pyroclastic material ○ lava is usually andesitic or rhyolitic + explosive reactions (high viscosity), long periods of repose, which makes them more dangerous 3. Cinder cones ○ small volume cones made of ash (basaltic-andesitic) ○ form after an explosive eruption ○ are usually lateral vents on composite volcanoes 4. Craters ○ circular depressions, <1km of diameter ○ are result of explosions that emit gasses / ash 5. Calderas ○ larger depressions, 1-50 km of diameter ○ are result of a collapse of a volcanic structure Volcanic eruptions 1. magmas begin to rise because they are less dense than the surrounding solid rock 2. when the gas can no longer be held in the magma, it begins to make bubbles and grows in size a. if the liquid part of the magma has low viscosity, the gas can expand easily; effusive or non-explosive eruption occurs b. if the liquid part of the magma has high viscosity, the gas cannot expand easily; pressure builds up and it bursts explosively Effusive eruptions ● they occur with low gas content and low viscosity ● they usually start with a fire fountain ● lava flows run down slope ● if the viscosity is high, but low gas content → lava dome Explosive eruptions ● they occur with high gas content and high viscosity ● magma fragments and forms clots that fall in the air ● solid particles become pyroclasts and ash 1. Plinian eruptions ○ clouds of gas and tephra produce an eruption column ○ can rise up for distance (45km) and then falls back to the surface 2. Strombolian eruption ○ an eruption column form for only some km ○ materials will fall close to the vent and create cinder cones 3. pyroclastic flow deposits ○ when the column collapses, a pyroclastic flow is produced ○ moves very rapidly → very dangerous 4. lateral blast ○ gas pressure is directed outwards ○ the pyroclastic flow is called a glowing avalanche Lahars ● debris mixes with water = mudflow ● can occur during an eruption or afterwards ● they move at high velocity and devastate everything in their path
