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Volcanoes & Plate Tectonics CH 6 Prentice Hall p.178-181 Characteristics of Magma • The force of an eruption depends partly on the amount of gases dissolved in the magma. • The temperature and the silica content are also important factors. – Temp- partially determines whether it is thick or fluid. (Hotter- the more fluid it is.) Characteristics of Magma • Silica- a material found in magma composed of the elements silica and oxygen. (The more silica- the thicker the magma.) • High Silica- light colored lava – Rhyolite an extrusive igneous rock – Pumice spaces left in the rock by gases bubbling out. – Obsidian- cools very quickly giving a glassy texture. • Low Silica- dark colored rocks – Basalt dark colored extrusive igneous rock Types of Volcanic Eruptions • A volcano’s magma influences how the volcano erupts. • The silica content helps determine whether the volcanic eruption is quiet or explosive. Quiet Eruptions • Magma flows easily. • Dissolved gases in the magma bubble out easily. • Thin runny lava oozes from the vent and flows. • Examples • Iceland and Hawaii aa • Cooler, slow moving lava. • Forms jagged chunks of lava. Pahoehoe • Fast moving, hot lava. • Surface of lava looks like a solid mass of wrinkles, billows and ropelike coils. Explosive Eruptions • Magma is thick and sticky. • Thick magma builds up in the pipe, plugging it like a cork in a bottle. • Pressure builds up due to the gases in the magma until the volcano explodes. • The gases push the magma out with incredible force. Explosive Eruptions • Fragments: • Volcanic Ash – fine, rocky, grain of sand sized particles. • Cinders– pebble sized particles • Bombs– large pieces of rock (baseball to the size of a car) • Pyroclastic Flow • Occurs when an explosive eruption hurls out ash, cinders and bombs as well as gases. (400mph, 1,800˚F) Other types of Volcanic Activity • Hot Spring- Forms when groundwater heated by a nearby body of magma rises to the surface and collects in a natural pool. Other types of Volcanic Activity • Geyser- A fountain of water and steam that erupts from the ground. – Forms due to rising hot water and steam that become trapped underground in a narrow crack. – Builds up pressure until it sprays out of the ground. Other types of Volcanic Activity • Geothermal Energy- A clean, reliable energy source provided by water heated by magma. • Hot water can be piped into homes. • Used as a source of electricity – Steam is piped into a turbine that spins and generates electricity. Monitoring Volcanoes • Geologists are more successful with predicting a volcanic eruption. • Changes around a volcano shows signs of an eruption and allow for a short warning time. – Monitor a volcano with: • Tiltmeters and Laser ranging devices to see ground changes due to shifting magma. • Monitor escaping gases and water level in a volcano's crater lake. Monitoring Volcanoes – Monitor a volcano with: • Check the temperature of the ground water to see if the magma is heating it. • Monitor the many small earthquakes triggered by magma moving from the chamber to the pipe. – Unable to predict the strength of the eruption Volcano Hazards • Time between eruptions can span hundreds of years. (Dormant Volcano) – People may be unaware of the danger. – Mt. St Helens 123 years. Volcano Hazards • Quiet Eruptions: – Lava flows- set fire and bury everything in their path. • Explosive Eruptions: – Hot clouds of volcanic gases, cinders and bombs • Pyroclastic Flow Volcano Hazards • Volcanic Ash: – Bury towns, damage crops, and clog car engines. – Wet ash is heavy and can cause roofs to collapse. • Plane engines can fail if pulled into the engine. • Lahar: – A mud flow of caused by melted snow. • Carries ash, trees and rocks. Volcano Hazards • Pompeii