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Understanding Volcanic Eruptions: Magma Composition and Effects

This article explains the different types of magma and how they influence volcanic eruptions, including factors such as temperature, pressure, dissolved gases, and viscosity. It covers basaltic, andesitic, and rhyolitic magma types, as well as the explosive nature of certain eruptions and the hazards associated with tephra and pyroclastic flows.

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Understanding Volcanic Eruptions: Magma Composition and Effects

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  1. Section 18.2 - Eruptions 8th Grade Earth and Space Science Class Notes

  2. Making Magma • The type of eruption depends on the composition of the magma. • Temperature – Most rocks melt between 800-1200 degrees C; temperature increases with depth; pressure and water affect magma formation • Pressure – Pressure increases with depth; melting points of rocks increases with depth

  3. Composition of Magma • Explosivity – how a volcano erupts and how its magma flows (ultimately determined by the make-up of the magma) • Factors - • Interaction with overlying crust • Temperature • Pressure • Dissolved gases • Silica content

  4. Dissolved Gases • As the amount of gases increases, the magma’s explosivity increases • Important gases: • Carbon dioxide • Water vapor (most common) • Sulfur dioxide • Hydrogen sulfide

  5. Viscosity • Viscosity – physical property that describes a material’s resistance to flow • Cooler magma = higher viscosity • High silica = higher viscosity • Tends to trap gases and produces explosive eruptions

  6. Types of Magma • The silica content of magma determines not only its explosivity and viscosity, but also which type of volcanic rock it forms as it cools • Three types: • Basaltic • Andesitic • Rhyolitic

  7. Basaltic Magma • Usually forms from rock in the upper mantle • Less than 50% silica – low viscosity • Gases escape easily • Quiet eruptions • Examples – Mauna Loa, Kilauea, Surtsey

  8. Basaltic Magma Basaltic lava flow from Kilauea in Hawaii

  9. Andesitic Magma • 50-60% silica • Found along oceanic-continental subduction zones • Forms from oceanic crust or oceanic sediments • Intermediate viscosity • Intermediate explosivity • Examples – Colima, Tambora

  10. Andesitic Magma Andesitic lava flow from Washburn in Yellowstone

  11. Rhyolitic Magma • Molten material that rises and is mixed with continental crust (rich in water and silica) • More than 60% silica • High viscosity • Large amount of trapped gases • Very explosive • Example – Yellowstone National Park

  12. Rhyolitic Magma

  13. Explosive Eruptions • When lava is too viscous to flow freely from the vent, pressure builds up in the lava until the volcano explodes • Tephra – erupted materials given off by the volcano; can be pieces of solidified lava or pieces of crust

  14. Tephra • Classified by size • Smallest – ash • Can rise very far in the air • Threatens aircrafts • Can affect weather • Largest – blocks • Can be as large as a car

  15. Pyroclastic Flows • Pyroclastic flow – rapidly moving clouds of tephra mixed with hot, suffocating gases • Can reach temperatures of 700 degrees C • Can move at more than 200 km/h

  16. Pyroclastic Flows

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