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Chapter 18 Notes

Chapter 18 Notes. Volcanism. Volcanoes. The location of volcanoes on Earth is not random. Volcanoes at Convergent Boundaries. Most volcanoes on land are at subduction zones, characterized by explosive eruptions Circum-Pacific belt (Ring of Fire; ex: Pinatubo, St. Helens)

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Chapter 18 Notes

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  1. Chapter 18 Notes Volcanism

  2. Volcanoes • The location of volcanoes on Earth is not random

  3. Volcanoes at Convergent Boundaries • Most volcanoes on land are at subduction zones, characterized by explosive eruptions • Circum-Pacific belt (Ring of Fire; ex: Pinatubo, St. Helens) • Mediterranean Belt (ex: Vesuvius, Etna)

  4. Volcanoes at Divergent Boundaries • About 2/3 of the Earth’s volcanic activity occurs underwater at divergent boundaries • Mid-ocean ridges • Non-explosive, produces large amounts of lava

  5. Hot Spot Volcanoes • Some volcanoes form over stationary magma plumes (hot spots), not at plate boundaries • As a plate moves over the hot spot, a chain of islands is formed

  6. Flood Basalts • Hot spots beneath continental crust can form flood basalts

  7. Anatomy of a Volcano • Magma travels from the magma chamber, through a conduit and lava emerges through a vent • Craters form around the vent

  8. Volcano Comparison • Appearance of a volcano depends on the type of eruptions and the type of material forming the volcano • Shield: non-explosive • Cinder: small, steep • Composite: explosive

  9. Eruptions • Characteristics of an eruption are determined by: • Temperature • Pressure • Magma composition (silica content) • Dissolved gases (think about opening a can of pop that has been shaken) • Viscosity (determined by temperature and composition)

  10. Types of Magma: Basaltic • Silica content: low (less than 50%) • Viscosity: low • Eruptions: quiet, very frequent • Example: Kilauea, Hawaii

  11. Types of Magma: Andesitic • Silica content: 50 to 60% • Viscosity: intermediate • Eruptions: intermediate explosivity • Examples: Tambora, Indonesia; Colima, Mexico

  12. Types of Magma: Rhyolitic • Silica content: high (more than 60%) • Viscosity: high, lots of dissolved gas • Eruptions: explosive, very infrequent • Example: Yellowstone

  13. Explosive Eruptions • Tephra: solid fragments of rock ejected from a volcanic vent • Ash (less than 2mm) • Bombs • Blocks • Pyroclastic flows: clouds of tephra mixed with hot gases

  14. Pyroclastic flow

  15. Intrusive Activity • Plutons form from volcanic activity below the Earth’s surface • Batholiths: large, irregular shape • Stocks: small irregular shape • Laccoliths: round top, flat bottom • Sills: parallel to existing rock • Dikes: cut across existing rock

  16. Plutons

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