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Chapter 13 - Volcanoes

Chapter 13 - Volcanoes. Section 2: Volcanic Eruptions. Section 2 Objectives. Explain how the composition of magma affects volcanic eruptions and lava flow. Describe the five major types of pyroclastic material. Identify the three main types of volcanic cones. Describe how a caldera forms.

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Chapter 13 - Volcanoes

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  1. Chapter 13 - Volcanoes Section 2: Volcanic Eruptions

  2. Section 2 Objectives • Explain how the composition of magma affects volcanic eruptions and lava flow. • Describe the five major types of pyroclastic material. • Identify the three main types of volcanic cones. • Describe how a caldera forms. • List three events that may signal a volcanic eruptions.

  3. Windows to Earth’s Interior • 2 general types of lava • Mafic: magma or rock rich in Mg and Fe; commonly dark in color • Commonly makes up oceanic crust • May also make up continental crust • Felsic: magma or rock that is rich in light-colored silicate materials • Makes up continental crust

  4. Types of Eruptions • Viscosity – the resistance of a liquid to flow; determined by magma’s composition • This can affect the force with which a volcano may erupt • Mafic magmas are runny with low viscosity and typically cause QUIET eruptions (typically oceanic volcanoes – Hawaii) • Felsic magmas are very sticky and therefore highly viscous. • They can trap large amounts of dissolved gases, which leads to more EXPLOSIVE eruptions.

  5. Lava Flows • Pahoehoe – mafic flows that cool & form a crust, continues to flow causing wrinkles in crust; “ropy” lava flows • Aa – forms if crust deforms rapidly or grows too thick to wrinkle, and surface breaks into jagged chunks. • Same composition but different textures due to gas content, rate and slope of flow • Blocky – Higher silica and is more viscous, forming large blocks of volcanic rock when cooled

  6. Explosive Eruptions • Felsic lavas of continental volcanoes such as Mount St. Helens tend to be heavier and stickier (more viscous) • Contain large amounts of trapped gases, such as carbon dioxide and water vapor • During an eruption, gases escape and molten and solid particles are thrown into the air • Pyroclastic material: fragments of rock that form during a volcanic eruption

  7. Types of Pyroclastic Material • Particles are classified according to size • Volcanic ash: particles < 2mm in diameter • Volcanic dust : particles < 0.25 mm in diameter • Lapilli: “little stones” < 64 mm in diameter • Generally fall near the vent • Volcanic bombs: clumps of red-hot lava that spin and cool in the air; round or spindle • Volcanic blocks: solid rock blasted from the vent; may be as large as a small house

  8. Types of Volcanoes • Table 1, page 328 • Lava &/or pyroclastic material ejected during both quiet &/or explosive eruptions build up around the vent • Forms 3 Types of Volcanic Cones • Shield volcanoes: Broad at base with gentle sloping sides; quiet eruptions; mafic lava • Cinder cones: Very steep slope; explosive eruptions with cones of pyroclastic material • Composite volcanoes: alt. layers of hardened lava flows and pyroclastic material; a.k.astratovolcanoes; lg. volcanic mtns.

  9. Calderas • Large circular or basin-shaped depressions in the ground • Form when the magma chamber below a volcanic cone collapses • Figure 4, page 329 • Eruptions discharge large amounts of magma can cause a magma chamber to collapse • Krakatau: volcanic island in Indonesia exploded in 1883 forming a caldera with diameter of 6km • May fill with water to form lakes

  10. Predicting Volcanic Eruptions • Earthquake activity may signal a volcanic eruption before it occurs • Due to increased pressure, increased temps, or fracturing of surrounding rock • Also increased strength and frequency of quakes • Patterns in Activity • Sides of volcano may bulge as magma moves • Instruments measure tilt of ground on slopes • Knowledge of previous eruptions • Daily measurements vs. past measurements

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