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Chapter 6. Volcanoes and Igneous Rocks. VOLCANOES AND IGNEOUS ROCKS. Objectives. Identify several different categories of volcanic eruptions. Identify the volcanic hazards. Describe how temperature, pressure, and water content affect a rock’s melting point.
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Chapter 6 Volcanoes and Igneous Rocks
Objectives Identify several different categories of volcanic eruptions. Identify the volcanic hazards. Describe how temperature, pressure, and water content affect a rock’s melting point. Identify three properties that distinguish one lava from another. Distinguish between and identify volcanic and plutonic rocks. Describe the most common plutonic formations.
Volcanoes and volcanic hazards Volcano A vent through which lava, solid rock debris, volcanic ash, and gasses erupt from Earth’s crust to its surface Can be explosive or nonexplosive
Lava Molten rock that reaches Earth’s surface Magma Molten rock, which may include fragments of rock, volcanic glass and ash, or gas Volcanoes and volcanic hazards
Eruptions, landforms and materials Eruption types Shield volcano A broad, flat volcano with gently sloping sides, built of successive lava flows Produce flood basalts or basalt plateaus
Eruptions, landforms and materials Eruption types Strombolian More explosive than Hawaiian Create loose volcanic rock called spatter cones or cinder cones
Eruptions, landforms and materials Eruption types Vulcanian More explosive than Strombolian and, as a result, can generate billowing clouds of ash up to 10 km Produce pyroclastic flows Hot volcanic fragments (tephra) that, buoyed by heat and volcanic gases, flow very rapidly
Eruptions, landforms and materials Eruption types Plinian Named after Pliny the Elder, who died during eruption of Mount Vesuvius Most violent eruptions, generating ash columns the can exceed 20 kilometers Produce steep sided volcanoes, called stratovolcanoes Composed of solidified lava flows interlayered with pyroclastic material. Steep sides that curve upward
Eruptions, landforms and materials Viscosity The degree to which a substance resists flow, Less viscous liquid is runny More viscous liquid is thick Volcanic materials Pyroclasts Tephra Ash Agglomerates Tuff
Eruptions, landforms and materials Other volcanic features Craters Resurgent dome Thermal spring Geysers Fumaroles
Volcanic hazards Primary effects Pyroclastic flows Volcanic gases Secondary effects Related to, but not a direct result of, volcanic activity Fires Flooding Mudslides Debris avalanche
Volcanic hazards Tertiary and beneficial effects Change a landscape Affect climate on regional and global scale Renew mineral content and replenish fertility Geothermal energy Provide mineral deposits
Predicting Eruptions Establish a volcano’s history Active Dormant Monitor changes and anomalies Earthquakes Changes in shape or elevation Volcanic gases Changes in ground temperature Composition of water
How, Why and Where Rocks Melt Heat and pressure inside Earth Continental crust: temperature rises 30°C/km, then about 6.7°C/km Ocean crust: temperature rises twice as rapid
How, Why and Where Rocks Melt Effect of temperature and pressure on melting
How, Why and Where Rocks Melt Heat and pressure inside Earth Fractional melt A mixture of molten and solid rock Fractionation Separation of melted materials from the remaining solid material during the course of melting
How, Why and Where Rocks Melt Magma Molten rock below surface Lava Magma when it reaches the surface Differ in composition, temperature and viscosity
How, Why and Where Rocks Melt Lava Composition 45-75% of magma by weight is silica Water vapor and carbon dioxide Temperature Lavas vary in temperature between 750°C and 1200°C Magmas with high H2O contents melt at lower temperatures Viscosity Lavas vary in their ability to flow Influenced by silica content and temperature
How, Why and Where Rocks Melt The tectonic setting Lava characteristics influenced by location Oceanic, divergent margins Lava is thin with a steep geothermal gradient Subduction zones Typically have high water content and melt at lower temperatures Hot- spots Lava tends to be hot and basaltic Build giant shield volcanoes Continental divergent margins are all different High silica lava
Cooling and Crystallization Crystallization The process whereby mineral grains form and grow in a cooling magma (or lava) Classified as: Volcanic Plutonic
Rate of Cooling Rapid cooling: Volcanic rocks and textures Volcanic rock An igneous rock formed from lava Glassy Aphanitic Porphyritic Pumice Vesicular basalt
Rate of Cooling Slow cooling: Plutonic rocks and textures Plutonic rock An igneous rock formed underground from magma Phaneritic-a coarse grained texture Can have exceptionally large grains
Chemical composition Igneous rocks subdivided into three categories based on silica content Felsic Intermediate Mafic
Fractional Crystallization Separation of crystals from liquids during crystallization Bowens reaction series Predictable melting and cooling of minerals
Plutons and Plutonism Plutons Any body of intrusive igneous rock, regardless of size or shape Batholith A large, irregularly shaped pluton that cuts across the layering of the rock into which it intrudes
Plutons and Plutonism Dikes Forms when magma squeezes into a cross cutting fracture and solidifies Sills Magma that intrudes between two layers and is parallel to them
Critical Thinking What factors might prevent magma from reaching Earth’s surface? What reasons can you think of for living near a volcano? Do you think the advantages outweigh the disadvantages? If you were to heat up a glass beaker full of crushed rock, the beaker would melt before you could finish studying the rock melting process. How do you think geologists study rock melting?
