Chapter 6: Volcanoes and Igneous Rocks © 2012 John Wiley & Sons, Inc. All rights reserved.

© 2012 John Wiley & Sons, Inc. All rights reserved. Learning Objectives Volcanoes and volcanic hazards Contrast the different types of volcanic eruptions and their effects. How, why, and where rock melts Describe how temperature, pressure, and water conditions produce magma. Cooling and crystallization Compare different types of igneous and volcanic rocks and their formation. Plutons and plutonism Describe the types of plutonic rock and plutons and their formation. © 2012 John Wiley & Sons, Inc. All rights reserved.

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. Figure 6.15 Geothermal gradient © 2012 John Wiley & Sons, Inc. All rights reserved.

How, Why, and Where Rocks Melt Effect of temperature and pressure on melting © 2012 John Wiley & Sons, Inc. All rights reserved.

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 © 2012 John Wiley & Sons, Inc. All rights reserved.

How, Why, and Where Rocks Melt © 2012 John Wiley & Sons, Inc. All rights reserved.

How, Why, and Where Rocks Melt Magma and Lava Molten rock below surface Lava Magma when it reaches the surface Differs in composition, temperature, and viscosity Figure 6.18b Two types of lava flows © 2012 John Wiley & Sons, Inc. All rights reserved.

How, Why, and Where Rocks Melt Magma and Lava Composition 45% to 75% of magma by weight is silica. Water vapor and carbon dioxide are usually present. 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. © 2012 John Wiley & Sons, Inc. All rights reserved.

Magma and Lava Figure 6.18a Viscosity of lava © 2012 John Wiley & Sons, Inc. All rights reserved.

How, Why and Where Rocks Melt Tectonic setting and volcanism 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: Lava is high in silica. © 2012 John Wiley & Sons, Inc. All rights reserved.

Lava types and tectonic settings © 2012 John Wiley & Sons, Inc. All rights reserved.

Lava types and tectonic settings Figure 6.19a Midocean ridge: submarine basaltic pillow lavas Figure 6.19b Continental rift: rhyolitic and lavas with unusual composition © 2012 John Wiley & Sons, Inc. All rights reserved.

Lava types and tectonic settings Figure 6.19c Ocean-continent subduction zone Figure 6.19d Shield volcano © 2012 John Wiley & Sons, Inc. All rights reserved.

Lava types and tectonic settings Figure 6.19e Ocean-ocean subduction zone © 2012 John Wiley & Sons, Inc. All rights reserved.

Cooling and Crystallization The process whereby mineral grains form and grow in a cooling magma (or lava) Classified as: Volcanic Plutonic © 2012 John Wiley & Sons, Inc. All rights reserved.

Cooling and Crystallization Rate of Cooling Rapid cooling: Volcanic rocks and textures Volcanic rock An igneous rock formed from lava Glassy Aphanitic Porphyritic Pumice Vesicular basalt Figure 6.20a Glassy texture © 2012 John Wiley & Sons, Inc. All rights reserved.

Cooling and Crystallization Rate of Cooling Figure 6.20b Aphanitic texture Figure 6.20c Porphyritic texture © 2012 John Wiley & Sons, Inc. All rights reserved.

© 2012 John Wiley & Sons, Inc. All rights reserved. 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 Figure 6.21 Plutonic rock textures © 2012 John Wiley & Sons, Inc. All rights reserved.

Cooling and Crystallization Chemical Composition Igneous rocks subdivided into three categories based on silica content: Felsic Intermediate Mafic © 2012 John Wiley & Sons, Inc. All rights reserved.

© 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanoes and Volcanic Hazards Lava Molten rock that reaches Earth’s surface Magma Molten rock, which may include fragments of rock, volcanic glass and ash, or gas Figure 6.1a Lava © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanoes and Volcanic Hazards Volcanic materials Pyroclasts Tephra Ash Agglomerates Tuff Figure 6.1b Volcanic bombs Figure 6.1D Volcanic ash Figure 6.1c Lapilli © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanoes and Volcanic Hazards 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 © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanoes and Volcanic Hazards Eruptions, Landforms, and Materials Strombolian eruptions More explosive than Hawaiian Create loose volcanic rock called spatter cones or cinder cones Figure 6.2d Strombolian eruption Figure 6.2e Cinder cones in Arizona © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanoes and Volcanic Hazards Eruptions, Landforms, and Materials Vulcanian eruptions 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), buoyed by heat and volcanic gases, flow very rapidly. Figure 6.2b Mt. Mayon in Philippines © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanoes and Volcanic Hazards Eruptions, Landforms, and Materials Plinian eruptions Named after Pliny the Elder, who died during eruption of Mount Vesuvius Most violent eruptions, generating ash columns that can exceed 20 kilometers Produce steep-sided volcanoes, called stratovolcanoes Figure 6.2c Mount Saint Helens © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanoes and Volcanic Hazards Eruptions, Landforms, and Materials Stratovolcanoes Composed of solidified lava flows interlayered with pyroclastic material. Steep sides curve upward. Viscosity Degree to which a substance resists flow. A less viscous liquid is runny, whereas a more viscous liquid is thick. © 2012 John Wiley & Sons, Inc. All rights reserved.

Eruptions, Landforms, and Materials Figure 6.2a Stratovolcano © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanoes and Volcanic Hazards Eruptions, Landforms, and Materials Shield volcanoes Broad, flat volcanoes with gently sloping sides, built of successive lava flows Produce flood basalts or basalt plateaus Figure 6.2f Shield volcanoes Figure 6.2h Flood basalts © 2012 John Wiley & Sons, Inc. All rights reserved.

Eruptions, Landforms, and Materials © 2012 John Wiley & Sons, Inc. All rights reserved.

Eruptions, Landforms, and Materials Figure 6.2g Fissure eruptions © 2012 John Wiley & Sons, Inc. All rights reserved.

Eruptions, Landforms, and Materials Figure 6.3 Crater Lake © 2012 John Wiley & Sons, Inc. All rights reserved.

Eruptions, Landforms, and Materials Figure 6.4 Resurgent Dome of Mt. St. Helens © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanoes and Volcanic Hazards Eruptions, Landforms, and Materials Other volcanic features: Craters Resurgent dome Thermal spring Geysers Fumaroles Figure 6.5 The Great Geysir © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanoes and Volcanic Hazards 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 Figure 6.6 Kalapana, Hawaii lava flow © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanic Hazards © 2012 John Wiley & Sons, Inc. All rights reserved. Figure 6.7 Victim of poisonous gases of eruption of Mt. Vesuvius © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanic Hazards © 2012 John Wiley & Sons, Inc. All rights reserved. Figure 6.8 Volcanic hazards © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanic Hazards Figure 6.9 Deadly eruptions © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanoes and Volcanic Hazards 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 © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanoes and Volcanic Hazards Volcanic Hazards Figure 6.11 Fertile but dangerous © 2012 John Wiley & Sons, Inc. All rights reserved.

Predicting Eruptions Figure 6.12 Volcano monitoring from the ground © 2012 John Wiley & Sons, Inc. All rights reserved.

Volcanoes and Volcanic Hazards Predicting Eruptions Establishing a volcano’s history Active Dormant Monitoring changes and anomalies Earthquakes Shape or elevation Volcanic gases Ground temperature Composition of water Figure 6.13 Monitoring volcanoes from orbit © 2012 John Wiley & Sons, Inc. All rights reserved.

Cooling and Crystallization Fractional Crystallization Separation of crystals from liquids during crystallization Bowen’s reaction series Predictable melting and cooling of minerals © 2012 John Wiley & Sons, Inc. All rights reserved.

Fractional Crystallization Figure 6.22a Filter pressing Figure 6.22b Crystal settling Figure 6.22c Crystal flotation © 2012 John Wiley & Sons, Inc. All rights reserved.

© 2012 John Wiley & Sons, Inc. All rights reserved. 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 © 2012 John Wiley & Sons, Inc. All rights reserved.

© 2012 John Wiley & Sons, Inc. All rights reserved. Plutons and Plutonism Figure 6.24 How magma rises © 2012 John Wiley & Sons, Inc. All rights reserved.

© 2012 John Wiley & Sons, Inc. All rights reserved. Plutons and Plutonism Batholiths are so huge that map views give us the best perspective. Stocks are smaller batholiths. Figure 6.25 Batholiths and stocks © 2012 John Wiley & Sons, Inc. All rights reserved.

Plutons and Plutonism Dikes and Sills Dikes form when magma squeezes into a cross-cutting fracture and solidifies. Sills form when magma intrudes between two layers and is parallel to them. © 2012 John Wiley & Sons, Inc. All rights reserved.

© 2012 John Wiley & Sons, Inc. All rights reserved. Plutons and Plutonism Volcanic neck Remnant of a volcanic pipe that once fed the magma to the volcanic vent Figure 6.26c Devil’s Tower, Wyoming © 2012 John Wiley & Sons, Inc. All rights reserved.

Amazing Places: Mount Saint Helens A May 1980 C Largest debris avalanche in recorded history B Forest flattened by blast D Mt. St. Helens today © 2012 John Wiley & Sons, Inc. All rights reserved.

© 2012 John Wiley & Sons, Inc. All rights reserved. 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? © 2012 John Wiley & Sons, Inc. All rights reserved.