chapter 6: volcanoes and igneous rocks © 2012 john wiley & sons, inc. all rights reserved

Chapter 6: Volcanoes and Igneous Rocks

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

Learning ObjectivesVolcanoes 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.


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



Effect of temperature and pressure on melting


How, Why, and Where Rocks MeltHeat and Pressure Inside Earth

Fractional melt•A mixture of molten and solid rockFractionation•Separation of melted materials from the remaining solid material during the course of melting


How, Why, and Where Rocks MeltMagma and Lava

Magma•Molten rock below surfaceLava•Magma when it reaches the surface•Differs in composition, temperature, and viscosity

Figure 6.18b Two types of lava flows


How, Why, and Where Rocks MeltMagma 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.


Magma and Lava

Figure 6.18a Viscosity of lava


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.


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



Figure 6.19c Ocean-continent subduction zone

Figure 6.19d Shield volcano



Figure 6.19e Ocean-ocean subduction zone



Cooling and Crystallization

Crystallization•The process whereby mineral grains form and grow in a cooling magma (or lava)•Classified as:

• Volcanic• Plutonic


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


Cooling and CrystallizationRate of Cooling

Figure 6.20b Aphanitic texture Figure 6.20c Porphyritic texture


Cooling and CrystallizationRate of Cooling

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


Igneous rocks subdivided into three categories based on silica content:•Felsic•Intermediate•Mafic


Cooling and CrystallizationChemical Composition

Volcanoes and Volcanic Hazards

Lava•Molten rock that reaches Earth’s surfaceMagma•Molten rock, which may include fragments of rock, volcanic glass and ash, or gas

Figure 6.1a Lava


Volcanoes and Volcanic HazardsVolcanic materials•Pyroclasts•Tephra•Ash•Agglomerates•Tuff

Figure 6.1b Volcanic bombs

Figure 6.1D Volcanic ash

Figure 6.1c Lapilli


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


Volcanoes and Volcanic Hazards

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


Volcanoes and Volcanic HazardsEruptions, 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



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



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.



Eruptions, Landforms, and Materials

Figure 6.2a Stratovolcano


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



Figure 6.2g Fissure eruptions



Figure 6.3 Crater Lake



Figure 6.4 Resurgent Dome of Mt. St. Helens



Other volcanic features:•Craters•Resurgent dome•Thermal spring•Geysers•Fumaroles

Figure 6.5 The Great Geysir



Volcanoes and Volcanic HazardsVolcanic 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


Volcanic Hazards

Figure 6.7 Victim of poisonous gases of eruption of Mt. Vesuvius


Volcanic Hazards

Figure 6.8 Volcanic hazards


Volcanic Hazards

Figure 6.9 Deadly eruptions


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



Figure 6.11 Fertile but dangerous



Predicting Eruptions

Figure 6.12 Volcano monitoring from the ground


Volcanoes and Volcanic HazardsPredicting 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


Fractional crystallization•Separation of crystals from liquids during crystallizationBowen’s reaction series•Predictable melting and cooling of minerals


Cooling and CrystallizationFractional Crystallization

Fractional Crystallization

Figure 6.22a Filter pressing

Figure 6.22b Crystal settling Figure 6.22c Crystal

flotation


Plutons and Plutonism

Plutons•Any body of intrusive igneous rock, regardless of size or shapeBatholith•A large, irregularly shaped pluton that cuts across the layering of the rock into which it intrudes



Figure 6.24 How magma rises



•Batholiths are so huge that map views give us the best perspective.•Stocks are smaller batholiths.

Figure 6.25 Batholiths and stocks


Plutons and PlutonismDikes 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.



Volcanic neck•Remnant of a volcanic pipe that once fed the magma to the volcanic vent

Figure 6.26c Devil’s Tower, Wyoming


Amazing Places: Mount Saint Helens

A May 1980

B Forest flattened by blast

C Largest debris avalanche in recorded history

D Mt. St. Helens today


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?


chapter 6: volcanoes and igneous rocks © 2012 john wiley & sons, inc. all rights reserved

Documents

john wiley sons

lava figure

types of lava

lava types

viscosity of lava

lava composition

tectonic settings slide

surface lava magma