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.• 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

Eruptions, landforms and materials

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

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

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

Predicting Eruptions

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

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

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

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

How, Why and Where Rocks Melt

How, Why and Where Rocks Melt

How, Why and Where Rocks Melt

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

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

Fractional Crystallization

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

Mount Saint Helens

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?