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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

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• Lava

– Molten rock that reaches

Earth’s surface

• Magma

– Molten rock, which may

include fragments of

rock, volcanic glass and

ash, or gas

• Gases

Volcanoes and volcanic hazards

Eruptions, landforms and materials• Eruption types

– Hawaiian type; fluid eruptions, least explosive

– Shield volcano

• A broad, flat volcano with gently sloping sides, built of successive

lava flows

• Produce flood basalts or basalt plateaus (from fissures instead of

central vents)

Shield Volcano-Iceland

Lava flows and shield

volcanoes

Iceland

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Mauna Loa from Kilauea

Mauna Loa, as seen from Kilauea

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

– Strato (composite)

volcano; mixture of

lava and pyroclastics

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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

Mt. Fuji

Mt. St. Helens, 1980

Lateral Blast

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Eruptions, landforms and materials

Shield volcano

Eruptions, landforms and materialsStratovolcano

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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: General term, all sizes

– Ash: smallest size

– Agglomerates: welded larger particles

– Tuff: welded smaller particles

Eruptions, landforms and materials

• Other volcanic

features

– Craters

– Resurgent dome

– Thermal spring

– Geysers

– Fumaroles

1 2

3 4

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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

Lahars

Mt. Rainier

Orting, Washington

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Mt. Pelee, 1902

Town of St. Pierre, Carribean Island of Martinique 30,000 killed

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

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Predicting Eruptions

• Establish a volcano’s history

– Active (eruption in recorded

history)

– 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 (geothermal gradient)

– Ocean crust: temperature rises twice as rapid

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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

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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

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How, Why and Where Rocks

Melt• The tectonic setting

– Lava characteristics influenced by location

• Oceanic, divergent margins

– Lithosphere (crust) is thin with a steep geothermal gradient

• Subduction zones

– Aleutian Islands; Andes

– Typically have high water content and melt at lower temperatures

• Hot- spots; Hawaiian Islands

– Lava tends to be hot and basaltic

– Build giant shield volcanoes

– Continental; Yellowstone

• Continental divergent margins are all different

– High silica lava

How, Why and Where Rocks

Melt

Pillow lavas, mid-ocean ridgeContinental hot spot; silica rich

How, Why and Where Rocks Melt

Continental-oceanic

subduction zone: andesite

Oceanic-oceanic subduction

zone: andesite

Oceanic hot spot:

basalt

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How, Why and Where Rocks

Melt Yellow-hot spot; red,

volcano

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

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Rate of Cooling

Aphanitic, vesicular Porphyritic; 2 sizes of crystals

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 (pegmatite)

Chemical composition

• Igneous rocks subdivided into three categories

based on silica content

– Felsic-high

– Intermediate

– Mafic-low

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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