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IGNEOUS ROCKS AND

PROCESSES

Rocks from a Melt

Igneous Rocks

• Named for “Fire” (ignis L.)

• Fire = Oxidation

• No Oxidation Involved in Melting

Rocks

• Fire may occur as lava burns

plants, but this is secondary.

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Creation of Magma

• Magma is generated from

• Heat

• Geothermal gradient p. 73

• Friction between lithospheric plates

• Pressure release

• Pressure gradient p. 74

• Decompression melting

• Water and other volatiles

• Wet melting points p. 75

• Subduction

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Heating from a mantle plume coming

from below the asthenosphere (Hot Spot)

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Spreading releases pressure on the

asthenosphere, causing melting and

lowering of magma density.

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Partial melting occurs as water, forced out of the subducting

plate drops the melting point of minerals in the surrounding

mantle. See p. 75

Geologic setting for volcanism

Magmas produced by:

World examples: Geologic setting for volcanism

Magmas produced by:

World examples:

Hotpots

Subduction zones

Divergent plate boundaries

Geologic setting for volcanism

Magmas produced by:

World examples:

Hotpots Increased heat Hawaii, Yellowstone, Iceland

Subduction zones

Divergent plate boundaries

Geologic setting for volcanism

Magmas produced by:

World examples:

Hotpots Increased heat Hawaii, Yellowstone, Iceland

Subduction zones

Addition of water causes melting point of mantle to drop.

Cascade Mtns, Andes Mtns, Indonesia, Aleutian Islands, Japan, Philippines

Divergent plate boundaries

Geologic setting for volcanism

Magmas produced by:

World examples:

Hotspots

Increased heat Hawaii, Yellowstone, Iceland

Subduction zones

Addition of water causes melting point of mantle to drop.

Cascade Mtns, Andes Mtns, Indonesia, Aleutian Islands, Japan, Philippines

Divergent plate boundaries

Drop in pressure causes melting point of mantle to drop

Iceland, Oceanic ridges everywhere, East African Rift Zone, Long Valley

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Motion of Magma

Viscosity – a fluid’s resistance to flow

– Temperature – Hotter magma is lower in viscosity (more fluid)

– Composition – Magma with more silica is more viscose

– Which kind of magma is more likely to reach the surface of the Earth?

– Why is a magma of mafic composition (Basalt) more often a lava while a felsic composition more often a plutonic rock (Granite)?

Crystalization of Magma

As a magma cools

–High temperature (high melting point) minerals crystallize first.

– Lower temperature (low melting point) minerals crystallize last.

–Proportions of these minerals change as the parent magma crystallizes

–Poker chip demonstration

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How can a magma change over time?

In the top figure, we have a homogeneous

magma with an initial composition (A).

However, as it cools and begins to

crystallize, the more dense crystals may

sink and settle on the bottom of the

magma chamber as illustrated in the

middle figure. These crystals are

effectively removed from the magma.

The remaining magma is depleted in the

chemical components that are required to

form these first crystals - i.e., the magma

has changed composition.

In figure B, the remaining magma has a

different composition than it initially had

in A.

In C, a different mineral begins to form as

the magma cools further, changing the

composition of the remaining magma in a

different way.

This process is called magmatic

differentiation and results in the

formation of one or more secondary

magmas from an initial parent magma.

There are a variety of ways that a magma

can change its composition - we have

only considered a case involving a

process called crystal settling.

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Norman Bowen (1887-1956) demonstrated the

complicated chemical behavior of crystallizing magmas

in the laboratory.

He showed that as a magma cools, certain minerals

crystallize first at high temperatures and that as the

magma continues to cool the identity of the minerals that

crystallize change. This is known

as the Bowen’s

reaction series

and explains

change in

magma

composition in

the example of

crystal settling

that we just

considered.

Bowen’s reaction series indicates that we can produce a magma of

nearly any composition from an initial ultramafic or mafic (basalt)

magma through magmatic differentiation.

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Igneous Rocks are classified by …………..

Composition and Texture

Composition = Mineral Content

Texture = Grain/Crystal Size

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Compo-

sition

Name Mineral

composition

Intrusive

rock

names

Extrusive

rock

names

< 45%

silica

45 –

55%

silica

55 –

65%

silica

> 65%

silica

Compo-

sition

Name Mineral

composition

Intrusive

rock

names

Extrusive

rock

names

< 45%

silica

Ultramafic

45 –

55%

silica

Mafic

55 –

65%

silica

Interme-

diate

> 65%

silica

Felsic

Compo-

sition

Name Mineral

composition

Intrusive

rock

names

Extrusive

rock

names

< 45%

silica

Ultramafic Mostly Olivine

Minor Pyroxene

Minor Plagioclase

45 –

55%

silica

Mafic

55 –

65%

silica

Interme-

diate

> 65%

silica

Felsic

Compo-

sition

Name Mineral

composition

Intrusive

rock

names

Extrusive

rock

names

< 45%

silica

Ultramafic Mostly Olivine

Minor Pyroxene

Minor Plagioclase

45 –

55%

silica

Mafic Pyroxene

Ca-rich Plagioclase

Minor Olivine

Minor Hornblende

55 –

65%

silica

Interme-

diate

> 65%

silica

Felsic

Compo-

sition

Name Mineral

composition

Intrusive

rock

names

Extrusive

rock

names

< 45%

silica

Ultramafic Mostly Olivine

Minor Pyroxene

Minor Plagioclase

45 –

55%

silica

Mafic Pyroxene

Ca-rich Plagioclase

Minor Olivine

Minor Hornblende

55 –

65%

silica

Interme-

diate

Hornblende

Biotite

Na-rich Plagioclase

Minor Pyroxene

> 65%

silica

Felsic

Compo-sition

Name Mineral composition

Intrusive rock names

Extrusive rock names

< 45% silica

Ultramafic Mostly Olivine

Minor Pyroxene

Minor Plagioclase

45 – 55% silica

Mafic Pyroxene

Ca-rich Plagioclase

Minor Olivine

Minor Hornblende

55 – 65% silica

Interme-diate

Hornblende

Biotite

Na-rich Plagioclase

Minor Pyroxene

> 65% silica

Felsic Quartz

K-Feldspar

Muscovite

Minor Biotite

Minor Hornblende

Minor Na-rich Plagiocl.

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Compo-sition

Name Mineral composition

Intrusive rock names

Extrusive rock names

< 45% silica

Ultramafic Mostly Olivine

Minor Pyroxene

Minor Plagioclase

Peridotite Komatiite (almost non-existent)

45 – 55% silica

Mafic Pyroxene

Ca-rich Plagioclase

Minor Olivine

Minor Hornblende

Gabbro Basalt, Scoria

55 – 65% silica

Interme-diate

Hornblende

Biotite

Na-rich Plagioclase

Minor Pyroxene

Diorite Andesite, Pumice, Tuff

> 65% silica

Felsic Quartz

K-Feldspar

Muscovite

Minor Biotite

Minor Hornblende

Minor Na-rich Plagiocl.

Granite Rhyolite, Pumice, Tuff, Obsidian

Crystallization of Magma

Magma usually contains some suspended crystals and dissolved gases

such as H2O and CO2.

As magma cools, the random motion of the ions begin to slow down

and arrange themselves into orderly crystalline structures.

Crystallization — process by which magma cools to form crystals

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Depending upon the cooling rate and mode of formation, igneous

rocks can possess a variety of textures.

It is important to understand that formation history can be

inferred from texture. For instance, slow plutonic cooling may result in a "coarse-grained"

igneous rock. There are several common igneous textures:

1. fine-grained (aphanitic)

2. course-grained (phaneritic)

3. porphyritic

4. glassy

5. frothy

Types of Igneous Textures

Texture

Composi-tion

Phaneritic Aphanitic Glassy Frothy (glassy & vesicular)

Pyroclastic or

fragmental

Ultramafic Peridotite

Mafic Gabbro Basalt Scoria Volcanic Tuff

Inter-mediate

Diorite Andesite Pumice Volcanic Tuff

Felsic Granite Rhyolite Obsidian Pumice Volcanic Tuff

Pegmatitic texture is a subcategory of Phaneritic

Porphyritic texture is a subcategory of Aphanitic

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Contrasting Textures

Phaneritic Texture

Granite – slow cooling

Aphanitic Texture

Ryolite – rapid cooling

Porphyritic Texture – 2 cooling times

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Grand Canyon Pegmatite

(at the bottom!)

Slow cooling with many volatiles

Some Volcanic Igneous Rock Textures

Rapid cooling – high silica Rapid cooling – high volatiles

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Igneous Compositions

Igneous rocks are mainly composed of silicate

minerals.

Mafic composition

Felsic composition

Ultramafic rocks are very rich in magnesium

and iron. Although these rocks are rare at the

Earth’s surface, peridotite is the main

constituent of deep interior portions of the Earth

(upper mantle).

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You can see from this diagram that

gabbro is the plutonic/intrusive

analog to basalt. Gabbro forms by

the slow cooling of a basaltic (mafic)

melt deep in the Earth.

Coast Range Ophiolite- Gabbro, Basalt, Diabase

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Basaltic Rocks

Basalt is dark green/gray to black in

color and is fine-grained (aphanitic)

in texture. The fine-grained texture

indicates that basalt is volcanic in

origin.

Basalt is the most common

volcanic/extrusive igneous rock - the

seafloor, many volcanic islands and

large portions of the continent are

composed of basalt.

Mafic rocks like basalt tend to be more

dense than felsic rocks.

Basaltic Lava

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Basalt commonly has a vesicular

texture - that is, it contains holes

where gas bubbles escaped during

the cooling and crystallization of

the basalt.

This variety of basalt is known as

vesicular basalt; when it is coarsely

vesicular, it may be called scoria.

Owens Valley – Basaltic Lava Flow

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Underwater formation of Pillow Basalt

Miocene Pillow Basalts

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Intermediate rocks fall between felsic

(granitic) and mafic (basaltic) igneous rocks.

Andesite is a common volcanic rock that

occurs along the margins of continents.

Mt. St. Helens Eruption – Andesite Volcano

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Andesite

Devil’s Post Pile, California

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Diorite

Diorite Pluton

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Granite is a coarse-grained (phaneritic) felsic igneous rock. It forms

by slow cooling at depth in the Earth and is exposed at the surface by

mountain building and erosion.

Granitic Rocks

Granite is a familiar rock that is

composed of predominantly light-

colored silicate minerals such as

quartz and feldspar .

Geologists refer to the composition

of granitic rocks as felsic - meaning

that it contains a lot of feldspar and

silica.

Granite is a common building material

and decorative material.

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Granitic Pluton

Sierra Nevada Batholith

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Igneous Rock Formations

Concordant and Discordant Plutons

• Parallel to beds/layers – Sill

• Near surface, tabular

• Crossing existing beds/layers – Dike

• Can be deeper, tabular

• Often vertical or nearly so, rising from magma

chamber

• Solidified in a network of fractures

• Volcanic neck

Types of Plutons

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Whereabouts of Igneous Rocks

Ultramafic – Mantle peridotite, sometimes at convergent boundaries, thrust up.

Mafic- Basalt and Gabbro – Compose oceanic crust, flood basalts from Hot Spots

Intermediate – Andesite and Diorite – compose island arcs and continental arcs from subduction zones

Felsic – Compose continental volcanics and plutons, batholiths, from mixing and partial melting near subduction zones

Whereabouts of Igneous Rocks