Transcript Ch04%20outline

Igneous Rocks
Chapter 4
Igneous Rocks
Igneous rocks form as molten rock
(magma) cools and solidifies
 General characteristics of magma

• Parent material of igneous rocks
• Forms from partial melting of rocks
• Magma at surface is called lava
General Characteristics
of Magma
Rocks formed from lava = extrusive,
or volcanic rocks
 Rocks formed from magma at depth
= intrusive, or plutonic rocks

General Characteristics
of Magma, cont’d
Magma consists of three components:
Liquid portion = melt
 Solids, if any, are silicate minerals
 Volatiles = dissolved gases in the melt,
including water vapor (H2O), carbon
dioxide (CO2), and sulfur dioxide (SO2)

General Characteristics
of Magma, cont’d
Crystallization of magma
Cooling of magma results in the
systematic arrangement of ions into
orderly patterns
 Silicate minerals result from
crystallization in a predictable order
 Texture - size and arrangement of
mineral grains

Igneous Textures
Texture - overall appearance of a
rock based on the size, shape, and
arrangement of interlocking
minerals
 Several factors affect crystal size

Igneous Textures

Factors affecting crystal size
• Rate of cooling
• Slow rate = fewer but larger crystals
• Fast rate = many small crystals
• Very fast rate forms glass
• % of silica (SiO2) present
• Presence of dissolved gases
Types of Igneous Textures

Aphanitic (fine-grained) texture
• Rapid rate of cooling
• Microscopic crystals
• May contain vesicles (holes from gas
bubbles)
Types of Igneous Textures

Phaneritic (coarse-grained) texture
Slow cooling
 Large, visible crystals

Types of Igneous Textures

Porphyritic texture
Minerals form at different temperatures
 Large crystals (phenocrysts) are
embedded in a matrix of smaller
crystals (groundmass)

Types of Igneous Textures

Glassy texture
Very rapid cooling of lava
 Rock is called obsidian or pumice

Types of Igneous Textures

Pyroclastic
texture
• Fragmental
appearance
produced by
violent volcanic
eruptions
• Often appear
more similar to
sedimentary
rocks
Types of Igneous Textures

Pegmatitic texture


Exceptionally
coarse grained
Forms in late
stages of
crystallization of
granitic magmas
Igneous Compositions
Igneous rocks are composed primarily
of silicate minerals

Dark (or ferromagnesian) silicates
• Olivine, pyroxene, amphibole, and biotite
mica

Light (or nonferromagnesian) silicates
• Quartz, muscovite mica, and feldspars
Igneous Compositions,
cont’d
Granitic versus basaltic compositions

Granitic composition
• Light-colored silicates
• Termed felsic (feldspar and silica) in
composition
• High silica (SiO2) content
• Major constituent of continental crust
Igneous Compositions,
cont’d
Granitic versus basaltic compositions

Basaltic composition
• Dark silicates and Ca-rich feldspar
• Termed mafic (magnesium and ferrum, for
iron) in composition
• Higher density than granitic rocks
• Comprise the ocean floor and many
volcanic islands
Igneous Compositions,
cont’d
Other compositional groups

Intermediate (or andesitic) composition
• Contain 25% or more dark silicate minerals
• Associated with explosive volcanic activity

Ultramafic composition
• Rare composition that is high in magnesium
and iron
• Composed entirely of ferromagnesian
silicates
Igneous Compositions,
cont’d

Silica content as an indicator of
composition
• Crustal rocks exhibit a considerable range-45% to 70%

Silica content influences magma
behavior
• Granitic magmas = high silica content and
viscous
• Basaltic magmas = much lower silica
content and more fluid-like behavior
Naming Igneous Rocks
Granitic rocks

Granite
• Phaneritic
• Over 25% quartz, about 65% or more
feldspar
• Very abundant - often associated with
mountain building
• The term granite includes a wide range of
mineral compositions
Naming Igneous Rocks,
cont’d
Granitic Rocks

Rhyolite
• Extrusive equivalent of granite
• May contain glass fragments and vesicles
• Aphanitic texture
• Less common and less voluminous than
granite
Naming Igneous Rocks,
cont’d

Granitic rocks
• Obsidian
• Dark colored
• Glassy texture
• Pumice
• Volcanic
• Glassy texture
• Frothy appearance with numerous voids
Naming Igneous Rocks,
cont’d

Intermediate rocks

Andesite
• Volcanic origin
• Aphanitic texture

Diorite
• Plutonic equivalent of andesite
• Coarse grained (phaneritic texture)
Naming Igneous Rocks,
cont’d

Basaltic rocks
• Basalt
• Volcanic origin
• Aphanitic texture
• Composed mainly of pyroxene and calcium-rich
plagioclase feldspar
• Most common extrusive igneous rock
Naming Igneous Rocks,
cont’d

Mafic rocks
• Gabbro
• Intrusive equivalent of basalt
• Phaneritic texture consisting of pyroxene and
calcium-rich plagioclase
• Significant % of the oceanic crust
Naming Igneous Rocks,
cont’d

Pyroclastic rocks
• Composed of fragments ejected during a
volcanic eruption
• Varieties
• Tuff = ash-sized fragments
• Volcanic breccia = particles larger than ash
Origin of Magma

Generating magma from solid rock
• Role of heat
• Temperature increases in the upper crust
(geothermal gradient) average between 20oC to
30oC per kilometer
• Rocks in the lower crust and upper mantle are
near their melting points
• Additional heat may induce melting
Origin of Magma, cont’d
• Role of pressure
• Increases in confining pressure increases a rock’s
melting temperature
• When confining pressures drop, decompression
melting occurs
• Role of volatiles
• Volatiles (primarily water) cause melting at lower
temperatures
• Important factor where oceanic lithosphere
descends into the mantle
Evolution of
Magmas

A single
volcano may
extrude lavas
exhibiting
very different
compositions
Bowen’s Reaction Series
Minerals crystallize in a systematic
fashion based on their melting
points
 During crystallization, the
composition of the liquid portion of
the magma continually changes

Evolution of Magmas, cont’d

Processes responsible for changing a
magma’s composition
• Magmatic differentiation
• Separation of a melt from earlier formed crystals
• Assimilation
• Changing a magma’s composition by
incorporating surrounding rock bodies into a
magma
• Magma mixing
• Two chemically distinct magmas may produce a
composition quite different from either original
magma
Partial Melting and Magma
Formation
Incomplete melting of rocks is known
as partial melting
 Formation of basaltic magmas

• Most originate from partial melting of
mantle rocks at oceanic ridges
(decompression melting) or at subduction
zones (presence of water)
• Large outpourings of basaltic magma are
common at Earth’s surface
Partial Melting and Magma
Formation, cont’d

Formation of andesitic magmas
• Produced by interaction of basaltic magmas
and more silica-rich rocks in the crust
• May also evolve by magmatic
differentiation
Partial Melting and Magma
Formation, cont’d

Formation of granitic magmas
• Most likely form as the end product of
crystallization of andesitic magma
•Granitic magmas are
more viscous than
other magmas – tend
to lose their mobility
before reaching the
surface
•Produce large
plutonic structures
End of Chapter 4
04.01
Lava flows are typically finer
grained than intrusive igneous
rocks. Why?
1. Intrusive magma is cooler because it is well insulated by the
surrounding rock.
2. Intrusive magma flows onto the Earth's surface and cools
very slowly, allowing many small mineral grains to grow.
3. The extrusive magma cools quickly so the mineral grains do
not have time to grow.
4. The extrusive magma, because it is deep below the surface,
cools very slowly, producing very small mineral grains.
What does Bowen’s
Reaction Series describe?
04.02
1. The pressures that different minerals are formed in
metamorphic rocks
2. Which minerals are recrystallized in a sedimentary rock
3. The temperatures at which different minerals crystallize out of
a melt
4. All of the above
5. None of the above
Which of the following describes
best the difference between
magma and lava?
04.03
1. Upon eruption, magma becomes “lava,” the form of the
magma that flows out of the volcano
2. When magma gets to the top of the volcano, it releases gas
found within the magma, and the lava that escapes the
volcano has therefore lost those gases.
3. Lava is the name given to the ejected magma from a passive
(fluid) volcano, but it is still called “magma” if it is erupted
explosively, like at Mt. St. Helens.
4. If the magma has no crystals or gases within it, it is called
“lava.”
5. Both 1 and 3
6. Both 3 and 4
7. All of these.
Which of the following rocks is
likely to have the most quartz
within it and why?
04.04
1. Granite; intrusive rock that formed from cooling of relatively
high silica magma.
2. Rhyolite; extrusive rock that formed from cooling of relatively
low silica magma.
3. Diorite; intrusive rock that formed from the cooling of
relatively intermediate silica magma.
4. Granite; intrusive rock that formed from cooling of relatively
intermediate silica magma.
5. Basalt; extrusive rock that formed from cooling of relatively
low silica lava.
6. Basalt; extrusive rock that formed from cooling of relatively
high silica lava.
04.05What
is the most important factor for
whether magma cools slowly or
quickly?
1. Pressure of the environment – higher P = slower magma
cooling.
2. The presence or absence of volatiles (gases) – less gases =
slower magma cooling.
3. Temperature of the environment – lower T = slower magma
cooling.
4. The presence or absence of volatiles (gases) – more gases =
slower magma cooling.
5. Temperature of the environment – higher T = slower magma
cooling.
6. Pressure of the environment – lower P = slower magma
cooling.
Igneous rocks are produced
largely by ________.
04.06
1.
2.
3.
4.
5.
the changing of a rock from one set of minerals to another
the compaction of metamorphic rocks
the melting of sedimentary rocks
the cooling of magma
Both c and d are correct.
Glassy igneous rocks form
when the magma _______.
04.07
1.
2.
3.
4.
5.
cools so fast that mineral grains cannot crystallize and grow
cools so slowly that only one mineral is formed
is composed of basalt
is a rhyolitic type
cools at an extremely high temperature
04.08
Quartz is ________.
1. resistant to weathering and is an important component of
sand in river beds and beaches
2. a main constituent of many igneous rocks
3. a main constituent of many sedimentary rocks
4. the most stable of all minerals at Earth’s surface
temperatures and pressures
5. all of the above