Chapter 4
Rocks & Igneous Rocks
Rock Definition
A naturally occurring, solid
Composed of one or more minerals
Exceptions to the Definition
Some rocks are not formed of minerals
Noncrystalline, glassy volcanic rocks such as
obsidian and pumice
Coal which is formed from compacted plant remains
Appearance of a Rock
The appearance of a rock is determined by
two major factors
Mineralogy
Texture
Appearance of a Rock
Mineralogy refers to the relative proportions of the
constituent minerals
Texture is determined by the size, shape and
arrangement of the mineral crystals
Appearance of a Rock
Mineralogy and texture are related
to how and where a rock was
formed and what has happened to
the rock since it was created
Three Families of Rocks
Igneous- from magma/lava
Sedimentary-compaction/cementing
of sediments
Metamorphic- changed by heat and
pressure
Three Types of Rocks
Igneous Rocks
Rocks formed by the solidification of magma
(molten rock)
Granite
Two Types of Igneous Rocks
Extrusive igneous rocks
form when magma erupts
at the surface (i.e., above
ground) and rapidly cools
Intrusive igneous rocks
form when magma
intrudes into bedrock and
slowly cools (i.e., below
ground)
Igneous Rocks
Most Igneous Rock are Silicates
Quartz
Feldspar
Mica
Pyroxene
Amphibole
Olivine
Basalt
Granite
Sedimentary Rocks
Rocks formed by consolidation of sediments
Classified by the type of sediments
Three Major Types of Sediments
Clastic Sediments are physically deposited
particles derived from weathered rocks
Chemical Sediments include minerals carried in
solution such as calcite and halite
Organic Sediments are composed of living
materials (plant/animal material and fossils/shells)
From Sediments to Solid Rocks
Lithification is the process of converting sediments
into solid rocks
(will cover in detail in another lecture)
The Two Types of Lithification
Compaction – sediments are squeezed together by
the weight of overlying sediments into a solid mass
Cementation – minerals precipitate around the
sediments and bind them into a solid rock
Sedimentary Rocks
Sedimentary Rocks
5 % by volume of the upper crust
75 % by exposed surface area of continents
Sedimentary Rocks
Sedimentary rocks commonly contain fossils
In fact, some sedimentary rocks are almost entirely
composed of fossils
Can exhibit extensive horizontal layers called
bedding
Composition
Sedimentary rocks can be
composed of sediments
created from any of three
great families of rock
(igneous, sedimentary,
metamorphic)
Chemical cementation
commonly includes
calcite, gypsum and
halite
Metamorphic Rocks
Metamorphic rocks are formed by the transformation
of previously-existing rocks in the solid state due to
increased temperature and pressure
Metamorphic Rocks
Metamorphism can change the mineralogy, texture
and/or the chemical composition of a “parent rock”
while maintaining its solid form
Note that the rock does not melt (temperature range
250 to 700 C)
Regional and Contact
Metamorphism
Regional metamorphism occurs where high
temperature and pressures occur over large
region (plate tectonics)
Contact metamorphism is limited to smaller
areas such as around a magma intrusion into
bedrock
Regional Metamorphism
Contact Metamorphism
Other Forms of Metamorphism
Ultra-high pressure metamorphism occurs deep
in the continental and oceanic crust
High-pressure, low-temperature metamorphism
occurs where oceanic crust subducts under a
continental plate
Shock metamorphism at impact sites
Metamorphic Mineralization
Silicates are the most common mineral in
metamorphic rocks
Minerals can tell you the “grade”of metamorphism
Certain minerals are
uniquely
characteristic of
metamorphic rocks
(kyanite, staurolite,
some garnets)
Metamorphic Mineralization
Contact
Ultra-high pressure
Regional
High-press, low-temp
Proportions of the Rock Types
A Few More Facts…
Igneous processes create new rock
Metamorphic and sedimentary processes
re-work old rock into a new form
Silicate mineral are the most common
minerals found in all types of rocks
Common Minerals Found in Rocks
Five Very Different Locations
Rock Cycle
Rock Cycle
Rock Cycle
Subduction of an
oceanic plate beneath
a continental plate
uplifts a volcanic
mountain range
Magma
rises from
melting
plate and
intrudes or
extrudes in
the crust
Rock Cycle
Magmas cool
to make
igneous rock
Basalt versus
granite
Rock Cycle
Weathering and
erosion creates
sediments
Rock Cycle
Sediments
are carried to
the oceans
and lakes
Lithification
Rock Cycle
Metamorphism of subducted rock
Tectonic plates interact
Rock Cycle
Entire process starts
over as plates interact
Igneous Rocks
Igneous Rocks
How do igneous rocks differ from one another?
Where do they form?
How do rocks solidify from a melt (magma)?
Where do rock melt?
We Classify Igneous Rocks
by Appearance
Mineralogy (Chemistry)
Texture
Igneous Rocks
We Classify Igneous Rocks
by Genetics (Origin)
There are two classes of igneous
rocks
Intrusive
Extrusive
Genetic Classification of
Igneous Rocks
Intrusive igneous rocks crystallized from slowly
cooling magma intruded within the Earth’s crust,
such as granite and gabbro
Granite Intrusions
Genetic Classification of
Igneous Rocks
Extrusive igneous rocks crystallized from rapidly
cooling magma extruded on the surface of the Earth
as lava or erupted as pyroclastic material, such as
basalt
Extrusive Igneous Rocks
Rocks formed from the cooling of lavas extruded
onto the Earth’s surface or onto ocean floors
Rocks formed by the cooling of pyroclastic
material, such as fragmented pieces of magma and
material erupted into the air
We Can Also Classify Igneous
Rocks by Composition
Chemistry
Mineralogy
Chemistry
Modern classification of igneous rock is based
upon the silica (SiO2) content
The silica content is determined by the silicate
minerals that occur in the rock (i.e., the minerals
contain SiO2)
The percentage of silica ranges from about 40% to
about 70%
Rocks are referred to as “silica rich” or “silica poor”
Mineralogy
There are four major divisions of igneous rocks
based upon the content of elements in the
minerals:
Felsic
Intermediate
Mafic
Ultramafic
Felsic Igneous Rocks
Rich (high) in minerals containing silica
Poor (low) in iron and magnesium
They include:
Granite
Rhyolite
Intermediate Igneous Rocks
Intermediate in composition between
felsic and mafic igneous rocks
Less silica, more Fe & Mg than felsic
More silica, less Fe & Mg than mafic
They include:
Granodiorite
Dacite
Diorite
Andesite
Mafic Igneous Rocks
Poor (low) in minerals containing silica
Rich (high) in iron and magnesium
They include:
Gabbro
Basalt
Ultramafic Igneous Rocks
Very uncommon on the Earth’s surface
Very poor (lower) in minerals containing silica
Consist primarily of mafic minerals (olivine,
pyroxene)
The most common ultramafic rock is:
Peridotite
Felsic
Granite
Rhyolite
Intermediate
Granodiorite Diorite
Dacite
Andesite
Mafic
Gabbro
Basalt
Common Minerals of Igneous Rocks
When Do Rocks Melt?
Melting starts at ~700o C
When the temperature exceeds the melting point
of the rock or some minerals within the rock
Minerals melt at different temperatures
When Do Rocks Melt?
Three Factors Affecting Melting of Rocks
Pressure: Increased pressures raises melting
points
Water Content: Increased water content
lowers melting points
Composition: Felsic minerals melt at lower
temperatures than mafic minerals
Magma Differentiation
The process by which rocks of
various compositions can arise from a
uniform parent magma
The
Formation of
Magma
Chambers
Partial melting
Less dense magma
Partial melting
The
Formation of
Magma
Chambers
Magma rises
Less dense magma
Partial melting
The
Formation of
Magma
Chambers
Magma pools in
magma chamber
Magma rises
Less dense magma
Partial melting
The
Formation of
Magma
Chambers
Magma Differentiation
Occurs because different minerals crystallize
(solidify) at different temperatures
In other words, as the magma cools some minerals
form first, some form last
Fractional Crystallization
The process by
which crystals
forming in a cooling
magma are
segregated from the
remaining liquid
In a simple scenario,
the crystal settle to
the floor or adhere to
the walls of the
magma chamber
Fractional Crystallization
The elements (such as Fe and Mg) used to create
the newly formed crystals are now no longer
available for creating new minerals
Therefore the chemical composition of the magma
slowly changes as new minerals are continually
formed and the available matter is selectively used
up
Bowen’s Reaction Series
Experiments that determined the sequence
of crystallization of minerals from a
gradually cooling mafic (basaltic) magma
Conducted by Norman L. Bowen prior to
1916
These experiments totally
rewrote our understanding of
igneous rock formation
Bowen’s Reaction Series
Evidence of Fractional
Crystallization in the Palisades Sill
Evidence of Fractional
Crystallization in the Palisades Sill
first olivine
next pyroxene
& plag. Feldpar
pyroxene
finishes
finally plagioclase
Partial Melting
and the Origin of Magmas
Partial melting of upper mantle:
e.g. at divergent spreading
centers
Mafic
Magmas
Partial melting of continental
crustal rocks
Felsic
Magmas
Magmatic Stoping: Making Room
for the Intrusion of Magma
Wedging open overlying rock
Breaking off large blocks of rock
(remnants of which are called xenoliths)
Melting of surrounding country rock
Rising Magma Wedges Open and
Fractures Overlying Country Rock
Overlying rocks may bow up
Magma melts surrounding rock
…changing the composition of the
magma
Blocks of the Overlying Country
Rocks (Xenoliths) May Break Off and
Sink into the Magma
Plutons
Large igneous bodies
formed at depth in the
Earth’s crust
Types of Plutons
Batholith: Massive, discordant intrusive body
covering at least 100 km2
Stock: Massive, discordant intrusive body
covering less than 100 km2
Dike: Tabular, discordant intrusive body
Sill: Tabular, concordant intrusive body
Types of intrusive and extrusive
igneous structures
Sill
Dike
Where Do Most Magmas Occur?
Divergent Plate Margins
Convergent Plate Margins
Mantle Plumes/Hot Spots
Island Arc
Plate Subduction
(Japanese Islands)
Continental
Plate Subduction
(Mount St. Helens)
Hot Spot
Volcanism
(Hawaii)
Generation of Magmas at
Convergent Plate Margins
Subduction drags oceanic
lithosphere (including a veneer
of “wet” sediments) beneath
the adjacent plate
Generation of Magmas at
Convergent Plate Margins
The release of volatiles lowers the
melting point of the adjacent mantle,
causing fluid-induced melting to form a
mafic magma, which becomes more
intermediate in composition as it rises
through the overlying crust
x
Ophiolite Suites
Unusual assemblages of
rocks found on land that
had characteristics of
seafloor
Composed of deep-sea
sediments, basaltic lavas
and mafic igneous
intrusions
Fragments of ocean crust
moved onto land by plate
tectonics
Idealized Section
of an Ophiolite
Suite
?
Generation of Igneous Rocks at
Divergent Plate Margins
Some of the mafic magma cools in massive magma
chambers to form massive gabbros
Some of the magma is intruded as sheeted dikes
These dikes are feeder for basaltic lava flows which
form pillows as they extrude beneath the ocean