Download Chapter 22: Minerals, Rocks, and Volcanoes

4/12/2010
Geology
• Geology – the study of the the composition,
structure, physical/chemical processes, and
history of the Earth
Chapter 22: Minerals, Rocks,
and Volcanoes
– Geology is also related to the study of planets and
moons of our solar system
• Homework: All questions on the
“Multiple-Choice” and the oddnumbered questions on “Exercises”
sections at the end of the chapter.
• The basic concepts of geology are introduced
and briefly discussed in the next four chapter
so that we may better understand the
physical nature of the world that we live in.
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Intro
Minerals
Minerals
• Mineral – a naturally occurring, inorganic,
crystalline (solid) substance consisting of one
or more chemical elements in fairly specific
proportions with a distinctive set of physical
properties
• Minerals are around us everywhere – some
are quite valuable (diamond, sapphires,
emeralds) other minerals are very common
(calcite, quartz.)
• Mineralogy – the study of minerals
• We also speak of certain rock-types (ores) as
being rich in minerals.
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Section 21.1
– For example an iron ore, gold ore, or copper ore
– Rocks classified as ores have a commercially
valuable amount of a some type of element or
mineral.
• In many cases, mineral names have some
type of historical connotation and reflect the
name of a geographic locality or a person’s
name.
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Relative Abundance by Mass of
Elements in the Earth’s Crust
Mineral Composition
• Eight chemical elements are the major
constituents of most of the minerals.
• These elements are the most abundant
elements in the Earth’s crust and include O,
Si, Al, Fe, Ca, Na, K, and Mg.
• Over 2000 minerals have been identified in
the Earth’s crust.
• Only about 20 of these minerals are common.
• Less than 10 minerals account for more than
90% of the Earth’s crust by mass.
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Section 21.1
• Only two
elements, O
& Si, account
for 74% of
the elements
(by mass) in
the Earth’s
crust.
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Mineral Groups – the Silicates
Silicates
• Silicate structure is based on a network of
SiO4-4 tetrahedra.
• These basic building blocks of the silicates
are called the silicon-oxygen tetrahedra.
• Most minerals that form crustal rocks
have significant amounts of oxygen and
silicon.
• The mineral quartz (silicon dioxide or
silica) is entirely composed of O & Si,
and has the chemical formula, SiO2.
• Quartz and many other minerals that
contain O & Si are called silicates.
• Silicates are the most abundant family
of rock-forming minerals in the crust.
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Section 21.1
– Covalent bonding occurs within the
tetrahedron.
• A significant amount of variation occurs in
the structural arrangement of these
tetrahedra.
• The silicon-oxygen tetrahedra may exist
independently (no sharing of O) or they
may share oxygens in various ways.
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The SiliconOxygen
Tetrahedron
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• In addition to O & Si, most silicate minerals
also contain Al and one or more other
elements.
• The various silicate minerals result from the
different structural arrangement of the siliconoxygen tetrahedron building blocks and the
different metal ions within these structures.
• The silicon-oxygen tetrahedra may be
arranged independently, in single chains, in
double chains, as continuous sheets, and as
a 3-dimensional network.
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Molecular
Structures of
Several Common
Silicate Minerals
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Section 21.1
Silicates
• Basic Building
Block of the
Silicate
Mineral Family
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Feldspars – Most Common Silicate Minerals
• Feldspar group - a group of structurallyrelated (3-dimension network) minerals,
that as a set, are the most abundant
minerals in the Earth’s crust
• There are two basic types of feldspars:
• Plagioclase feldspars – O, Si, Al, and
Ca or Na
• Potassium feldspars – O, Si, Al, and K
• All silicates have Si and O in their
formulas.
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Nonsilicate Minerals
Carbonates
• All of the other minerals in the crust are
considered “nonsilicate.”
• Nonsilicate minerals comprise less than
10% of the minerals in the crust.
• Carbonates, oxides, and sulfides are
the most common nonsilicate minerals.
• Pure elements such as gold and silver,
and some gemstones (diamond, ruby,
sapphire) are also nonsilicates.
• Carbonate minerals form when the carbonate
ion (CO32-) bonds with certain metal ions.
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Section 21.1
– Ca, Mg, Mn, Fe, and others may bond with CO32-
• CaCO3, calcite, is a very common carbonate
mineral.
– CaCO3 readily reacts (dissolves) in acidic water.
• Calcite is a very common mineral component
of the the rock limestone.
– Limestones will dissolve over time in slightly acidic
ground waters, contributing to cave formation.
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Oxides & Sulfides
Section 21.1
• Mineral classification is based on both
physical and chemical properties.
• This is advantageous because there are
some minerals that have the same chemical
formula but different molecular structures.
• For example the two minerals graphite and
diamond are both made of pure C but they
are dramatically different minerals.
– Fe2O3, SnO2, and UO2 are all oxide ores for their
respective metallic ion.
• Sulfides minerals form through the bonding of
S ions with metallic ions.
– Fe, Pb, Cu, Zn, and others may bond with S2-.
– FeS2 is pyrite, commonly called “fool’s gold.”
– CuFeS2, PbS, ZnS are all sulfide ores for their
respective metallic ion.
– Pure diamond is very hard, clear, and crystalline.
– Pure graphite is soft and black (dry lubricant and
pencil lead.)
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Section 21.1
Identification of Minerals
Physical Properties – Crystal Form
• Certainly minerals can be identified by careful
and costly chemical analysis.
• Many minerals can be easily identified by
taking advantage of the distinctive physical
properties that each exhibits.
• These physical properties are easily learned
and well known by any serious rock and
mineral collector.
• Crystal form – the outward form of a crystal is
a visible representation of its internal
molecular arrangement
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– Many minerals have characteristic crystal forms.
– If crystallization occurs in unrestricted space, with
just the right components, and just the right P & T
conditions a perfect crystal forms. This is very rare.
• Usually an aggregate of crystals form with
none exhibiting a perfect crystal form.
– A particular physical property that helps ID one
mineral may not help with another, so the key is to
find which property ID’s which mineral.
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Identification of Minerals
• Oxide minerals form through the bonding of O
ions with metallic ions.
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Each of these minerals has an orderly
internal atomic structure that forms a cube.
Galena
(PbS)
Pyrite
(FeS2)
Physical Properties – Hardness & Cleavage
• Hardness – a mineral’s resistance to
scratching or abrasion
– Hardness is relative to other minerals or
substances.
• Cleavage – the tendency of some
minerals to break along distinct planes
Halite
(NaCl)
– These planes represent planes in the
crystal’s structure where the bonds are
weakest.
Fluorite
(CaF2)
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Physical Properties – Fracture & Color
Mohs
Hardness
Scale
• Fracture – irregular or random breakage of a
mineral
Friedrich Mohs
(1773-1839)
– Minerals that do not cleave have no planes of
weakness in their structure, therefore do not break
in a uniform manner.
• Color – the property of reflecting particular
light wavelength
– Although this is probably the first property noticed,
it is usually not reliable.
– Quartz, for example, occurs in many colors,
including clear, milky, smoky, yellow, purple, red,
orange, pink.
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Physical Properties – Streak & Luster
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Hematite – red brown streak
• Streak – the color of the mineral in powder
form
– Although an intact mineral sample my exhibit
several colors, the streak color is always the
same.
• Luster – how a mineral’s surface reflects light
– Minerals may exhibit either metallic or nonmetallic
luster.
– Metallic lusters appear like polished metal.
– Nonmetallic lusters can be vitreous (glassy),
adamantine (diamond), pearly (opal), greasy
(talc), or earthy/dull (clay.)
Source: Breck P. Kent
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Physical Properties – Specific Gravity
Rocks
• Specific gravity – ratio of a mineral’s weight to
the weight of an equal volume of pure water
• Many minerals have specific gravities
between 3-4 (in other words, 3 to 4 times as
heavy as water.)
• Some minerals have a significantly higher
specific gravity.
• Rock – a natural, solid, cohesive
aggregate of one or more minerals
• Different types of rocks comprise the
vast majority of the Earth’s crust.
• In general, when we look at a rock we
do not see the individual minerals.
• Rocks can be divided into three major
types as a result of the way they
originated.
– Galena (PbS) has a specific gravity of 7.6.
– Pure gold (Au) has a specific gravity of 19.3.
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Igneous Rocks
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Enchanted Rock, TX – Solidified Magma
• Igneous rock – a type of rock formed from a
molten material that has cooled and solidified
• Magma – molten rock material that originates
deep within the Earth
– Rocks that solidify from a magma, beneath the
Earth’s surface, are called “intrusive” igneous
rocks.
• Lava – molten rock material that reaches the
Earth’s surface due to a volcanic eruption
– Rocks that solidify from lava, at the surface, are
called “extrusive” igneous rocks.
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Kalapana Flow, HI – Solidified Lava
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Sedimentary Rocks
•
•
Sedimentary rock – rocks that form at or
very near the surface of the Earth due to
compaction and cementation of sediments
The sediments that comprise sedimentary
rocks come from three general sources:
1) Rock fragments due to the erosion of preexisting
(older) rocks
2) Minerals chemically precipitated from solution
3) Plants or animal remains (fossils)
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Grand Canyon, AZ –
Sedimentary Rock Layers
Metamorphic Rocks
• Metamorphic rock – forms by the
alteration of a preexisting rock due to
the effects of pressure, high
temperature, and/or a chemical change
• Metamorphism generally occurs well
below the surface of the Earth but at
shallower depths and temperatures than
would cause the rock to melt.
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El Paso, TX Contorted Metamorphic Rocks
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Uniformitarianism
• Beginning with the Scottish physician/scientist
James Hutton, scientists started to realize
that ancient rocks were formed the same way
as modern rocks.
– Since they were formed the same way, they can
be interpreted similarly.
• Unifomitarianism – geologic processes
occurring today operated similarly in the past
and can therefore be used to explain past
geologic events
– “The present is the key to the past”
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Hutton and the Rock Cycle
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The Rock
Cycle
• James Hutton recognized that rocks in
the Earth’s crust were continuously
being formed, broken down, and then
re-formed.
– These are the same processes that are
responsible for the three types of rocks –
igneous, sedimentary, and metamorphic.
• This model of the continuous cycling of
the crustal rock material is called the
rock cycle.
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Formation of Igneous Rocks
Hutton and Geologic Time
• Igneous rocks form from the solidification
of molten material that generally
originates far beneath the surface of the
Earth.
• Magma – molten material beneath the
Earth’s surface
• Lava – molten material a the Earth’s
surface
• Hutton also recognized that the processes
that form and breakdown the different rocktypes take enormous amounts of time.
– Thus he hypothesized the Earth to be very old.
• Central to the science of geology are the
following three concepts:
– The principle of uniformitarianism
– The rock cycle
– The recognition that the Earth is very old
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– The term “lava” is also used to describe the
resulting and cooled igneous rock.
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Igneous Rocks
Section 21.3
• Igneous rocks can be divided into two
basic categories:
Section 21.3
– Extrusive igneous – igneous rocks that
cool at the surface of the Earth, generally
due to some type of volcanic eruption
– Intrusive igneous – igneous rocks that
cooled somewhere beneath the surface of
the Earth
• Intrusive igneous rocks only appear at the
Earth’s surface due to erosion of the overlying
rocks or due to some type of uplift.
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Section 21.3
Plate Tectonics
Plate Tectonics
• The prediction of individual volcanic eruptions
is generally not possible.
• However we are aware of specific trends
where volcanic eruptions typically do occur.
• Most active volcanoes are located along
linear zones, particularly along the margins of
the Pacific Ocean.
• According to the Theory of Plate Tectonics
the solid outermost shell of the Earth is called
the lithosphere.
• The rigid lithosphere rests or “floats” on a
semimolten layer called the asthenosphere.
• We will study plate tectonics in greater detail
in the next chapter.
• According to plate tectonics the lithospheric
plates slowly move over the semimolten
asthenosphere.
• The theory of plate tectonics can explain why
volcanoes occur where they do.
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– The lithosphere is separated into several large
and small fragments, called plates.
– The so-called “ring of fire”
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Types of Igneous Rocks
• Most geologists think that the first rocks on
Earth were formed as the outer crust slowly
solidified billions of years ago.
• Therefore, the first rocks formed were
igneous.
• Since these initial rocks were formed,
geologic processes have modified, covered,
or eroded most of these ancient materials.
• It is estimated that approximately 80% of the
Earth’s crust is comprised of igneous rocks.
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Most Volcanoes and Earthquakes occur along Plate
Boundaries in thePacific Ocean – “Ring of Fire”
Plate Boundaries
• The lithospheric plates interact with
each other in three basic ways:
• Convergent boundary – two plates
move towards each other
• Divergent boundary – two plates move
away from each other
• Transform boundary – two plates slide
past each other
• The vast majority of the volcanoes on
Earth occur at convergent boundaries.
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Subduction forms a Volcanic Island Arc
Convergent Boundaries
The islands of Japan are a result of subduction.
• When both converging plates consist of
oceanic crust one plate will bend and
slide slowly underneath the other – a
process called subduction.
• During this process of subduction, an
enormous amount of friction is
generated, resulting in the melting of
rocks close to the subduction zone.
• The new magma rises to the surface
and forms a volcanic island arc.
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Convergent Boundaries
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Andes Mountains, South America
• Another type of convergent boundary exists
when an oceanic plate converges with a
continental plate.
• The oceanic plate is more dense and
therefore is subducted beneath the
continental plate.
– Once again friction between the two plates will
melt nearby rocks.
• The Andes and Cascade Mountains are both
volcanic mountain ranges formed by the
convergence of an oceanic & continental
plate.
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Igneous Rock Texture and Composition
Texture and Cooling Rate
• Recall, there are two types of igneous rocks:
• The texture (grain or crystal size) of an
igneous rock is determined primarily by the
cooling rate of the magma or lava.
• If a magma cools very slowly, deep within the
Earth, large crystals can form.
• If lava cools rapidly at the surface, large
crystals do not have sufficient time to form.
• If lava cools extremely rapidly (in water or
ice), no crystals form, resulting in volcanic
glass.
– Intrusive – cool slowly below Earth’s surface
– Extrusive – cool quickly at the Earth’s surface
• Igneous rocks are classified according to two
criteria:
– Texture – generally refers to the size of the
mineral grains (crystals) in the igneous rock
– Mineral Composition – refers to the specific type
of mineral crystals in the igneous rock
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Section 21.3
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Mount Rushmore, South Dakota – Granite
Visible Crystals – Slow Cooling
Mt. Rushmore, South Dakota
This rock originally cooled slowly at great depth.
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Composition and the Color of
Igneous Rocks
Effects of Cooling on the Textures of
Igneous Rocks
• In general, the color of an igneous rock gives
a clue as to its composition.
• The amount of silica (SiO2) within an igneous
rock can be used to classify it according to
composition.
• If the magma/lava is high in silica, minerals
form with abundant Si, Na, & K. These
minerals are usually light in color.
• When the magma/lava is low is silica,
minerals rich in Fe, Mg, and Ca form that are
dark in color.
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Stone Mountain, Georgia. Light Colored
Granite – High in Silica
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Dark Volcanic Basalt – Low in Silica
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Common Igneous Rocks
Organized by Composition
Density and Composition
• The dark low-silica rocks are more dense
than the light colored high-silica rocks.
• Continental plates are composed of less
dense high-silica rocks and therefore stand
high about sea level.
• Oceanic plates are composed of more dense
low-silica rocks and therefore rest lower,
usually below sea level.
• Andesite is the name given to many rocks of
medium-silica content.
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Plutons
Pluton
• A pluton is said to be discordant if it cuts
across the grain or layering of the
surrounding rock.
• Pluton – a large body of intrusive
igneous rock formed below the Earth’s
surface by solidification of magma
• Plutons are classified according to two
criteria:
– Batholiths and dikes are discordant
igneous bodies.
• A pluton is said to be concordant if it is
parallel to the grain or layering of the
surrounding rock.
– Size (exposed aerial extent) and shape of
the intrusive rock body
– Relationship of the intrusive rock body to
the surrounding rock that they penetrate
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Section 21.4
– Sills and laccoliths are concordant igneous
bodies.
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Plutonic Bodies
Batholiths
• Batholith – large exposed discordant
intrusive igneous body
– By definition, a batholith must have an
exposed area of at least 103 km2.
• They originally crystallize slowly at great
depths below the surface of the Earth.
• Batholiths only become exposed at the
Earth’s surface when powerful forces
push them up or when a very deep
canyon is carved by erosion.
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Yosemite National Park, CA
Part of the Sierra Nevada Batholith
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Dikes
• Dike – a discordant intrusive igneous
body formed from magma that filled a
vertical or near-vertical fracture
• Since most fractures are narrow, the
infilling intrusive igneous material
hardens into a thin vertical sheet of
igneous rock.
• The size and extent of individual dikes
is quite variable, depending on the
dimensions of the fractures they fill.
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Volcanic Dike Exposed in Colorado
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Sills & Laccoliths
The dike is more resistant to weathering.
• Sills are similar to dikes except they are
concordant to existing layering.
– Sills are injected between layers of
preexisting rock.
• Laccoliths are also concordant, and are
injected between layers of preexisting
rock.
• Laccoliths cause a noticeable blistering
of the overlying rock layers.
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Green Mountain, WY – Laccolith
Volcanoes
The preexisting layers of rock are blistered up.
• Volcano – a hill or mountain formed by
the accumulation of lava and volcanic
rock fragments ejected through a vent in
the Earth’s surface
• Three basic products are ejected from
active volcanoes:
– Gas
– Lava
– Solid rocks
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Tephra Emissions
Products of Volcanic Eruptions
Active Volcano near Huehuetenango, Guatamala
• Gases are generally expelled from a volcano
during its entire life cycle .
– Mainly H2O with lesser amounts of CO2 and H2S
• Lava is extruded from volcanoes in varying
amounts and varying viscosities.
• Some volcanoes also emit large volumes of
solid material, collectively known as tephra.
– Tephra is expelled in almost any size.
– Tephra includes material that is initially ejected
molten and hits the ground as a solid.
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Volcano – Eruptive Style
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Magma Viscosity
• Volcanoes display two basic eruptive styles:
explosive and peaceful (non-explosive)
• The viscosity of the magma largely
determines the eruptive style of a particular
volcano.
• Low viscosity magma can flow easily and
therefore is characterized by peaceful
eruptions.
• High viscosity magma has difficulty flowing
and only moves when subjected to great
pressure.
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Section 21.4
• Magma viscosity, in turn, is dependent on two
factors: temperature and silica content
• The higher the temperature the lower the
viscosity. (flows easier)
– In general magmas that originate deep within the
Earth have higher temperatures.
• The higher the silica content the higher the
viscosity. (more difficult to flow)
– In general magmas that originate at shallow
depths have a higher silica content.
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Peaceful Eruptive Style
Basaltic Hawaiian Lava – Peaceful Eruption
• Peaceful eruptions are involve basaltic
magmas.
• Basaltic magmas originate deep within
the Earth and therefore are very hot.
• Basaltic magmas also have a relatively
low silica content.
• Due to the combination of high
temperature and low silica content,
basaltic magmas flow easily, resulting in
peaceful eruptions.
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Hawaiian Islands &
Emperor Seamount Chain
Basaltic Eruptions
• Rising magma from a stationary “hot spot” in the
asthenosphere forms volcanoes as the Pacific plate
moves slowly over the hot spot.
• Most basaltic eruptions occur in two distinct
settings:
– Along the length of a divergent plate boundary
– In isolated areas, as a lithospheric plate moves
over an unusually hot and stationary zone in the
upper mantle, called a “hot spot” or mantle plume
• Hot spots appear at the surface as a line of
active, dormant, and extinct volcanoes.
– The Hawaiian Islands and Emperor Seamounts
are a continuous series of volcanoes formed over
the past 70 million years.
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Volcanic Features High in the Andes
Mountains Due to Subduction Beneath
South America
Explosive Eruptive Style
• Explosive eruptions generally occur along
subduction zones and involve silica-rich
magma.
• Magmas that originate within a subduction
zone are not deep, and are therefore cooler
in temperature.
• These magmas are also higher in silica
content.
• Due to the combination of ‘low’ temperature
and high silica content, these magmas are
very viscous, resulting in explosive eruptions.
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Explosive Eruption - Mt. Saint Helens
Volcanic Structures
Due to Subduction Beneath North America
• Molten volcanic material that reaches the
surface of the Earth creates a number of
dramatic surface expressions.
• The type of volcanic feature formed is largely
dependent on the composition and
temperature of the lava extruded.
• Specific and common volcanic structures
include flood basalts, shield volcanoes,
stratovolcanoes, cinder cones, and calderas.
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Fissure Eruptions and Flood Basalts
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Columbia Plateau Flood Basalts, Oregon
• Fissure eruptions take place when large
volumes of lava are extruded from long
fractures along the surface of the Earth.
• Significant portions of Washington, Oregon,
and Idaho are covered with a series of
ancient flood basalts called the Columbia
Plateau.
• The basaltic lava that formed these layers
was extremely fluid (high temperature & low
silica content), eventually covering an area of
576,000 km2.
– Average thickness of 150 meters
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Kilauea Crater, Hawaii- Shield Volcano
Shield Volcanoes
Note the very gentle slope away from the crater.
• Shield volcanoes form from lava that is
not quite as fluid as the lava that formed
flood basalts.
• The Hawaiian volcanoes are the classic
examples of shield volcanoes.
• Shield volcanoes have very gentle
slopes and were formed by repeated
basaltic flows.
• Eruptions are generally frequent but not
particularly violent.
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Stratovolcanoes
Mt. Shasta, CA – Stratovolcano
• Stratovolcanoes are composed of alternating
layers of medium- and high-silica content
lava and tephra.
• These volcanoes generally have violent
eruptions, although they usually erupt much
less frequently than shield volcanoes.
• Most of the majestic solitary mountains of the
world are stratovolcanoes.
– For example, Mount Fuji, Mount Shasta, Mt. St.
Helens, and Mount Hood are all stratovolcanoes
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Recent Volcanic Cinder Cone to the right of
an older ‘grown-over’ and eroded Cinder
Cone
Cinder Cones
• Some volcanic features are relatively
small.
• Cinder cones are built when a volcanic
eruption is particularly high in gas
content, and thus emits primarily tephra.
• Cinder cones rarely exceed 300 m in
height.
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Crater Lake occupies a Caldera at the top
of Mt. Mazama
Caldera
• Caldera – a large, roughly circular,
steep-walled depression at the
volcano’s summit, formed when the roof
of the magma chamber collapses after
all the volcanic material within has been
emitted
• Crater Lake in southern Oregon
occupies a caldera formed by the
collapse of the magma chamber of a
once active stratovolcano.
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Devastation Due to 1980 Mt. St. Helen Eruption
Historic Eruptions
Over 8 km from the volcano’s summit
• Mount Saint Helens 1980 – a stratovolcano
located in southern Washington state that
had been dormant since 1857
– This eruption devastated more than 400 km2 of
area and resulted in the death of more than 60
people.
• Mount Pinatubo 1991 – located in the
Philippines
– Propelled ash into the atmosphere to a height of
over 24 km
– Destroyed thousands of acres of cropland and
forced the abandonment of Clark Air Force base
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Section 21.4
Why do People Continue to live in the
Shadow of a Dangerous Volcano?
Sediments & Sedimentary Rocks
• An eruption by Mt. Vesuvius in A.D. 79
buried 2000 residents of Pompeii.
• Yet the large and growing Italian city of
Naples lies only a few kilometers from
Mt. Vesuvius.
• Soils derived from volcanic ash are rich
and fertile, resulting in an extremely
productive agricultural area.
• Particles that are eroded from one place
are later deposited as sediments
somewhere else, usually along rivers, in
lakes, at the shore, or on the seafloor.
• Hutton also observed layered rocks in
the highlands that were composed of
cemented sand and other rock
fragments. He called these
sedimentary rocks.
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Sedimentary Rocks
21 | 94
Importance of Sedimentary Rocks
• Although sedimentary rocks only comprise
5% of the Earth’s crust, they are exposed
over approximately 75% of the Earth’s
surface.
• Hutton also realized that most sediments
were deposited in horizontal layers, or strata.
• Older layers are converted to sedimentary
rock as they are compacted by the weight of
the overlying and successively younger
layers.
• Later, some of these sedimentary layers are
pushed up by powerful forces within the Earth
to form mountain ranges.
• The process continues as the newly uplifted
areas are affected by weathering processes.
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Section 21.5
– The Earth’s crust is dominated by igneous and
metamorphic rocks with sedimentary rocks serving
as a thin veneer of only a few kilometers in
thickness.
• Many of the necessities of modern life come
directly from sedimentary rocks.
– Petroleum, coal, some metal deposits, and many
materials used in the construction industry come
from sedimentary rocks.
21 | 95
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The Grand Canyon – Erosion of
Sedimentary Rocks
Sedimentary Rocks - Landforms
• Some of the most picturesque spots on
Earth are composed of sedimentary
rocks.
• Sedimentary rocks occur in a number of
varieties and colors.
• The Colorado Plateau of the
southwestern U.S. offers a particularly
outstanding display of sedimentary
rocks and their erosional products.
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Section 21.5
Origins of Sedimentary Rocks
Sedimentary Rock Classification
• Lithification – the process of transforming
sediments into sedimentary rocks
• Compaction and cementation of the
loose sediments are the dominantly
responsible for lithification.
• Calcium carbonate (CaCO3), silica
(SiO2), and iron oxides that are dissolved
in groudwater serve as the dominant
cementing agents.
• Sedimentary rocks can be classified
according to the origin of their
constituents.
• There are two main types of sediments:
detrital and chemical.
• Detrital sediments originate from the
solid fragments (detritus) that erode
from preexisting rocks.
• Chemical sediments originate as
dissolved minerals in solution.
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Detrital Sedimentary Rocks
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Classification of Detrital Sedimentary Rocks
• Detrital sedimentary rocks are classified
according to the grain size of its components.
• Shale – composed of very fine-grained
particles that were initially mud
• Sandstone – composed of sand-sized
particles
• Conglomerate – composed of rounded
pebbles
• Breccia – composed of angular pebbles
• Detrital rock particles are generally held
together by CaCO3 or SiO2.
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Chemical Sedimentary Rocks
Types of Chemical Sedimentary Rocks
• Chemical sedimentary rocks are divided
into two basic types: organic and
inorganic.
• In both cases the chemical sediments
are composed of minerals that were
transported in solution to their eventual
deposition site.
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Organic Chemical Sedimentary Rocks
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Organic Limestone, full of marine fossils,
Everglades, Florida
• Organic sediments are formed largely
by the action of organisms that remove
the minerals out of solution.
• For example, many large and
microscopic organisms in the ocean
extract CaCO3 out of seawater to
construct skeletal and shell material.
• As these organisms die, their hard parts
come to rest on the bottom and
eventually lithify into organic limestone.
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Coal
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Inorganic Chemical Sedimentary Rocks
• Although coal is not formed from
minerals carried in solution, it is still
classified as an organic sedimentary
rock.
• Coal is formed from the lithified remains
of ancient plants.
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• Inorganic chemical sedimentary rocks
are commonly formed by the
evaporation of water.
• As the water evaporates, dissolved
minerals in solution will precipitate out.
• Examples of inorganic chemical
sedimentary rocks include halite (HCl
rock salt), gypsum (CaSO4), and
various cave deposits (CaCO3).
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Sedimentary Rock Characteristics
Rock Color Can be Quite Varied
• Many inherent characteristics of sedimentary
rocks are used by geologists to gain a better
understanding of the conditions under which
these rocks were deposited.
• Characteristics such as rock color, grain
rounding, grain sorting, bedding, fossil
content, ripple marks, mud cracks, footprints,
and even raindrop prints within the rock
reveal the rock’s origin and history.
• Gray shades - common in sedimentary rocks
– This color indicates that the rock was deposited in
shallow, well aerated marine waters.
• Shades of yellow, brown, and red indicate
that the rock was deposited on land, above
sea level in the presence of abundant
oxygen.
– These bright shades indicate an iron oxide.
• Dark gray to black rocks contain abundant
amounts of organic carbon.
– Accumulated in stagnant, oxygen-poor areas
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Bright Colors Indicate Terrestrial Deposition
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Grain Rounding and Size
• Angular grains indicate that these sediments
were not transported very far.
• Rounded grains indicate great transport
distances and vigorous water/wind action that
wore away the sharp edges.
• Large grains indicate that strong currents
deposited the sediments.
• Small grains indicate that a very quiet
environment existed when the sediments
were deposited.
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Glacial Moraine, WY
Sorting
Glacial deposits are typically poorly-sorted
• Sorting is the degree to which sediments are
separated according to size.
• Well-sorted sediments are comprised of
grains all about the same size.
– Well-sorted sediments have been transported
great distances over long periods of time.
• Poorly-sorted sediments are comprised of
grains a vastly different sizes.
– Poorly-sorted sediments indicate a short
transportation distance and/or time.
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Bedding/Stratification
• Bedding – layering of the rock that develops
as the sediment is deposited
• Bedding may be very obvious or subtle. It
may be very thick (massive) or it may be thin.
• Most bedding is horizontal, since most
sediments come to rest due to gravity.
• In some instances, where strong water/wind
currents are present, bedding is tilted. This
type of bedding is called cross-bedding.
Cross-bedding
within a Sandstone
Zion National Park,
Utah
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Dinosaur tracks and
fossilized dinosaur
bones are both
considered fossils.
Fossils
• Perhaps the most distinctive and
interesting feature of many sedimentary
rocks is the fossils it may contain.
• Fossil – the remains, an imprint, or any
type of trace of an ancient organism,
preserved in the rock record
• Later in Chapter 24 we will discuss in
detail the use of fossils in the
determination of geologic age.
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Metamorphic Rocks
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Metamorphism
• As we have already learned, igneous rocks
form by the crystallization of magma that
generally forms deep within the Earth.
• Sedimentary rocks are deposited at or near
the surface of the Earth.
• Metamorphic rocks form in conditions below
where sedimentary rocks form and above
where igneous rocks form.
– Metamorphic rocks form where pressure and
temperature conditions lie between the other two
rock types.
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• Metamorphism – conditions within the Earth
that result in the changing of the structure
and mineral content of a solid rock without
melting it
– Any type of rock may be metamorphosed igneous, sedimentary, or even metamorphic.
• Heat, pressure, and chemically active fluids
are the ‘agents’ of metamorphism.
– Heat and pressure break some of the mineral
bonds in the original mineral, with the fluids
serving to allow movement of the ions.
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Classification of Some Common
Metamorphic Rocks
Parent Rocks
• Parent rock – the original rock before
metamorphism takes place
• In addition to heat, pressure, and fluid activity,
the composition of the parent rock helps
determine what metamorphic rock forms.
• The overall process of metamorphism can
change both the texture (crystal size) and the
mineral composition of the parent rock.
– Or metamorphism could only change one of these
characteristics of the parent rock
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Parent Rock  Metamorphic Rock
– Both of these rocks are composed of CaCO3.
• When limestone is metamorphosed into
marble, the ‘structure’ of it is changed but not
the chemical composition.
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Fossiliferous Limestone & Marble
As the limestone recrystallizes the fossils are destroyed
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• In general, if the parent rock only contains
one mineral. The metamorphic rock will also
be composed of that same mineral.
• For example limestone (parent rock) will
metamorphose into marble.
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Fossiliferous Limestone
Marble
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Parent Rock  Metamorphic Rock
• The change from sandstone to quartzite
is another example of the
metamorphism of a rock containing only
one mineral.
– Both sandstone and quartzite are
composed of SiO2.
• When sufficient heat and pressure are
applied to a sandstone, the grains fuse
together to form the very resistant rock
quartzite.
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Parent Rock  Metamorphic Rock
• If the parent rock is composed of several
minerals, the process of metamorphism will
create a new suite of different minerals.
– These new minerals are formed in response to the
escalated heat and pressure conditions.
• Shale is a very fine-grained mixture of several
minerals: quartz, clay, mica, and chlorite.
• Depending on the degree of metamorphism,
shale will be progressively changed into slate,
schist, or gneiss.
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Effects of Increasing Temperature and
Pressure on the Metamorphism of Shale
Types of Metamorphism
• Three basics types of metamorphism are
recognized: contact, shear, and regional.
• Contact metamorphism occurs when magma
comes into direct contact with the parent rock
– Changes in the parent rock are primarily due to
very high temperatures but not increased
pressures.
• Immediately next to the magma, intense
metamorphism results in the formation of
coarse-grained crystals.
– Moving away from the magma, the resulting rock
becomes progressively finer-grained.
• A contact metamorphic zone occurs when a
parent rock is intruded by molten magma.
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Types of Metamorphism
Types of Metamorphism
• Shear metamorphism results from the
intense pressures that exists along
active fault zones, where rock units
slide past each other.
• Mechanical deformation and
recrystallization of the minerals result
from the heat, pressure, and movement
of fluids as rock units slide or shear past
one another.
• Regional metamorphism affects
extremely large areas and is caused by
a combination of both high temperature
and high pressure.
• Most areas that are affected by regional
metamorphism are areas undergoing
intense deformation due to mountain
building processes.
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– Convergent plate boundaries are common
zones of regional metamorphism.
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Foliation
Foliation
• The enormous pressures that accompany
regional metamorphism cause the newly
formed mineral grains to align themselves in
a distinctly parallel arrangement.
• Foliation – the prominent layering in a
metamorphic rock
• Only rocks that are metamorphosed under
intense pressure will exhibit foliation.
– Metamorphic rocks that form almost entirely due to
high temperatures will not have foliation.
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Section 21.6 21 | 131
• The progressive patterns of foliation that
develop in a metamorphosed shale serve as
a good example of foliation types.
• Slaty cleavage develops when a shale
undergoes mild metamorphism.
• Schists are a foliated metamorphic rock with
larger, visible grains (crystals.)
• Gneisses form under intense regional
metamorphism and display banding.
– The more intense the metamorphism, the larger
the crystal but less distinct the foliation.
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22