Formation of Sedimentary Rocks
Objectives
• Sequence the formation of sedimentary rocks.
• Explain the formation and classification of clastic
sediments.
• Describe features of sedimentary rocks.
Vocabulary
– sediment
– cementation
– clastic
– bedding
– deposition
– graded bedding
– lithification
– cross-bedding
Formation of Sedimentary Rocks
Formation of Sedimentary Rocks
• Much of Earth’s surface is covered with sediments.
• Sediments are pieces of solid material that
have been deposited on Earth’s surface by wind,
water, ice, gravity, or chemical precipitation.
• When sediments become cemented together,
they form sedimentary rocks.
• The formation of sedimentary rocks begins when
weathering and erosion produce sediments.
Formation of Sedimentary Rocks
Weathering
• Wherever Earth’s crust is exposed at the surface
it is subject to weathering.
• Weathering is a set of physical and chemical
processes that break rock into smaller pieces.
– Chemical weathering occurs when the minerals in a
rock are dissolved or otherwise chemically changed.
– Minerals remain chemically unchanged during
physical weathering.
Formation of Sedimentary Rocks
Weathering
Formation of Sedimentary Rocks
Weathering
• Clastic describes rock and mineral fragments
produced by weathering and erosion.
• Clastic sediments range in size from huge
boulders to microscopic particles.
Formation of Sedimentary Rocks
Erosion and Transport
• After rock fragments have been weathered out of
outcrops, they are transported to new locations.
• Erosion is the removal and movement of surface
materials from one location to another.
• The four main agents of erosion are wind, moving
water, gravity, and glaciers.
• Eroded materials are almost always
carried downhill.
Formation of Sedimentary Rocks
Erosion and Transport
Deposition
– Deposition occurs when sediments are laid down on
the ground or sink to the bottoms of bodies of water.
– Sediments are deposited when transport stops.
– As water or wind slows down, the largest particles
settle out first, then the next-largest, and so on, so
that different-sized particles are sorted into layers.
– Since wind can move only small grains, sand dunes
are commonly made of fine, well-sorted sand.
– Sediment deposits from glaciers and landslides are not
sorted because both move all materials with equal ease.
Formation of Sedimentary Rocks
Erosion and Transport
Burial
– Most sediments are ultimately deposited on Earth in
depressions called sedimentary basins.
– These basins may contain layers of sediment that
together are more than 8 km thick.
– As more and more sediment is deposited in an area, the
bottom layers are subjected to increasing pressure and
temperature which causes lithification.
– Lithification includes the physical and chemical
processes that transform sediments into
sedimentary rocks.
Formation of Sedimentary Rocks
Lithification
• Lithification begins as the weight of overlying
sediments forces the sediment grains closer
together, causing the physical changes.
• Layers of mud shrink as excess water is
squeezed out.
Formation of Sedimentary Rocks
Lithification
• Sand resists additional compaction during burial.
• Grain-to-grain contacts in sand form a supporting
framework that helps maintain open spaces
between the grains.
Formation of Sedimentary Rocks
Lithification
• The temperature in Earth’s crust increases with
depth by about 30°C per kilometer.
• Sediments that are buried 3 to 4 km deep
experience temperatures that are high enough to
start the chemical and mineral changes that
cause cementation.
• Cementation occurs when mineral growth
cements sediment grains together into solid rock.
Formation of Sedimentary Rocks
Lithification
• There are two common types of cementation.
– A new mineral, such as calcite
(CaCO3) or iron oxide (Fe2O3)
grows between sediment
grains as dissolved minerals
precipitate out of groundwater.
– Existing mineral grains grow larger as
more of the same mineral precipitates
from groundwater and crystallizes
around them.
Formation of Sedimentary Rocks
Features of Sedimentary Rocks
• Bedding, or horizontal layering, is the primary
feature of sedimentary rocks.
• The type of bedding depends upon the method
of transport.
• The size of the grains and the material within the
bedding depend upon many factors.
Formation of Sedimentary Rocks
Features of Sedimentary Rocks
• Graded bedding is bedding in which the particle
sizes become progressively heavier and coarser
towards the bottom layers.
• Graded bedding is often observed in marine
sedimentary rocks that were deposited by
underwater landslides.
Formation of Sedimentary Rocks
Features of Sedimentary Rocks
• Cross-bedding is formed as inclined
layers of sediment move forward across a
horizontal surface.
Formation of Sedimentary Rocks
Features of Sedimentary Rocks
• Small-scale cross-bedding can be observed at
sandy beaches and along sandbars in streams
and rivers.
• Most large-scale cross-bedding is formed by
migrating sand dunes.
• Small sedimentary features such as ripple marks
are also preserved in sedimentary rocks.
• If a rippled surface is buried gently by more
sediment without being disturbed, it might later
be preserved in solid rock.
Formation of Sedimentary Rocks
Features of Sedimentary Rocks
Evidence of Past Life
– Fossils are probably the best-known features of
sedimentary rocks.
– Fossils are the preserved remains, impressions, or any
other evidence of once-living organisms.
– Fossils are of great interest to Earth scientists because
fossils provide evidence of the types of organisms that
lived in the distant past, the environments that existed in
the past, and how organisms have changed over time.
Formation of Sedimentary Rocks
Section Assessment
1. Match the following terms with their definitions.
___
C clastic
___
D deposition
___
B lithification
___
A bedding
A. horizontal layering of
sedimentary rocks
B. the physical and chemical
processes that transform
sediments into sedimentary
rocks
C. rock and mineral fragments
produced by weathering
and erosion
D. occurs when sediments are laid
down on the ground or sink to
the bottoms of bodies of water
Formation of Sedimentary Rocks
Section Assessment
2. How is cross-bedding formed?
Cross-bedding is formed as inclined
layers of sediment move forward across
a horizontal surface.
Formation of Sedimentary Rocks
Section Assessment
3. Identify whether the following statements are
true or false.
________
true
Cementation is a form of lithification.
________
false
During deposition, the largest particles end up in
the top layer.
________
true
Graded bedding is often observed in marine
sedimentary rocks.
________
true
Clastic sediments can range in size from
microscopic particles to huge boulders.
Types of Sedimentary Rocks
Objectives
• Describe the types of clastic sedimentary rocks.
• Explain how chemical sedimentary rocks form.
• Describe organic sedimentary rocks.
• Recognize the importance of sedimentary rocks.
Vocabulary
– clastic sedimentary rock
– porosity
– evaporite
Types of Sedimentary Rocks
Types of Sedimentary Rocks
• The classification of sedimentary rocks is based
on how they were formed.
• There are three main groups of sedimentary
rocks: clastic, organic, and chemical.
Types of Sedimentary Rocks
Types of Sedimentary Rocks
Types of Sedimentary Rocks
Clastic Sedimentary Rocks
• Clastic sedimentary rocks, the most common
type of sedimentary rocks, are formed from the
abundant deposits of loose sediments found on
Earth’s surface.
• Clastic sedimentary rocks are further classified
according to the sizes of their particles.
Types of Sedimentary Rocks
Clastic Sedimentary Rocks
Course-Grained Clastics
– Sedimentary rocks consisting of gravel-sized rock
and mineral fragments are classified as
coarse-grained clastics.
– Conglomerates are coarse-grained sedimentary rocks
that have rounded particles, whereas breccias contain
angular fragments.
– Conglomerates, such as gravel, are transported by highenergy flows of water and it becomes abraded and
rounded as the particles scrape against one another.
– The angularity of particles in breccias indicates that the
sediments did not have time to become rounded.
Types of Sedimentary Rocks
Clastic Sedimentary Rocks
Medium-Grained Clastics
– Sedimentary rocks that contain sand-sized rock and
mineral fragments are classified as medium-grained
clastic rocks.
– Sandstone is formed when these medium-sized
sediments are buried and lithified.
– Sandstone has high porosity of up to 30 percent.
– Porosity is the percentage of open spaces between
grains in a rock.
– Sandstone layers are valuable as underground
reservoirs of oil, natural gas, and groundwater.
Types of Sedimentary Rocks
Clastic Sedimentary Rocks
Fine-Grained Clastics
– Sedimentary rocks consisting of silt and mud are
called siltstone and mudstone.
– Siltstone is mostly composed of silt-sized grains,
while shale is composed mostly of silt and
clay-sized particles.
– Shale has very low porosity and often forms barriers
that hinder the movement of groundwater and oil.
Types of Sedimentary Rocks
Chemical Sedimentary Rocks
• During chemical weathering, minerals can be
dissolved and carried into lakes and oceans.
• As water evaporates from the lakes and oceans,
the dissolved minerals are left behind.
• In arid regions, high evaporation rates can
increase the concentration of dissolved minerals
in bodies of water.
Types of Sedimentary Rocks
Chemical Sedimentary Rocks
Rocks Formed from Evaporation
– When the concentration of dissolved minerals in a body
of water reaches saturation, crystal grains precipitate
out of solution and settle to the bottom.
– Evaporites are the layers of chemical sedimentary
rocks that form as a result of the precipitation of
crystal grains.
– Evaporites most commonly form in arid regions, in
oceans and in drainage basins on continents that have
low water flow.
– The three most common evaporite minerals are calcite
(CaCO3), halite (NaCl), and gypsum (CaSO4).
Types of Sedimentary Rocks
Chemical Sedimentary Rocks
Rocks Formed from Evaporation
Types of Sedimentary Rocks
Chemical Sedimentary Rocks
Organic Sedimentary Rocks
– Organic sedimentary rocks are formed from the remains
of once-living things.
– The most abundant organic sedimentary rock is
limestone, which is composed primarily of calcite.
– Calcite comes from the calcium carbonate that some
organisms use to make their shells.
– Calcium carbonate precipitates out of the water and
crystallizes between the grains of carbonate sediment
to form limestone.
– Limestone is common in shallow water environments.
Types of Sedimentary Rocks
Chemical Sedimentary Rocks
Organic Sedimentary Rocks
– Another type of organic sedimentary rock, coal, forms
from the remains of plant material.
– Over long periods of time, thick layers of vegetation
slowly accumulate in swamps and coastal areas and
are buried and compressed.
– Coal is composed almost entirely of carbon and can be
burned for fuel.
Types of Sedimentary Rocks
Importance of Sedimentary Rocks
• The characteristic textures and features of
sedimentary rocks provide a geologic “snapshot”
of surface conditions in Earth’s past.
• By considering all of this information, geologists
can better understand how geologic changes
occur over time.
Types of Sedimentary Rocks
Importance of Sedimentary Rocks
Energy Resources
– The study of sedimentary rocks has great practical
value because many of the natural resources used
by humans come from sedimentary rocks.
– Oil, natural gas, coal, uranium, phosphate, and iron
are found in sedimentary rocks.
– Limestone is processed to make cement for the
construction industry.
– Sandstone and limestone are often cut into blocks for
use in walls and buildings.
Types of Sedimentary Rocks
Section Assessment
1. What is porosity and how is it a valuable
characteristic?
Porosity is the percentage of open spaces
between grains in a rock. It is a valuable
characteristic because a rock with high porosity,
such as sandstone, can be an underground
reservoir for oil, natural gas, and groundwater.
Types of Sedimentary Rocks
Section Assessment
2. The following are which type of
sedimentary rock?
C coal
___
A sandstone
___
E limestone
___
A. clastic
A shale
___
A conglomerate
___
B rock gypsum
___
D. A and B
A breccia
___
B. chemical
C. organic
E. B and C
Types of Sedimentary Rocks
Section Assessment
3. What are the three most common
evaporite minerals?
The three most common evaporite minerals
are calcite (CaCO3), halite (NaCl), and
gypsum (CaSO4).
Metamorphic Rocks
Objectives
• Compare and contrast the different types and
causes of metamorphism.
• Distinguish among metamorphic textures.
• Explain how mineral and compositional changes
occur during metamorphism.
• Understand how rocks continuously change from
one type to another in the rock cycle.
Metamorphic Rocks
Vocabulary
– regional metamorphism
– contact metamorphism
– hydrothermal metamorphism
– foliated
– nonfoliated
– porphyroblast
– rock cycle
Metamorphic Rocks
Causes of Metamorphism
• Metamorphic rock forms when high temperature
and pressure combine to alter the texture,
mineralogy, or chemical composition of a rock
without melting it.
• The high temperatures ultimately are derived
from Earth’s internal heat.
• The high pressures can be generated in two ways:
– From vertical pressure caused by the weight of
overlying rock
– From the compressive forces generated as rocks are
deformed during mountain building
Metamorphic Rocks
Types of Metamorphism
• Different combinations of temperature and
pressure result in different types of
metamorphism.
• Large belts of regional metamorphism are
produced when high temperature and pressure
affect large regions of Earth’s crust.
• Regional metamorphism can be low grade,
intermediate grade, and high grade.
• The grade of regional metamorphism reflects the
relative intensity of temperature and pressure.
Metamorphic Rocks
Types of Metamorphism
Metamorphic Rocks
Types of Metamorphism
• Geologists have
divided the regional
metamorphic belt
that has been
mapped in the
northeastern
United States belt
into zones based
upon the mineral
groups found in
the rocks.
Metamorphic Rocks
Types of Metamorphism
• Knowing the
temperatures that
certain areas
experienced
when rocks were
forming can help
geologists locate
economically
valuable
metamorphic
minerals.
Metamorphic Rocks
Types of Metamorphism
• Some key minerals are used to map
metamorphic zones.
Metamorphic Rocks
Types of Metamorphism
• A local effect called contact metamorphism
occurs when molten rocks, such as those in an
igneous intrusion, come in contact with solid rock.
• High temperature
and moderate-tolow pressure
form the mineral
assemblages that
are characteristic
of contact
metamorphism.
Metamorphic Rocks
Types of Metamorphism
• Because temperature decreases with distance
from an intrusion, metamorphic effects also
decrease with distance.
• Contact
metamorphism
from extrusive
igneous rocks
is limited to
thin zones.
Metamorphic Rocks
Types of Metamorphism
• Hydrothermal metamorphism occurs when very
hot water reacts with rock and alters its chemistry
and mineralogy.
• Hydrothermal fluids can dissolve some minerals,
break down others, and deposit new minerals.
• Hydrothermal metamorphism is common around
igneous intrusions and near active volcanoes.
Metamorphic Rocks
Metamorphic Textures
• Metamorphic rocks are classified into two
textural groups: foliated and nonfoliated.
– Foliated metamorphic rocks are characterized by wavy
layers and bands of minerals.
– High pressure during metamorphism causes minerals
with flat or needlelike crystals to form with their long
axes perpendicular to the pressure.
Metamorphic Rocks
Metamorphic Textures
– Nonfoliated metamorphic rocks lack mineral
grains with long axes in one direction.
– Nonfoliated rocks are composed mainly of minerals
that form with blocky crystal shapes.
– Quartzite and marble are two common examples of
nonfoliated rocks.
Metamorphic Rocks
Metamorphic Textures
Porphyroblasts
– Under certain conditions, new metamorphic minerals
can grow quite large while the surrounding minerals
remain small.
– Porphyroblasts are large crystals, which can range in
size from a few millimeters to a few centimeters.
– Porphyroblasts are found in areas of both contact and
regional metamorphism.
Metamorphic Rocks
Mineral Changes
• Minerals are stable at certain temperatures and
crystallize from magma at different temperatures.
• During metamorphism, the minerals in a rock
change into new minerals that are stable under
the new temperature and pressure conditions.
• Minerals that change in this way are said to
undergo solid-state alterations.
Metamorphic Rocks
Compositional Changes
• When hot fluids migrate in and out of the rock
during metamorphism the original composition of
the rock can change.
• Chemical changes are especially common during
contact metamorphism near igneous intrusions.
• Valuable ore deposits of gold, copper, zinc,
tungsten, and lead are formed through the
invasion of hydrothermal fluids.
Metamorphic Rocks
The Rock Cycle
• Metamorphic rocks are formed by the changing
of other rocks.
• Any rock can be
changed into any
other type of rock.
• The rock cycle
is the continuous
changing and
remaking of rocks.
Metamorphic Rocks
Other Possible Paths
• There is more than one path in the rock cycle.
• The rocks of Earth’s
crust are constantly
being recycled
from one type
to another.
• The processes
that help shape
Earth’s landscapes
are also part of
the rock cycle.
Metamorphic Rocks
Section Assessment
1. Match the following terms with their definitions.
___
C foliated
___
A nonfoliated
___
D porphyroblasts
___
B rock cycle
A. metamorphic rocks that lack
grains with their long axes in
one direction
B. the continuous changing and
remaking of rocks
C. metamorphic rock with wavy
layers and bands of minerals
D. large crystals in metamorphic
rocks that are surrounded by
smaller crystals
Metamorphic Rocks
Section Assessment
2. Identify what causes each type of
metamorphism.
___
B regional
___
D contact
___
A hydrothermal
A. very hot water reacts with
rock and alters its chemistry
and mineralogy
B. high temperatures and
pressures affect large areas of
Earth’s crust
C. molten lava hardens on
Earth’s surface
D. molten rocks come in contact
with solid rock
Metamorphic Rocks
Section Assessment
3. How can sedimentary rocks skip the metamorphic
and igneous stages in the rock cycle?
Rocks can take many different paths through
the rock cycle. If a sedimentary rock is raised
through uplifting and exposed to weathering
and erosion it breaks into sediments, possibly
to be reformed into new sedimentary rock.
Chapter Resources Menu
Study Guide
Section 6.1
Section 6.2
Section 6.3
Chapter Assessment
Image Bank
Section 6.1 Study Guide
Section 6.1 Main Ideas
• The processes of weathering, erosion, deposition, burial, and
lithification form sedimentary rocks.
• Clastic sediments are rock and mineral fragments produced
by weathering and erosion. They are classified based on
particle size.
• Sediments are lithified into rock by the processes of
compaction and cementation.
• Sedimentary rocks can contain depositional features such as
horizontal bedding, cross-bedding, and ripple marks.
• Fossils are the remains or other evidence of once-living things
that are preserved in sedimentary rocks.
Section 6.2 Study Guide
Section 6.2 Main Ideas
• There are three main classes of sedimentary rocks: clastic,
which are formed from clastic sediments; chemical, which are
formed from minerals precipitated from water; and organic,
which are formed from the remains of once-living things.
• Clastic sedimentary rocks are classified by particle size
and shape.
• Evaporites are chemical sedimentary rocks that form
primarily in restricted ocean basins in regions with high
evaporation rates.
• Limestone, composed primarily of calcite, is the most
abundant organic sedimentary rock. Coal is another
organic sedimentary rock.
• Sedimentary rocks provide geologists with information about
surface conditions that existed in Earth’s past.
Section 6.3 Study Guide
Section 6.3 Main Ideas
• Metamorphic rocks are formed when existing rocks are
subjected to high temperature and pressure, which cause
changes in the rocks’ textures, mineralogy, and composition.
• The three main types of metamorphism are regional, contact,
and hydrothermal.
• Metamorphic rocks are divided into two textural groups:
foliated and nonfoliated.
• During metamorphism, minerals change into new minerals
that are stable under the conditions of temperature and
pressure at which they formed.
• The rock cycle is the set of processes whereby rocks
continuously change into other types of rock.
Chapter Assessment
Multiple Choice
1. What is it called when sediments are laid down
on the ground or sink to the bottoms of bodies
of water?
a. erosion
c. deposition
b. transport
d. lithification
Erosion is the removal of surface. Transport is the
movement of eroded materials from one location to
another. Lithification is the chemical and physical
processes that transform sediments into
sedimentary rocks.
Chapter Assessment
Multiple Choice
2. Oil and natural gas reserves will most likely be
found in what type of rock?
a. sandstone
c. limestone
b. shale
d. gypsum
Sandstone has a high porosity, or percentage of open
spaces between the grains of the rock. Porosities of
sandstone can be as high as 30 percent.
Chapter Assessment
Multiple Choice
3. What are the chemical sedimentary rocks that
form as a result of crystal grains precipitating out
of a solution called?
a. porphyroblasts
c. clastic
b. evaporites
d. sediments
Evaporite refers to the evaporation that helps a solution
(body of water) become supersaturated. When a solution
is supersaturated, crystal grains can precipitate out and
collect at the bottom where they become evaporite
sedimentary rock.
Chapter Assessment
Multiple Choice
4. Which of the following is an example of a
nonfoliated metamorphic rock?
a. schist
c. gneiss
b. slate
d. quartzite
Unlike foliated rocks, nonfoliated metamorphic rocks lack
mineral grains with long axis in one direction. Schist,
slate, and gneiss are all foliated rocks.
Chapter Assessment
Multiple Choice
5. What is the horizontal layering that is a primary
characteristic in sedimentary rocks called?
a. bedding
c. metamorphism
b. cementation
d. ripple marks
Bedding can range from a millimeter thick layer of shale
to sandstone deposits several meters thick. The type of
bedding depends upon the method of transport.
Chapter Assessment
Short Answer
6. What are the three grades of regional
metamorphism and how do they relate to
temperature and pressure?
The three grades of regional metamorphism
are low grade, intermediate grade, and high
grade. The grade of regional metamorphism
reflects the relative intensity of temperature
and pressure, with low grade metamorphism
reflecting the lowest temperature and pressure.
Chapter Assessment
Short Answer
7. What is the geological significance of
sedimentary rocks in relation to Earth’s past?
The characteristics of textures and features
in sedimentary rocks, such as cross-bedding,
ripple marks, layering, and fossils, provide a
geologic “snapshot” of surface conditions in
Earth’s past.
Chapter Assessment
True or False
8. Identify whether the following statements are
true or false.
______
true Contact metamorphism is a local effect.
______
false Clastic sediments are created through a
metamorphic process.
______
true Cementation is part of the lithification process.
______
true Coal forms from the remains of plant material.
______
false Hydrothermal metamorphism does not alter the
chemistry or mineralogy of a rock.
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