Download Formation of Sedimentary Rocks

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 and move
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.
Types of Weathering
Physical Weathering (Four Types) –
Processes in which rock is broken into smaller
fragments by physical means. The chemical
composition of the weathered rock remains the
same as the parent material.
• Abrasion
• Frost Action (Frost/Ice Wedging)
• Root Pry
• Exfoliation
Physical Weathering: Abrasion
Abrasion –
• Occurs when
objects rub against,
or collide, with one
another
• Rounding and a
reduction in
sediment size are
the results of
abrasion
Physical Weathering: Frost Action
Frost Action –
• Occurs when water
repeatedly freezes,
expands, and thaws in
the fractures of rock
• This is one of the most
common forms of
physical weathering.
Physical Weathering: Frost Action
Physical Weathering: Root Pry
Root Pry –
• Occurs when the roots
of plants begin to grow
into the fractures of
rock
• The pressure created
as the root grows
wedges the rock apart
Physical Weathering: Root Pry
Physical Weathering: Exfoliation
Exfoliation – Occurs in two manners:
•
occurs when rocks that have been formed under
conditions of great pressure are brought to the
surface. The “sudden” change in pressure causes
the surface of the rock to expand very quickly
fracturing the rock. Gravity and erosion then can
cause the rock to be removed.
•
Occurs when there are large shifts in the daily
temperature. The surface of a rock will heat and cool
much more quickly than the interior the repeated,
rapid expansion and contraction cycle cause
fractures to form in the surface layer of the rock.
Physical Weathering: Exfoliation
Types of Weathering
Chemical Weathering (Three Types) –
Processes in which the chemical composition of
the rock material is changed by chemical
means. All of these processes involve, either
directly or indirectly, the presence of water.
• Oxidation
• Hydrolysis
• Acidification of Water/Acid
Precipitation
Chemical Weathering: Oxidation
Oxidation – Reaction of minerals with
atmospheric oxygen
Chemical Weathering: Hydrolysis
Hydrolysis – Reaction of minerals with water to
for new minerals
• Water typically acts only as a solvent is chemical
reactions, not as a reactant. However there are
conditions under which water will act as a
reactant and hydrolysis takes place.
Chemical Weathering: Hydrolysis
Chemical Weathering: Acidification of Water/
Acid Precipitation
• Acidification of Water – Reaction of
atmospheric gases with atmospheric water;
primarily by carbon dioxide
• Acid Precipitation – Same as above except it is
considered to be a form of pollution
Chemical Weathering: Hydrolysis and
Acidic Solutions
Factors Affecting the Rate of Weathering
• Climatic Factors
• Characteristics of the Parent Rock
• Surface Area
• Slope
• Vegetation
Rate of Weathering: Climatic Factors
• Temperature – The
higher the average
temperature the faster
the rate of chemical
weathering
• Precipitation – The
greater the amount of
precipitation the faster
the rate of chemical
weathering
Rate of Weathering: Rock Characteristics
• Hardness/Resistance to Abrasion – Some rock types
are more susceptible to physical weathering due to the
method of rock formation. In general sedimentary rocks
are less resistant to physical weathering than are
metamorphic or igneous rocks
• Chemical Composition – The chemical composition of
rocks also can affect the type and rate of chemical
weathering. For example carbonates are susceptible to
water that has been acidified, whereas silicate based
rocks are not generally susceptible to this form of
weathering.
Rate of Weathering: Rock Characteristics
Rate of Weathering: Surface Area
Since all chemical reactions take place at the
surface of materials, any increase in surface
area increases the rate of chemical weathering.
Rate of Weathering: Slope
Slope:
Since soils and therefore vegetation has a more
difficult time remaining on a steep slope; the
steeper the slope the greater the rate of
chemical and physical weathering. This is due to
the fact that as sediment is produced on a steep
slope it is continuously being removed by
erosion to a lower elevation. This process
continuously exposes new parent material to the
elements.
Rate of Weathering: Vegetation
Vegetation:
•
The presence of vegetation can both increase and
decrease the rate of weathering.
•
Since the presence of vegetation inhibits the flow of
water across the surface of a slope this tends to
increase the rate of deposition of sediments (or from
another perspective it decreases the rate of erosion).
This in turn exposes less of the parent material to the
elements thus slowing physical weathering.
•
However, as vegetation undergoes the decay process it
naturally releases acids which can affect an increase in
the rate of chemical weathering.
Rate of Weathering: Vegetation
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.
• All agents of erosion move sediments in the down
hill direction due to the pull of gravity. Wind is an
exception. It has the ability to move sediments up
slope
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).