Vocabularybedfordfms.sharpschool.net/UserFiles/Servers/Server... · Weathering •Wherever Earth’s crust is exposed at the surface it is subject to weathering. Formation of Sedimentary

Objectives

• Sequence the formation of sedimentary rocks.

Formation of Sedimentary Rocks

• Explain the formation and classification of clastic sediments.

• Describe features of sedimentary rocks.

– sediment

– clastic

– deposition

– lithification

– cementation

– bedding

– graded bedding

– cross-bedding

Vocabulary

http://www.earthgeu.com/

• When sediments become cemented together, they form sedimentary rocks.

• The formation of sedimentary rocks begins when weathering and erosion produce and move sediments.

• Sediments are pieces of solid material that have been deposited on Earth’s surface by wind, water, ice, gravity, or chemical precipitation.


• Much of Earth’s surface is covered with sediments.



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


Weathering



Weathering

• Clastic describes rock and mineral fragments produced by weathering and erosion.


• Clastic sediments range in size from huge boulders to microscopic particles.


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



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.



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.


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.


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.


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.


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.


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.





• 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.





• Cross-bedding is formed as inclined layers of sediment move forward across a horizontal surface.






• 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.







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.




Section Assessment

1. Match the following terms with their definitions.

___ clastic

___ deposition

___ lithification

___ 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

C

D

B

A


Section Assessment

2. How is cross-bedding formed?


Cross-bedding is formed as inclined layers of sediment move forward across a horizontal surface.


________ Cementation is a form of lithification.

________ During deposition, the largest particles end up in the top layer.

________ Graded bedding is often observed in marine sedimentary rocks.

________ Clastic sediments can range in size from microscopic particles to huge boulders.

Section Assessment

3. Identify whether the following statements are true or false.


true

false

true

true


End of Section 1

Objectives

• Describe the types of clastic sedimentary rocks.

• Explain how chemical sedimentary rocks form.

• Describe organic sedimentary rocks.

• Recognize the importance of sedimentary rocks.

– clastic sedimentary rock

– porosity

– evaporite

Vocabulary

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.





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.



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 high-energy 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.



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.



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.


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.



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).



Rocks Formed from Evaporation




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.



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.


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.


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.


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.


Section Assessment


2. The following are which type of sedimentary rock?

___ coal

___ sandstone

___ limestone

___ shale

___ conglomerate

___ rock gypsum

___ breccia

A. clastic

B. chemical

C. organic

D. A and B

E. B and C

C

A

E

A

A

B

A


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).


End of Section 2

Vocabularybedfordfms.sharpschool.net/UserFiles/Servers/Server... · Weathering •Wherever Earth’s crust is exposed at the surface it is subject to weathering. Formation of Sedimentary

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