Weathering and Erosion

CHAPTER 1

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Weathering, Erosion, andDeposition

Weathering: Weathering is the breakdown of rocks. There are two types of weathering Mechanical/Physical and Chemical.Mechanical/Physical Weathering this is the physical breaking apart of the rock into smaller pieces.Chemical Weathering this is the process of breaking rocks down atom by atom through chemical changes.Types of Mechanical Weathering:Frost wedging

When water gets into cracks of rocks, freezes and expands making the crack larger.

ExfoliationWhen sheets of rock ‘peel off’. It is caused when surface rock weathers and falls off thus releasing pressure on underlying rock allowing that rock to expand and ‘peel off’.

Thermal Expansion and ContractionCrystals in rocks will expand and contract as they are heated and cooled, as this process happens it can cause for the rock to begin to fracture/break. The process is slow. Desert regions with large temperature differences experience mechanical weathering of rock through expansion and contraction due to large temperature variances.

Crystal GrowthCommon when salt water environments seep into rock fractures and the water then evaporates allowing for crystals to grow and then expand the fracture as they grow.

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Plant ActionTree roots grow into cracks of rocks breaking the rock apart.

AbrasionWhen rocks rub against other rocks as they fall or move leaving marks on the rock.

Types of Chemical Weathering:Dissolution/Carbonation

When mineral grains are torn apart and ions are carried away in solution. Caves are a result of dissolution.Cave formation: rainwater contains dissolved CO2 (carbon dioxide). The CO2 pro-duces carbonic acid (a weak acid). The rainwater will become more acidic from or-ganic acids created from plant decomposition as it filters down through soil. If the bedrock or underlying rock is limestone (a carbonate rock which means it will dissolve in acid) the acidic water will ‘eat’ away and dissolve atom by atom of mate-rial giving rise to open spaces/caverns/caves in the rock. The process takes a long time. Rapid/quick in cold moist areas.

OxidationWhen minerals react to the air. Most know this type as ‘rust’.

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Hydrolysis (hydro water; lysis reaction)Involves direct reaction with water molecules.Controlled by moist climates since water is needed.Affects most silicate minerals – especially feldspars which are the most common in earth’s crust.

Click the link below to see what types of climates produce the different types of mechanical weathering and chemical weathering:

http://www.as.uky.edu/sites/default/files/elearning/module07swf.swf

Factors that Affect the Rate of Weathering:

• Surface Area more surface area the greater the weathering rate• Mineral Composition some minerals are more resistant than other. For example:

Quartz is resistant to chemical and physical weathering.• Climatic Conditions Cold and/or dry climates favor mechanical weathering; Warm

and wet climates favor chemical weathering. Frost action works best where tempera-tures fluctuate.

• Pollution sulfuric and nitric acid (from pollution) mix with rain creating acid rain which enhances chemical weathering.

Erosion: the movement/transportation of weathered materials.

Four Agents of Erosion:• Gravity• Water• Wind• Glaciers

Gravity:Pulls weathered particles downhill and causes Mass Movement

Types of Mass Movement:1. Creep

Very low velocity eventCaused by heating/cooling, freezing/thawing, wetting/drying

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2. SlumpHigh velocity eventMaterial breaks offRotationalCreates scarps

3. LandslidesHigh velocity eventMaterial is looseMaterial is dryResults in Talus (big pieces of fallen rock)

! 4. Mudslides landslide with water

! 5. Avalanche landslide of snow

Click the link below to learn about Mass Movements: Make sure to read and go through all buttons!

http://www.as.uky.edu/sites/default/files/elearning/module11swf.swf

Water:Most common type of erosion (water is everywhere)Causes abrasion, rounded pebbles, forms rivers

Stages of River Development:1. Youthful

Has a rapid flowSteep slopesNarrow V-shape ValleyCan carry gravel and boulders

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2. MatureSteady flowGentler slope and flattened land surfacesSand-sized particles

3. Old AgeSlow flowingAlmost no slopeSilt

Carrying Capacity of Streams and Rivers:The diagram below shows which size particles drop out (settle/deposit), in order, as a stream’s velocity slows down or empties into a larger body of water such as a lake or ocean.

Features that Form From a River Flowing Down a Mountain Along a Floodplain, then into a Large Body of Water:1. Alluvial Fan sediments deposited at the base of a mountain2. Floodplain the area to where water will flow during a flood. They extend for miles on either side of the river.3. Delta sediment deposited in a fan shape when it hits a larger body of water. Refer to above picture for profile view.

Oxbows and River Turns:The diagram below illustrates where deposition (D) and erosion (E) will take place as the water flows in the river. The movement of water is from left to right. Eventually the mean-ders (bends in the river) will pinch off and create an Oxbow Lake. The velocity is faster at the outside of the meander (bend) and slower at the inside of the meander.

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Wind:Occurs mainly in dry climates. When sand is blown around it can ‘pit’ other rocks (mechanical weathering).

Types of Wind Erosion:Saltation particles bounce which loosens particles (also occur in rivers as water moves particles in the river flow)

Deflation the blowing of loose sediments that leave the surface

Sandblasting/Abrasion when particles of sand carried by wind rub against the surface of other materials scouring them away.

Glaciers:Glaciers are rivers of slowly flowing ice. The glaciers will break up rock and transport it to new locations. Create a U-shaped valley. Leaves striations (linear scratches on the surface below the glacier).Two Types of Glaciers:Alpine: individual glaciers found on mountains (example: The Alps)Continental: thick ice sheets that cover large areas of land (example: Antarctica)

Alpine Glacier:

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Continental Glacier: showing features created after the glaciers retreats (gets smaller)

Click the link below to run glacier simulations to learn how they work:

http://phet.colorado.edu/en/simulation/glaciers

Soil: is a mixture of minerals, water, gases and humus. Humus is decaying or decayed ani-mal and plant material. Soil develops as weathering of bedrock occurs. There are two ways soil form. The first is residual soil, this soil has bedrock as its parent material (where it comes from). The second type is transported soil, which is formed from parent material that has been eroded (transported) by winds, glaciers, or rivers and then left/deposited.A layer of soil is called a horizon. Horizons are categorized by the type of material in them. Below is a Soil Profile of the different layers.:

There are three main horizons in a soil profile. They are: Horizon A topsoil (hums), sometimes Horizon A is broken into an additional layer called Horizon O organic layer.Horizon B subsoil (minerals leached from layer A, and Horizon C partially weathered bedrock (usually the zone of saturation). The bottom most and oldest layer is the parent material (layer R) from which the soil hori-zons and soil profile formed.

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Karst Topography: landforms made from the dissolving of limestone by carbonic acid in groundwater. Limestone is a sedimentary rock that commonly has certain features associ-ated with it due to the dissolving of it from carbonic acid. The following is a list of features with a diagram below representing these features:

• Caverns caves (1)• Sinkholes form when the roof of a cave collapses (2)• Stalactites when water with dissolved minerals drips down and solidifies before fal-

ling all the way to ground. Looks like an icicle (3)• Stalagmites when the dissolved calcite that drips down from the ceiling of the cave

falls to the floor of the cave and hardens and then begins to build a mound upwards. (4)• Columns when a stalactite and stalagmite join together after 10,000’s of years form-

ing creating one connecting piece from the ceiling to the floor. (5)• Disappearing Streams when stream go into a sinkhole and continue to flow under-

ground eventually reappearing above ground downstream (6)• Underground Stream created from the disappearing river (7)• Limestone sedimentary rock, has the brick pattern (8)

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Deposition: when eroded (transported) materials are dropped (deposited) in a new loca-tion.

Factors Affecting Rate of Deposition:

1. Size: larger particles settle out first when running water or wind slow down2. Shape: the rounder the particle the faster it will settle.3. Density: the higher density particle will settle faster than a lower density particle.

Gradual sorting: When moving water begins to slow down, particles are deposited.

Rapid Sorting: When sediments are deposited fast.

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Over time rocks transported by water will become smaller and rounded.

Click the link to observe how sediments are deposited when a river or stream enters into a larger body of water. Pay attention to the particle sizes.

http://www.classzone.com/books/earth_science/terc/content/visualizations/es0604/es0604page01.cfm?chapter_no=visualization

Erosion and Deposition by a Glacier:A glacier is a naturally occurring mass of ice and snow that forms overtime as snow falls, builds up, and doesn’t melt. As snow falls onto the previous fallen snow it adds weight turning the lower layers of snow into ice by compressing it. Glaciers move due to gravity.

How Glaciers Move:Moves faster at the center near the top due to less frictionMoves slower at the bottom and sides due to friction with the bottom and sides of the land

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Erosion Landscapes from a Glacier:• U-shaped valley• Hanging Valley• Cirques• Arete! ! ! ! ! ! !• Horn• Polished Rock (from constant rubbing/friction of glacier)• Striated Rock (has linear scratches in the rock form the glacier going over it)• Abrasion (wearing, scraping, or rubbing away of rock surfaces by friction)• Plucking (processes that cause rocks to become loose, picked up, and carried away by

glaciers)! ! !

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U-Shaped Valleys and Hanging Valleys:

A glacier will leave behind sediments that are unsorted (glacial till). These deposits are usu-ally found at the front of the glacier 9Terminal Moraine). Deposits left at the side of the gla-cier are called lateral moraines.

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