Topics 9 & 10



Topics 9 & 10

Weathering, Erosion, and Deposition

( Weathering

- The processes that produce changes in the surfaces of rocks that are exposed to the atmosphere are called weathering.

- Weathering can be described as the combined effect of the atmosphere and hydrosphere on the lithosphere.

- The result of weathering is a breakdown of the rock material which eventually helps to form soil

Two types of weathering:

Physical/Mechanical Weathering

- Any process that causes a rock to crack or break into pieces is called physical weathering.

( Frost action / ice wedging -- water enters cracks and pores in rock then freezes and expands, puts pressure on the rock, causes the crack to widen, and the process keeps repeating. Constant freezing and thawing eventually breaks the rock. This is common on mountaintops.

( Abrasion -- rocks rub against each other or bounce off each other in streams or during landslides

( Exfoliation -- gravity causes large sheets of loose material to “peel away” from rock

( Plant action -- roots force cracks to widen as plants grow larger

Chemical Weathering

- Chemical weathering changes the chemical composition of the minerals in rocks, forming new substances and weakening the rock

- This change occurs when one of the minerals in the rock combines or reacts with a substance from the environment

( Oxidation -- minerals react with oxygen

EX/ iron in rocks reacts with oxygen to form rust (iron oxide)

( Hydration -- minerals react with water

EX/ water dissolves halite (salt)

Hydration also speeds up other processes

( Carbonation -- minerals react with carbon acid (forms from carbon dioxide in water)

EX/ carbonic acid dissolves calcite in limestone, creates caverns

( Acids from decaying organic matter -- react with rainwater and groundwater to dissolve minerals

* Man’s activities release an alarming amount of weathering agents into the environment

Rates of Weathering

Factors which affect the rate of weathering:

1. Climate -- the rate is increased in areas that are moist (and warm)

EX/ Florida has more chemical weathering than NY

2. Particle size and surface area -- smaller rock particles have more surface area than an equal mass of large particles; more exposure increases rate of weathering

EX/ a pile of pebbles weathers faster than a boulder

3. Mineral composition -- different minerals weather at different rates

EX/ sandstone weathers faster than granite

Results of Weathering

Physical and chemical weathering break rocks down into particles of many sizes:

boulders ( cobbles ( pebbles ( sand ( silt ( clay ( colloids ( ions

The end result of weathering is soil, but soil is also made up of organic material called humus.

A side-view of soil is called a soil profile. The different layers of soil in a profile are called soil horizons. The profile shows the weathering history of the area.

[pic]

Immature soils will not have topsoil or have a very thin layer

Soil characteristics are often uniform over large areas so they can be classified by region (desert, forest, etc.)

( Erosion

- Sediments that have been moved into a region from another place by natural processes are called transported sediments.

- Sediments formed in their present location from original bedrock are called residual sediments.

- Transported sediments are far more common than residual ones.

- Any natural process that removes sediments from one place and carries them away to another placed is called erosion.

- Gravity is the main driving force of all erosional processes.

- The greatest angle at which loose material will rest on a surface is called its angle of repose [For most Earth materials, this angle is 35(]

- Water may increase this angle for some materials; however, too much water may produce slumping (slipping) of material downhill

- On slopes that are close to the angle of repose, gravity can cause hillside creep (slow slippage of material) or possibly landslides, both forms of “mass wasting.”

Streams

- Gravity is the main driving force for erosion but the predominant agent of erosion on Earth is running water

- Raindrops fall and either become runoff or infiltrate the soil; eventually they end up in streams

- Small streams join to form larger streams and so on until they form a river

- Every stream and its branches make up a single system that collects all the runoff within a definite area, called the drainage basin or watershed of the area

Characteristics of Streams

( Velocity -- as slope increases, velocity increases

( Curves -- velocity is greater on outside of curve so erosion is greater on outside of curve

( Discharge -- as discharge increases, velocity increases

( Ability to transport sediments:

- ions are carried in solution, maybe indefinitely

- colloids, clays, and silt are kept suspended by motion

- sand and small pebbles bounce along the bottom

- large pebbles, cobbles, and small boulders move by sliding, rolling, and dragging along the bottom

* The size of the sediments a stream can transport increases as stream velocity increases [see ESRT]

( Quantity of sediments transported -- quantity increases as discharge increases

Stages of Stream Development

(“stages” do NOT refer to a stream’s age in years)

Most streams begin on steep slopes between mountain peaks and begin to carve valleys between them

( Youthful Stream

- steep gradient, high velocity, able to cut through solid bedrock

- carries large sediments

- able to erode obstacles so stream valley tends to be straight

- V-shaped profile

- rapids and waterfalls are common

( Mature Stream

- as a youthful stream continues to cut its bed downward, it approaches its base level (usually sea level)

- gradient is decreased so velocity is decreased

- more smaller streams are entering (called tributaries)

- U-shaped profile

- carries more sediments but of smaller size

- stream is affected by obstacles so now it moves around them forming loops called meanders

( Old Stream

- very small gradient, slow velocity

- carries only the smallest particles

- shallow profile (squashed U)

- tends to overflow and flood the valley leaving a layer of silt and clay which creates fertile farmland; this area that gets flooded is referred to as the floodplain

- meanders get cut off forming oxbow lakes

( Rejuvenated Stream

- a stream in any stage can become a youthful stream again if the land undergoes uplifting (folding or faulting, volcanism)

Most streams are not in just one stage from beginning to end, but rather have different areas in different stages, mostly depending on the type of bedrock it’s moving through.

( Other Agents of Erosion

Glaciers

- Glaciers form by the steady accumulation of snow which compresses the lower portions and turns them to ice

- Weathered particles from mountain sides continually slide and roll down the slopes, falling on top of snow. The heavier particles sink to the bottom. As the glacier moves down the mountain, these sediments grind the floor and erode the land.

- The result is a carved out U-shaped valley.

- When the advancing edge of a glacier reaches areas of warmer temperatures, it “retreats” -- ice at the leading edge melts, creating streams of water carrying dislodged sediments

- When the glacier is advancing, it pushes large loads of sediments ahead of it like a bulldozer

Wind

- Wind erosion occurs with loose dry sediments

- Wind can transport sediments of sand size or smaller

- Wind can also erode solid rock by the grinding action of windblown sand, which remains close to the ground

Ocean Waves

- The force of waves alone can break up rock

- The sand and pebbles carried by the waves also weather rock

- Broken rock particles are carried out to sea

( Deposition

Weathering breaks down rocks into smaller particles, erosion carries them away.

The processes by which transported materials are left in new locations are called deposition.

A great deal of sedimentation occurs in water that is moving. Since erosion occurs whenever the medium is gaining speed, deposition occurs whenever the medium is losing speed.

Factors Affecting Deposition Settling Rates in Water

1. Particle size

- smaller particles settle more slowly

[as particle size increases, settling time decreases]

2. Particle shape

- spherical particles settle first, then oval ones, then flat ones

[the more spherical the particle, the faster the settling rate]

3. Particle density

- higher density particles settle first

[as density increases, settling time decreases]

The longer a sediment is carried by water, the more rounded and smoother it becomes.

When several events of deposition occur in quiet water, each involving a mixture of sediments, several distinct layers develop. The separation of particles during deposition is called sorting. When it occurs from bottom to top, it’s called vertical sorting. If the particles sink only a short distance they don’t have much time to get sorted so the top layer tends to be more unsorted than lower ones.

In a stream that is gradually slowing down, larger and heavier particles are deposited first, then smaller ones. This takes place over a horizontal distance so the sediments are graded (sorted) horizontally, creating horizontal sorting. When a river enters a lake or ocean, the velocity suddenly drops and particles are released. The particles accumulate and eventually form a bed of sediment above the water’s surface called a delta.

Deposition from Glaciers

Sediments deposited from glaciers are divided into two types:

1. direct -- sediments are dropped directly from the ice as it melts. Sediments are unsorted, particles are scratched and polished from carving the landscape. This material is called till.

2. indirect -- particles are carried by the ice then carried further by the water in streams formed from the melting ice front. Sediments can show scratches, some sorting and rounding of particles. This material is called outwash.

Deposition from Wind

Deposition from wind is similar to water but layers often rest upon one another at angles (due to changing direction of wind) and sediments are said to be cross-bedded. Particles are somewhat rounded, sorted, and have a “frosted” appearance.

Deposition from Gravity

Gravity causes streams to flow, glaciers to move, etc., but it can also act alone on surface slopes (mountain sides) in the form of mass wasting (landslides, hillside creep, slumping, etc.). Particles end up at the bottom unsorted, fragmented, and angular. This material is called talus.

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