Chapter 11 Erosion and Deposition

Geologic Processes 2- Weathering, Erosion and Deposition How can wind and water change our landscapes?

Weathering- the destructive processes by which materials at or near the Earths surface are changed in color, texture, composition, firmness or form. Weathering is the beginning of erosion. There are two types of weathering.

Mechanical or physical weathering- a physical change in which rocks are broken into different shapes and smaller pieces

Six main processes can cause mechanical weathering

(1) Impact and Abrasion- moving materials can cause rocks to fracture, flake or lose small particles

(2) Frost Action- water seeps into an opening or crack in a rock and then freezes when temperatures drop. Water expands when it freezes and causes rocks to break into pieces.

(3) Temperature- The cycle of heating and cooling of rocks causes a cycling of expansion and contraction that causes particles on the rock surfaces to crack or flake off.

(4) Organic Activity- The roots of plants can often loosen and change rock material.

(5). Gravity can pull loosened rocks downhill resulting in a landslide causing rocks to break into smaller pieces.

(6) Exfoliation- Rocks which form under the surface form under pressure - confining pressure. When that pressure is released, the rock will expand. As it expands the outer layers break free in sheets, and is called exfoliation.

All causes of mechanical weathering result in the breaking down of rocks into smaller pieces called sediments.

Chemical Weathering- changes in the chemical makeup or mineral composition of rocks. Five main processes can cause chemical weathering

(1) Water- can dissolve minerals holding rocks together, or form acids when mixing with gases in the atmosphere to form acid rain (speeds up decomposition of rocks)

(2) Oxidation- oxygen can chemically combine with substances resulting in the formation of a new substance (Ex. iron in rocks forming rust)

(3) Carbonation- carbon dioxide in the air can dissolve in rain forming carbonic acid which can dissolve certain rocks (Ex. limestone and feldspar)

(4) Sulfuric acid- sulfur oxides can combine with water producing acid rain which can chemically break down rocks, metals and other materials

(5) Plant acids- some plants produce weak acids that can dissolve minerals in rocks (Ex. mosses)

As chemical changes take place minerals can be added or removed from rocks.

Erosion and Deposition

1. Erosion is the process by which weathered rock and soil are moved from one place to another.

2. Erosion changes landscapes by wearing down mountains, filling in valleys, and making rivers appear and disappear.

3. Deposition is the process by which sediments are laid down in new locations.

4. Weathering, erosion, and deposition act together in a cycle to wear down and build up Earths surface.

5. The agents of erosion are gravity, wind, running water, glaciers, and waves.

C. The Five Agents of Erosion and their Effects on Landscape

1. Gravity erosion or mass wasting- the downhill movement of sediments caused by gravity.

a. Landslides- the tumbling of soil, rocks and boulders down a slope

b. Mudflows- usually occur after heavy rains carrying large amounts of mud downhill

c. Slump- materials such as rock or soil on a steep slope move downhill as a coherent unit to decrease the slope angle of land

d. Creep- the slow movement of sediments downslope, caused by repeated expansion and contraction of materials (Examples: freezing and melting, growth and decay of roots, burrowing animals)

Mass Wasting

Running water- Runoff, Rivers, and Streams

1. The amount of runoff is affected by the amount of rainfall, plant growth, and shape of the land.

2. Urban storm water runoff effects water quality, water quantity, habitats, biological resources, public health and the aesthetic appearance of waterways.

(a) After storms there is often a temporary increase in pollutants, toxins, and bacteria levels.

(b) Increased erosion and deposition results from large amounts of urban runoff which alters aquatic habitats.

b. Streams

(1) Streams usually cause erosion by abrasion (sediments colliding with rocks and wearing the rocks down over time)

(2). Load- the soil particles and rocks carried by the stream

(3) If a stream flows first over a hard rock layer, then a soft rock layer that is easily eroded, a waterfall forms.

(4) Downcutting represents downward erosion of the stream channel. It occurs when the stream energy (capacity) exceeds the stream's load, and causes valleys to be narrow and steep-sided (canyons and gorges).

(5) Lateral erosion undermines part of the bank or valley wall, leading to mass wasting and widening of the valley.

(6) Stream deposition results from decreasing water velocity or from chemical changes. The coarsest material is deposited first as water velocity decreases. Stream deposited material is called alluvium.

c. River Systems- streams that join form rivers, which usually begin in mountains or hills

(1) Larger steams flowing into a main river are called tributaries.

(2) Eventually a river empties into another river, a lake, or an ocean at the mouth.

(3) The drainage basin is all of the area drained by a river and its channel.

Drainage basins are separated by divides (high ground between two drainage basins). The Continental Divide is the largest divide, running north and south (close to Denver, CO).

Which river basin do YOU live in?

(4) Deposits made by rivers are constantly changing the landscape of surrounding areas with sediments being deposited where a river curves, or meanders.

(5) Oxbow lakes are small lakes formed from a U-shaped bend that erosion and deposition of sediments cuts off from the river.

River meander and Oxbow Lake(6) Deltas are formed as large amounts of sediments are deposited at the mouth of a river.

(7) Flood Plains are flat areas on both sides of a mature river or stream. Fine sediments are deposited on flood plains after heavy rains or spring thaws. Larger sediments are deposited and form ridge-like deposits called levees.

Mississippi River Delta

3. Glacial Ice- erodes by abrasion and plucking (picking up rocks) and wearing away underlying rocks.

a. As glaciers move they carry rock materials and wear down (erode) the land surface it passes over usually forming U-shaped valleys.

b. The material deposited by a glacier is called till. The till left behind after a glacier melts forms a ridge called a moraine.

c. Glaciers can also form lakes by leaving behind till that keeps water from flowing away from the area (Ex. Great Lakes).

4. Wind- active agent of erosion in deserts, plowed fields and beaches (a) Wind erodes Earths surface in two ways- abrasion and deflation.

(b) Abrasion occurs as larger particles carried by wind act as sandblasters, cutting and polishing rocks.

(2) Deflation occurs as loose materials such as clay, silt, and sand are carried away by the wind.

b. The amount of wind erosion is dependent upon four factors.

(1) Size of particles being carried

(2) Speed of the wind

(3) Length of time the wind blows

(4) Resistance of rocks exposed to wind

3. Deposits by wind form dunes or loess deposits (layers of fine sand and silt deposited in an area).

5. Waves- their powerful force is constantly eroding and shaping shorelines

a. Waves receive their energy from winds that blow across the water surface. The size of a wave depends on how fast, how far, and how long the wind blows.

2. Deposition of sediment forms our beaches. Some of the sediments comes from surrounding shorelines, while others are carried by rivers from inland areas. 3. Deposition by a longshore current (water moving parallel to the shoreline) creates sand bars or barrier islands (large semi-permanent bars) like the Outer Banks

The NC Outer Banks4. The shape of a shoreline often results from changes in sea levels. Increased sea levels create bays and harbors while decreasing sea levels creates many sea cliffs and terraces (flat platforms at the base of a cliff).

1. The key to stopping erosion is to anchor the soil in place and to slow down the water moving across it.

2. The best way to hold the soil in place is to plant vegetation in the soil. Establishing a ground cover has many benefits besides controlling erosion. Grass, shrubs and trees are all effective ground covers.

D. Controlling Erosion

3. Terracing is an effective treatment to control erosion on sloping areas. The level steps of terraces reduce the velocity of water, stopping the water from flowing down hill. This allows time for water to seep into the ground.

4. Strip-cropping is when farmland is plowed so that plowed strips are separated by planted strips, helping to reduce soil loss.

5. Contour farming is a method of plowing along the contours of the land rather than up and down a slope. It creates furrows between crops to collect water.

6. Construction sites can use sediment fences, temporary vegetation, and grass covered drainage ditches to reduce erosion.

7. Collect and direct urban runoff to protect aquatic ecosystems from sedimentation.