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Chapter 7- Weathering, Erosion and Soil

 

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I. Weathering

• Where rocks on or near Earth’s surface break down and change is called weathering.

 

• Removal and transport of weathered material from one location to another is erosion

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Two Types of Weathering

1. Mechanical or Physical

2. Chemical

Mechanical Weathering

Where rocks and minerals break down into smaller pieces.

The only thing that changes is the rock’s size and shape.

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Cleopatra’s Needle in NYC.

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2 Factors involved in mechanical weathering:

1. Temperature

2. Pressure

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Temperature

• When water freezes, it expands and increases in volume.

• Water collects in the cracks of rocks.

• It temperatures drop to freezing, it expands and exerts pressure on the rocks therefore causing them to split.

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This is called “Frost Wedging.”

The repeated thawing and freezing

of water in the cracks of rocks.

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Pressure

Rocks build up on top of one another causing pressure. When overlying rocks are removed, the pressure on the bedrock below is reduced.

 

Pressure continued….

Over time, the outer layers of rock are stripped away in succession, similar to an onion being peeled. This is called “exfoliation.”

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Chemical Weathering• Where rocks and minerals undergo

changes in their composition as the result of chemical reactions.

 

• Some agents include: water, oxygen, carbon dioxide, and acids.

• Temperature is involved also.

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The Agent: Water

Water can dissolve many kinds of minerals and rocks. Water has an active role in some reactions.

 

The reaction of water with other substances is known as hydrolysis

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The Agent: Oxygen

Oxygen can combine with other substances. This chemical reaction of oxygen with other substances is called oxidation.

 

The Agent: Carbon Dioxide

This is produced by living organisms during the process of respiration.

When carbon dioxide combines with water in the atmosphere, it forms a weak carbonic acid that falls to Earth’s surface as precipitation.

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Limestone Cave in Guatemala produced when carbonic acid

Dissolved the calcite in the limestone rock.

The agent: Acid Rain

Caused by the oxidation of sulfur dioxide and nitrogen oxides that are released into the atmosphere by human activities.

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What affects the rate of weathering? Occurs very slowly

 1. The climate of an area is a major influence on the rate of chemical weathering of earth materials. (Temperature, precipitation, and evaporation)

2. Rock type and composition

3. Surface Area

4. Topography

Chemical weathering occurs…

In areas that are tropical! Warm, abundant rainfall, lush vegetation.

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Mechanical or physical weathering occurs…

In cool, dry climates. Where water freezes and thaws repeatedly.

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7-2 Erosion and Deposition

As you recall… erosion is the process that transports Earth materials from one place to another.

Rivers, running water in streams, glaciers, wind, and ocean currents can pick-up and carry materials.

 

At some point though…

The movement of transported materials will slow down.

When this happens, the materials are dropped in another location in a process called DEPOSITION (final stage of erosional process).

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What can pick-up and carry materials?

1. Gravity: Being pulled downhill

2. Running water moving downhill 

Rill Erosion

Erosion by running water in small channels, on the side of a slope.

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Gully Erosion occurs when:

•  Channels becomes deep and wide. (These can cause big trouble for farmers)

 

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Gully Erosion

Water erosion also includes:

Coastal deposition and erosion. Current, waves, and tides carve out cliffs, arches, and other features along the continent’s edges.

Sand particles accumulate on shorelines and form dunes and beaches.

Sandbars form as a result of being picked up and moved from the shoreline. 27

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What else can pick-up and move materials?

•Glaciers  •Wind  •Humans, plants and animals

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Glaciers

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Wind and planting a garden