Understanding Weathering and Soil Formation: A Look at the Great Sphinx and Desert Soil, Schemes and Mind Maps of Topography

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Mechanical

Weathering

Objectives

Students will be able to:

• Explain how weathering breaks down rock.
• Explain how a rock’s surface area affects the rate of weathering.

Weathering

• The mechanical and chemical processes that change objects on Earth’s surface over time are called weathering.
• Over thousands of years, weathering can break rock into smaller and smaller pieces, such as sand, silt, and clay.

Mechanical Weathering

• When physical processes naturally break rocks into smaller pieces, mechanical weathering occurs.
• The chemical makeup of a rock stays the same during mechanical weathering.
• Mechanical weathering can be caused by ice wedging, abrasion, plants, and animals.

Chemical

Weathering

• Do you think the structures in these images have always looked like this?
• What do you think caused these structures’ appearances to change?

Chemical Weathering

• Chemical weathering changes the materials that are part of a rock into new materials.
• These granite obelisks show how chemical weathering can affect some rock.

Chemical Weathering by Acids

• Acids are also agents of chemical weathering and cause more chemical weathering than pure water does.
• Scientists use pH to determine if a solution is acidic, basic, or neutral.
• The pH of a solution is rated on a scale from 0-14.
  • Acidic: pH between 0 and 7.
  • Basic: pH between 7 and 14.
  • Neutral: pH 7
• Vinegar has a pH of 2 to 3. What kind of solution is vinegar?

Chemical Weathering by Rain

• Normal rain is slightly acidic, around 5.6.
• Carbon dioxide in the air reacts with rain to form a weak acid.
• When coal burns, sulfur oxides enter the atmosphere.
• When these oxides dissolve in rain, acid rain is produced.
• Acid rain has a pH of 4.5 or less
• Which type of rain causes more chemical weathering?

Weathering rates

• Weathering depends on water and temperature.
• Mechanical weathering occurs fastest in areas that have a lot of temperature changes.
• Chemical weathering is fastest where the climate is warm and wet , near the equator.

Soil

Components

• What is soil?
• Why do you think the soil in this image is so red?

Engagement Which student do you agree with? Explain.