Chapter 7—Weathering,

soil & erosion

EARTH STUDIES

Geoscience 1000

June 9, 2010

Queen’s Garden Trail below Sunset PointBryce Canyon National Park, Utah

Weathering versus Erosion

Weathering –rocks and minerals breaking down at or near the Earth’s surface

Mechanical weatheringChemical weathering

Erosion –moving water, wind, or ice carrying away pieces of rock and soil

Streams, rivers, glaciers, wind and/or waves carry sediment (loose, fragmented surface material that makes up many sedimentary rocks)

Weathering

Positive

Breaks down minerals to become part of soils and water

Enriches foods (plants, etc.) and water with essential nutrientsProduces soil, so we can grow crops, etc.

Negative

Can weaken building, roads, etc

Can destroy monuments, grave markers, etc.

Weathering

•Mechanical Weathering –rocks are broken into smaller pieces without changing the chemical makeup of the individual pieces

–Examples: Frost wedging, thermal expansion, abrasion

•Chemical Weathering –rocks and minerals that are decompose and are chemically changed

–Examples: oxidization, dissolving

Water expands ~9% when it freezes, wedging open water-filled cracks in rocks & roads!

Frost wedging leads to talus formation

Plant roots cause mechanical weathering

Plants (trees and grasses) are agents of weathering

Acid rain weathered away the detail from this marble statue

Soils and soil formation

• Regolith: The layer of weathered pieces of rocks that covers the land surface nearly everywhere. In Michigan, that averages about 200 ft thick of mostly glacial deposits

• Soil: That portion of the regolith that supports plant growth; A combination of 1) mineral and 2) organic matter, 3)

water, 4) air and 5) living organisms; A valuable resource for agriculture & life on earthSoil is the living skin of planet Earth

Composition of a “good” soil: 45% mineral matter, 25% each water & air, & 5% humus

Soil

• Soils build up due to mechanical & chemical weathering.

• Over hundreds and even thousands of years, they can build up & become deeper and more fertile.

• The factors that control soil formation are: 1) climate 2) some rocks weather faster 3) vegetation and animals4) time.

Variations in soil development

Typical soil profile

Soil profile showing different horizons

Which soil below would you want in your garden, farm, etc? The clod of loam on the left is largely without of organic matter. The nice dark loose soil on the right has the same loamy texture combined with lots of organic matter, or humus. Where did the organic matter come from? From dead plants, and from organisms that processed those plant residues.

The soil ecosystem converts dead organisms into humus. Soil is nature’s recycling plant, a thin compost heap, part of “dust to dust”.

Soil recycles numerous wastes that otherwise would build up to poison our world. These wastes become resources and nutrients. Astronauts will not be able to live long in space unless they take a soil with them.

Soil structure affects soil productivity

Soil structure depends on soil horizon. Soil structure can be disrupted by over farming, compaction, & bad farming.

What would be the best agricultural soil?You may work together on this.

1. A heavy clay loam.

2. A sandy soil often found in SW Michigan

3. A peat composed of decaying vegetation

4. A silty soil like those along the Ohio & Mississippi River valleys.

5. A humus-rich loam aggregated in crumbs

What would be the best agricultural soil?

1. A heavy clay loam. Drains slowly; hard to plow

2. A sandy soil often found in SW Michigan. Requires irrigation because it doesn’t hold water

3. A peat composed of decaying vegetation. Likely acid, with improper texture and structure.

4. A silty soil like those along the Ohio & Mississippi River valleys. Easily erodible.

5. A humus-rich loam aggregated in crumbs. This rich, fertile soil would be a farmer’s dream.

Soil erosion

• Soil erosion is part of the natural recycling of earth materials. Almost all soils will be eroded. That doesn’t mean erosion is good for us.

• Natural rates of soil erosion depend on

– Soil characteristics

– Climate

– Type of vegetation

• How land is used by man largely controls erosion rates

Soil erosion is a real threat to our ability to feed ourselves

This field in Wisconsin can’t be farmed like this—you can’t drive a tractor over it. You can’t plow or harvest this field.

This soil in Columbia will never be farmed, & can’t be used for much else due to this severe gully erosion. This damage is irreversible.

This abandoned Oklahoma farm had been excellent prior to the Dust Bowl conditions (wind erosion &

deposition) of the late 1930’s.

Soil concerns

• In many regions soil erosion is much greater than the rate of soil formation

• Soil particles may be contaminated with pesticides, herbicides, fertilizers, or toxic chemicals

• Groundwater quality is affected by the soils through which rain seeps into the ground.

Soil’s essential roles

• Soil is the base of the ecosystem which sustains life and is the source of significant biodiversity on Earth.

• Soil is essential for producing food, feed, fiber, and biofuel feedstock production.

• Survival of past and future civilizations is linked to sustaining the soil resource.

Which of the following statements about soils is not true? Work together on this.

1. Erosion is the ultimate fate of virtually all soils.

2. Soil erosion rates were higher before our ancestors started farming

3. Many civilizations have disappeared because they mismanaged their soils.

4. Soil is a natural resource which cannot be renewed economically in a few centuries.

Which of the following statements about soils is not true?

1. Erosion is the ultimate fate of virtually all soils.2. Soil erosion rates were higher before our

ancestors started farming False: soil erosion was much less before our ancestors started farming.

3. Many civilizations have disappeared because they mismanaged their soils.

4. Soil is a natural resource which cannot be renewed economically in a few centuries.