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.

The Great Sphinx

Of all the amazing statues built in ancient times, the Great Sphinx of Giza is one of the few that still stands. For many years it was partially buried in sand, which helped protect it. Even after many restoration projects, the statue still lacks some of its original features. It surely had a nose when it was first built and perhaps a beard as well.

1. How did being buried in sand help protect the Great Sphinx?

2. How might the statue have lost its nose and beard?

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.

Surface Area

– When something is broken into smaller pieces, it has a greater surface area.

– Surface area is the amount of space on the outside of an object.

Chemical

Weathering

Objectives

Students will be able to:

• Identify evidence of chemical weathering.

• Explain how chemical weathering changes rock.

• Differentiate between chemical and mechanical 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 Water

– Water is important in chemical weathering because most substances dissolve in

water.

– The process of dissolving breaks up the minerals in the rock into small pieces.

– The small pieces mix with water to form a solution and are washed away from the

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?

Chemical Weathering by

Oxidation

– Oxidation combines the element oxygen with other elements or molecules.

– The product of oxidation is called an oxide.

– Oxides are useful because they form ores, which people use to obtain metals.

– Do you know what the common term for iron oxide is?

– Do all parts of an iron-containing rock oxidize

at the same rate?

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

Today’s Objectives

Students will be able to:

• Identify organic and inorganic parts of soil.

• Explain how decomposition is related to organic matter.

• Describe soil composition by how it feels.

• What is soil?

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

Engagement

Which student do you agree with?

Explain.

What is Soil?

– Soil is a mixture of weathered rock, decayed organic matter, mineral

fragments, water, and air.

– About half the volume of soil is solid materials and the other half is liquids and

gases.

– Soil contains gases that fill the soil pores–the small holes and spaces in soil.

– Why are pore spaces in soil needed to support the animals and plants that

live in it?

What is Soil?

– We discussed before that weathering gradually breaks down rocks into smaller

fragments.

– What is the difference between sediment and soil?

The Organic Part of Soil

– These fragments don’t become good soil until organic matter is added to them.

– What does organic mean?

– What is organic matter?

– Organic matter is the remains of something that was once alive.

– Decomposition is the process of changing once-living material into dark-colored

organic matter.

– Which parts of soil are inorganic?

The Inorganic Part of Soil

– Soil scientists classify soil fragments according to their sizes.

– Rock fragments can be boulders, cobbles, gravel, sand, silt, or clay.

– Between large particles are large pores, which affect soil properties such as

drainage and water storage.

Soil Formation

& Horizons

Objectives

Students will be able to:

• Identify evidence of chemical weathering.

• Explain how chemical weathering changes rock.

• Differentiate between chemical and mechanical weathering.

Wildflowers can bloom in all sorts of places, including the desert. Plants need

many things to survive, including water and sunlight. But in the desert, as

elsewhere, the composition of soil determines whether or not plants will grow.

The plants shown here can grow in desert soil, but not on solid rock.

1. Describe the soil of a desert. How does it compare to the soil where you live?

2. Why is it difficult for most plants to grown on a solid rock?

3. Do you think that all plants could grow well in desert soil if you added water?

Formation of Soil- Parent Material

• Why are there so many different types of soil?

• The kinds of soils that form depend on five factors of soil formation.

• The starting material of soil is called parent material.

• Parent material is made of the rock or sediment that weathers and forms soil.

• The particle size and type of parent material can determine the properties of the soil

that develops.

Factors that Affect Soil Formation:

Climate

• The average weather of an area is its climate.

• If the parent material is in a warm, wet climate, soil

formation can be rapid.

Factors that Affect Soil

Formation: Topography

• Topography is the shape and steepness of the

landscape.

• The topography of an area determines what

happens to water that reaches the soil surface.

• Water running downhill can carry soil with it,

leaving some slopes bare of soil.

Factors that Affect Soil

Formation: Biota

• Biota is all of the organisms that live in a region.

• Biota in the soil help speed up the process of soil formation in various ways.

• Organisms can be involved in decomposition of organic matter or form

passages in soil for water to move through.

• Rock and soil are affected by organism activity.

• Mature soils develop layers as new soil forms on top of older soil.

Factors that Affect Soil

Formation: Time

• As time passes, weathering is constantly acting on rock and sediment.

• This means soil formation is a constant, but slow process

Horizons

• Horizons are layers of soil formed from the movement of the products

of weathering.

• There are three horizons common to most soils.

• Each horizon has characteristics based on the type of materials it contains.

• The A-horizon is the part of the soil you are most likely to see when you dig a shallow

hole; it contains most of the organic matter in the soil.

• The B-horizon usually contains a great deal of clay particles.

• The C-horizon consists of parent material.

• The top, organic layer is called the O-horizon and the unweathered, bedrock layer is

the R-horizon.