Mechanical Weathering Objectives Students will be able to: - - PowerPoint PPT Presentation

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

• ver time are called we

weath atheri ering ng. – Over thousands of years, we weather athering ing 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, mecha hanical nical we weath atheri ering ng occurs. – The chem emical cal makeup of a rock stays the same during mechanical weathering. – Mechanical weathering can be caused by ice we wedging dging, abrasion rasion, plants ants, and anim imal als.

Surface Area

– When something is broken into smaller pieces, it has a greater surfac face area ea. – Surface area is the amount of space on the outsi tside de 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

– Chem hemical ical we weath atheri ering ng 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

– Wat ater is important in chemical weathering because most substances disso ssolve 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 ds are also agents of chemical weathering and cause more chemical weathering than pure water does. – Scientists use pH 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: c: pH between 7 and 14. – Neutral al: : 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 dation combines the element oxygen with other elements or molecules. – The product of oxidation is called an oxide de. – 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?

En Engageme gagement nt

Which student do you agree with? Explain.

What is Soil?

– Soil l is a mixture of weathered rock, decayed organic matter, mineral fragments, water, and air. – About half the volume of soil is solid id materials and the other half is liqui uids ds and gase ses. – Soil contains gases that fill the soil pores es–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. – Deco compositio mposition is the process of changing once-living material into dark-colored

• rganic 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

ve factors of soil formation.

• The starting material of soil is called par

arent nt materi erial. al.

• Parent material is made of the rock or sedim

diment ent that weathers and forms soil.

• The particle size

ze and d type pe 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 climat

ate. e.

• 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

ta 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

• Horiz

rizons

• ns are layers of soil formed from the movement of the products
• f weathering.
• There are three

ee horizons common to most soils.

• Each horizon has characteristics based on the type of materials it contains.
• The A-hori

rizon zon 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

rizon usually contains a great deal of clay particles.

• The C-horizon
• rizon consists of parent material.
• The top, organic layer is called the O-hori
• rizon
• n and the unweathered, bedrock layer

is the R-horizon horizon.