Unit 2: Surface Processes and the Hydrosphere

Lesson 5: Weathering and Erosion

(Heath Earth Science – Pg. 130-147)

Today’s Objectives Relate the processes associated with

weathering and erosion to the resulting features, including: Distinguish between weathering and

erosion Use examples (eg. Exfoliation) to

distinguish between mechanical and chemical weathering (including weathering by biological organisms)

Describe the origin and function of soil

Where is this?

Shilin Stone Forest, Yunnan

Weathering and Erosion Weathering and Erosion Songs What do you think weathering means? What do you think erosion means? Weathering:

Is the break-up of rock due to exposure to the atmosphere

Erosion: Is the removal and transport of earth materials by

natural agents (such as wind, water, ice, tides, or gravity)

Weathering an Erosion – working together – Grand Canyon

Weathering Why do rocks weather? Partly due to the difference between

conditions deep beneath the surface and the conditions at the surface: Deep underground, where many rocks

are formed, pressure and temperature are very high

When these rocks rise to the surface, pressures and temperatures are much less: rock expands

Water and oxygen, also not present at depth, exist on the surface: break rock down

Types of Weathering Weathering includes many processes,

but these processes are grouped into two types of weathering: mechanical and chemical weathering

Mechanical (Physical) Weathering – Also called disintegration Takes place when rock is split or broken

into smaller pieces of the same material without changing its composition

Ex.)The breaking of a rock cliff into boulders and pebbles

Types of Weathering Chemical Weathering –

Also called decomposition takes place when rock’s minerals are

changed into different substances Water and water vapor are important

agents of chemical weathering Ex.) Clay being formed from feldspar

Types of Weathering Mechanical and Chemical weathering

processes are often studied separately but they seldom occur alone

Because water vapor is present almost everywhere, chemical weathering happens almost everywhere

In different parts of the world, one process may be more important than the other, based on climate

However, mechanical and chemical weathering almost always act together

Working Together

Practice

Topic Questions, pg. 137 #1-2

Types of Mechanical Weathering Mechanical weathering happens in

many different ways. Frost action, wetting and drying, action of plants and animals and the loss of overlying rock and soil all play a role in mechanical weathering.

Common processes include: 1) Ice (Frost) wedging 2) Exfoliation (sheeting) 3) Root wedging

1) Ice Wedging Water takes up about 10% more space when it

freezes This expansion puts great pressure on the walls

of it’s container Example) Put a completely full bottle of water into

your freezer and leave it there for 2 or 3 days, then check it out!

Water held in the cracks of rocks wedges the rocks apart when it freezes, a process called frost wedging, or frost action

Often occurs in areas where daytime temperature is > 0˚ C, and night temperatures are < 0˚ C (such as southern Canada)

Most damaging of all weathering processes in these areas

1) Ice Wedging

• Ice wedging is very common on bare mountain tops and in porous rocks or rocks with cracks in them

2) Exfoliation (sheeting) Some rock is formed far below Earth’s surface,

for example, Granite (rock that makes up Huangshan)

It is exposed when large masses of rock are lifted to form mountains and the rocks above the granite are worn away

The removal of this rock reduces the pressure on the granite, and it begins to expand

Upward expansion leads to long, curved breaks, or joints. The joints are parallel to the surface (sheet jointing)

When these sheets break away, it is called exfoliation

2) Exfoliation (sheeting)

3) Root Wedging Small plants or even

trees grow on rocks When they are very

small, they wedge their tiny roots into pores or crevices in the rock

As the roots grow, the rock splits apart

Practice

Topic Questions, pg. 137 #3

Types of Chemical Weathering Chemical weathering of rock results

mainly from the action of rainwater, oxygen, carbon dioxide, and acids of plant decay

Typical processes include: 1) Oxidation 2) Hydrolysis 3) Carbonic acid dissolving

1) Oxidation The chemical reaction of

oxygen with other substances is called oxidation

Iron-bearing minerals are the ones most easily attacked by oxygen

Oxidation of these minerals results in rust, or iron oxides

When water is present, oxidation occurs more quickly

2) Hydrolysis The chemical reaction of water with other

substances is called hydrolysis Common minerals that undergo hydrolysis

include feldspar, hornblende, and augite When these minerals are exposed to water,

they slowly unite with it, and form clay

2) Hydrolysis

3) Carbonic Acid Dissolving Carbon dioxide dissolves easily in water

forming carbonic acid This is the same acid found in soft drinks The acid dissolves out elements such as

potassium, sodium, magnesium, and calcium from minerals

When this occurs, the original mineral is changed into a clay mineral

The greatest effect is on the mineral calcite Carbonic acid completely dissolves this

mineral, leaving behind great underground caverns in limestone bedrock

3) Carbonic Acid Dissolving

Where is this?

Yangshuo, Guangxi

Practice

Topic Questions, pg. 137 #4

Weathering and Erosion Video

Differential Weathering

Some rocks are more resistant to weathering than other rocks.

This can lead to very strange looking landscapes such as in this photo or the photo from the title page

Differential Weathering - Pedestals Where is this?

Zhangjiajie, Hunan

Soil Weathering has attacked the rocks of Earth’s

surface since the beginning of time It has helped wear down mountains and to

shape countless landforms This weathering has led to valuable mineral

deposits and to form a priceless resource – Earth’s life supporting soil

Soil is made of loose, weathered rock and organic material in which plants with roots can grow

Without soil, there could be no life on land The rock material in soil contains three

noticeable parts: sand, silt, and clay

Soil Content

• There are many different soil classifications, based on the amount of sand, silt, and clay particles present

Soil Content These particles only

make up about half of the material in soils

Organic matter (decayed plant and animal material), air, and water also make up substantial portions of soil

Soil Origins The material from which soil is formed is

called its parent material Often this material is the bedrock beneath

the soil Soil that has bedrock as its parent material is

called residual soil In some places, deposits left by winds,

rivers, and glaciers have covered over the bedrock

Soils formed from such materials are called transported soils

Soil Origins

Importance of Soil Soil has many important functions that permit

life on Earth: 1) Soil provides a stable medium from which

plants can grow 2) Soil provides habitat for numerous organisms 3) Soil is porous and permeable, allowing water

to reach roots beneath the surface 4) Soil acts as a filter helping to clean water

before it reaches the ocean or drinking wells 5) Organic matter in soil provides nutrients for

plants and detrivores

Practice

Topic Questions, pg. 143 #7

Review What is weathering?

What is erosion?

What is mechanical weathering?

What is chemical weathering?

Where does soil come from?

Why is soil important?