Unit 2: Surface Processes and the Hydrosphere Lesson 5: Weathering and Erosion ( Heath Earth Science – Pg )

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?