Unit 11 external forces

Chong Boon Secondary SchoolGeography Unit Sec 1 Geography

Unit 11 Investigating External Forces

Unit Objectives(by the end of this unit you will be able to…)

1. Identify the 2 key External Forces that change the earth’s surface.

2. Describe and Compare Weathering and Erosion

Weathering

•The process by which rocks break down in situ.

•Occurs at or near the Earth’s surface.

* in situ(broken materials remain in place, not moved away)

Types of Weathering

2 main types of weathering

1.Mechanical Weathering2.Chemical Weathering

Mechanical Weathering

•Exfoliation•Freeze-Thaw Action

•Plants and Animals

Exfoliation•Exposed rocks to high temperature changes.

•Rapid expansion and contraction of the rocks.

•Rates of expansion differ between the minerals found in the rock, causing cracks.

•Layers of rock crack and peel off.

Frost shattering

•Water is trapped in cracks of rocks.•When temperature drops below freezing, water freezes and expands

•Cracks widen and over time, pieces are fractured off.

•Occurs in areas with low temperatures and high mountains.

Plants and Animals action

•Roots of plants and animal actions (burrowing of underground tunnels)

•Cracks that exist may be further widened and eventually rocks are fractured.

Root burrowing through rock cracks.

2 forms of Chemical Weathering

1.Weathering by Solution

2.Weathering by oxidation

Solution

•Rainwater combining with Carbon dioxide forms a mild carbonic acid.

•Soluble minerals and weaker rocks (limestone) can be dissolved by the acid.

•Limestone landscapes.

Oxidation

•Iron within rocks can be decomposed through oxidation.

•Iron turns into iron oxide and breaks down.

•Reddish brown ‘rust’ found on the rocks.

Erosion

•Wearing away of Earth’s surface by agents of erosion.

•Erosion shapes and creates landforms.

•3 key agents of erosion1 Water2 Wind3 Waves

Water

•Running water carries sediments

•Sediments are rubbed against the rocks as the water flows

•Prolonged abrasion cuts the rock surface forming rivers, valleys, forges and canyons

Wind

•Strong wind is able to force itself into cracks in the rock surface and enlarge cracks.

•Particles carried in the wind accelerate the process.

•Dramatic features formed.

Waves•Kinetic energy of waves when they hit the coast cause erosion.

•With prolonged impact, cracks in cliffs may enlarge and eventually break off

•Cliffs, Stacks, Stumps, Arches are all formed in this manner.

•Waves carrying eroded materials accelerate the process.

Recap

•What is the key difference between weathering and erosion?

•What scale are we looking at for each process?

•Can you list the examples for each process?

Assessment of learning

•Let’s take a look at the exercise for this unit

•Complete the exercise and login to your e-learning platform to continue your pre-learning for the next unit.