1. Weathering and Erosion

2. Learning Objectives
Explain the differences in chemical and physical weathering
Explain how weathering rates are affected by a variety of factors including climate, topography and rock composition
Compare erosion by water, wind, ice, and gravity and the effect on various landforms

3. Weathering
Weathering is the process by which rocks on or near Earth’s surface break down and change
There are two types of weathering:
Mechanical Weathering
Chemical Weathering

4. Mechanical Weathering
Occurs when physical forces break rock into smaller and smaller pieces without changing the rock’s mineral composition Definite change in size and shape Occurs mostly where it is cool and dry

5. Factors that Affect Mechanical Weathering
There are processes in nature that play a vital role in mechanical weathering:
Temperature / Frost Wedging
Unloading / Exfoliation
Biological Activity

6. Frost Wedging
When water seeps into existing cracks in rock it will freeze if the temperature is low enough
Once the water freezes, it expands beyond the space provided by the crack
The expansion causes the rock to break

7. Unloading / Exfoliation
Unloading reduces pressure on igneous rocks, causing it to expand, which allows slabs of outer rock to break off in layers in a process called exfoliation
Exfoliation is the process by which outer rock layers are stripped away

8. Biological Activity
The activity of organisms, including plants, burrowing animals, and humans, can also cause mechanical weathering
For example, tree and plant roots wedge into cracks of rocks and cause them to split

9. Weathered Rock: Talus
As sections of rock are wedged loose they may tumble into large piles called talus, which typically form at the base of steep, rocky cliffs

10. Chemical Weathering
Chemical weathering is the transformation of rock into one or more new compounds (new chemical composition)
Process by which rocks and minerals undergo changes in their composition as the result of chemical reactions
Reactions result in the formation of new minerals and release of dissolved substances
Occurs greatest where it is warm and wet

11. Factors that Affect Chemical Weathering
Water – the reaction of water with other substances is called hydrolysis
Oxygen – The chemical reaction of oxygen and other substances is called oxidation
Carbon Dioxide – combines with water in the atmosphere to form weak carbonic acid which dissolves calcite found in limestone and other minerals
Acids – caused mainly by the oxidation of sulfur dioxide and nitrogen oxides emitted by cars. (Many times this leads to acid rain.)

12. What Affects the Rate of Weathering?
Climate Rock Type and Composition Surface Area Topography and Other Variables
What Affects the Rate of Weathering?

13. Rock Type and Composition
Climate
Rock Type and Composition
Climate affects areas precipitation, temperature, and evaporation
Chemical Weathering occurs greatest in warm/wet climates
Mechanical Weathering occurs greatest in cool/dry climates
Characteristics of rocks, including how hard or resistant they are, depends on their type and composition
Sedimentary rock are more easily weathered than igneous or metamorphic rocks
Due to differential Weathering, which is caused by variations in composition, unusual and spectacular rock formations and landforms are created

14. Spheroidal Weathering
Spheroidal weathering is a form of chemical weathering that affects jointed bedrock and results in the formation of concentric or spherical layers of highly decayed rock within weathered bedrock
The spherical layers of highly decayed rock are referred to as saprolite
Ultimately, the process causes the corners and edges of rock to be more rounded

15. Surface Area Mechanical weathering breaks rocks into smaller pieces
As these pieces get smaller, their surface area increases
As result, more surface area is available for chemical weathering
So, the greater the surface area… the more weathering occurs

16. Topography / Other Variables
Materials on level surfaces remain in place as they undergo changes
Materials on sloped surfaces tend to move as result of gravity
As materials move downward, they expose underlying rock surfaces and provide more opportunities for weathering to occur
Animals also increase weathering through the release of carbon dioxide during decay
Living plants release carbon dioxide as well, which combines with water to produce acid

17. Erosion and Deposition
Erosion is the process that transports the Earth’s materials from one place to another At some point, the movement of materials slows and the minerals are “dropped” in a new location (deposition) Gravity is a huge factor in many erosional agents, such as streams and glaciers, because the force of gravity pulls materials downslope

18. What is Erosion?
Erosion Video: What is Erosion

19. Erosional Agents Water
More power to move large particles than most winds
Greater the volume of water, the more material that can be eroded
The steeper the slope, the increase of erosional ability
Small channels (rill erosion) of water run downward to meet larger channels (gully erosion) draining into an area called a watershed

20. How Does the Force of Water Shape the Earth?
Water Erosion

21. Erosional Agents Glacial Erosion
Although glaciers cover less than 10% of the Earth’s surface, their erosional effects are large-scale and dramatic
They scrape out and gouge large sections of landscape
They are so powerful, they have the ability to pick up large rocks and move them over great distances

22. How Do Glaciers Shape the Earth?
Glacial Erosion

23. Erosional Agents
Wind
Major erosional agent in areas on Earth that experience limited precipitation, high temperatures, and therefore minimal vegetation to hold soil in place
As result, wind can easily pick up and transport fine, dry particles
The effects of wind erosion can be damaging and dramatic, although wind erosion is relatively insignificant to erosion by water and glaciers
Wind barriers can be created as a farming method to reduce soil erosion

24. Erosional Agents Living Variables
Plants, animals and humans all have activities in their daily lives that affect and move Earth’s materials from one place to another
For example, plants move surface materials as they carry on their life process (roots grow)
Animals burrow into soil – relocating the Earth’s materials
Humans relocate soil every time they plant a garden or build a new development

25. How do Human Forces Shape the Earth?
Erosion Video: Human Impact