1. Introduction
Erosion and weathering are part of the forces of gradation which do battle with tectonic forces
Tectonic forces: strive to build up rock structures
Gradational forces: strive to bring rock structures to a level or a uniform slope; this can be done in two ways--by tearing down (i.e. degradation or erosion) or filling in (i.e. aggradation or deposition)

2. Introduction Erosion can be divided into two processes:
a) the breaking up of rock masses (i.e. weathering )
b) the carrying away of the weathered rock fragments (i.e. transportation )
Cycle of gradation: weathering, transportation, and deposition

3. Cycle of gradation
weathering
transportation
deposition

4. Weathering vs Erosion
if a particle is loosened, chemically or mechanically, but stays put, call it weathering.
Once the particle starts moving, call it erosion.

5. Regolith
The broken rock particles that are the result of weathering

6. Bedrock
solid unweathered rock lying beneath surface deposits of soil

7. Physical Weathering Usually occurs in northern climates
due to temperature fluctuation above and below freezing
more rock is exposed because soil is not very deep
Agents of Physical Weathering: rain, wind, heat cold

8. Physical Weathering Types: Frost Shatter/Freeze Thaw Thermal Expansion
Exfoliation/Pressure release
Wetting Drying
Salt Crystallization

9. Frost Shatter Water enters cracks in rock and freezes
As water expands it expands the crack
Over time, it will break the rock apart

11. Salt Crystallization All water has salt
When water evaporates it leaves salt crystals behind.
This fill pores and cracks in rock and works in a similar fashion to frost shatter.
It is normally associated with arid climates where strong heating causes strong evaporation and therefore salt crystallization. It is also common along coasts.

14. Thermal Expansion
Usually an arid region that has hot days and cold nights (deserts)
Usually impacting just the top few centimetres
The constant expansion and contraction weakens the rock leading to it breaking apart

16. Exfoliation/Pressure release
As a rock is exposed at the surface of the earth (due to erosion), pressure is released allowing layers of rock to ‘peel’ off
Also known as ‘unloading’ and ‘sheeting’
Eventually, rock becomes what is called an exfoliation dome

18. Plants and Roots
Tree roots, for example, growing down into cracks can exert powerful forces
Even mosses and lichens can help to split rocks apart

20. Wetting/Drying All rocks absorb water, some more than others
This causes the rock to swell
During the drying out process, the outer surface of the rock shrinks- this weakens rock
The weakened rock cracks and eventually breaks apart.
Usually near the coast (tides) and tropical climates where rocks could dry out quickly

22. Chemical Weathering
Usually happens in warm moist climates (equatorial regions) where there is a lot of rainfall and most rock is buried under deep soil.
Greater chemical weathering takes place below the surface

23. Chemical Weathering Main Types: Solution Hydrolysis Oxidation
Hydration

25. How water becomes acidic
As water droplets fall through the atmosphere they absorb carbon dioxide creating carbonic acid
Normal pH of water is 5.6- the lower the number the more acidic it is- ex. Acid rain is ~4.1
When water hits the earth and combines with decomposing plant and animal matter

26. Solution Some rocks are made of minerals that are completely soluble.
By the time rain water reaches the earth, it is a mild carbonic acid.
This acid works to break down the rock
Ex. Clints and grikes

28. Hydrolysis
“-lysis” means to split apart- the H and O2 molecules split and attach to other minerals in the rock
Carbonic acid works on silicate minerals
Water ions replace the silicate ions and they get washed away
Clay minerals are not soluble so areas that have a high clay content in the soil are areas where hydrolysis is prominent
Ex. Spheroidal weathering

30. Hydration
Entire H2O molecule bonds with mineral in rock causing it to become soluble.

31. Oxidation Oxygen binds with iron to form iron oxide
When this contacts water it can be dissolved and carried away
Sign of oxidation in a rock is the rust colour of the iron oxide

33. Notice the difference between the upper oxidized layer and the grey unweathered bedrock

34. Biological/Biotic weathering

35. Complete the handout