1. Mechanical and Chemical
Weathering
Mechanical
and
Chemical

2. What Caused This?

3. What is Weathering? First step to forming soil and sedimentary rock
Breakdown of rock into smaller pieces
The smaller pieces do not move to a new location until erosion carries them away
Many types of weathering

4. What is Mechanical Weathering?
Breakdown of rock into smaller pieces without any change in the chemical composition of its minerals
Sometimes called “physical” weathering
Rock is torn apart by physical force, rather than by chemical breakdown
Smaller pieces do not move to a new location until erosion carries them away

5. Mechanical - Ice Wedging
Water fills joints of rocks and freezes
Water expands 10% when it freezes, pushes rock apart
Repeated freeze and thaw cycles over the years causes rock to break along joint

6. Mechanical - Exfoliation
Exfoliation or unloading
Rock breaks off into sheets along joints which are parallel to the surface
Caused by expansion of rock due to uplift and removal of surface material that originally buried the rock

7. Mechanical - Thermal Thermal expansion
Repeated daily heating and cooling of rock
Heat causes expansion; cooling causes contraction
Different minerals expand and contract at different rates causing the rock to split

8. Mechanical - Biotic Biotic – means life
Weathering caused by living organisms
Plant roots act as a wedge and widen cracks
Other causes of biotic weathering are digging animals, microscopic plants and animals, algae and fungi.

9. What is Chemical Weathering?
Breakdown of rock into smaller pieces because of change in the chemical composition of its minerals
Chemical reactions break down the bonds holding the rocks together, causing them to fall apart
Chemical weathering occurs in all types of rock
Rock reacts with water, gases and solutions

10. Chemical - Oxidation
Oxidation - oxygen combines with other elements in rocks to form new types of rock
New substances are usually much softer than original, easier for other forces to break rock apart
Causes a “rusting” of the rock, often causes a color change in the rock

11. Chemical - Carbonation
Carbonation – Carbon dioxide (CO2) is dissolved in water making carbonic acid
Weak acid is formed when CO2 in the air mixes with rain. This is the same acid found in soft drinks. 
Acid is too weak to harm plants and animals but slowly causes feldspars and limestone to decompose