1. Weathering Unit 4: Lesson 2
Mechanical
and
Chemical

2. TEKS:
Analyze the effects of weathering, erosion, and deposition on the environment in Texas’ Eco-regions.

3. What Caused This?

4. 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

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

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

7. 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

8. 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.

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

10. What is Chemical Weathering?
Chemical reactions break down the bonds holding the rocks together, causing them to fall apart.
Occurs in all types of rock.
Rock reacts with water, gases, and solutions.

11. Chemical - Oxidation
Oxidation - oxygen combines with other elements in rocks to form new types of rock
New substances due to oxidation:
rock becomes softer
easier to break apart
“rusting” of the rock
color change in the
rock

12. Chemical - Carbonation
Carbonation – carbon dioxide (CO2) is dissolved in water making carbonic acid
Acid rain forms with it mixes with air.
Acid is too weak to harm plants and animals but slowly causes feldspars and limestone to decompose.