1. Weathering 1

2. True or False: The Earth’s surface has stayed the same for thousands of years

3. The Earth’s surface is always changing!
True or False: The Earth’s surface has stayed the same for thousands of years
False
The Earth’s surface is always changing!

8. Weathering
takes place as rocks are broken down into smaller pieces over time by the effects of weather
Weathering causes
soil formation

9. What is Weathering? First step to forming soil and sedimentary rock.
It is the breakdown of rocks into smaller
pieces by physical and chemical
processes.
No movement is involved in weathering.
The smaller pieces do not move to a new
location until erosion carries them
away. 9

10. 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.
breakdown of rock by
physical means

11. Mechanical Weathering
Ice wedging – water seeps into cracks in rocks, freezes and expands, and eventually cracks the rock

12. Ice Wedging Water water seeps into cracks in rocks and freezes.
Water expands 10% when it freezes, pushing rock apart.
Repeated
freeze and
thaw cycles
over the
years causes
rock to break

13. Ice causes weathering

14. Abrasion – grinding and wearing down of rock surfaces by other rocks
Example: rock bump against each other at the beach
Example: pebbles bump against each other in a river bottom

15. Wind causes weathering
Why wasn’t this mass of land weathered away?
What evidence of weathering do you see in this picture?

16. 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

17. Plants –
roots can crack rock

18. Mechanical Weathering – breakdown of rock by physical means
Animals – human traffic or worms and other animals digging

19. 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

20. Thermal Expansion and Contraction
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.

21. What is Chemical Weathering?
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.

22. Chemical Weathering Chemical Reactions change rock composition.
They break down rock and minerals into
new substances.
Chemical reactions happen
faster in warm,
wet conditions

23. Big Bend National Park, West Texas
breaks down rock and minerals into new substances
Chemical Weathering -.
4 Big Players
1. Water : will dissolve rock, but it takes
1000’s of years.
Big Bend National Park, West Texas
Grand Canyon National Park, Arizona.

24. Water causes weathering
What evidence of weathering do you see in this picture?

25. Chemical Weathering Acid precipitation 2. Carbon Dioxide:
breaks down rock and minerals into new substances
Chemical Weathering
2. Carbon Dioxide:
Dissolves in water
to form a weak
acid that reacts
with calcite,
limestone, and
marble
Ex.
Acid precipitation
(acid rain) breaks down rock.
Carbonation – Carbon dioxide (CO2) is dissolved in water making carbonic acid

26. Chemical Weathering
breaks down rock and minerals into new substances
There are acids in ground water that dissolve rock underground, creating caves.
Limestone being eroded by water that is high in carbonic acid
(formed by carbon dioxide). Natural Bridge Caverns, San Antonio, TX.

27. Chemical Weathering
Organic matter decays,
makes water more
acidic and reactive.
Ex.
Living things like algae, lichen, and humans dissolve rock with the acids they contain.

28. Chemical Weathering (oxidation)
4. Oxygen: reacts with iron and makes rust
(oxidation)
Oxidation – chemical reaction in which an element (iron) combines with oxygen to form an oxide (rust),
rust = iron oxide
Palo Duro Canyon, Texas
Monument Valley, Utah.