The document discusses the destructive forces that weather landforms over time. It describes weathering as the process where rocks are broken down into smaller pieces by forces like water, freezing and thawing, plant and animal activity, and acid in water. Weathering destroys existing landforms and creates new ones through erosion and deposition. The two main components of destructive forces are weathering, which breaks rocks into smaller pieces, and erosion, which transports weathered material.

1. WEATHERING
Destructive Forces
in Nature..
Those that destroy landforms.
By Moira Whitehouse PhD

2. How are Landforms Made?
• The forces that create the different
landforms are, broadly speaking:
• Constructive forces
• Destructive forces

3. Constructive forces
• Are those that build up the land.
• Landforms such as mountains ranges,
volcanoesand plateaus are built by the
movement of the Earth’s plates
• Landforms such as deltas, plains and sand
dunes are created when rocks and soil
resulting from weathering and erosion are
carried away and deposited in new areas.

4. Destructive forces
• Those that wear down the land, like weathering
and erosion.
(Don’t let the name “destructive forces” mislead you.
Destructive forces create landforms like canyons,
valleys, etc. but to do so they first had to destroy
some other landforms, mountains, plateaus, etc.)

5. The two players in the destructive process are:
Weathering
and
Erosion

7. All rocks do not weather at the same rate.
Further we will find that parts of some
rocks weather faster than other parts of
the same rock. Why?
Whatever the reason, one can find some
very odd looking weathered rock:

8. Why?
Grand canyon
Image courtesy of National Geographic

9. Yes, the rock on the bottom is softer than the
rock on top of this formation. The top rock
weathers more slowly.

10. Why?
Is it because the
rock on top is
harder than the
rock below?
http://www.flickr.com Wolfgang Staudt

11. What causes weathering; that is, what
causes rocks to break into smaller and
smaller pieces?

12. • Nonliving things and living things can
break bigger rocks into smaller pieces.
We will look first at non living things that
break up rock.

13. Nonliving things that break rocks into pieces.
1. Water running over the rock
2. Water freezing in cracks in the rocks
3. The temperature of rocks changing
from hot to cold
4. The abrasion of rock by the blowing
wind carrying sand
5. Water with acid in it

14. Running water

15. Rapidly moving water particularly high up in
the mountains or a canyon can break off pieces
of rock.
http://www.ngu.no/en-gb/hm/

16. Seen here, a small,
fast running
stream of water is
dramatically
cutting through
the rock in the side
of this mountain.

17. Rocks carried by fast moving water hit other rocks
breaking them into pieces. Moving sand acts like
sandpaper on the larger rocks in the river bed
rubbing off pieces of rock.
These
smallerpieces
are then
carried
downstream
by thefast
movingriver.
http://www.flickr.com kia4067

18. Here you can see pieces of rock created by
fast moving water.
http://www.flickr.com Randy OHC

19. Rocks that have
been tumbled for
a long time in
rivers and streams
become smooth
and rounded.
http://www.flickr.com
Dawn

20. Freezing water

21. Here we can see
cracks in large
mountain rocks.
Copyright 2004 by Andrew Alden,
geology.about.com, reproduced under
educational fair use http://www.ct.gov/

22. Rain fills these cracks with water.
When the water freezes, it expands within
the crack pushing the rock apart.
Image courtesy of the geology department umd

23. As the ice melts, the water flows
deeper into the crack.
Then the water freezes again, further
cracking the rock.

24. This repeated of
freezing and
thawing forces the
crack open more
and more,
eventually
shattering the large
rock into smaller
pieces.

25. Changing temperature

26. 3. Changing temperature also causes
weathering.
As rocks heat up, they expand. As they cool,
they contract.
This process is repeated over and over again
in nature.
Eventually this process causes them to
break apart.

27. Here we see
large rock in a
desert
environment
that has
probably
been exposed
to the freeze-
melt cycle.
http://www.flickr.com
Hoggheff aka Hank Ashby aka Mr. Freshtags'

28. Abrasion

29. 4. Abrasion by windblown sand also
weathers rocks particularly in deserts. It
is similar to rubbing sandpaper over a
piece of wood or sandblasting concrete.
http://www.flickr.com beige alert

30. Frequent sandstorms in the desert
weather exposed rocks.
http://www.flickr.com nukeit1

31. The scouring of the rock by the wind
carrying sand wears off fragments of rock.
http://www.flickr.comlumierefl

32. Water with acid in it

33. Carbonic acid is very common in nature. It is
produced when carbon dioxide combines
with water.
When this weak carbonic acid trickles
into cracks in limestone, it dissolves the
rock and eats “holes” in it.

34. The mildly acidic rain
water flows into
cracks in the ground.
Sometimes it eats
huge holes in the
rock--caves.
http://www.esi.utexas.edu

35. The same acid that made this rock “holy”
when it was buried in the ground, also
works to make caves

36. stalactites
stalagmites
This is a picture of a cave with stalactites
and stalagmites.

37. When the acid water dissolved the rock
evaporates, crystals of calcite are left behind.
When the water from many, many drips at the
top of a cave evaporates, a stalactite forms. (the
one on the ceiling stuck tight ...stalactite)
Drips that fall on the cave floor cause stalagmites
to grow. (The stalagmitesmight have stuck to the
ceiling but they didn’t.)

38. More pictures of
stalactites and
stalagmites, do
you remember
which is which?

39. Sometime other minerals in rocks react with
the weak acid in water to form other weaker
substances.
These weaker substances are then more easily
worn away by weathering.
Feldspar changes
to clay.

40. Living things
that cause
weathering

41. These are some of the living things
that break rocks into smaller pieces:
1. Plant roots—particularly tree roots
2. Lichen growing on rocks
3. Burrowing animals

42. Plant roots

43. The roots of plants, particularly tree roots,
are amazingly strong. When they start
growing as tiny root hairs they can fit into
the smallest of cracks.
As these tree roots continue to grow,
cause the cracks to get bigger and bigger
breaking the rock apart.

44. Here the
roots of the
tree are
growing in
the cracks in
the rocks
making the
cracks larger.
http://www.flickr.com
Chazz Layne

45. http://media.photobucket.com/ city bumpkins

46. Lichen growth

47. Lichens appear in the form
of small patchy crusty
colors of green, brown,
and orange patches. They
often grow on rocks and
break them apart.
http://www.flickr.combrian http://www.flickr.comSeaDavid

48. Burrowing animals

49. 3. Burrowing animals
When animals burrow in rocks or between
the rocks, they carry seeds which germinate
in the cracks in the rocks.

51. How about a little review.
Do you remember what the word
weathering of rock means?
Yes, it means breaking rock into smaller and smaller pieces.
But, with weathering, it is not a hammer or man who
breaks the rock. It is forces in nature.

52. What are the main forces in nature that break rock into
smaller and smaller pieces?
Yes, there is:
• fast running water
• wind carrying sand
• water in the cracks freezing, melting and freezing
• rocks heating up in daytime and cooling down at night
• acid in water “eating” holes in rock and forming caves
• plant roots
• burrowing animals

53. What force in nature is
causing the rock to
weather in the pictures
on the left forming a
deeper and deeper
valley?

54. In the desert, there
is very little running
water. What force
in nature causes
rock formations like
the one seen in the
pictures on the
left?

55. What force in
nature has caused
the rock in the
picture on the left
to break into two
pieces?

56. What will happen
to this rock if water
gets into the cracks
and freezes, melts,
freezes and melts
over and over
again?

57. After millions and
millions of years
the mountains on
the left will finally
look like the ones
shown below.
Weathering is
constantly changing
the surface of the
Earth.