9_Weathering_and_Mass_Wasting Grade 11 earth Science

SHS Earth Science

1. Chapter 9: Weathering and Mass Wasting

2. WEATHERING AND MASS WASTING
Pressure release and expansion have caused an outer
shell of this granite outcrop to break parallel to the
main mass and then into smaller pieces (J Petersen).

3. INTRODUCTION
Exogenic processes:
Break down rocks
erode rock materials
Transport them from high energy locations to
low energy locations
Relocation of rock by:
Gravity
Geomorphic Agents
Flowing water, wind
moving ice, waves

4. NATURE OF EXOGENIC PROCESSES
 Weathering
breakdown of rock materials
at and near Earth’s surface
Other exogenic processes

Erosion

Transportation

Deposition

Often occurs with assistance
from geomorphic agent (water,
ice, wind, and waves)

Why are some exposed
parts of the boulder
darker than others?

5. NATURE OF EXOGENIC PROCESSES

Mass wasting
Gravity-induced
downslope movement of
rock material that occurs
without a geomorphic
agent
Rock falling
High energy locations to
low energy

6. Endogenic processes compared with exogenic
processes


NATURE OF EXOGENIC PROCESSES

7. Weathering, mass wasting, and geomorphic agents
usually do not work alone

NATURE OF EXOGENIC PROCESSES
How does the cross-sectional profile of the
valley change at each phase?

8. Can you identify evidence of the three phases
shown in the figure below (Figure 15.3) in Idaho?


NATURE OF EXOGENIC PROCESSES

9. WEATHERING
 Two types of
weathering
Physical weathering

Disintegrates rocks
breaking smaller
fragments from a large
block or outcrop of rock
Chemical weathering

Decomposes rock through
chemical reactions that
remove ions from the
original rock

10. Physical weathering
Unloading
Thermal expansion and
Contraction
Freeze-thaw
Salt crystal growth
Hydration

WEATHERING
How might an animal
cause physical weathering?

11. Unloading
High elevation helps drive
this process
Erosional stripping of
overlying rocks, and
ultimate removal of
overlying weight
Upper granite is exposed
where it experiences lower
atmospheric pressure as
compared to the rock at
depth

WEATHERING

12. WEATHERING
Unloading
As outer sheet continues to
weather, segments of it may
slide off
Exfoliation
Successful removal of these
outer rock
Exfoliation sheet
Exfoliation dome
Stone Mountain, Georgia
Half Dome (Yosemite)
Sugar Loaf Mountain, Rio de
Janeiro, Brazil
Enchanted Rock, Texas

Why is granite so
susceptible to unloading
and exfoliation?

13. WEATHERING
Thermal expansion
and contraction
Granular
disintegration

Differential thermal
expansion and
contraction of
individual mineral
grains in coarse
crystalline rocks


What other evidence exists on
the boulder to suggest that it
has been subjected to
considerable weathering?

14. WEATHERING
Freeze-thaw
weathering
Also called frost
weathering or ice
wedging
Water freezes and
expands 9% causing
large pressure
differences in the
crack, widening it,
leading to rock
breaking off

How important if
freeze-thaw weathering
where you live?

15. WEATHERING
Freeze-thaw
weathering
Pipes bursting
Not significant at
lower latitudes
except high
elevations
Especially noticeable
in mountainous
regions near tree line Why are these rocks angular
in shape rather than
rounded?

16. WEATHERING

Salt crystal growth
Water with dissolved salt
accumulates in these
spaces and then
evaporates, and growing
salt crystals wedge rocks
apart
Hydration
Water molecules attach
to the crystalline
structure of a mineral
A mineral expands
when hydrated and
shrink when dehydrated
Clay minerals
Once a small hollow is formed,
how might it affect further
weathering at this site?

17. WEATHERING
Chemical weathering
Ions from a rock are
released into water or
combine with other
substances to form new
materials (clay minerals)
Most important catalysts
and reactive agents:

Water

Oxygen

Carbon dioxide

18. WEATHERING
Chemical weathering
Three types of chemical
weathering:

Oxidation

Chemical union of oxygen
atoms with another
substance to create a
new product

Solution and carbonation

Hydrolysis

What is a likely chemical
formula for the reddish-
orange substance?

19. WEATHERING
Chemical weathering
Solution and carbonation

Carbonation

Carbon dioxide and
water molecules
reacting with, and
decomposing rocks

Most effective on
carbonate rocks
(limestone)
Hydrolysis

Water alone reacts with
rocks

How does the fact that
lichens retain moisture also
contribute to weathering?
Why does the rock near the
bottom of the outcrop seem
to be more weathered than
that at the top?

20. VARIABILITY IN WEATHERING
Effectiveness of
Weathering
Climate
Type of rock
Nature and amount
of fractures

21. VARIABILITY IN WEATHERING
Climate
Chemical weathering
dominates humid
regions
Slopes in wetter
climates have a
rounded appearance

What kind of chemical
weathering has impacted
the iron fence?

22. VARIABILITY IN WEATHERING
Rock type
Differential weathering
and erosion
Strong rock may be
easily eroded in a
different environment
Structural weakness
Joints and fractures
Exposed rock, collect
salts, accumulation of
water, and plants

23. VARIABILITY IN WEATHERING
Structural weakness
Joint set

Two joint sets will cross
each other at an angle
Spheroidal weathering
With north at the top of
this photo (top right),
what directions do the
two most apparent joint
sets trend?

24. VARIABILITY IN WEATHERING
Differential weathering and erosion
Grand Canyon

25. VARIABILITY IN WEATHERING
Differential weathering
and erosion
Appalachian ridge and
valley region
Resistant rocks form
ridges and weaker
rocks form valleys

26. MASS WASTING
Mass movement
Downslope transport of
material due to gravity
What other kinds of
problems on roads are
related to mass
wasting?

27. MASS WASTING
Mass movement
Slope angle
Gravity is the principal
force
Water is also a
contributing factor
How might vegetative cover or
moisture content affect the
potential for downslope
movement of soil?

28. MASS WASTING
Materials and motion
Types of Earth material

Involves almost all kinds of
surface materials

Rock, snow, ice, soil, earth,
debris, and mud

Soil – thin, and
unconsolidated

Earth – thicker soil

Debris – wide range of
grain sizes

Mud – saturated sediment
Speed of motion

Slow mass wasting

Fast mass wasting

29. MASS WASTING
Slow mass wasting
Creep

slow migration, but most
persistent

Heaving process
Are there places near
where you live that
show evidence of soil
creep?

30. MASS WASTING
Slow mass wasting
Creep
What other
constructed features
might be changed by
creep?

31. MASS WASTING
Slow mass wasting
Solifluction

Most common in high
latitudes or high
elevation tundra regions

Permafrost

Active layer

Summer thaw

Tundra landscapes

How does solifluction differ
from soil creep?

32. MASS WASTING
Fast mass wasting
Four major types:

Falls

Avalanches

Slides

Flows
Effects more dramatic than slow mass wasting

33. MASS WASTING
Falls
Rockfalls
Due to:

Weathering weakens the
bonds between clasts
and cliff

Large rock masses that
fall from a cliff face or an
overhanging ledge
What weathering
processes might be acting
on the sandstone cliff at
the base of the ledge
when it becomes wet?

34. MASS WASTING
Falls
Talus slope
Talus cone
Angle of repose

How would the angle of
repose of rounded
particles differ from that
of angular particles of the
same size?

35. MASS WASTING
Falls
Yosemite Valley, California
Serious hazard

36. MASS WASTING
Avalanches
Material is pulverized
(broken down into
small powdery
fragments) and then
flows rapidly
Snow
Rock
debris

37. MASS WASTING
Slides
Water plays biggest role
Rockslides
Threaten lives and property

38. MASS WASTING
Slides
Rockslides
Debris slide
Mudslide
Slumps
Landslide
Earthquake lake
rockslide

Southwestern Montana
(1959)

killed 28 people

Why can earthquakes
trigger landslides?

39. MASS WASTING
Slides
Mudslide (California)
Landslides

May contain more than
category of motion or
material

40. MASS WASTING
Slides
Slumps – common when it is wet

How does the earthflow component differ from the
slump component?

41. MASS WASTING
Flows
Earthflow
Slump-earthflow
Debris flows
Mudflows
Flow levees
What evidence is there
to indicate this is a sit
of repeated debris
flows?

42. MASS WASTING
Flows
Debris flow in La
Conchita, CA.
Lahars


Why might a specific site
experience repeated slope
failures over time?

43. MASS WASTING
Flows
Lahars
Why might a specific
site experience
repeated slope
failures over time?

44. WEATHERING, MASS WASTING, AND THE
LANDSCAPE
Weathering and mass wasting typically determined by the
properties of the rocks and the local climate factors
Slow weathering of resistant rocks leaves steeps slopes
Rapid weathering of weak rocks produces gentle slopes
Weathering proceeds rapidly in warm, humid climates
Rocks in arid and cold climates weather slower