Mass Wasting
Erosion
Soils
Weathering
Weathering:
the physical and chemical
breakdown of rock – exposed to
wind, water, ice, and living
organisms
Regolith:
loose or broken rock layers
resulting from weathering
Regolith covers most of the earth.
Soil:
the upper-most layer of
regolith that supports life
Do “things” last for ever ? Consider :
new concrete
a 1955 Chevrolet
Rocks can be broken down or chemically altered on the
earth’s surface.
Mechanical Weathering:
Rocks are physically broken down by various
surface processes into smaller parts without
changing physical properties.
Chemical Weathering:
Rocks are altered from one form to a
completely new form with a different
set of physical properties.
I
weathering
Discuss with a friend:
1. Describe the difference between
mechanical and chemical weathering.
2. Give two examples of MW and CW you
have observed.
I will get an A on my exams and quizzes
Mechanical Weathering – the breaking down of rocks
How do we expose “more surface” area for chemical
weathering to do its job ?
Let’s show some surface
Rock
Less surface area
More surface area
Increased mechanical weathering accelerates
chemical weathering.
Types of Mechanical Weathering Processes
Frost Wedging: repeated cycles of freezing and thawing
Water
rock
rock
Water Freezes (expands)
pieces break off
(watch for falling rock)
Dominantly takes place in freezing / thawing areas
Unloading – reduction of pressure from
overlying rocks -- take the pressure off me.
overlying rocks
granite pluton
granite pluton
eroded and uplifted
Frost Wedging – rocks are broken into
plates and split apart.
Exfoliation Dome – granite is
“sheeting” from loss of pressure due
to unloading processes.
Types of Mechanical Weathering Processes
Biological Activity – breaking down rocks through
• animal burrowing
• humans
• plant roots
Human intervention
Animal burrowing
Plant root weathering
I
mechanical weathering
Discuss with a friend:
1. Why is MW important?
2. Briefly describe how frost wedging,
unloading and biological activity
contributes to mechanical weathering
I will get an A on my exams and quizzes
Chemical Weathering
Rocks are chemically altered producing a new
compound
Why would a perfectly good rock want to change
it’s appearance?
STABILITY! Rocks require stability and achieve
equilibrium with it’s surroundings
•Most rocks are not chemically stable at the
earth’s surface
•Minerals (compositions) change to become stable
at surface conditions
What common agent can begin the process of chemical alteration?????
•The Universal Solvent – WATER (H2O)
How would we speed up the action of water? Rain + ?????
What in the atmosphere is added to water to create a weak acid?
H2O (rain) + CO2 (atmosphere) = H2CO3 (carbonic Acid)
Let’s weather some granite with the natural acid H2CO3
Granite – the most abundant continental rock
weathering the potassium feldspar (orthoclase)
orthoclase
carbonic acid
water
2KAlSi2O8 + 2(H+ + HCO3) + H2O
kaolinite clay
Al2Si2O5(OH)4 + 2K + 2HCO3 + 4SiO2
Granite has been
chemically weathered
and broken down into
individual grains.
in solution
Reduced to Clay
I
Chemical weathering
Discuss with a friend:
1. What is the universal solvent?
2. How is carbonic acid naturally
Produced?
3. How would you describe the “last” or
most reduced material ----- the end
product of chemical weathering?
I will get an A on my exams and quizzes.
Soil –
product of weathering containing solids,
liquids, and gas.
What’s in your soil?
CLAY
Loam – dominant soil fraction
Humus – partially decayed
organic material
CO2/Air – fills the soil voids
Water – wets the soil
SAND
and carries high
levels of dissolved
substances
SILT
Soil Profiles:
• horizons: a layer of soil with
distinctive physical and chemical
properties.
• sequence of soil horizons from
the surface to the underlying
bedrock
O- Organic matter (humus)
A- Dark layer mixed with mineral
and organic matter
E- light colored layer produced
from removal of soluble material
“leaching”
B- accumulation of clay
C- weathered parent rock
O
A
E
B
C
Factors that influence soil formation:
• Parent Material:
• residual regolith – soil forms emplace or “insitu”
Soil forms and
weathers emplace
from parent
material.
• transported regolith – regolith that is moved by rivers,
wind, and ice
transported river
transported
deposits
regolith on Mars
Factors that influence soil formation:
• Climate – soil profiles differ around the world
which is directly influenced by climate
arid soils vs. tropical soils
• Living organisms – a HUGE influence on soils
Plants make organic matter.
(humus) – animals mix the soil
and recycle air and water.
• Topography – the contour or “slope” of the land
• Time – the longer the time the more mature a
a soil becomes --- 1000’s of years
Mass Wasting
Mass Wasting
What do these pictures have
common?
What is Mass Wasting ?
• down-slope movement of rock
and soil under the direct
influence of GRAVITY
What force acts alone – to create
movements of the earth?
The role of Mass wasting– What’s it good
for?
• Responsible for the evolution of
landforms – features of the earth
• weathering weakens rock – does
not create significant landforms
• need mass wasting to impact
weathering and shape significant
earth features
Example:
River Valley
from weathering
River Valley
widened by
mass wasting
I
mass wasting
Discuss with a friend:
1. What is mass wasting ?
2. What single “force” acts alone to
produce mass wasting processes?
3. What is the sole purpose of mass
wasting?
I will get an A on my exams and quizzes.
So, what “triggers” a mass
wasting episode allowing gravity to
move the mass downward?
•
•
•
•
The role of water
Slopes to steep
removal of vegetation
Earthquakes
The Role of Water
• Water saturates surface materials.
• Particles loose their cohesion (can’t
stick together).
• Particles slide past one another and
slip downward under gravity forces.
Slopes to Steep
• steeper slopes exceed
the “angle of
repose” and material moves downward to
stabilize or reach equilibrium.
Angle of repose:
• the angle at which particles are overtaken by
gravity and move downward
• angle depends on size, shape, and moisture content
of accumulated particles
Dry Sand
Wet Sand
Removal of Vegetation
• where plants are lacking – mass wasting
attacking
• root systems bind soil
• where soil is removed frequent
down slope movements occur
The Role of Earthquakes
• Areas that have not moved in “years”,
are activated by seismic waves.
Mass slide produced by an earthquake
dammed a river forming a lake.
I
Mass Wasting
Discuss with a friend:
1. Describe the 4 triggers that enable
mass wasting process.
2. What the angle of repose?
3. What is the most dominant “trigger”
causing the onset of mass wasting?
I will get an A on my exams and quizzes
How do geologists classify various
mass wasting processes?
• Classification is based on
Material
Unconsolidated:
debris, mud, earth
Bedrock:
rock
Fall: freefall of material
Motion
Slide: material moves cohesively along
well defined surface
Flow: material moves down-slope as
a viscous fluid
I
Mass Wasting.
Discuss with a friend:
1. What two criteria do geologist use to
classify mass wasting movements?
I will get an A on my exams and quizzes
Classification
of
Mass Wasting Processes
SLUMP
• downward movement of a sliding
mass, rock or unconsolidated material
that moves as a single unit along a
curved surface
• moves very slow
• over-steepened slopes (T)
• water saturated (T)
Rockslide
• blocks of bedrock break loose and slide
down a slope
Over steepness (T)
Debris slide
• unconsolidated material break loose and
slide down a slope
Debris flow
Rockslide
Debris Flow
• rapid mass movement containing soil
and regolith with large amounts of
water
Water saturation (T)
Lahars
• debris flows composed largely of volcanic
material from the flanks
• flows down existing stream valleys
• extremely destructive events’
• water saturation (T)
Lahars buried
a village
Earth Flow
• form on hillsides in humid areas with
abundant precipitation
• water saturated – hill may break away
under gravity and weight of soil
• water saturated (T)
• rich in clays and silt
• slow moving flows
Soil Creep
• the gradual downhill movement of soil and
regolith.
• soil moves extremely slow
• soil disturbed by rain, roots
• water saturation (T)