Earth’s External Processes
• Weathering – disintegration of rock at or
near the earth’s surface
• Mass wasting – transfer of material down
slope in response to gravity
• Erosion – transportation of material by a
mobile agent such as water, wind, or ice
These are non–stop, never-ending
processes!
Which comes first
Erosion or weathering?
Weathering must happen before erosion
can take place!
Mechanical Weathering
•Breakdown of bigger pieces into smaller
pieces
•This is also referred to as physical
weathering
•Only size changes occur in this process
•No chemical composition change occurs
in mechanical weathering
Weathering
Mechanical weathering – the physical breaking or
fragmentation of rock into smaller pieces
What things can affect
mechanical weathering?
-Temperature
-Most of the temperature influence
comes from the freezing of H2O
-Ice expands by 9% over the volume of
liquid H2O
-Freezing H2O cracks rocks
Frost wedging
-Repeated freezing and thawing of H2O in rocks
-Can form potholes in cold climate areas
-Can be responsible for splitting trees as well
Biological Activity
Plant roots in
search of
minerals and
water grow into
fractures, and as
the roots grow,
they wedge the
rocks apart. This
is physical
weathering.
Pressure
-Reduced pressure is what we are talking
about
-Bedrock moves upward as overlying
rock is removed
-Bedrock expands upward
-Cracks form as pressure is reduced
-Cracks are called joints
-Joints/cracks form parallel to the ground
Mechanical Weathering
Exfoliation:
Rock breaks apart in layers that are parallel to the
earth's surface; as rock is uncovered, it expands
(due to the lower confining pressure) resulting in
exfoliation.
Weathering
Mechanical Weathering
Sheet Joints
(Exfoliation)
Stone Mountain, GA
Half Dome,
Yosemite, CA
Exfoliated Domes, Yosemite
Stone Mountain, Georgia, showing the product of exfoliation due
to unloading
Stone Mountain, GA
Chemical Weathering
• Oxygen dissolved in
water will oxidize
some materials like
nails and rocks!
(Rust)
• Carbon dioxide
dissolved in water
forms a weak acid
called carbonic acid,
which helps dissolve
rock material.
Chemical Weathering of Statues, Bath, UK
The Result of Chemical Weathering
-New Minerals Are Formed and Dissolved
Particles Are Released
-The Amount of Chemical Weathering Is
Determined by the Composition of the
Parent Rock
-Temperature increases leads to an
increase chemical reactions
How Granite Weathers
Chemical weathering – the alteration of the internal
structure of minerals by chemical reactions
Potassium Feldspar + carbonic acid + water 
Clay mineral + Potassium bicarbonate + Silica
Spheroidal Weathering
Fluids enter
rock along
joints or
fractures and
the edges
become
rounded.
Rates of Weathering
Factors
influencing
weathering:
-Mineral
makeup
-Climate
-Differential
Weathering
Granite versus Marble
Differential Weathering
Rocks weather at different rates because of different
mineral makeup, degree of jointing, and exposure to the
elements.
More resistant rock protrudes as ridges and pinnacles.
Bryce National Park, Utah
What Is Soil?
Soil is the layer of rock and mineral fragments along with
organic matter, water and air that supports the growth of
plants.
The four
components of
soil:
Decomposed rock
Humus
Air
Water
What is Dirt?
Dirt is what gets on
your clothes and
into your house
Controls on Soil Formation
• Parent material – Bedrock
vs. Unconsolidated
• Time – longer time, more
soil
• Climate – temperature and
precipitation
• More plants and animals =
more organic matter
• Poorly developed soils on
steep slopes
Soil Texture and Structure
Point A:
10% silt
40% clay
50% sand
Clay loam has
no single
particle size
percentage
that
dominates!
Controls of Soil Formation
If the parent material is
bedrock, then we get residual
soils
On unconsolidated sediments,
we get transported soils
Soil composition
-Soil forms in layers
-Larger materials are found on the bottom
How quickly is soil formed?
-It may take several hundred years for 1cm to
form
Soil profile
A vertical column
showing the
sequence of layers
of particles in the
soil
Soil Horizons
-A distinct layer or zone within a profile
-There are four major zones (O, A, B, C)
O zone
A Zone
-Leaf litter, high decomposition
-High organic matter
-Generally the darkest in color
-High humus content
B Zone
-Less developed
-Lighter in color
-Zone of accumulation (highly soluble mineral build-up)
-Red or brown in color
-Clay accumulated (Hard pan formation)
C Zone
-Contains weather parent material
Soil Horizons
O zone
-Leaf litter, high decomposition
A Zone
-High organic matter
-Generally the darkest in color
-High humus content
B Zone
-Less developed
-Lighter in color
-Zone of accumulation (highly
soluble mineral build-up)
-Red or brown in color
-Clay accumulated (Hard pan
formation)
C Zone
-Contains weather parent material
Rill and Gully Erosion
Eroded channels ranging in size from rivulets to small gullies
caused mainly by runoff waters rather than raindrop dislocation.
Gully erosion in Tanzania. What is the main factor for it’s cause?
Who cares about soil erosion?
Mass Wasting: The Work of
Gravity
Mass wasting
refers to the
down slope
movement of
rock and soil
under the
influence of
gravity.
Hurricane Mitch in 1998 caused severe mud
flows in Central America
Triggers of Mass Wasting
• The Role of
Water
• Oversteepened
Slopes
• Vegetation
• Earthquakes
Flows, Slides, and Falls
(All types of Mass Wasting)
• Type of material, the kind of motion, and the
velocity of movement dictate kind of mass
movement
• Falls involve the free-fall of debris or rock
• Slides occur when the material remains coherent
and moves along a well-defined surface
• Flow occurs when material moves down slope as
a viscous fluid (liquidy)
Mass Wasting Processes
A debris flow
involves a
flow of soil
and regolith
containing a
large amount
of water.
Mass Wasting Processes
Earthflows
form on
hillsides in
humid areas
during times of
heavy rainfall.
Slow Movements
Creep involves
the gradual
downhill
movement of
soil and
regolith.
Freeze and
thaw cycles
contribute to
creep.
Slow Movements
Slump
Landslide movement along
a curved understructure
Occur on thick soils with
moderate slopes
Common after rainfalls
Mixing of debris
Crescent shaped scar on the
landscape
Triggered by rain or EQ
Solifluction
Solifluction occurs
in regions of
permafrost when
the thawed upper
region flows down
slope.
The frozen bottom
layer stays in
place.
Rock Avalanches
The high
speed of rock
avalanches
suggests that
trapped air
has helped
them attain
velocities of
125 miles per
hour.
What does this coin
have to do with
Rock Falls?
"The Old Man of the Mountain" is a
rock formation that can be found on
Mt. Cannon in the Franconia Notch
gateway to Northern New
Hampshire. From the right view, this
unique rock formation, comprised of
five layers of Conway red granite,
depicts the distinct profile of an
elderly man gazing eastward.
Geographers believe that the layers of
granite were positioned by the
melting and slipping away action of
an ice sheet that covered the
Franconia Mountains at the end of
the glacial period - some 2,000 to
10,000 years ago. Today, the
formation, measuring over 40 feet
high with a lateral distance of 25 feet,
is held in place by cables and
turnbuckles to prevent further
slipping and possible destruction.
The Old Man is no more!!