Transcript Document

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
•The physical breaking or fragmentation of rock 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
What things can cause
mechanical weathering?
1.
2.
3.
4.
5.
6.
Ice
Abrasion
Crystal Growth
Biological Activity: Plants and Animals
Temperature changes
Pressure changes
1. Ice or 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
2. Abrasion
• Abrasion is the
weathering of rock
due to impact and
wearing down.
• This can happen
because of water,
wind, waves or ice.
• Abrasion happens
when rocks get hit
over and over and
over and over
3. Crystal Growth
Weathering starts
when water
brings dissolved
minerals to the
rock surface.
When the water
dries, the
minerals form
crystals that force
small particles to
break off.
4. Biological Activity
Plant roots in search of minerals and water
grow into fractures, and as the roots grow,
they wedge the rocks apart.
Burrowing of Animals
5. Temperature Changes
When heated up by
solar radiation each
different mineral will
expand and contract a
different amount at a
different rate. With time,
the stresses produced
are sufficient to weaken
the bonds, and thus
breaking of fragments.
6. 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
Jointing in upper Navajo Sandstone, Grand
Staircase-Escalante National Monument
Chemical Weathering
• The process that breaks
down rock through
chemical changes.
• The agents of chemical
weathering
–
–
–
–
–
Water
Oxygen
Carbon dioxide
Living organisms
Acid rain
Water
• Water weathers rock by dissolving it
• Water is by far the most important agent of chemical
weathering.
• Pure Water is nonreactive, yet a small amount of dissolved
material is generally all that is needed to activate a
chemical reaction.
Oxygen
• Oxygen dissolved in water will
oxidize some materials.
• Iron combines with oxygen in the
presence of water in a processes
called oxidation
• The product of oxidation is
rust
Carbon Dioxide
• CO2 dissolves in rain water and creates carbonic acid
• Carbonic acid easily weathers limestone and marble
Pollution…Acid Rain
• Compounds from burning coal, oil and gas react
chemically with water forming acids.
• Acid rain causes very rapid chemical weathering
Living Organisms
• Lichens that grow on rocks produce weak acids that
chemically weather rock.
• Also, decaying organic material will produce CO2. As
water moves through the soil, it can become slightly acidic.
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
Pinnacles in
Australia
Pinnacles in
Bryce
Canyon NP
Arches
National
Park, Utah:
Hoodoos have a cap of harder less eroded rock:
Canada
Utah
Chemical Weathering of Granite
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
• Oversteepene
d 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!!