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Geologic Processes 2Weathering, Erosion and
Deposition
How can wind and water change our landscapes?
Weathering- the destructive processes by
which materials at or near the Earth’s
surface are changed in color, texture,
composition, firmness or form. Weathering
is the beginning of erosion.
There are two types of weathering….
Mechanical or physical weathering- a physical
change in which rocks are broken into
different shapes and smaller pieces
Six main processes can cause mechanical
weathering
(1) Impact and Abrasion- moving materials
can cause rocks to fracture, flake or lose small
particles
(2) Frost Action- water seeps into an
opening or crack in a rock and then freezes
when temperatures drop. Water expands
when it freezes and causes rocks to break
into pieces.
(3) Temperature- The cycle of heating and
cooling of rocks causes a cycling of
expansion and contraction that causes
particles on the rock surfaces to crack or
flake off.
(4) Organic Activity- The roots of plants
can often loosen and change rock
material.
(5). Gravity can pull loosened rocks downhill
resulting in a landslide causing rocks to
break into smaller pieces.
(6) Exfoliation- Rocks which form under the
surface form under pressure - confining
pressure. When that pressure is released, the
rock will expand. As it expands the outer
layers break free in sheets, and is called
exfoliation.
All causes of mechanical weathering result
in the breaking down of rocks into smaller
pieces called sediments.
Chemical Weathering- changes in the
chemical makeup or mineral composition
of rocks. Five main processes can cause
chemical weathering
(1) Water- can dissolve minerals
holding rocks together, or form acids
when mixing with gases in the
atmosphere to form acid rain (speeds
up decomposition of rocks)
(2) Oxidation- oxygen can chemically
combine with substances resulting in the
formation of a new substance (Ex. iron in
rocks forming rust)
(3) Carbonation- carbon dioxide in the air can
dissolve in rain forming carbonic acid which
can dissolve certain rocks (Ex. limestone and
feldspar)
(4) Sulfuric acid- sulfur oxides can combine
with water producing acid rain which can
chemically break down rocks, metals and
other materials
(5) Plant acids- some plants produce weak
acids that can dissolve minerals in rocks
(Ex. mosses)
As chemical changes take place minerals
can be added or removed from rocks.
Erosion and Deposition
1. Erosion is the process by which weathered
rock and soil are moved from one place to
another.
2. Erosion changes landscapes by wearing
down mountains, filling in valleys, and making
rivers appear and disappear.
3. Deposition is the process by which
sediments are laid down in new locations.
4. Weathering, erosion, and deposition act
together in a cycle to wear down and build up
Earth’s surface.
5. The agents of erosion are gravity, wind,
running water, glaciers, and waves.
C. The Five Agents of Erosion and their
Effects on Landscape
1. Gravity erosion or mass wasting- the
downhill movement of
sediments caused by gravity.
a. Landslides- the tumbling of soil, rocks and
boulders down a slope
b. Mudflows- usually occur after heavy rains
carrying large amounts of mud downhill
c. Slump- materials such as rock or soil on a
steep slope move downhill as a coherent unit
to decrease the slope angle of land
d. Creep- the slow movement of sediments
downslope, caused by repeated expansion
and contraction of materials (Examples:
freezing and melting, growth and decay of
roots, burrowing animals)
Mass Wasting
Running water- Runoff, Rivers, and Streams
1. The amount of runoff is affected by the
amount of rainfall, plant growth, and shape
of the land.
2. Urban storm water runoff effects water
quality, water quantity, habitats, biological
resources, public health and the aesthetic
appearance of waterways.
(a) After storms there is often a temporary
increase in pollutants, toxins, and bacteria
levels.
(b) Increased
erosion and
deposition results
from large amounts
of urban runoff
which alters aquatic
habitats.
b. Streams
(1) Streams usually cause erosion by
abrasion (sediments colliding with rocks and
wearing the rocks down over time)
(2). Load- the soil particles and rocks carried
by the stream
(3) If a stream
flows first over a
hard rock layer,
then a soft rock
layer that is easily
eroded, a waterfall
forms.
(4) Downcutting
represents
downward erosion of
the stream channel.
It occurs when the
stream energy
(capacity) exceeds
the stream's load,
and causes valleys
to be narrow and
steep-sided (canyons
and gorges).
(5) Lateral erosion undermines part of the
bank or valley wall, leading to mass wasting
and widening of the valley.
(6) Stream deposition results from decreasing
water velocity or from chemical changes. The
coarsest material is deposited first as water
velocity decreases. Stream deposited material
is called alluvium.
c. River Systems- streams that join form
rivers, which usually begin in mountains or
hills
(1) Larger steams flowing into a main river are
called tributaries.
(2) Eventually a river empties into another
river, a lake, or an ocean at the mouth.
(3) The drainage basin is all of the area
drained by a river and its channel.
(a)Drainage basins are separated by divides
(high ground between two drainage basins).
(b) The Continental Divide is the largest divide,
running north and south (close to Denver,
CO).
Which river basin do YOU live in?
(4) Deposits made by rivers are constantly
changing the landscape of surrounding areas
with sediments being deposited where a river
curves, or meanders.
(5) Oxbow lakes are small lakes formed from
a U-shaped bend that erosion and deposition
of sediments cuts off from the river.
River meander and Oxbow Lake
(6) Deltas are formed as large amounts of
sediments are deposited at the mouth of a
river.
(7) Flood Plains are flat areas on both sides of
a mature river or stream. Fine sediments are
deposited on flood plains after heavy rains or
spring thaws. Larger sediments are deposited
and form ridge-like deposits called levees.
Mississippi River Delta
3. Glacial Ice- erodes by abrasion and
plucking (picking up rocks) and wearing away
underlying rocks.
a. As glaciers move they carry rock materials
and wear down (erode) the land surface it
passes over usually forming U-shaped valleys.
b. The material deposited by a glacier is
called till. The till left behind after a glacier
melts forms a ridge called a moraine.
c. Glaciers can also form lakes by leaving
behind till that keeps water from flowing
away from the area (Ex. Great Lakes).
4. Wind- active agent of erosion in deserts,
plowed fields and beaches
(a) Wind erodes Earth’s surface in two waysabrasion and deflation.
(b) Abrasion occurs as larger particles carried
by wind act as “sandblasters”, cutting and
polishing rocks.
(2) Deflation occurs as loose materials such
as clay, silt, and sand are carried away by the
wind.
b. The amount of wind erosion is dependent
upon four factors.
(1) Size of particles being carried
(2) Speed of the wind
(3) Length of time the wind blows
(4) Resistance of rocks exposed to wind
3. Deposits by wind form dunes or loess
deposits (layers of fine sand and silt
deposited in an area).
5. Waves- their powerful force is constantly
eroding and shaping shorelines
a. Waves receive their energy from winds that
blow across the water surface. The size of a
wave depends on how fast, how far, and how
long the wind blows.
2. Deposition of sediment forms our
beaches. Some of the sediments
comes from surrounding shorelines,
while others are carried by rivers from
inland areas.
3. Deposition by a longshore current
(water moving parallel to the
shoreline) creates sand bars or barrier
islands (large semi-permanent bars)
like the Outer Banks
The NC Outer
Banks
4. The shape of a shoreline often results
from changes in sea levels. Increased sea
levels create bays and harbors while
decreasing sea levels creates many sea
cliffs and terraces (flat platforms at the base
of a cliff).
D. Controlling Erosion
1. The key to stopping erosion is to anchor the
soil in place and to slow down the water
moving across it.
2. The best way to hold the soil in place is to
plant vegetation in the soil. Establishing a
ground cover has many benefits besides
controlling erosion. Grass, shrubs and trees
are all effective ground covers.
3. Terracing is an
effective treatment
to control erosion on
sloping areas. The
level steps of
terraces reduce the
velocity of water,
stopping the water
from flowing down
hill. This allows time
for water to seep
into the ground.
4. Strip-cropping
is when farmland
is plowed so that
plowed strips are
separated by
planted strips,
helping to reduce
soil loss.
5. Contour farming is a method of plowing
along the contours of the land rather than up
and down a slope. It creates furrows between
crops to collect water.
6. Construction sites can use sediment fences,
temporary vegetation, and grass covered
drainage ditches to reduce erosion.
7. Collect and direct urban runoff to
protect aquatic ecosystems from
sedimentation.