Transcript Chapter 18 Lecture 1

Wind and Erosion
A lesser known problem
Geo 18b
“Assess to what extent wind erosion is a
natural phenomenon, but the surfaces it acts
upon may be made susceptible to active wind
shaping and transport by human actions”
Definition: Wind erosion is the process of
detachment, transportation, and
deposition of soil material by wind
Source: Wind in the Environment,
1994
The Mechanisms of Wind Erosion
There are 3 major processes involved in wind
erosion. These are:
•Saltation
•Suspension
•Creep
Saltation involves small particles being lifted into the
air by strong wind currents then blown back onto the
surface.
Suspension is the process immediately after saltation
where turbulent airflow keeps the (fine) sediment in the
air. These grains travelling by suspension may remain
airborne until rain washes them out of the air.
Creep occurs when larger grains of material cannot be
picked up and are pushed downwind by the turbulence
caused in suspension..
Soil grain motion or grain transport depend on particle size
and rely on interactions between the grains and the passing
airflow
Here man copies nature by
using wind abrasion in a
technique called sandblasting
Abrasion is the direct result of wind erosion.
It is the process by which landforms in arid
environments are shaped. Abrasion is the
wearing away of (relatively) solid rocks by
suspended particles being thrown against a
surface at high velocity (aka sand blasting).
Types of Erosion Damage
Loss of Soil – General Effects
Textural Change – Movement of fertile soil
Nutrient Losses – Loss of fertility
Productivity Losses – Nutrients and water holding
capacities
Air Pollution – Discomforts caused.
Deposition - Sedimentation
Wind Erosion
• Wind erodes dry land much more
effectively than it does moist land.
• As the wind erodes land it carries rock
particles along with it, mostly sand, silt,
and clay.
• Sand material is moved along by a number
of jumps and bounces, much how a pebble is
moved along the bottom of a stream bed.
• The grains do not rise higher than
about 1 meter, and they move in the
same direction the wind is blowing.
• Dust particles (silt and clay) can be
carried along great distances and at
greater heights than sand particles
Wind Erosion: Experience in the
Great Plains
Wind Erosion: Experience in the
Great Plains
• What is the Dust Bowl?
- The Dust Bowl extends from southwestern Great Plains including parts of
new Mexico and Colorado. Its an area
well known for the severe wind erosion
that affects it from time to time.
• What Caused it?
- It was an interaction between natural
and human factors but primarily due to
human induced factors.
Effects of wind erosion
• Abrasion is the weathering of rock
particles by the impact of other rock
particles.
• In areas where there are strong,
steady winds, large amounts of loose
sand, and relatively soft rocks,
abrasion causes a great amount of
erosion.
• Pebbles and small stones exposed to
wind abrasion show surfaces that are
flattened and polished on two or
three sides.
• Rocks smoothed this way are called
ventifacts.
• Particles that have
been moved by the
wind are well
rounded by the
repeated impact of
grain against grain,
and when examined
with a magnifier,
their surfaces have
a frosted
appearance.
• Outcrops of rocks in arid regions
undergo the greatest amount of
erosion close to their base, because
particles transported by the wind
remain close to the ground.
Deflation
• Wind
removes top
layer of fine,
dry
particles,
leaving
larger rocks
• This is known
as desert
pavement
Deflation hollow
Dune Formation
• Need sand
• Need wind
• That’s it!
Sand
• The majority of the sand you'll see in
temperate regions is composed of minerals
eroded from bedrock. It is generally a
combination of many mineral types. The
primary component of rock-derived sand is
silica, what both quartz and glass are made
of, and is very resistant to erosion. It
remains after other minerals have been
broken down mechanically and altered
chemically by their environments.
Sand
• Another mineral can also be found in sand calcium carbonate, or CaCO3, in higher
percentages the closer to the equator the
sample comes from. Calcium carbonate is a
biotic by-product - it is grown as a shell or
skeleton by marine organisms suck as
molluscs and corals. Many other minerals
are also present in beach sand; among
these black sand and green sand
Dunes
• Typically dunes have a low slope face,
called the windward face, that faces the
wind direction and a steeper face, called
the slip face, which points downwind, as
shown below. Dunes move by wind eroding
sand from the windward face by pushing it
and bouncing the sand along the windward
face until it reaches the top of the dune.
It is deposited along the slip face, which in
effect moves the slip face forward in the
direction of the wind
E. Star dune
Types of Dunes
• Barchan dunes are crescent-shaped
dunes that are concave downwind, as
shown below. Barchan dunes form in
places where there is limited sand
and a constant wind direction
Types of Dunes
• The parabolic dune is a crescent
shaped dune that is concave upwind
and forms in areas in which there is
some vegetation and a good supply of
sand.
Types of Dunes
• The longitudinal dune is a linear dune that
is parallel to the direction of the wind and
forms in areas in which the wind direction
is not constant and the supply of sand is
moderate to good. Longitudinal dunes can
be kilometers in length, and the formation
of them is not entirely understood. They
are formed in areas in which there is a
desert pavement and variable winds.
Types of Dunes
• Transverse dunes are linear dunes
that are perpendicular to the
direction of the wind and are not as
long as longitudinal dunes. Transverse
dunes form in areas with abundant
sand and little vegetation.
Star Dunes
• Star dunes, which are one of the
characteristics of The Namib desert,
result where winds converge from
several directions. Buffeted by the
wind from all sides, this dune remains
largely immobile. Its arms have sharp
ridges, sometimes over 250 feet in
height, that radiate from a central
point.
Dune Migration
• Dunes migrate by a eroding from the upwind side
and depositing on the downwind side, as illustrated
below. Strong winds pick up sand grains from the
slope of the dune (upwind or windward side) and
blow them across the crest, where they fall onto
the slip face. Protected from the wind, they
accumulate on the upper slip face to a low, broad
mound. Eventually the cornice deposit grows too
steep and its sand begins to flow as an avalanche
down the slipface. The process repeats over and
over causing the dune to advance with the wind.
Loess
• Loess is a geologic term that refers
to deposits of silt (sediment with
particles 2-64 microns in diameter)
that have been laid down by wind
action (aeolian activity to geologists).
• Extensive, thick loess deposits
generally formed in areas bordering
large, continental glaciers. Large
volumes of meltwater flowed from
the edges of these glaciers during
the summer. This meltwater carried
large amounts of sediments that
formed as the glacier ground the
bedrock over which it moved.
• Much of this sediment was silt-sized
material known as rock flour. During
the winter, when the glacier did not
melt, the area where the water
flowed was primarily dry. The winter
winds would pick up the rock flour
from these dry areas and carry it
long distances in huge dust storms.
• When the wind slowed, the silt would
fall out and blanket the area.
Frequently the resulting loess
deposits are several meters thick
(tens of feet). Because the source of
the silt is the outwash from the
glaciers, loess deposits are frequently
most extensive and thickest
downwind from large river valleys.
Puget Sound glaciation led to the
loessal deposits we know as the
Palouse Hills.
Deposits were
often very
deep and
obviously well
spread out
The Palouse
What factors
erosion?
1.
control
wind
Heavy grazing of the original grass
cover:
• Decreased the residual organic matter
present on the ground.
2. Cultivation:
• Broke the soil aggregates of the land
increasing potential of wind erosion.
3.
Successive planting & harvesting of
crops:
• Affected soil moisture and left the
ground unprotected by vegetation.
Controlling wind erosion on agricultural land.
Need for erosion control:
Plant cover to reduce wind speed,
protecting soil.
Protecting Summer fallow fields.
Livestock management to prevent
overgrazing.
Shelterbeds, Tree lines: reduce wind
speed, trap soil moisture.
Maintain crop residue cover to protect
Irrigated fields.
Emergency Control using manure and
straw