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What are the impacts of wind on desert landscapes?
Deserts are associated with high-pressure sinking air.
That sinking air has little water, and that’s what leads to the desert environment.
Once the air reaches the surface, it moves horizontally along the surface away from the
high pressure zone and towards the nearest low pressure systems, which can be hundreds
of kilometers away.
That surface air motion is the wind, and as the differences btween the pressure of the
sinking air and somewhere nearby rising air intensify, the winds will get stronger.
Day after day in the deserts, winds blow across unvegetated surface rocks and pick up the
tiniest grains carrying them in suspension.
Winds can also cause some slightly larger grains to pop up a bit before gravity returns
them back to the surface.
In this way, wind can push grains along the surface in small jumps called saltation.
Winds can also just push larger grains along the surface in a rolling motion known as creep.
The main result of all these types of movement is the removal of the finest grain sand grains
from one area of the desert and the deposition of those somewhere else.
The areas of the desert that have had the continual removal of fine sand grains over
time will then drop in elevation (we call that deflation).
They will also end up with a surface of just the larger grains left behind, which can produce
a hard surface or lid that preserves the rest of the sediment underneath.
We call this surface desert pavement.
What happens to the sand that was carried away in suspension in the wind?
If the wind collides with rock outcrops or buildings or people (think of a sand storm),
the sand in the wind will hit and scour the surfaces – sand blast them – just like
we do to frost glass.
The sandblasting produces tiny etches and marks as the sand grains abrade the surface.
When this happens in nature, it creates shapes and pits on the surfaces of rocks.
We call those ventifacts.
The continued abrasion caused by the sandblasting contributes to the physical breakdown of the
rock.
Where there are large deposits of sediment (like at the mouths of river canyons at the
base of mountains), the wind will collect the sand, remove it, and then somewhere else
drop it and disperse it into continually moving piles of sand known as dunes.
In a dune, the wind is continually pushing the sand grains up the front of the dune through
saltation and creep, and once they reach the top, gravity helps the grains settle down
the back.
In this way the dunes migrate according to the direction of the wind, and the deposits
left behind show inclined sand layers that change direction as the winds change direction.
This type of layering produced during the formation of sand dunes is called cross-bedding.
There are a few locations around the world where large dune fields form, including the
Imperial Sand Dunes found to the east of the Salton Sea in California.
Dunes also form along coastlines when rivers rich in sediment reach the ocean and dump
that sediment on the beaches, winds can blow the sands back onto the land.
The shape of sand dunes formed in this region and other regions world wide come from a combination
of the prevailing wind direction and speed as well as the presence of vegetation that
can trap the sand.
Common shapes of dunes include crescent-shaped, dome shaped, and ridge shaped.
The finest grains, the mud-sized grains, that winds can carry, can also blanket a large
area of land, often behind mountains or large rocks that create a wind barrier or shadow.
These deposits we call Loess.
The Loess Plateau in China is one of the largest and thickest loess deposits in the world and
consists of fine mud-sized sediments that accumulated in a 200 to 300-meter thick pile
about 2.4 million years ago..
These deposits sit just on the back side of a mountain, where the winds that had picked
up dust on the other side would reach the back side of the mountain, lose the wind’s
energy, and drop the dust in the landscape
Pause now.
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