Transcript Deserts and Wind Action

Deserts and Wind Action
Prepared by Betsy Conklin
for
Dr. Isiorho
Deserts
desert: any region with low rainfall
World distribution of nonpolar deserts. Most deserts lie in two bands near 30o N and 30oS.
Distribution of Deserts
the location of most deserts is related to
descending air - the equator receives the
sun’s heat more directly than the rest of the
earth, the air warms and rises then moves
both northward and southward to sink near
30oN and 30oS
Distribution of Deserts
(cont.)
rain shadow: deserts resulting from mountain
ranges - moist air is forced up to pass over a
mountain range, it expands and cools, losing
moisture as it rises, dry air coming down on
the other side of the mountain compresses
and warms, bringing high evaporation with
little or no rainfall to the downwind side of
the range
Distribution of Deserts
(cont.)
great distance from the ocean: since most
rainfall comes from water evaporated from
the sea, a great distance from the ocean
is another factor that can create deserts
Characteristics of Deserts
lack through-flowing streams
internal drainage - the streams drain
toward landlocked basins instead of
toward the sea
flash floods - because of the lack of
vegetation, heavy rainfall runs rapidly
over the surface and can create sudden
local floods of high discharge
Characteristics of Deserts
(cont.)
arroys or dry washes: channels created
by the rapid downcutting by sedimentladen floodwaters which tend to
produce narrow canyons with vertical
walls and flat, gravel-strewn floors
Desert Landforms
 plateaus: a broad, flat-topped area elevated
above the surrounding land and bounded, at
least in part, by cliffs
 mesa: a broad, flat-topped hill bounded by
cliffs and capped with a resistant rock layer
 butte: a narrow hill of resistant rock with a flat
top and very steep sides
Desert Landforms (cont.)
monocline: bends in rock layers
hogback: a sharp ridge that has steep
slopes
cuesta: a gently tilted resistant layer
with one steep side and one gently
sloping side
Desert Landforms (cont.)
 playa lake: a shallow temporary lake (following a rainstorm)on a
flat valley floor in a dry region
 playa: a very flat surface underlain by hard, mud-cracked clay
 bajada: a broad gently-sloping depositional surface formed by
the coalescing of individual alluvial fans
 pediment: a gently sloping surface, commonly covered with a
veneer of gravel, cut into the solid rock of the mountain
Wind Action
wind can be an important agent of
erosion and deposition in any climate, as
long as sediment particles are loose and
dry
wind differs from running water in two
ways:
because air is less dense than water, wind
can erode only fine sediment - sand, silt and
clay
wind is not confined to channels as running
Wind Erosion and
Transportation
dust storms: when loose silt and clay are easily
picked up from barren dry soil, such as in a
cultivated field - silt and clay can remain
suspended in turbulent air for a long time, so a
strong wind may carry a dust cloud hundreds of
meters upward and hundreds of kilometers
horizontally
An approaching dust storm in Prowers Country, Colorado (1930s)
Wind Erosion and
Transportation (cont.)
volcanic ash: ash from a volcanic eruption that
can be carried by wind for a very great distance
- the ash can be caught in high altitude jet
streams which have been known to carry ash as
far as 3,000 miles
jet stream: a narrow belt of strong winds with
velocities sometimes greater than 200 mph
saltation: a mode of transportation that carries
sediment down current in a series of short leaps
or bounces
Wind Erosion and
Transportation (cont.)
sandstorms: clouds of sand moving
rapidly near the land surface
ventifacts: rocks with flat, wind-abraded
surfaces
Ventifacts eroded by blowing sand
Wind erosion near the ground has sandblasted the lower 3 ft.
of this basalt outcrop, Death Valley, CA. (Hammer for scale)
Deflation
deflation: the removal of clay, silt, and sand
particles from the land surface by wind
blowout: a depression on the land surface
caused by wind erosion
pillar: erosional remnant of the former land
that may be left at the center of a blowout
Deflation by wind erosion can form a blowout in loose,
dry sediment. Deflation stops at the water table. A pillar, or
erosional remnant, may be found in the center of a blwout
Large blowout near Harrison, Nebr.
Wind Deposition
loess: a deposit of wind-blown silt and clay
composed of unweathered, angular grains of
quartz, feldspar, and other minerals weakly
cemented by calcite - have a high porosity,
typically near 60%
Definite
Possible or
probable
Major loess-covered areas in the world
Vertical road cuts in loess - Vicksburg, Mississippi
Sand Dunes
sand dunes: mounds of loose sand grains
heaped up by the wind, most likely to
develop in areas with strong winds that
generally blow in the same direction
sand grains found in dunes are commonly
well-sorted and well-rounded because
wind is very selective as it moves
sediment
Sand Dunes (cont.)
slip face: the steep downwind slope of a
dune
wind ripples: small, low ridges of sand
produced by saltation of the grains
Wind ripples on sand surface - Monument Valley, Utah
Types of Dunes
barchan: a crescent-shaped dune with a
steep slip face on the inward or concave
side which is formed where the sand
supply is limited
Barchans
These barchan dunes are advancing as much as 50 ft.
A year over this barren valley floor in southern CA
Types of Dunes (cont.)
transverse dune: a relatively straight,
elongate dune oriented perpendicular to
the wind direction
Transverse dunes
Transverse dunes, Oregon
Types of Dunes (cont.)
parabolic dune: somewhat similar in
shape to a barchan dune, except that it is
deeply curved and is convex in the
downwind direction
Parabolic dunes
Parabolic dunes near Pismo Beach, central California.
Wind blows from left to right. The ocean and a sand
beach are just to the left of the photo
Types of Dunes (cont.)
longitudinal dune: one of the largest types
of dunes which is a symmetrical ridge of
sand that forms parallel to the prevailing
wind direction
Longitudinal dunes
Longitudinal dunes in the Sahara Desert, Algeria.
Photo from Gemini spacecraft at an altitude of
about 65 miles
Dunes Types(sketches)
Pictures
All pictures used in this power point presentation were
taken from the following:
Carlson, Diane H., David McGeary and Charles C.
Plummer. Physical Geology: Updated Eighth Edition. New
York City, McGraw-Hill Higher Education, 2001.