esas1001temp_Deserts-stu

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Unformatted text preview: What
is
it
that
awakens
in
my
soul
when
I
walk
in
the
desert; when
I
catch
the
scent
of
rain,
when
I
see
the
sun
and
moon
rise
and
set on
all
the
colors
of
the
earth, when
I
approach
the
heart
of
wilderness? N.
Scott
Momaday Dunes
in
the
Gran
Desierto,
Sonora,
Mexico,
Basin
and
Range Dunes
in
the
Gran
Desierto,
Sonora,
Mexico,
 Photo
by
Peter
Kresan The
geology
of
deserts EARTH
SCIENCE
in ARIZONA
and
the SOUTHWEST cover
about
30%
of
the
land
surface
on
Earth …most
of
any
climatic
zone Death
Valley
Natl.
Park,
California

































San
Juan
Basin,
New
Mexico Basin
and
Range


























































Colorado
Plateau Drylands P.
Kresan 




Desert


 Arid Steppe
or
semi‐desert 



 Semi‐arid Precipitation
is
“scarce,
unreliable,
intense” 

<
25
cm
(10
in)/y






 






25‐63
cm
(10‐25
in)/y The
geology of
deserts 3 Why
deserts
exist: 1.

High
pressure,
low
humidity
in
subtropics
(around
30°
N
and
S) Warm
equatorial
air rises,
sheds
moisture as
it
cools Cool
air
descends at
30°N
and
S, warms,
soaks
up moisture
and
clouds Grotzinger
et
al.
(2007) The
geology of
deserts 4 Warm
air gathers moisture over oceans Why
deserts
exist: 2.

The
rain
shadow
effect
of
mountains Air
cools
and
loses moisture
as
it
rises over
mountains Windward
 slope is
wet Descending
air sinks,
warms 3.

Great
distance
from
ocean Leeward
 slope is
dry Grotzinger
et
al.
(2007) The
geology of
deserts 5 Drylands
of
the
Earth (not
including
polar
deserts) Colorado Plateau Sonoran Chihuahuan Grotzinger
et
al.
(2007) The
geology of
deserts 6 Topography
influences
climate Intermountain West The
geology of
deserts 7 Precipitation
patterns
across
the
Southwest The
geology of
deserts 8 Desert
ecoregions
of
the
Southwest The
geology of
deserts 9 In
deserts,
where
precipitation
is
scarce
and
infrequent, wind
becomes
nearly
as
important
in
gradation Dry
streambed
near
Winslow, northern
AZ The
geology of
deserts Dust
devil
in
the Chihuahuan
Desert, southwest
NM, Basin
and
Range 10 A
fluid
that
can
erode,
transport,
and
deposit
sediment An
effective
agent
of
gradation 
where
it
is
steady,
or
where
rain
is
scarce Wind:
flowing
air The
geology of
deserts Near
Red
Mesa,
Navajo
Nation,
AZ,
Colorado
Plateau 11 
Wind
direction
and
speed
are
variable
in
the
short
term, but
consistent
in
the
long
term Uneven
heating
of
the
atmosphere
drives
winds; Grotzinger et al. (2007) The
geology of
deserts 12 Wind
as
a
transport
agent Wind
obeys
laws
of
fluid
flow,
but
is usually
unconfined. Wind
nearly
always
flows
turbulently because
of
its
low
density
and viscosity. Wind
is
generally
less
powerful
than flowing
water. Capacity
of
wind
to
carry
sediment
depends
on
wind
strength,
particle
size,
and nature
of
area
over
which
the
wind
flows. The
geology of
deserts 13 Windblown
sediments Dust Dry
silt‐
and
clay‐sized
particles Minerals,
organics,
soot
and
ash Can
significantly
affect
climate Sand Frosted,
rounded
windblown
sand Saudi
Arabia
Grotzinger
et
al.
(2007) April
2003
dust
storm,
Red
Valley,Navajo
Nation,
AZ,
Colorado
Plateau The
geology of
deserts 14 Wind
is
an
agent
of
erosion: Sandblasting
and
deflation Ventifact Grotzinger
et
al.
(2007) Deflation
hollow, San
Luis
Valley,
Colorado The
geology of
deserts 15 Formation
of
desert
pavement Windblown
dust
 becomes
trapped
 between
pebbles Rain
and
microbes in
dust
produce bubbles
that
lift pebbles Grotzinger
et
al.
(2007) Desert
pavement
near Kofa
Mountains,
Arizona, Basin
and
Range The
geology of
deserts 16 Desert
pavement in
the
Sonoran
Desert,
AZ, Basin
and
Range The
geology of
deserts 17 P.
Kresan When
wind
dies
down,
it
drops
its
sediment
load. Eolian
sand
forms
dunes Wind
is
also
a
depositional
agent Windripples and
dunes Stovepipe
Wells, Death
Valley National
Park, CA, Basin
and
Range Grotzinger
et
al.
(2007) The
geology of
deserts 18 Dunes Gran
Desierto,
Sonoran
Desert,
Sonora,
Mexico

















(P.
Kresan) The
geology of
deserts 19 Dunes The
geology of
deserts Great
Sand
Dunes
National
Monument,
CO, Southern
Rocky
Mountains 20 Formation
of
a
sand
dune Grotzinger
et
al.
(2007) The
geology of
deserts 21 Dynamics
of
a
sand
dune Grotzinger
et
al.
(2007) The
geology of
deserts 22 Sand
dunes
vary
according
to
sand
supply
and
wind
velocity Barchans:
limited
sand
supply Blowout:
abundant
sand, strong
winds Transverse:
abundant
sand, strong
winds The
geology of
deserts Linear:
moderate
sand
supply, steady
winds Grotzinger
et
al.
(2007) 23 Eolian
sediments
preserved
in
the
geologic
record E.
Riggs S.
Semken Rhizolith Cross‐bedded
eolian (desert
dune)
sandstone Desert
siltstone Desert
siltstone (loessite) Upper
Permian,
Canyon
de
Chelly, Navajo
Nation,
AZ,
Colorado
Plateau Upper
Permian,
Valley
of
the
Gods, UT,
Colorado
Plateau But
streams
are
the
most
powerful
agents
of
gradation
in
deserts, even
though
they
flow
infrequently The
geology of
deserts Kaiparowits
Basin,
UT, 25 Colorado
Plateau
(USGS
photo)
 Most
erosion
is
by
flash
flooding Eagle
Nest
Arroyo,
Shiprock,
Navajo
Nation, NM,
Colorado
Plateau


September
2002 Saguaro National
Monument, Arizona P.
Kresan The
geology of
deserts 26 Steep‐walled
drainages:
Arroyos
and
cañons Malpais
Arroyo,
NM



























Goosenecks
of
the
San
Juan,
UT The
geology of
deserts 27 Alluvial
fans Tucki
Wash,
Death
Valley
National
Park,
CA‐NV,
Basin
and
Range
 Grotzinger
et
al.
(2007) The
geology of
deserts 28 Alluvial
sediments,
McCoy
Mtns,
AZ,
Basin
and
Range The
geology of
deserts 29 When
water
evaporates,
playas
form Racetrack Playa, Death Valley National
 Park CA‐NV Evaporite
sediments
and
muds Mudcracks,
Malpais
Arroyo,
NM Salar,
Bolivia The
geology of
deserts P.
Kresan 30 Evaporite
sedimentation Littlewater, Navajo
Nation,
NM, Colorado
Plateau The
geology of
deserts 31 Rocks
gradually
build
up
coats
of clay
minerals
stained
by
Mn
and
Fe
oxides Desert‐varnished
sandstone Aneth,
Navajo
Nation,
UT,
Colorado
Plateau Desert
varnish Petroglyphs
in
 desert
varnish
on
sandstone, Newspaper
Rock,
UT Grotzinger
et
al.
(2007) The
geology of
deserts 32 Summarizing weathering
and
erosion
in
deserts Arid;

physical
weathering
>>
chemical
weathering Soils
thin,
sparse,
often
high
in
soluble
salts Water,
wind
are
the
two
major
agents Ferric
(Fe3+)
iron
oxides
stain
many rocks
and
sediments Monument
Valley,
Navajo
Nation,
AZ‐UT,
Colorado
Plateau Press
et
al. (2004) The
geology of
deserts 33 Summarizing
desert
sedimentation Three
major
types
of
sedimentary
environments: 
Alluvial

















Eolian
















Lake
(Playa) 




Saguaro
National
Park,
AZ

















Gran
Desierto,
Sonora

















Death
Valley
Natl.
Park,
CA‐NV 





(P.
Kresan) Channel
sands
 
and
gravels Alluvial
fans Floodplain
sands,
 
silts,
clays The
geology of
deserts Well‐sorted,
 
cross‐bedded
 
eolian
sands Loess Evaporites Muds 34 Erosional Desert landforms Depositional The
geology of
deserts Gary
Hincks
(in
Marshak,
2001) 35 Stairstep
topography Marshak
(2001) The
geology of
deserts 36 Retreat
of
cliffs Plateau Mesas Buttes Marshak
(2001) The
geology of
deserts 37 Mesas,
buttes,
and
pinnacles Monument
Valley,
Colorado
Plateau,
Navajo
Nation,
AZ‐UT


















(P.
Kresan) The
geology of
deserts 38 Hoodoos Bistahi
(“among
adobe
formations”,
Bisti)
Badlands, San
Juan
Basin,
New
Mexico,
Colorado
Plateau The
geology of
deserts 39 Cuestas
and
hogbacks
formed
on
monoclines Marshak
(2001) The
geology of
deserts 40 Hogback
east
of
Shiprock,
Navajo
Nation,
NM,
Colorado
Plateau





















(L.
Maher) The
geology of
deserts 41 mountains
erode
into broad
aprons
of
alluvium Pediments Mountain
remnants Cima
Dome,
Basin
&
Range,
Mojave
Desert,
California Grotzinger
et
al.
(2007) The
geology of
deserts 42 Desert
landscapes
of
the
Southwest Gary
Hincks
(in
Marshak,
2001) The
geology of
deserts 43 Desertification Near
Red
Mesa,
AZ, Colorado
Plateau Spring
2003 Expansion
of
arid
regions
as
a
result
of: 



Climate
change
(e.g.,
drought) 



Human
activities
(e.g.,
livestock
overgrazing) The
geology of
deserts 44 The
geology
of
deserts: essential
ideas Drylands,
where
precipitation
is
“scarce,
unreliable,
and
intense,”
cover
about
30%
of
the
Earth’s
land surface,
the
most
of
any
climatic
type. Deserts
are
arid
places
that
receive
less
than
25
cm
(10
in)
of
precipitation
per
year
on
average,
and steppes
are
semi‐arid
places
that
receive
25‐63
cm
(10‐25
in)
precipitation
per
year
on
average. Most
drylands
occur
in
subtropical
regions
where
air
pressure
is
high
and
humidity
low.

Some
form downwind
of
high
mountains
as
a
result
of
the
rain
shadow
effect,
or
because
of
their
great distance
from
any
ocean. Water
(runoff
and
streams)
is
the
most
powerful
agent
of
gradation
in
deserts,
but
wind
also becomes
nearly
as
important
as
precipitation
is
infrequent. Because
water
is
scarce,
chemical
weathering
is
typically
very
slow
in
dryland
environments.

Desert soils
are
thin,
sparse,
and
often
full
of
soluble
salts. The
predominant
sedimentary
environments
in
drylands
are
alluvial,
eolian,
and
lake
or
playa (evaporites),
reflecting
brief
but
intense
periods
of
precipitation
and
abundant
winds. Gradation
in
drylands
forms
classic
desert
landforms
such
as
mesas,
buttes,
pinnacles,
cañons, arroyos,
dunes,
alluvial
fans,
playas,
pediments,
and
hogbacks. The
expansion
and
degradation
of
drylands
as
a
result
of
drought,
or
human
misuse
of
land,
is
called desertification,
and
is
a
serious
environmental
concern
worldwide. The
geology of
deserts 45 ...
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