Transcript Life of Sand Dunes

Life of Sand Dunes
Wind
Wind and sand create majestic
dunes that are constant but ever
changing.
They move across the deserts,
sing to the wind and inspire our
creativity
• The accumulation of windblown
sand marks the beginning of one
of nature's most interesting and
beautiful phenomena.
• Sand dunes occur throughout
the world, from coastal and
lakeshore plains to arid desert
regions.
• In addition to the remarkable
structure and patterns of sand
dunes, they also provide
habitats for a variety of life which
is marvelously adapted to this
unique environment.
Origins of Sand Dunes
Three essential prerequisites:
• 1. An abundant supply of loose sand
in a region generally devoid of
vegetation (such as an ancient lake
bed or river delta)
• 2. A wind energy source sufficient to
move the sand grains
• 3. A topography whereby the sand
particles lose their momentum and
settle out. Any number of objects,
such as shrubs, rocks or fence posts
can obstruct the wind force causing
sand to pile up in drifts and ultimately
large dunes.
Mountains out of Ant hills
• There are even reports of ant hills forming the nucleus
upon which sand dunes are built.
• The direction and velocity of winds, in addition to the
local supply of sand, result in a variety of dune shapes
and sizes.
• The wind moves individual grains along the inclined
windward surface until they reach the crest and cascade
down the steep leeward side or "slip face," piling up at
the base and slowly encroaching on new territory. Some
California dunes with crests only 30 feet high may
advance 50 feet a year, posing a serious threat to
nearby farms and roads.
Building of a Dune
• If the wind direction is fairly uniform over the years, the
dunes gradually shift in the direction of the prevailing
wind.
• Vegetation may stabilize a dune, thus preventing its
movement with the prevailing wind. Along the Oregon
coast, entire forests may cover sand dune areas.
Sometimes severe storms or other disturbances can
destroy the forest canopy allowing sand from nearby
dunes to move into the disturbed area. In fact, I have
stood at the crest of a shifting dune where the tip of a
sitka spruce (Picea sitchensis) was protruding from the
sand.
Geology of Sand Dunes
The structure and
mineral composition
of sand grains
depends on the
geology
of the mountains
that have been
eroded away by
wind and water.
Coral Sands
• Gleaming white sands of tropical coral beaches
and atolls are composed of a glistening,
microscopic assortment of reef animals and
algae, including wave-worn fragments of
brightly-colored corals, minute one-celled
foraminiferans, fragments of sea shells and starshaped sponge spicules.
• Sand grains of some tropical beaches are
composed of fragments from a common
calcareous green alga (Halimeda) that grows
among submarine seagrass meadows and coral
reefs.
• Evidence of abrasion on sandblasted
surfaces of telephone poles and
posts reveals that sand grains
seldom travel more than a few feet
above the ground.
• Myriads of sand grains bouncing
and rolling up the windward surface
of a dune often form a series of
ridges and troughs called wind
ripples. Bouncing sand grains tend to
land on the windward side of each
ripple, thus producing a low ridge.
Jumping Grains of Sand
• Without getting too complicated, the
spacing of ripples is related to the average
distance grains jump. This in turn, is
related to the wind velocity and size of the
grains. Wind ripples are often very
spectacular and photogenic, especially
when the thousands of tiny ridges catch
the shadows of early morning or late
afternoon.
Where do they bounce?
Bouncing sand grains
tend to land
on the windward
side of each ripple,
thus producing a
low ridge.
1/5 of earth is sand
• Many people associate deserts with vast areas
of drifting sand, as portrayed by a number of
Hollywood films depicting the French Foreign
Legion, battles of World War II and other
dramas.
• In fact, less than 20 percent of the earth's total
desert area is covered with sand, and sand
dunes only account for about two percent of
the surface of North American deserts. One of
the largest dune systems in the United States
is the Algodones Dunes.
Algodones Dunes.
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It extends southeasterly for more than
40 miles (64 km), from north of Glamis
in Imperial County, California to the
southwestern corner of Arizona and into
Sonora, Mexico.
In California, the dunes range from two
to six miles in width, with crests rising
200 to 300 feet (91 m) above the
surrounding landscape.
Other large dunes occur in Death
Valley and Eureka Valley, in the Mojave
Desert near Kelso, and along the
California coast just south of Pismo
Beach.
The Eureka dunes rise to nearly 700
feet (200 m) and the Great Sand Dunes
in Colorado rise to nearly 800 feet (240
m).
Booming Dunes
Acoustical Dunes
• Some of the earliest references about "acoustical" dunes
are found in Chinese and Mideastern chronicles dating
back more than 1500 years. Marco Polo described weird
sounds on a journey through the Gobi Desert, and Charles
Darwin mentioned it while traveling through Chile.
• The sounds have been variously described as singing,
whistling, squeaking, roaring and booming. Some accounts
compare the sounds with distant kettle drums, artillery fire,
thunder, low-flying propeller aircraft, bass violins, pipe
organs and humming telegraph wires.
Acoustical Dunes
What makes the sound?
 Low frequency sounds are produced when closely
packed sand grains slide over each other, such as an
avalanche down the slip face of a dune.
 The stationary sand underneath apparently acts as a
giant sounding board or amplifier to produce the
enormous volume of sound. The sand must be very dry
for sound production, and under a microscope the grains
appear more rounded and finely polished compared with
ordinary (silent) sand.
 Astronomers and geologists have speculated that this
remarkable phenomenon may be common in the windy
and nearly waterless sand dunes on Mars!
Where does sand boom?
• Acoustical "booming" dunes are rather widespread on
earth, including the Sahara Desert, Middle East, South
Africa, Chile, Baja California and the Hawaiian Islands.
California has at least two documented areas with
booming dunes, the massive Kelso Dunes of San
Bernardino County and the scenic Eureka Dunes of Inyo
County.
• Dryness is essential for sound production in booming
sand. Rain or high humidity will eliminate booming
completely. Hot, dry days are best to experience this
remarkable phenomenon.
Best Place to hear the Boom
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One of the best places to observe booming
dunes in the western United States is Sand
Mountain, about 16 miles southeast of
Fallon, Nevada. A short dirt road north of
Highway 50 leads to the base of the
massive white dunes.
Sand Mountain is composed of two "seif"
(sword-shaped) dunes whose summits
stand about 390 feet (120 m) above the
desert floor.
To really appreciate this acoustical
phenomenon you must climb to the crest of
a dune and then slide down the steep slip
face. Going down with an avalanche of
sand is sort of like riding down an
escalator, ankle deep in sand. As the sand
begins to vibrate the sound becomes quite
loud, like a low-flying B-29 bomber or
squadron of World War II vintage fighter
planes.
The lost dinosuar
• There are several interesting legends about the
mysterious moaning of Sand Mountain.
According to Mary Holliday (Nevada Official
Bicentennial Book, page 137), a large sea
dinosaur or plesiosaur once lived and frolicked
with its mate in ancient Lake Lahontan.
• Strong winds piled the lakebed sediments into
what is now called Sand Mountain, completely
burying the dinosaur under hundreds of feet of
sand. Today the dinosaur moans for its mate
and the deep blue waters of Lake Lahontan.
• To stand before an enormous, gleaming white sand
dune and realize that all of this was once an ancient lake
bed or coastal plain is quite astonishing.
• The incredible roaring sounds of distant dunes is an
unforgettable experience, particularly during the quiet
hours of darkness and daybreak.
• Starting with the wind and tumbling particles of sand and
culminating in picturesque drifts of rippled sand with an
entire, dynamic, living community of plants and animals;
this is one of nature's most remarkable cycles, and it is
truly a phenomenon of wind
Pink sand verbena (Abronia villosa) and white dune evening-primrose
(Oenothera deltoides), two common wildflowers in sand dune areas of
the Colorado Desert in the southwestern United States.
Adapting to drifting sand
• During years with favorable winter rains carpets of pink
sand verbena (Abronia villosa), white dune eveningprimrose (Oenothera deltoides) and yellow sunflower
(Geraea canescens) may extend for miles, and the air is
filled with the sweet aroma of fragrant blossoms.
• Deep-rooted shrubs, such as mesquite (Prosopis
glandulosa), creosote bush (Larrea tridentata) and
desert buckwheat (Eriogonum deserticola) provide vital
shade and food for a host of animals, from insects and
reptiles to birds and mammals.
• Often entire rodent condominiums are constructed
beneath the protective cover of dune shrubs.
Dune Lizards and Snakes
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Two remarkable dune residents, the fringetoed lizard (Uma notata) and shovelnosed sand snake (Chionactis
occipitalis), are well adapted for rapid
burrowing through sand, a phenomenon
known as "sand swimming."
At depths of only 12 inches (30 cm), the
temperature may be 50 Fahrenheit
degrees cooler. Fringe-toed lizards are
often seen scurrying over sand dunes
during mild spring days and even in the hot
summer.
Their toes are fringed with elongate,
pointed scales which provide traction in the
sand (like extra-grip tread). They can run
with amazing speed across steep dunes
and then stop suddenly and wriggle out of
sight into the cooler layers of sand. Its
special eyelids and countersunk lower jaw
keeps out sand grains
Shovel-nosed Sand Snake
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The shovel-nosed sand snake and
banded sand snake
(Chilomeniscus cinctus) are
also well-adapted for burrowing
through sand with their
streamlined head and smooth
scalation which minimize friction.
The way they glide swiftly through
the sand is reminiscent of the
giant "sand worms" in the sci-fi
movie "Dune," and the terrifying
sand creatures in the movie
"Tremors."
Although it is harmless, the
colorful banded sand snake is
sometimes mistaken for the
poisonous coral snake; however,
the coral snake has a black-tipped
head and red bands that
completely encircle its body,
bordered by yellow or white
bands.
Sand Food/Sand Plant
• One of the most
interesting of all dune
plants, and certainly
one of the most
bizarre wildflowers in
North America is
"sand food"
(Pholisma sonorae),
formerly placed in the
genus Ammobroma.
Parasitic flowers
• This amazing parasitic flowering plant grows in
the Algodones Dunes of southeastern California
and adjacent Arizona, and in the sand dunes of
El Gran Desierto in Sonora, Mexico (north of
Bahia Adair in the Gulf of California).
• The southernmost extent appears to be the
region around Bahia Adair on the Sea of Cortez
coast of Sonora, Mexico. Within this area, the
plants grow on sand dunes produced by wind
transport of sand from the beaches of ancient
Lake Cahuilla and the Colorado River delta.
Sand Parasites
• Sand food is a root parasite with a thick, scaly
stem that may extend 6 feet (2 m) or more
into the dune where it attaches to the roots of
nearby shrubs deep in the soft sand.
• The entire plant lives below the surface of the
sand, with only the flower head pushing
above sand during early spring. The scaly
stem is without chlorophyll and is
nonphotosynthetic, and all of its vital organic
nutrients (amino acids and carbohydrates)
come from nearby host shrubs.
• An old, dried flower head with a long,
subterranean scaly stem superficially
resembles a sand dollar attached to a long
piece of seaweed
Sand Foot Flowers
Sand food flowers are apparently insect-pollinated, the flowers of A.
culiacana have a faint, sweet odor and are visited by flies, beetles and
small butterflies.
Sand food plants on the Algodones Dunes are sometimes infested with
small ants and mealybugs. The ants may be after the small seeds, or
perhaps the sweet nectar secretion from the mealybugs known as
"honeydew." Following pollination, each flower gives rise to a small dry
fruit (capsule) composed of 12 to 20 minute seeds arranged in a circle
like wedges of cheese. Depending on favorable winter rains and
pollination, each flower head may produce hundreds of seeds.
The scaly stems extend more than 6 feet (2 m) into the sand where they
are attached to the roots of dune buckwheat, or another host shrub. It is
difficult to trace the origin of these strange plants because the sand
caves in as fast as you can shovel it out. Native Indians, including the
Sand Papagos and Cocopas, ate the fleshy stems of sand food, either raw
or roasted over a campfire. The stems were also dried in the sun and
ground on a metate with mesquite beans, forming a flour called "pinole."
• How the seedlings of these unusual root parasites are able to find
the host root buried deep in the sand is truly remarkable. The sand
plant (Pholisma arenarium) sends out "pilot roots" two feet (0.6 m)
below the surface of the sand. When they reach the vicinity of a host
shrub, the pilot roots send out special "haustorial roots" which
connect and penetrate the host root.
• The haustorial connection (haustorium) absorbs carbohydrates and
amino acids manufactured by the photosynthetic host shrub. Other
factors may also be involved in finding the host roots.
• The small seeds may move downward through the sand or may be
buried by continually shifting sand dunes which are subsequently
colonized by new host vegetation. They may also be carried deep
into the sand by harvester ants and by rodents (kangaroo rats) that
burrow into the dunes under host shrubs
Antlions
• The antlion resembles a grotesque small scale
version of the sand creature in the Star Trek II
film, "The Wrath of Khan." It builds a circular
crater in the sand and then waits patiently under
the sand at the bottom of the pit for a hapless
passerby. The steep crater walls make escape
by small crawling insects virtually impossible.
Struggling victims are literally pulled into the
sand and sucked dry by the hollow fangs (jaws)
of the antlion. If antlions were 6 feet (2 m) long,
they would be a formidable desert sci-fi monster
that could easily grab humans!
Sand Wasps
• Sand wasps (Bembix species) dig tunnels in soft sand
where they live in elaborate "condominiums." They are
readily distinguished from other wasp species by their
elongate, triangular labrum (lip). Sand wasps differ from
spider wasps, mud daubers and many other digging
wasps that provide their larvae with a single cache of
food that must last throughout the larva's development.
Sand wasps continually catch insects, such as flies, and
bring them home to their burrows. They are often seen
hovering over dunes in search of small insects to feed
their young. Although they are not social insects like
hornets, yellow jackets and honey bees, they do nest in
the same vicinity, and tend to develop a primitive type of
colony.
Seeking Shade
A well-camouflaged desert iguana (Dipsosaurus
dorsalis) peering out of its burrow in the sand. Tolerant
of high temperatures, this lizard can be seen scurrying
across sand hummocks and roads on hot, sunny days
when most other lizards seek shelter.
Sand Dune Preservation
• Many people ask why sand dunes should be
preserved; "they are just piles of sand devoid of
plants and animals."
• Nothing could be further from the truth. There
are hundreds of dune species throughout the
southwestern United States and Mexico, some
of which are considered rare and endangered by
the United States Fish and Wildlife Service. This
is because their limited and specialized habitats
are threatened by urbanization and various
motorized off-road vehicles.