Transcript WIND, WAVES AND CURRENT

WIND, WAVES AND
CURRENT
and their effects
WIND
• Winds pick up and move sediment
• Winds drive sediment against rock and
other materials, causing weathering
• Wind erosion is most effective in areas of
loose sand, silts and clay
• Common in deserts, beaches and semiarid regions
Abrasion
• Sand grains are larger (than silts and clay) and
tend to be more abrasive
• Desert sands blast and grind boulders and rocks
into ventifacts (side of rock that faces wind
wears into a smooth, flat surface or facet)
Deflation
• Deflation is the removal of loose rock particles
by the wind
• When the surface particles are removed (sands,
clays), pebbles and rocks are left behind, called
desert pavement.
SAND DUNES
• Sand dunes are hills of sand deposited by
winds and are formed when the sand piles
up against shrubs, boulders and other
obstructions
• They occur where there are strong winds
and enough loose sand (deserts, sandy
flood plains and beaches)
• Dune sands are usually made of quartz,
but sometimes gypsum, calcite or others.
Dune Shapes/Types
Dune Shapes/Types
BARCHAN: cresent shaped dunes, where
the ends point downwind.
TRANSVERSE: long continuous sand
ridges, that form at right angles to the wind
PARABOLIC: u-shaped dunes (like
barchans) but their ends face upwind
LONGITUDINAL: long and straight dunes
that are parallel to the wind direction
Dune Migration
• Each time the wind blows against the windward
side of a sand dune, sand is blown over the top,
and falls down onto the leeward side (called the
slip face)
• As a result the dune moves!
WAVES
Waves can be produced in one of three ways:
1. Wind
2. Undersea earthquakes
3. Effects of the moon (tides)
Waves begin as ripples and as they grow larger
they become waves.
The height of a wave depends on:
1. The length of time the wind blows
2. The fetch or length of open water over which
the wind blows.
Anatomy of a Wave
• Wave height is the difference between its
crest and trough.
• Wavelength is the distance from one crest
to the next (strong winds make waves with
long wavelengths)
• The period of a wave is the time it takes
one wavelength to pass a given point
SPEED=WAVELENGTH
PERIOD
Wave Motion
• Water is not carried along with the motion
of a wave….each water molecule moves
in place in a circular motion….and bumps
into the next molecule which passes along
the energy
• Water molecules move in smaller and
smaller circles as depth increases.
• Waves bend (refract) when they approach
a shoreline
Breakers
• As the wave scrapes the
bottom, the circular
motion of the wave is
distorted (lower part
slows down, upper part
moves ahead)
• The crest falls over and
breaks into surf that
washes onto the beach
• The surf is a powerful
agent of erosion.
Shoreline Currents
• Waves can come from any direction and
thus strike shorelines at angles
• When waves break, large amounts of
water and sand are pushed up the beach
(swash)
• Most of the water runs back down the
beach under the next wave in a gentle
current called backwash (strong
backwash is called an undertow!)
Longshore and Rip Currents
• Water past the breakers is also pushed
towards the shore by waves and pulled
back by backwash, and the movement
forms a longshore current, which runs
parallel to the shore.
• Rip currents are strong surface currents
that flow away from the beach, which can
pose a serious problem for swimmers.
Erosion of Shorelines
• Breakers easily remove large masses of
sand and clay
• Bedrock is scoured away by sand and
pebbles
• Seawater dissolved minerals from rocks
such as limestone
• Sea cliffs, sea caves, sea arches and
stacks may form
SANDBARS
• Many times a longshore current carries
enough sand to form a sandbar across
the mouth of a bay.
• A spit is a sandbar attached at one end
• A baymouth bar is one that has grown
completely across the bay
• A spit with a curved end is called a hook
• Sandbars protect the water behind them
from wind and waves (form lagoons)
Spits & Hooks
Baymouth Bars
Barrier Islands
• Sandbars not
attached to the
shoreline
• Run parallel to it
Beaches and Shorelines
• A beach is the area between high-tide
level and low-tide level
• Beaches can be sandy, pebbly or rocky
• If the seafloor slopes gently, only clay
washes out (by backwash) and sand
remains.
• If the seafloor is steep, sands and clay are
washed out to sea and pebbles remain.
Other Shoreline Features
• Fiords are long, deep-sided bays formed when
glacial troughs were flooded.
• Irregular shorelines zig zag
• Regular shorelines are fairly straight
Corals and Reefs
• Corals are tiny sea animals that live in
colonies, which have shells made of
calcium carbonate
• When they die, their shells are left behind
and new corals grow on top
• Large buildups of coral shells are called
coral reefs
Fringing and Barrier Reefs
• Coral colonies growing close together form
fringing reefs
• A fringing reef slowly moves ocean ward,
because the coral grows on the ocean
side where ocean waves bring food
• A barrier reef forms when a fringing reef
moves away from shore
The Great Barrier Reef-Australia