Erosion by Wind and Waves

Ch. 16

Wind Deposition

when the wind stops, it drops its load

Dunes: mounds of wind blown sand

•form where soil is dry and unprotected and the wind is strong

How dunes form: when a barrier slows the speed of the wind, it causes sand to accumulate on the sheltered side, sand accumulates on exposed side, and eventually buries the barrier

Wind causes saltation, or jumping grains, on the windward side of sand dunes

windward

crest

slipface

barchan dune: crescent shaped, open side faces away from wind, common, found in deserts

parabolic dune: crescent-shaped, open sided faces into the wind

transverse dune: series of ridges of sand in long, wave-like patterns- form at a right angle to the wind

longitudinal dune: ridges of sand in long, wave-like patterns- lie parallel to the wind direction

This Landsat image reveals sand dunes advancing on Nouakchott, the capital of Mauritania.

Alien dunes. NASA's Mars Global Surveyor photographed these sand dunes on Mars

Shoreline erosion is caused by:

1.waves weathering rock

2. chemical weathering

3. storms

sea stack: offshore, isolated columns of rock which was once connected to the headland

sea cliff: high, nearly vertical,steep structure that forms where waves strike directly against the rock

terraces: a nearly level platform that is found at the base of a sea cliff

•wave-cut: formed when a sea cliff is worn back

•wave-built: forms when eroded material is deposited

sea cave: large holes found in a sea cliff

spit: long, narrow deposit of sand connected at one end to the shore

The formation of a spit by sand movement in response to longshore currents.

tombolo: deposits that connect an offshore island to the mainland

sea arch: enlarged sea cave, produced when waves cut through the headland

beach: a deposit of sand or rock fragments along an ocean shore

•form where amount of fragments moving toward shore is greater than amount moving away

size and kinds of materials found on beaches vary widely

Boulders more than 256mm

Cobbles 64 to 256 mm

Gravel 2 to 64 mm

Sand .06 to 2 mm

Silt less than 0.06mm

A close-up view of ordinary sand

Atlantic Coast

•white sand with quartz and feldspar

•erosion from Appalachian Mountains

Hawaii

•black/gray sand from basalt

•erosion from dark volcanic igneous rock

each wave reaching the shore moves individual sand grains forward total action of thousands of waves moves sand a great distance sand piled up on shore producing sloping surface

berm: raised section at the back of the beach

•results from high tides or large storm waves

•In winter, large storm waves remove sand from the beach and deposit it offshore

•sandbar: long ridge of sand deposited offshore

•In summer, waves return the sand to the beach

BEACH

direction of sand drift

LONGSHORE CURRENT

SWASH BACKWASH

•the direction a wave approaches the shore determines sand grain movement

•swash: motion of water up a beach

•backwash: water that runs back down the beach under the next wave

•strong backwash is called an undertow

longshore current: movement of water parallel to and near the shore

•transport sand in direction parallel to shore

•may produce a spit or a tombolos

Processes that Affect Coastlines

1.long term rising and falling of the sea level

2.long term uplifting or sinking of land that borders water

3.wave erosion and deposition

The sea level is now rising at a average rate of about 1mm/yr

SUBMERGENT COASTLINES

•estuary: bay where freshwater and saltwater mix

•forms when sea level rises and sinks

EMERGENT COASTLINES

barrier islands: long, narrow offshore ridges of sand many over 100 km long

•lie parallel to the shore about 3 km to 30 km offshore

•lagoon: narrow region of shallow water between barrier islands and shore

sometimes a hazardous place to live because it is severely eroded by storms

coral reefs: ridge-like features made from millions of coral skeletons- form around tropical volcanic islands

•fringing reef: coral reef formed around the coast of a volcanic island

•barrier reef: coral reef that forms around the remnant of an island

•atoll: nearly circular coral reef surrounding a shallow lagoon where a volcanic island used to be

PRESERVING THE COASTLINE

(two things that damage the coastline)

1. recreation, commercial fishing, shipping, industrial and residential development

2. pollution