Chapter 7: Erosion and Deposition

Chapter 7: Erosion and Deposition

7.1: Forces wear down and build up Earth’s surface7.2: Moving water shapes land

7.3: Waves and wind shape land7.4: Glaciers carve land and move sediments

7.3: Waves and wind shape land

Before, you learned: Stream systems shape Earth’s surface Groundwater creates caverns and

sinkholes Now, you will learn:

How wave currents shape shorelines How wind shapes land

Book, p.158: Pillars of Rock, Port Campbell, Australia: Why do you think these are left over from large areas of rock?

The pillars are made of hard rock that was among softer or fractured rock that was easier to weather and disintegrate

Waves and currents shape shorelines

Port Campbell pillars formed by movement of waterContinuous action of waves breaking against the

cliffs2 cm/year, and still eroding

Force of wave: powered by wind, can wear away rocks and transport sand

Force of wind: can change the look of land

Shorelines

Can be flat, low, high, steep, rocky, sandy…High rock cliff: waves crash and wear away

bottom rocksLow coastlines build up

Similar to when a stream flows into an ocean or lake, sediment is deposited near its mouth

This sediment mixes with sediment from waves beating against the coast, building up beaches

Describe movement of sediment and water along the shore:Longshore driftLongshore current

Shorelines – Longshore driftLongshore drift: zigzag movement of sand along a beach

Wind blows across water and may hit the shoreline at an angleCarries sand to shore, and gravity pulls the water and sand back out

to the waterThe sand gradually moves down the beachMoves large amounts of sand along a beach: shrink/grow

Shorelines – Longshore current

Longshore current: movement of water along a shore as waves strike the shore at an angleDirection may change from day to day as wave direction

changes

Oceans, aside

Ocean waves are a movement of energy, not water

A cork in water impacted by a wave moves - it rises and falls on the wave but does not move forward

Water waves move energy, not materialA message in a bottle thrown to sea may

arrive cross the sea by currents, rather than waves

Sandbars and Barrier Islands

Longshore currents can deposit sand along shorelines This sand builds up to form sandbars

A ridge of sand built up by the action of waves and currentsIf the sandbar is built up above the water’s surface and is joined to the

land, it is called a “spit” Strong longshore currents that mostly move in one direction over

time build up the sandbar into barrier islandsA long narrow island that develops parallel to a coastForms a barrier between the ocean waves and the shore of the

mainlandCommon along gently sloping coasts: New Jersey, North Carolina,

Gulf of MexicoConstantly change shape

Wind shapes landSand dunes…far from a desert or ocean!

Created by wind:Dune: a mount of sand built up by wind

Like water, wind can transport and deposit sedimentEspecially powerful in dry regions where soil is not

held in place (few plants)

Near Lake Michigan

Wind Shapes LandDune formation:

Strong wind picks up and transports sand particles – deposits the sand which builds to dunes

Can start as a ripple then grow largerCan form as wind-carried sand settles around a rock,

log, or other obstacleNeed: strong winds AND constant supply of loose sandVary in size an d shape

up to 300 meters (1000 ft), curved, straight, moundsTypically have a gentle slope facing the wind, and steep

on the other side

Loess

Wind also changes soil by depositing dustLoess: deposits of fine wind-blown sedimentCan build up over thousands or millions of

yearsForms good soil for growing cropsSome deposits 300 meters thick!

Ex: China: deposits are 2 million years oldCentral U.S.: between 8-30 m (25-100 ft) deep

Desert Pavement

Wind can also remove dustIt blows away sand, silt, and

gravel, leaving behind a layer of stones and gravelCalled “desert pavement”

because it looks like a cobblestone pavement

What remains is too heavy to be picked up by wind

Coastline video: http://scienceblips.dailyradar.com/video/coastal-erosion-video/

Videos:http://gatm.org.uk/geographyatthemovies/

coasts.html

Snow Line Elevation and Latitude p.251

0 10 20 30 40 50 60 70 80 90 1000

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Y-Value 1

Y-Value 1

1. Mount kenya is very close to the equator. Estimate the snow line elevation on mount kenya.

2. Mount rainier is at 47 degrees north latitude and is 4389 meters tall. Can there be glaciers on mount rainier? If so, estimate the elevation above which the glaciers form.

3. Mount washington in new hampshire is at 45 degrees north latitude and is 1917 meters tall. Can there be glaciers on mount washington? If so, estimate theeir lowest elevation.

Challenge: temperatures are hotter at the equator than at 28 degrees north latitude. Why is the snow line lower at the equator in Ecuador?