Chapter 8: Erosional Forces

Aim: What is the difference between erosion and deposition?

Erosion is a process that wears away surface

materials and moves them from one place to another

Agents of erosion include gravity, water, wind and glaciers

Water and wind need to have large amounts of energy to move materials

Glacial erosion moves sediments trapped in the ice by melting

Deposition Agents of erosion drop sediments they

are carrying as they lose energy Eroded sediments are not lost from

earth They are moved by erosional forces

and relocated to other areas

Aim: How does gravity act as an erosional force?

Mass Movement Is an type of erosion that happens as

gravity moves materials down slope Some are so slow you can’t see them

happening Other happen rapidly and you can see Landslides are one of the 5 types or a

combination of them 5 types: slump, creep, rock falls, rock

slides and mudflows

Slump Is when a mass of material slips down

slope as one large mass 3 ways this can happen:1. when a slope becomes too steep and

the base can no longer support the sediments above

2. When water weakens the base and the slipping sediments move downhill

3. Strong rock layer lays on top of a weaker layer and it can not support the strong rock

Creep Occurs when sediments slowly shift

their positions downhill Is common in places where there

freezing and thawing are common Evidence of creep includes: leaning

trees or human-built structures

Rockfalls Happens when rocks break loose from

a steep slope and fall to the ground As they fall they knock other rocks

loose Falling rocks can cause serious damage

to structures on the ground This is caused by the ice wedging On roadways signs warn of falling rock

areas

Rock Slides Occurs when layers of rocks slide

downslope They move quickly and can be

destructive Occur in areas with steep,

mountainous slopes Mostly happen after heavy rainfall or

during earthquakes

Mudflows Occur in places that have thick layers

of loose sediments, like soil Happen after vegetation has been

removed because of fires With the lose of vegetation and heavy

rainfalls the loose sediments are carried downhill by gravity

While it moves down slope it gains energy and can destroy anything in its path

Aim: How can we slow down erosion on steep slopes?

Preventing Erosion on Slopes

Building on steep slopes, you must worry about erosion

By building we quicken the process of erosion by removing vegetation and makes slopes steeper

There are ways to prevent erosion By planting vegetation on slopes we

keep soil in place, which reduces the risk of mass movements

Construction of draining systems prevents water from building up

These methods help build a stronger stability of the slope

We can also help stability by building walls to hold soil in place

Large amounts of money are spent on trying to prevent mass movements

Earthquakes and rain make it difficult to completely prevent erosion on slopes

Eventually materials will weaken and gravity takes over causing mass movements

Aim: How do glaciers cause erosion?

Glaciers Glaciers form in regions where snow

accumulates As snow builds up the weight of snow

compresses the lower layers into ice With enough pressure the mass of ice

and snow begins to move, which are known as glaciers

Are agents of erosion When moving over land they erode, and

change features of earth While the move the take eroded

materials and deposit them somewhere new

Plucking Glaciers weather and erode solid rock When glaciers melt the water flows

into cracks in rocks Water refreezes , expands and pieces

of rock are plucked out by the ice During this process, boulders, gravel,

and sand are added to bottom and sides of the glacier

Transporting and Scouring

As the glacier moves large amounts of sediments are transported

Sand and plucked rock fragments scour and scrape the soil and bedrock

Glaciers cause grooves to form along bedrock

Grooves are deep, long, parallel scars Striations are shallower scars Grooves and striations show the

direction that the glacier is moving

Aim: How do glaciers deposit sediments?

Types of Deposits As glaciers melt boulders, sand, clay and

silt are left behind1. Till Deposits Mixture of different sized sediments Can cover huge areas of land Till areas in the US span from northwestern

Iowa and northern Montana, which are wheat fields

Some farmland in Ohio contain till deposits Rocky pastures of New England have till

deposits

Moraine Deposits Form at the end of glaciers Type of till deposit This deposit doesn’t cover a large area

of land Materials pile up and form a ridge

known as a moraine Can also form at the sides of glaciers

2. Outwash Deposits Materials deposited my the meltwater

from a glacier is called outwash Carries sediments and deposits them in

layers Heavier materials drop first Can form a fan-shaped deposit

Eskers Type of outwash deposit Looks like a winding ridge Forms in a melting glacier when

meltwater forms a river within the ice River carries sand and gravel and

deposits them within their channel

Aim: What are the 2 types of glaciers?

Continental Glaciers Are huge masses of ice and snow Are thicker than some mountain ranges In the past covered as much as 28% of

earth Today they cover 10% of the earth Many of them melted after the last ice

age

Valley Glaciers Occur in mountains where the

temperature is very low all year round To find evidence of glaciers look for

striations and plucking They erode bowl-shaped basins called

cirques Arete forms when 2 valley glaciers side

by side erode a mountain Horns form when the glacier erodes the

mountain from several directions Valleys eroded by glaciers have a U-

shape

Aim: How does wind cause erosion?

Wind Erosion Wind picks up loose sediments and

materials It can’t pick up very large sediments Wind erodes earth’s surface by

deflation and abrasion1. Deflation wind blows across loose sediment,

removing small particles like silt and sand

Larger materials are left behind Occur mostly in deserts, plowed lands

and beaches

2. Abrasion Windblown sediments strike rocks and

change the surface and shape of the rocks

The impact of the sand is so strong it breaks off small fragments

Occurs mostly in beaches, deserts and plowed lands

Aim: How can we reduce wind erosion?

Sandstorms occur when wind blows forcefully in deserts

Dust storms occur when soil dries out Soil particles weigh less than sand-sized

particles and they move higher through the air

Dust storms can cover hundreds of kilometers

Blow soil from places where vegetation has been removed

Reducing Wind Erosion Planting vegetation is one of the best

ways to prevent wind erosion Farmers plant trees along their fields to

act like windbreaks The trees reduce the energy of the wind

and it is unable to pick up loose sediments

Tree belts also trap snow, which makes the soil more moist

Moist soil helps prevent erosion

Vegetation is planted along seacoasts and deserts too

Grasses, which have fibrous root systems are the best at stopping wind erosion

Their roots are shallow and twist between particles to hold them in place

planting vegetation is a good way to reduce the effects of abrasion and deflation, but can’t complete stop it

Aim: What do wind deposits develop into?

Loess Loess is deposits of fine-grained

sediments Sediments settle on hilltop and in

valleys The particles pack together and create

a thick, yellowish deposit Is as fine as talcum powder Many farmlands have fertile soil that

developed from loess deposits

Dunes Dune is a mound of sediments drifted by the

wind Common in desert regions Sediments being blown by the wind build up

against a rock or bundle of vegetation Side of dune facing the wind has a gentler

slope Side away from the wind is steeper Shape of a dune depends on the amount of

sediment, wind speed and direction and amount of vegetation present

Crescent-shaped dune (barchan dune): open side faces the direction that the wind is blowing

This type of dune forms on hard surfaces where the sand supply is limited

Transverse dune: forms where sand is abundant

The direction of this dune is perpendicular to the wind direction

Star dune form in areas where the wind direction changes and form pointed structures