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: when a slope becomes too steep and the base can no longer support the sediments above When water weakens the base and the slipping sediments move downhill 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 behind 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

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 abrasion 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

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