Erosional ForcesErosional Forces

Chapter 8Chapter 8

EROSION BY GRAVITYEROSION BY GRAVITY

Erosion & DepositionErosion & Deposition

Erosion- the removal or movement of surface Erosion- the removal or movement of surface material from one place to another.material from one place to another.

Gravity plays a big part in erosion. It is the Gravity plays a big part in erosion. It is the force of attraction that pulls all objects toward force of attraction that pulls all objects toward Earth’s center. We call gravity an erosional Earth’s center. We call gravity an erosional agent, along with wind, water and glaciers.agent, along with wind, water and glaciers.

As these erosional agents lose energy they As these erosional agents lose energy they drop or deposit the sediments that they are drop or deposit the sediments that they are carrying. This is called deposition. carrying. This is called deposition.

The greater an objects mass, the greater its The greater an objects mass, the greater its gravitational pull is. And rock and other sediment, gravitational pull is. And rock and other sediment, especially ones that sit on top of steep slopes are especially ones that sit on top of steep slopes are pulled toward the center of the Earth by gravity.pulled toward the center of the Earth by gravity.

Mass movement is any type of erosion that occurs Mass movement is any type of erosion that occurs as gravity moves materials down slope.as gravity moves materials down slope.

These can occur quickly or so slowly that you cant These can occur quickly or so slowly that you cant even see it.even see it.

Ex: Landslide, Slump, Creep, Rock fall, Rock Ex: Landslide, Slump, Creep, Rock fall, Rock Slide & mud flows. Slide & mud flows.

Slump & CreepSlump & CreepSlump is when a mass of material slips Slump is when a mass of material slips down along a curved surface. It occurs down along a curved surface. It occurs when the supporting rock cannot hold the when the supporting rock cannot hold the rock and sediment above it, and it collapses rock and sediment above it, and it collapses and slides down the side of the slope.and slides down the side of the slope.

Sometimes the accumulation of water at the Sometimes the accumulation of water at the base of a slipping mass can cause base of a slipping mass can cause weakening and slipping of a mass.weakening and slipping of a mass.

Creep is the slow shift of sediments Creep is the slow shift of sediments downhill, like on the side of a highway.downhill, like on the side of a highway.

Rock falls & Rock slidesRock falls & Rock slidesA rock fall is when large pieces of rock break A rock fall is when large pieces of rock break loose from a steep slope and tumble down a loose from a steep slope and tumble down a slope. slope. These rocks usually crash into other rocks on These rocks usually crash into other rocks on the way down and cause more rock falls and the way down and cause more rock falls and cause serious damage to things at the bottom of cause serious damage to things at the bottom of the slope.the slope.Rock slides are when layers of rock slip down Rock slides are when layers of rock slip down hill suddenly.hill suddenly.Both happen quickly and cause a lot of Both happen quickly and cause a lot of destruction. They usually occur in areas of destruction. They usually occur in areas of steep cliffs and happen often after heavy rains steep cliffs and happen often after heavy rains or during earthquakes.or during earthquakes.

MudflowMudflow

A mudflow os a thick mixture of sediments and A mudflow os a thick mixture of sediments and water flowing downhill. water flowing downhill. They occur in areas that have a lot of loose They occur in areas that have a lot of loose sediment and often occur after vegetation is sediment and often occur after vegetation is removed and heavey rain fall mixes with the removed and heavey rain fall mixes with the sediment and allows gravity to pull this mass sediment and allows gravity to pull this mass down a slope.down a slope.When these reach the bottom the spread out in When these reach the bottom the spread out in a fan shape and deposit whatever sediment they a fan shape and deposit whatever sediment they held.held.

Rock falls, slides, slumps, and creeps all Rock falls, slides, slumps, and creeps all have things in common:have things in common:

1.1. They occur more often after rain.They occur more often after rain.

2.2. They all occur on steep slopes.They all occur on steep slopes.

3.3. They all depend on gravity for them to They all depend on gravity for them to occur.occur.

Making slopes safeMaking slopes safe

The best way to prevent erosion and The best way to prevent erosion and slumps, slides, etc… is by planting on a slumps, slides, etc… is by planting on a slope.slope.The roots of plants will allow the soil to be The roots of plants will allow the soil to be stronger because the roots allow the soil stronger because the roots allow the soil to bind and become thicker.to bind and become thicker.Also, inserting drainage pipes into slopes Also, inserting drainage pipes into slopes will not allow excess water to build up in will not allow excess water to build up in an area.an area.Building walls around the slope will also Building walls around the slope will also help by holding back the above sediment.help by holding back the above sediment.

GLACIERSGLACIERS

GlaciersGlaciers

A glacier is a large mass of snow and ice A glacier is a large mass of snow and ice that moves on land under its own wieght.that moves on land under its own wieght.Glaciers form from the build up of snow Glaciers form from the build up of snow layers. The newly added snow will cause layers. The newly added snow will cause the bottom layers to compress and the bottom layers to compress and become ice.become ice.When the weight of the glacier becomes to When the weight of the glacier becomes to heavy it will start to slide away from its heavy it will start to slide away from its origin.origin.

2 types 2 types There exists two types of glaciers: There exists two types of glaciers: Valley Valley GlaciersGlaciers and and Continental GlaciersContinental Glaciers. .

Valley Glaciers form in mountainous regions and Valley Glaciers form in mountainous regions and are sometimes called alpine glaciers. They flow are sometimes called alpine glaciers. They flow downhill following mountain valleys. The other downhill following mountain valleys. The other type is called a Continental Glacier. type is called a Continental Glacier.

Continental glaciers are usually much larger and Continental glaciers are usually much larger and more slowly moving than valley glaciers. These more slowly moving than valley glaciers. These glaciers can cover most of a continent. glaciers can cover most of a continent.

Two very large continental glaciers are the Two very large continental glaciers are the Greenland and Antarctica glaciers (sometimes Greenland and Antarctica glaciers (sometimes called ice sheets).called ice sheets).

A glacier is another agent of erosion. A glacier is another agent of erosion. When they pass over land they erode it When they pass over land they erode it and change the surface features.and change the surface features.

They than carry the eroded material along They than carry the eroded material along and deposit it somewhere else.and deposit it somewhere else.

The sheer size and weight of a glacier has The sheer size and weight of a glacier has the ability to move extremely large rocks, the ability to move extremely large rocks, boulders and sediments.boulders and sediments.

PluckingPluckingGlaciers weather and erode solid rocks Glaciers weather and erode solid rocks when glacial ice melts and re freezes when glacial ice melts and re freezes inside of rock and cause them to crack.inside of rock and cause them to crack.

These rock pieces can then be carried These rock pieces can then be carried awy by the glacier one it starts to move, awy by the glacier one it starts to move, and can be deposited else where.and can be deposited else where.

This is called plucking.This is called plucking.

This results in boulders, sand and gravel This results in boulders, sand and gravel being deposited on the sides of glaciers.being deposited on the sides of glaciers.

PluckingPlucking

Scouring Scouring

When the plucked rock is moved along by When the plucked rock is moved along by the glacier it scours and scrapes the land the glacier it scours and scrapes the land and soil below it. ( Like sandpaper) and soil below it. ( Like sandpaper) Deep, long, parallel scars on rocks are Deep, long, parallel scars on rocks are called grooves.called grooves.Shallow marks on rocks are called Shallow marks on rocks are called striations.striations.Both of these marks indicate the direction Both of these marks indicate the direction the glacier was moving.the glacier was moving.

Glacial sedimentGlacial sediment

When glaciers begin to melt they can not When glaciers begin to melt they can not carry as much sediment so it becomes carry as much sediment so it becomes deposited on the land.deposited on the land.

This melting of a glacier is called retreat, This melting of a glacier is called retreat, and the sediments that are left behind are and the sediments that are left behind are called till deposits, and they can cover called till deposits, and they can cover huge areas of land.huge areas of land.

Till deposits are very fertile responsible for Till deposits are very fertile responsible for a lot of farm land in Iowa and Montana, a lot of farm land in Iowa and Montana, Ohio, and Indiana.Ohio, and Indiana.

Moraines and Outwashes & EskersMoraines and Outwashes & EskersA moraine is a large pile of deposited A moraine is a large pile of deposited sediments that has built up at the front of a sediments that has built up at the front of a glacier. Picture a big pile of dirt that has glacier. Picture a big pile of dirt that has been moved by a bulldozer.been moved by a bulldozer.Outwash is the material deposited by the Outwash is the material deposited by the melt water of a glacier. Usually beyond melt water of a glacier. Usually beyond the end of the glacier.the end of the glacier.An esker is a type of outwash. It is a An esker is a type of outwash. It is a winding ridge of sand and gravel left winding ridge of sand and gravel left behind by a glacier. Form around bodies behind by a glacier. Form around bodies of meltwater.of meltwater.

Moraine, EskerMoraine, Esker

EskerEsker

Evidence of Valley glaciersEvidence of Valley glaciers

In mountains they cause bowl shaped In mountains they cause bowl shaped basins, called cirques.basins, called cirques.

An arete is formed when 2 valley glaciers An arete is formed when 2 valley glaciers erode a mountain along a ridge.erode a mountain along a ridge.

A horn, or a sharp peak of a mountain is A horn, or a sharp peak of a mountain is formed when a glacier erodes a mountain formed when a glacier erodes a mountain in several different directions.in several different directions.

WINDWIND

Deflation and AbrasionDeflation and Abrasion

Deflation: wind removes small particles, Deflation: wind removes small particles, leaves heavier particles behindleaves heavier particles behind

Abrasion: scraping and wearing away of Abrasion: scraping and wearing away of surface by wind blown particlessurface by wind blown particles– Similar to sandblastingSimilar to sandblasting

Occurs mostly in deserts, beaches, and Occurs mostly in deserts, beaches, and plowed fieldsplowed fields

Sandstorms and Dust StormsSandstorms and Dust Storms

Sandstorm: Wind blows forcefully, picks Sandstorm: Wind blows forcefully, picks up sand grains as it blows forming a low up sand grains as it blows forming a low cloud just above the groundcloud just above the ground– Occurs in deserts and other arid regionsOccurs in deserts and other arid regions

Dust storm: fast wind picks dust/silt/clay Dust storm: fast wind picks dust/silt/clay particles high up into the air particles high up into the air – Occurs in arid regions and regions with little Occurs in arid regions and regions with little

or no vegetationor no vegetation

Reducing Wind ErosionReducing Wind Erosion

TreesTrees– Provide windbreaksProvide windbreaks– Traps snow/rain to moisten soilTraps snow/rain to moisten soil

GrassGrass– Fibrous roots grip soilFibrous roots grip soil

Deposition by WindDeposition by Wind

Sediment blown away eventually Sediment blown away eventually deposited somewhere elsedeposited somewhere else

LoessLoess– Sediment deposited on tops of hilltops and in Sediment deposited on tops of hilltops and in

valleysvalleys– Pack together, forming a thick, unlayered, Pack together, forming a thick, unlayered,

yellowish-brown deposit that is as fine as yellowish-brown deposit that is as fine as baby powder; fertilizes soilbaby powder; fertilizes soil

DunesDunes

Sediment deposited by wind that builds up Sediment deposited by wind that builds up over time and becomes a moundover time and becomes a mound

Common near desert regionsCommon near desert regions

Two sides:Two sides:– Gentler side facing the windGentler side facing the wind– Steeper side facing away from the windSteeper side facing away from the wind

Travelling dunesTravelling dunes– Dunes that migrate rapidly across desert Dunes that migrate rapidly across desert

areasareas

DunesDunes

Shape of dunesShape of dunes– Depend on amount of sand or other sediment, Depend on amount of sand or other sediment,

wind speed and directionwind speed and direction– Common shapesCommon shapes

Barchan duneBarchan dune– Crescent shaped; open side of crescent faces towards windCrescent shaped; open side of crescent faces towards wind

– Forms on hard surfaces where sediment is limitedForms on hard surfaces where sediment is limited

Transverse duneTransverse dune– Perpendicular to general wind directionPerpendicular to general wind direction– Forms where sediment is abundantForms where sediment is abundant

Star dunesStar dunes– Form pointed structuresForm pointed structures

Shifting SedimentsShifting Sediments

Dunes and loess form = landscape Dunes and loess form = landscape changeschanges

Like erosion, these cause constant Like erosion, these cause constant reshaping of the landreshaping of the land