Weathering and Erosion 2013

Weathering and Erosion

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The surface of the Earth is constantly being shaped and reshaped by a number of mechanical and chemical processes.Although these processes occur naturally, human activity can cause or accelerate changes in the land.

Introduction

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The rock that makes up the Earth’s surface is constantly being broken down, moved, and built up by different processes.The process which breaks rock down is called weathering. There are two main types of weathering: Physical (or mechanical) Chemical

Weathering

Working together

• As mechanical weathering breaks rock into pieces, more surface area becomes exposed to chemical weathering. So, they also work together.

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Mechanical weathering occurs when rocks are broken down without a change in the chemical make up of the rock. It is usually the effect of water, wind, ice, pressure or heat.

Mechanical weathering usually results in the production of sand and small fragments of rock.

Mechanical Weathering

• Freezing and thawing• Release of pressure• Biological action (plants/animals)• Abrasion

Agents of mechanical weathering

When water gets into cracks or holes in rock and freezes, the water expands.This can force the rock apart. Repetition of this process, known as ice wedging, can result in the rock breaking.

Physical Weathering – Water and Ice

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Wind can be responsible for weathering by breaking down rocks.

Wind carrying fine particles, such as sand, can rub away the surface of a rock, forming even more fine particles.

Physical Weathering – Wind Abrasion

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Wind will wear away soft rock more quickly than hard rock. As wind usually blows horizontally, weathering by wind can easily be recognized.

WindRain

Compare this formation to water weathering...

Wind

Recognizing Wind and Water Weathering

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When rocks get hot they expand.

When rocks get cold they contract.

Repetition of heating and cooling will eventually cause the rocks to crack.

Physical Weathering – Hot and Cold

(Heating and cooling has been exaggerated for effect)

Chemical weathering occurs when the chemical make up of rock changes, causing it to deteriorate.

The agents of chemical weathering Water Oxygen Carbon dioxide Living organisms Acid rain

.

For example, water can contain dissolved gases or materials that cause rock to break down.

Chemical Weathering

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Water

• Water weathers rock by dissolving it

Oxygen• Iron combines with

oxygen in the presence of water in a processes called oxidation

• The product of oxidation is rust

Carbon Dioxide

• CO2 dissolves in rain water and creates carbonic acid

• Carbonic acid easily weathers limestone and marble

Living Organisms

• Lichens that grow on rocks produce weak acids that chemically weather rock

Acid Rain• Compounds from burning coal, oil and gas

react chemically with water forming acids.• Acid rain causes very rapid chemical

weathering

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Human activity can contribute to weathering of the Earth’s surface.

Walking or driving off-road can also cause weathering of the land.

You may have noticed that when a large number of people walk across grass away from a designated path, the grass is worn away and the soil broken up, leaving the ground vulnerable to erosion.

Weathering - Human Activity

Factors Affecting Weathering• Type of material – both are from 1780’s; one is

slate, the other is marble. Which is which? Why is there a difference?

Factors Affecting Weathering• Climate – Both

mechanical and chemical weatering occur faster in wet climates. Chemical reactions occur faster in higher temperatures, so weathering will occur faster in hot and wet climates.

Factors Affecting WeatheringSurface Area – • Smaller particles,

more surface area • Examples

– Crushed ice melts faster

– Granulated sugar dissolves faster

– More exposed rocks weather more

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Question 1

"If the water held in a crack in a rock freezes it will expand and could break the rock apart." What type of weathering does this statement describe?A) Chemical

B) Physical

C) Both of the above

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Question 1

"If the water held in a crack in a rock freezes it will expand and could break the rock apart." What type of weathering does this statement describe?A) Chemical

B) Physical

C) Both of the above

"Pollution can cause or accelerate chemical weathering." Is this statement true or false?

Answer True or False.

Question 2

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True

Question 2

"Pollution can cause or accelerate chemical weathering." Is this statement true or false?

Answer True or False.

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Soil Formation• Soil is loose,

weathered material on Earth’s surface in which plants grow.

Soil Texture

-depends on the size of the particles - Is important for plant growth; soil that is too high in clay has a dense, heavy texture- Clay soil holds a lot of water so plants can “drown” for lack of air- Soil that is too course however, drains to quickly

Soil Composition

• Soil is a mixture of rocks particles, minerals, decayed organic material, air and water.

• Soil that is made up of about equal parts sand, clay, and silt is called loam

• Decayed organic material is called humus NOT hummus

Soil Profiles and HorizonsWhat is a soil profile?• A vertical sequence of soil

horizons• A soil horizon is a distinct layer,

or zone, within a soil profile. • There are three major soil

horizons: A, B, and C.– Horizon A contains high

concentrations of organic matter and humus aka topsoil.

– Horizon B contains subsoil that are enriched with clay and minerals.

– Horizon C, below horizon B and directly above solid bedrock, contains weathered parent material.

• The O horizon is organic material, that may or may not be present.

Soil Formation

Step 1:C Horizon forms as bedrock weathers and rock breaks up into soil particles.

Soil Formation

Step 2:The A horizon develops from the C horizon when plant roots weather the rocks mechanically and chemically. The plants also add organic material

Soil Formation

Step 3:The B horizon develops as rainwater washes clay and minerals from the A horizon to the B horizon.

Factors that affect soil typesClimate

Topography

The topography of a region affects the thickness of developing soil.

Soils on slopes tend to be thin, coarse, and infertile. Soils formed in lower areas, such as in valleys, are

thick and fertile.

Plants and Animals

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When solid particles (sediment), are moved by wind, water or ice it is called erosion.

Erosion can dramatically change land features. This can be quick...Deserts are formed by the surface layers being eroded by the wind; with sand dunes shifting on a daily basis.

Erosion

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…or may take thousands of years.

The Grand Canyon was formed by the Colorado River gradually eroding the land over a very long period of time.

Some valleys have even been formed by glacial erosion.

Erosion

• Gravity is the force that moves rock and other materials downhill.

• Gravity causes mass movement, any one of several processes that move sediment downhill.

• Mass movement can be rapid or slow.

Mass MovementsThe different types of mass movement

include: • Landslides – rock and soil slide quickly

down a steep slope.• Mudslides -the rapid movement of a mixture

of water, rock, and soil.• Slump – a mass of rock and soil suddenly

slips down a slope in one large mass.• Creep – very slow downhill movement of

rock and soil.

Landslides

Landslide

Mudslide

Slump

Creep

Wind can be very effective at erosion however, it is the weakest agent of erosion.Wind gradually removes fine-grained particles, like sand, from the top surface and carries it away to be deposited elsewhere.

If the wind is really strong it can move lots of sand; creating a sand or dust storm.

Wind Erosion

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Wind Erosion•Greatest impact is on an area that has few plants to hold dry soil in place

Wave Erosion

• Energy in waves comes from wind that blows across the water’s surface

Gravity acts on water or ice that transports sediment, causing it to move down a slope.

In cold climates, glaciers shape valleys by erosion. As a glacier moves over the surface it carries loose rock and soil with it.

Water Erosion

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Water Erosion

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Run-off is water that flows over the land, draining into another body of water. Run-off can consist of rain water or melting snow. Run-off causes erosion. As water moves across the land, it picks up and transports sediment. When the flow of run-off is particularly strong, it can cut channels in soil. Over time this can lead to the development of streams and river systems.

Water Erosion – Run-off

Runoff

• Factors– Amount of rain– Vegetation– Type of soil– Shape of land– Human usage of land

Watershed aka Drainage Basin

• A watershed is the area of land where all of the water that is under it or drains off of it goes into the same place.

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Some of the water on the Earth’s surface gradually seeps into the ground through the soil and/or rock.

Caves can also be formed underground by erosion due to the movement of ground water.

Water Erosion – Ground Water

This water is known as ground water.

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The underground spaces created by ground water can weaken the rock or soil at the surface by removing support.

When this happens, the rock at the surface can collapse, forming a sinkhole.

Water Erosion – Ground Water

However, sinkholes can also be formed gradually, as rock at the surface is eroded over time.

Glaciers

Any large mass of ice that moves slowly over land.

Types of Glaciers

• Continental Glaciers– Covers much of a

continent or large island

• Valley/Alpine Glaciers– Forms high in mountains

and moves into valley

Glacial Erosion

• Plucking– Rocks stick to the

bottom of the glacier as it moves over them and breaks them apart

• Abrasion– As the rocks are dragged

along they abrade the land and rocks further ex. Glacial striations

U-Shaped Valleys

Fjord

Cirque

Horn

Arete

Kettle Lake

Finger Lakes

Middle Wisconsin Glacier

Ice / Snow

• The Finger Lakes in New York show the direction of glacial travel.

Human activity can contribute to erosion of the Earth’s surface.

For example, when trees are cut down, the land becomes more vulnerable to erosion.

Erosion – Human Activity

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This is because trees and plants shelter soil from the effects of wind and water.

Agricultural impact on land can be high if the land is not managed properly; plowing up and down a hill for example can increase soil erosion.

Erosion – Human Activity

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Helena

How does run-off cause erosion?

A) As the water flows over land, it picks up and transports sediment

B) It erodes rock underground, forming cavesC) It breaks rock down in its placeD) It changes the chemical make up of rock,

causing the rock to disintegrate

Question 3

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Question 3

How does run-off cause erosion?

A) As the water flows over land, it picks up and transports sediment

B) It erodes rock underground, forming cavesC) It breaks rock down in its placeD) It changes the chemical make up of rock,

causing the rock to disintegrate

Which of the following can cause erosion underground?A) Run-off

B) Ground water

C) Aquifer

D) Sediment

Question 4

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Question 4

Which of the following can cause erosion underground?A) Run-off

B) Ground water

C) Aquifer

D) Sediment

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Natural erosion by wind and water can be reduced by planting vegetation.

Reducing and Preventing Erosion

Trees and hedges for example will both slow the wind by acting as a windbreak. Their roots will bind the soil, thereby reducing water erosion.

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Groins extending from the beach into the sea reduce the amount of coastal erosion.

Building terraces into hillsides, turfing slopes or even plowing along contour lines can help reduce water erosion on agricultural land.

Reducing and Preventing Erosion

Taking care not to allow farm animals to over-graze an area of land can also help to prevent erosion.

Many natural areas that receive large numbers of visitors, such as forests or national parks, request that people keep to designated areas and paths. This helps to keep visitors safe, but also protects the rest of the area from the effects of human activity.

Reducing and Preventing Erosion

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Which of the following can prevent erosion?

A) Planting trees

B) Not allowing farmland to be over-grazed

C) Keeping to designated paths in forests and national parks

D) All of the above

Question 5

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Question 5

Which of the following can prevent erosion?

A) Planting trees

B) Not allowing farmland to be over-grazed

C) Keeping to designated paths in forests and national parks

D) All of the above

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Erosion transports sediment away from its original location. Eventually, the sediment is deposited in a new location creating land features.

Deposition

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Deposition

Sediment is usually deposited in layers.

Layers of sediment can contain fragments of rock that are very different from the native rock of the area.

Deposition

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This is because erosion may transport fragments over great distances before finally depositing it.

Glacial Deposition

• Erratic Boulders

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Rivers naturally carry sediment and other materials. These materials are called alluvium.

This can cause areas of mud, silt and rocks to build up, and in some cases, the river may change course.

If the speed or volume of water is reduced, the alluvium will be deposited on the river bed.

Deposition – Rivers

Sam Beebe

As water flowing from a river disperses into the sea or other body of water, it slows down, and as such the sediment it carries sinks to the bottom.This sediment deposition is called a delta, and usually forms a fan shape.Eventually the deposition will block the original channel of water flow, so new routes have to be forged; this is called a distributary network.

Deposition – Deltas

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As with water flow, when wind slows down it will also deposit any substance it is carrying.

Dunes are formed in deserts by the wind carrying sand particles, and then depositing them as it slows down.

This can cause the dunes to appear to shift through the desert, much like a wave on the sea.

Deposition – Wind

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Glacial Deposition

• Till

Glacial Deposition

• Moraine

Drumlin

Edwards Plateau / Llano Uplift

Located in Central Texas WHAT’S HAPPENING?• Exfoliation – mechanical

weathering has caused the rock to “peel” in sheets. This was caused by contracting and expanding during heating and cooling in the temperatureMechanical or chemical weathering? What station?

• The weathering of sedimentary rock has exposed the granite dome underneath

• Other types of mechanical weathering present – plant roots

Edwards PlateauRainwater mixes with carbon dioxide in the atmosphere and soils to form carbonic acid (H2CO3), which acts to dissolve away limestone.

Georgetown-

Inner Space Cavern

High Plains

• One of the windiest regions in the US

• Low vegetation, so high wind erosion

• Palo Duro Canyon• Formed by water

erosion

Trans Pecos / Chihuanhuan Desert

Davis Mountains – formed from volcanic activity

Guadalupe Mountains – formed by fossilized coral reef

Big Bend– Water erosion from the Rio Grande River

Big Bend– Ancient swamp, current desert

Trans Pecos / Chihuanhuan DesertBig Bend - the warm red and brown colors

of igneous rocks show oxidation, oxygen reacting with iron forming rust

Big Bend– limestone cliff eroded by carbonic acid

Water droplets capture carbon dioxide to form carbonic acid

Piney Woods Due to abundant vegetation

that holds the soil in place, there is very little erosion

Gulf Coast Plains and MarshesWave action, a cause of weathering, erosion and deposition, is constant along the Texas Coastline.

Sediments are weathered and eroded along beaches and carried out into the ocean to form sand bars and islands

Galveston Beach

Mustang Island was formed by erosion and deposition

Sand dunes in Padre Island

Before Katrina

After Katrina

After Katrina

Natural catastrophes

• A natural catastrophe/hazard is an event that results from Earth processes and that can cause damage and endanger human life.

Catastrophic events can affect Earth in several ways.

• Catastrophic events that result from surface processes may speed up erosion and deposition.

• Catastrophic events resulting from processes inside Earth change the crust itself (volcanoes).

And the natural catastrophes are…

• droughts• wildfires• floods• the different types of mass movement• tsunamis • hurricanes • tornadoes• earthquakes • volcanoes• other storms

Volcano

Earthquake

Tornado

Hurricane

People, People, People

• Where people locate their activities can increase the damaging effects of natural hazards.

• How people use land also alters Earth systems in ways that may cause catastrophic events.