Weathering, Erosion, & Deposition

Weathering, Erosion, & Deposition

The surface features of the Earth change as a result of observable physical and chemical processes

Weathering and ErosionThis is a monument called Cleopatra’s

Needle. It was carved in Egypt around 1450B.C.

The sides are carved with hieroglyphs, the writing

of ancient Egypt. It stood in the dry, hot

Egyptian desert for over 3000 years. During that

time, the hieroglyphs remained distinct.

WeatheringIn 1800, the

monument was moved to New York City. Almost immediately, the hieroglyphs began to fade. In only a few years in the wet and variable climate of New York, the Egyptian writing became indistinct!

ErosionCleopatra’s Needle was carved

from granite, a hard tough, crystalline rock.

Although it is tough, granite is changed by the atmosphere.

Some of the minerals that make up granite change to clay.

Chips and flakes of minerals break away from the granite surface.

The Cycle of Changing Earth’s Surface

Weathering, erosion, and deposition act together in a cycle that wears down and builds up Earth’s surface

Weathering-wears down the Earth’s surfaceErosion-moves the Earth’s surface (acts as a

bridge between the weathering and deposition)Deposition-builds up the earth’s surface

Weathering

ErosionDeposition

Weathering

The breaking down and/or wearing awayof earth materials (rocks) into smaller pieces

Must occur before erosion can happen

No movement is involved

Physical aka Mechanical Weathering

Breaks rocks down into smaller pieces (nothing changes but the size)

Sediments: naturally-occurring material that is broken down by processes of weathering; small/tiny pieces of rock

Sediments vary in sizes and are named depending on their size

WeatheringPhysical

Common weathering processes:Frost actionThermal Expansion and Contraction

(wetting and drying)Biological ActivityExfoliationAbrasion

Types of Mechanical Weathering

Thermal Expansion & Contraction (Wetting & Drying)

Especially effective at breaking up rocks that contain clay.

Clays swell up when wet and shrink when dry.

Causes rocks that contain clay, such as shale, to fall apart.

Types of Mechanical Weathering

Frost action or Ice Wedging: Water takes up about 10% more space when it

freezes.Expansion puts great pressure on the walls of

a container. Water in the cracks of rocks wedges the rock

apart when it freezes.Often occurs in places where temperatures

vary from below and above the freezing point of water.

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Abrasion

These mountains in Salt Lake City were weathered by abrasion.

Abrasion is the grinding action of other rock particles due to gravity or the motion of water, ice or air.

Most often seen in running water, the beach, or the desert

Types of Mechanical Weathering

Action of plants and animals (biological activity)Larger trees and shrubs may grow in

the cracks of boulders.Ants, earthworms, rabbits,

woodchucks, and other animals dig holes in the soil.These holes allow air and water to

reach the bedrock and weather it.

Types of Mechanical WeatheringGranite exfoliation

ExfoliationOccurs when thin layers of rocks

break off in sheets

Chemical Weathering

This cave was formed by slightly acidic water

dissolving the limestone

Occurs when chemical reactions dissolve or change the minerals in rocks (rusting, acid breakdown, etc)• Occurs most often in

warm and humid climates

Chemical WeatheringBreaks down rock through chemical

changes.The agents of chemical weathering

Water: dissolves rock over timeOxygen: combines with iron to form rust

(oxidation)Carbon Dioxide: dissolves in water to form

carbonic acidLiving Organisms: plant roots secrete acidsAcid Rain: from the burning of fossil fuels

Water

Living OrganismsLichens that grow on rocks produce weak acids

that chemically weather rock.

OxygenIron combines

with oxygen in the presence of water in a processes called oxidation.

The product of oxidation is rust.

Carbon DioxideDissolves easily in

water.Forms a weak acid

called carbonic acidThe same

compound that is in carbonated drinks.

Carbonic acid easily weathers limestone and marble.

Acid RainCompounds from burning coal, oil and gas

react chemically with water, forming acids.Carbon dioxide and sulfur compounds

released by industries unite with water in the atmosphere to form acid rain.

Increasing amounts of acid rain in the environment increase the rate of chemical weathering.

Acid rain causes very rapid chemical weathering.

Physical and Chemical Weathering Rate of weathering depends on:

Climate—warm and humid is fasterSize of the rock particleMineral composition—determines how fast a rock

weathersTime—most important factor

Given enough time all rocks with weatherRock Type—rock that is permeable (has holes

that let water in), weathers faster than rock that is impermeable.

Weathering creates either sediment or soilClassified by size—sand, silt, or clay

Check for KnowledgeHow does the size of the rock particle affect

weathering?

How does the mineral composition of the rock particle affect weathering?

Why is time the most important factor?

Lets comparePlease draw a Venn diagram and compare

chemical and physical weathering.How are they similar/different?

ErosionThe movement of weathered earth materials

by moving water, wind, gravity or iceEx: waves causing beach erosion (shoreline of Texas is

disappearing) running waterEx: sand storm windEx: rain causing a mudslide running water and gravityEx: landslide gravityEx: rain carrying soil and gravel down a driveway

running water and gravity

What are the 4 main ways water erodes?

RunoffStream ErosionWave Action Glaciers

ErosionThe Bernard Glacier in Alaska's Saint Elias Mountains looks like a huge alpine highway. Glaciers are slow but highly effective shapers of the land, essentially carrying away anything in their path—from soil and rocks to hills and even the sides of mountains

Why are glaciers called dirty snowballs? What is glacial till?

Glaciers are called dirty snowballs because as they move across the land they carve up the earth’s surface collecting rocks and sediments with the snow.

Glacial till is the jagged uneven particles the glacier deposits as it retreats (melts)

Mass Wasting and CreepMass wasting—downhill movement by

gravityPotential increases with slopeCauses rock slides, slump, mud

slides, or avalanches

LandslidesLandslides- A geological phenomenon• Includes a wide range of

ground movement, such as rock falls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments.

SlumpA steep hillside shows a spoon-shaped depression in which material has begun to slide downhill.

Occurs along a distinct fracture zone, often within materials like clay that may move rapidly downhill.

In some cases the slump is caused by water beneath the slope weakening it.

Soil CreepCan be almost undetectable.Normally found on weathered

mountain slopes. Gravity forces the material to

slowly slide downhill, and eventually the signs of creep become evident.

Tree trunks curve upward as creep caused them to tilt downhill

Fence posts and hydro poles may also begin to lean downslope

Why care about erosion?Human activities such as construction increase

the rate of erosion 200, even 2,000 times that amount. When we remove vegetation or other objects that hold soil in place, we expose it to the action of wind/water and increase its chances of eroding

The loss of soil from a construction site results in loss of topsoil, minerals and nutrients, and it causes ugly cuts and gullies in the landscape. Surface runoff and the materials it carries with it clog our culverts, flood channels and streams. Sometimes it destroys wildlife and damages recreational areas such as lakes and reservoirs.

Bare Slopes vs. Vegetation Stabilized Slopes•Mudslide danger•Loss of topsoil•Clogged storm drains•Flooding problems•Expensive clean up•Eroded or buried house foundations

•Soil in place•Less clean up•Minimum erosion•Protection for house foundations

DepositionThe process of eroded earth materials

(sediments) being deposited in a new locationEx: delta-where sediment is deposited where a

river flows into an ocean or lake builds up a landform

Ex: sand dune-a deposit of wind blown sand

Process of DepositionWhen sediments are released after being

transportedUsually when velocity decreases

Factors affecting deposition:Particle size—smaller, the slower the rateShape—more spherical, the faster the rateDensity—more dense, the faster the rate

Process of DepositionGrade bedding aka Vertical Sorting

Velocity of transporting agent reduced quicklySediments of mixed sizesSize of sediment increases with depth

Horizontal sortingVelocity decreases slow rate over long distanceLarger first, smaller last—mouth of river

Erosion and DepositionMeanders are

S shaped curves formed by weathering of one bank of a river and deposition of the weathered sediment on the opposite bank of the river.

This creates a loop-like bend in a river.

Erosion Control Methods:Riparian

BuffersLeaving

undisturbed plant life on either side of a body of water to prevent erosion

Erosion Control

silt fencing

terracing

Notes quiz1) Deposition isa) the movement of sediment or rockb) the settling out of sediment or rockc) the breaking down of sediment or rock

2) A meander is• a) a small river that runs into a larger river• b) a loop-like bend in a river• c) a lake that has been cut off from a river

• 3) Name 3 methods of erosion controla)b)c)

4) What are riparian buffer zones?