Weathering, Erosion, Deposition (WED)

Introduction

 

·       weathering, erosion, and deposition are a main force behind landscape types

 

·       But they are not the only reason for a plain, plateau, and mountain region to form

 

o     Other factors such as glaciation and tectonic activity lead to landscape building

 

§       In New York State, glaciation is one of the main causes of many types of landscape regions

 

o     the water cycle or hydrologic cycle (see graphic below) is the driving force behind WED (weathering, erosion, and deposition)

 

o     winds also help form the surface landscape

 

o     But without gravity and the sun this whole process would not occur

 

Water Cycle Diagram

 

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The water cycle process looks like this:

 

Evaporation ̃ condensation ̃ precipitation  ̃ infiltration  ̃ run-off

 

Without this process, weathering, erosion, and deposition would have a difficult time occurring on Earth.   

 

 

 

Weathering

 -- chemical and physical processes that change the characteristics of rocks on the Earth’s surface.

o     also known as the preparation for erosion

o     for weathering to occur, the rock sample must change and rock needs to be exposed to water and air

o     Human processes such as pollution, (like acid rain) along with the acts of other living organisms, can cause chemical weathering to occur at faster rates

Weathering Process
The weathering process occurs when rocks are exposed to the hydrosphere (water) and atmosphere (air).

·       These agents can change the physical and chemical characteristics of rocks.

·       As rocks are broken down (weathered), they can be classified as different types of sediments, which are:

o     boulders, cobbles, pebbles, sand, silt, clay, and colloids.

 

See page 6 of the Earth Science Reference Tables explains the sizes of each of these sediments.

 

 

 

Physical Weathering
Physical weathering -- when rocks are broken in to smaller pieces without changing the chemical composition of the rock.

·       Think of a physical change (e.g., ripping a piece of paper) -- will change size, but all other characteristics will remain the same

·       Types of physical weathering:

o     Frost action/ice wedging -- breakup of rock caused by the freezing and thawing (contracting and expansion) of water. A very similar process occurs on roads, which causes potholes.

Frost Wedging Diagram

 

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o     Abrasion --physical wearing down of rocks as they rub or bounce against each other--most common in windy areas, under glaciers, or in stream channels.

o     Exfoliation --peeling away of large sheets of loosened materials at the surface of a rock. Common in shale, slate, and mica.

Chemical Weathering
Chemical weathering --when a rock is broken down by chemical action resulting in a change in the composition of a rock.

·       Such as the change a piece of paper would go through after being burned.

·       Main agents of chemical weathering are oxygen, rainwater, carbon dioxide, and acids produced by decaying plants and animals that leads to the formation of soil.

·       There are a few types of chemical weathering such as:

o     Oxidation -- when oxygen interacts chemically with minerals. Ex.: when a nail rusts oxygen combines with the iron in the nail to form iron oxide.

o     Hydration -- when water interacts chemically with minerals. Ex., when hornblende and feldspar unite with water they eventually form into clay.

o     Carbonation -- when carbon dioxide interacts chemically with minerals.

§       carbon dioxide -- dissolved in water, forms weak carbonic acid.

§       Carbonic acid -- comes in contact with the surface of the earth dissolves large masses of limestone, creating caves and caverns.

§       Other common terms associated with carbonation are sink holes, karst topography, stalactites and stalagmites.

·       Weathering rates --depend on 3 different factors:

o     Particle size/surface area exposed to the surface

o     Mineral composition

o     Climate

Soil Formation

·       One of the major products of weathering is soil

·       Soil -- combination of particles of rocks, minerals, and organic matter

·       Soil contains the necessary nutrients to support plant and animal life

·       As a result of the weathering processes and biologic activity, soil horizons (layers) form

·       Soil horizons vary in depth depending on an areas climate and weathering rates

 

 

 

 

The below diagram is a mature soil profile common to New York State:

Soil Profile Diagram

 

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Erosion

 

Once a rock material has been weathered, it is ready to be transported, or eroded. Erosion refers to the transportation of rock, soil, and mineral particles from one location to another.

·       Erosion is different from weathering since erosion has the moving element.

·       The main driving force behind all agents of erosion is gravity.

·       Without gravity the other major natural agents of erosion such as: wind, running water, glaciers, waves, and rain would not occur.

·       usually takes many years for erosion to be noticed, such is the case with the Grand Canyon.

·       But it is common to see erosion take place in a quick fashion.

·       Ex. --Recent erosion along the California coastline. Many homes and land has eroded into the Pacific Ocean due to intense wave action and weak soil/bedrock compositions (erosion).

Factors Affecting Transportation of Sediments

·       Running water is the primary agent of erosion on Earth

·       Most running water is found in streams and rivers

·       Many factors affect the movement of sediments in a stream:

o     Gradient (slope), discharge, and channel shape influence a stream’s velocity and the erosion and deposition of sediments.

·       Sediments carried by a stream are almost always rounded due to the grinding action of the water on the rocks, a process called abrasion

·       Streams are usually formed in V-shaped valleys; and deltas, flood plains, and meanders are results of what a stream can form

·       The watershed of a stream is the area drained by a stream and its tributaries (smaller feeder streams).

·       The average velocity (speed) of a stream depends on its slope and discharge, which in turn can explain the carrying power of a stream.

o     As velocity of the stream water increases, size of the particles carried in the stream also increases, a direct relationship.

·       Streams carry materials in 4 distinct ways:

o     Floatation, solution (dissolved particles), suspension (within the water profile), and bed load (bouncing and dragging along the stream bed.

 

See the graph on page 6 of  the Earth Science Reference Tables to explain particle size to stream speed.

 

 

 

·       Meanders -- another very important part of stream erosion.

o     In a meandering stream velocity is lowest (High Potential Energy) along the inner banks

o     Along a straight channel segment, water moves the fastest in mid-channel, near the surface.

o     But as water moves around a bend, the zone of high velocity (High Kinetic Energy) swings to the outside of the channel.

o     another landform -- an oxbow lake -- can develop-- forming an independent loop that will become a lake (see below).

Oxbow Lake Formation Diagram

 

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Adapted from original illustration by Steven Fick, Canadian Geographic, July/Aug '93

 

 

 

 

Valleys Eroded by Streams and Glaciers

·       Valleys that have been eroded:

o     by streams are v-shaped

o     by glaciers are u-shaped

 

Effect of Humans on Erosion
Humans add greatly to the natural processes of land erosion through activities, such as:

·       highway and construction,

·       destruction of forests (clear-cutting),

·       set forest fires,

·       poor landfill projects, etc…

·       Today, human activities contribute more than ever to the erosion on the Earth’s surface

 

 

Deposition

 

-- Rock particles are deposited somewhere else --the final step in the erosional-depositional system.

·       agents of erosion become agents of deposition

·       Final deposition of particles (sediments) usually occurs at the mouth of a stream--a process called horizontal sorting takes place:

o     The sediments that were once carried down the stream are arranged from largest to smallest.

 

Stream Horizontal Sorting Diagram

 

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Factors Affecting Deposition
The major factors that affect the rate of deposition are:

·       particle size, shape, density, and the velocity of the transporting stream:

o     Size: smaller particles settle more slowly than the larger particles, due to gravity. The smaller particles tend to stay in suspension for longer periods of time. This form of deposition is called graded bedding or vertical sorting. The diagram below shows graded bedding.

 

Graded Bedding Diagram

 

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·       Shape: A round sediment compared to a flat (skipping stone) sediment of equal size will settle faster in a body of water. The graphic below shows the relationship.

Rate of Settling Diagram

 

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·       Density: If particles are the same size but have different densities the higher density particle will settle faster

 

·       Velocity:

o     If the stream slows down during a drought period, the carrying power will decrease and the particle sizes carried and deposited will also decrease.

o     If a stream is flowing faster due to flood conditions, then the carrying power of the stream will increase and the sizes of particles deposited will increase as well.

Glacial Deposition
Glacial ice deposits --very different from stream (water) deposits.

·       Glacial deposits of gravel, boulders, and sand are unsorted with no layer as in graded bedding.

·       Till which is the accumulation of sediments carried by a glacier is very sharp like broken glass.

The diagram below shows the unsorted nature of glacial deposits:

Glacial Deposits Diagram

 

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Adapted from: Regents Exam Prep Center  http://regentsprep.org/Regents/earthsci/earthsci.cfm

 

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