geological process pdf

7/25/2019 Geological Process PDF

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Geological Process

on the Earth

Dr. Tushara ChamindaDepartment of Civil and Environmental Engineering,Faculty of Engineering,

Uni. of Ruhuna


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Soil Composition Soil forms in layers during the process of its development.

The parent rock is the solid bedrock from which weathered

pieces of rock first break off.

The smallest pieces of weathered rock, along with living and

dead organisms, remain in the very top layer.

Rainwater seeps through this top layer of materials,

dissolves soluble minerals, and carries them into the lowerlayers of the soil.


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Residual soil is soil located above its

parent bedrock.

Transported soil is soil that has been moved to a

location away from its parent bedrock by agents of

erosion, such as running water, wind, and glaciers.

The parent bedrock determines what kinds of

minerals a soil contains.

The parent rock and climatic conditions of an area

determine the length of time it takes for

soil to form.


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STAGE 1 STAGE 2 STAGE 3

STAGE 4


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Definitions

Weathering, erosion, mass-wasting, and depositional processes occur at

or near theEarths surface and produce changes to the landscape thatinfluence surface and subsurface topography and landform development.

Weathering is the physical disintegration or chemical alteration of

rocks at or near theEarths surface.

Erosion is the physical removal and transportation of weathered

material by water, wind, ice, or gravity. Mass wasting is the transfer or movement of rock or soil down slope

primarily by gravity.

Deposition is the process by which weathered and eroded materials

are laid down or placed in a location that is different from their source.

These processes are all very important to the rock cycle because over

geologic time weathering, erosion, and mass wasting transform solid rock

into sediments and soil that result in the redeposition of material forming

new sedimentary rocks.


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Rock Cycle


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Types of Weathering

1) Mechanical (physical) weathering is the physical disintegration and

reduction in the size of the rocks without changing their chemicalcomposition.

Examples: exfoliation, frost wedging, salt wedging, temperature changes, and

abrasion

2) Chemical weathering decomposes, dissolves, alters, or weakens therock through chemical processes to form residual materials.

Examples: carbonation, hydration, hydrolosis, oxidation, and solution

3) Biological weathering is the disintegration or decay of rocks andminerals caused by chemical or physical agents of organisms.

Examples: organic activity from lichen and algae, rock disintegration by plant or

root growth, burrowing and tunneling organisms, and acid secretion

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I. Mechanical Weathering

Mechanical weathering is the process by which rocks are

broken into smaller and smaller pieces by physical forces.

These physical forces may be running water, wind, ocean

waves, glacier ice, frost action and expansion and contraction

caused by gain and loss of heat.


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The weathered fragments of rock break

apart from the exposed rock from

freeze-thaw action and collect as

angular blocks of talus material.

The rock fragments in the lower right side

of this image have weathered as a result

of extreme fluctuations in day and night

temperature changes.


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Abrasion processes in creek bedsproduce rounded boulders and cobbles.

Over time, abrasion processes will

eventually break these rocks into

progressively smaller particle sizes, such

as gravel, sand, silt, and clay.

Expansion by the freezing of water in pores and

fractures in cold regions


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The processes most commonly involved in mechanical weathering are:

Mechanical unloading- Vertical expansion due to the reduction of vertical load by

erosion. This will open existing fractures and may permit the creation of new

fractures.

Mechanical loading- Impact on rock, and abrasion by sand and silt size windborne

particles in deserts. Impact on soil and weak rocks by raindrops during in tense

rainfall storms.

Thermal loading - Expansion by the freezing of water in pores and fractures in coldregions, or by the heating of rocks in hot regions. Contraction by the cooling of rocks

and soils in regions.

Wetting and drying- Expansion and contraction associated with the repeated

absorption and loss of water molecules from mineral surfaces and structures.

Crystallization- Expansion of pores and fissures by crystallization within the minerals

that were originally in solution. Expansion is only severe when crystallization occurs

within a confined space.

Pneumatic loading- The repeated loading by waves of air trapped at the head of

fractures exposed in the wave zone of a sea cliff (steep rock stones face to sea).


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Chemical weathering is the break-down of minerals into new compounds by the

action of chemical agents. The original material may be changed to something

entirely different (e.g. chemical weathering of feldspar can produce clay minerals).Some chemical agents associate with chemical weathering are the acid in the air, in

rain and in river water.

2. Chemical weathering (decomposition):


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Solution -Separation of minerals in to ions. This is greatly aided by the presence of

Co2 in the soil profile, which forms carbonic acid (H2Co3) with percolating

rainwater.

Oxidation -The combination of oxygen with a mineral to from oxides and hydroxides

or any other reaction in which the oxidation number of the oxidized elements is

increased.

Reduction -Release of oxygen from a mineral to its surrounding environment (i.e.

iron leaves the mineral structure as the oxidation number is decreased).

Hydration -Absorption of water molecules into the mineral structure. This normally

results in expansion of material (some clay expand as much as 60%). By admitting

water accelerates the processes of solution, oxidation, reduction and hydrolysis).

Hydrolysis -Hydrogen ions in percolating water replace mineral cations (no

oxidation-reduction occurs)

Leaching -The migration of ions produced by the above processes. Ca, Mg, Na, K

are easily leached by moving water. Fe is more resistant, Si is difficult to leach and

Al is almost immobile.

Cation Exchange -Absorption onto the surface of negatively charged clay of

positively charged cations in solution (especially Ca, H, K, Mg).

Commonly occurring processes in chemical weathering are:


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Stalactite and stalagmite joining together in

Onondaga Cave State Park, Missouri.Limestone weathered by

carbonation processes

The weathering rinds shown on this sample of

amphibolite illustrate the effects of hydrolysis

weathering on deposited rock fragments.


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Hydration processes cause the formation of clays

and contribute to the reddish-tan color of the

saprolite.

The reddish-orange color of this sandstone is a

result of oxidation processes weathering the rock.


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Subsurface dissolution of halite has

caused overlying rocks to collapse

and form crater-like features.

This is an example of a limestone

solution karst feature found in

Florida's Everglades National Park.


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3. Biological Weathering

Biological weathering is the disintegration or decay of rocks and

minerals caused by chemical or physical agents of organisms.

Organic activity from lichen and algae

Rock disintegration by plant growth

Burrowing and tunneling organisms

Secretion of acids

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Lichen, Algae, and Decaying Plants Organisms such as lichen and algae often live on bare rock and extract minerals from

the rock by ion-exchange mechanisms.

This bio-chemical weathering process leaches minerals from the rock causing it to

weaken and breakdown.

The decaying of plant materials can also produce acidic compounds which dissolve

the exposed rock.

The presence of organisms growing, expanding, or moving across the surface of the

rock also exerts a small amount of abrasion and pressure that gradually cause the

mechanical weathering of the rock as the organisms extract various minerals.

This is an example of biological weathering

that is caused by mosses and lichen

growing on the face of a rock.

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Plant Roots

The most common form of biological weathering is when plant roots penetrate into

cracks and crevices of rocks and cause the rock to split or break into smaller particles

through mechanical weathering.

Although, this process is gradual, it can be fairly effective at breaking apart rocks that

may already have a pre-existing weaknesses such as fractures, faults, or joints.

This is an example of a tree thatis growing between a crevasse

in a rock. The tree is splitting the

rock along parallel planes of

alignment that are already

weakened by foliation

processes, a form of mechanical

weathering.

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Organism Activity

Burrowing, tunneling, and acid-secreting organisms are another form of biological

weathering that chemically or mechanically contribute to weathering.

Some animals may burrow or tunnel into rocks or cracks in rocks and cause the rockto break down and disintegrate. Small animals, worms, termites, and other insects,

often contribute to this form of biological weathering.

Some organisms, such as snails, barnacles, or limpets, attach themselves to rocks

and secrete acid acids that chemically dissolve the rock surface.

The periwinkle snails on this rock

are secreting acids that dissolve

the rock. This picture is taken from

a volcanic shoreline in Hawaii.

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Erosion

Erosion Process by which weathered rock and soil particles are

moved form one place to another Carries away products of weathering

Deposition Process by which sediments are laid down in new

locations Final stage in the erosion process

Erosion moves materials; deposition builds newlandforms


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Agents of Erosion

Gravity

Wind Running water

Glaciers

Waves

Running Water

Water has more power than wind to move particles (exceptions are

hurricane and tornado winds)

When water moves faster, erosion is greater

Erosion by running water in small channels on side of slope is rill

erosion

When channels become deep it evolves into gully erosion


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Gravity

Pulls rocks and soil down slopes

Agent of mass movements

Landslides Mudflows

Avalanches

Wind Major agent of erosion in hot, dry climate or places with little or no

vegetation

When wind erodes soil to depth that water is present,shrubs/grasses can growcalled oasis (happens in desert)

Wind barriers (windbreaks) are used to reduce effects of wind erosion

Trees, plants planted perpendicular to winds direction

Reduces soil erosion, can trap blowing snow, protect crops, etc.


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Deposition

When the sediment being carried is dropped (or

deposited) in a new area, it is called deposition.

Depositions can create new landforms like

beaches and sand dunes.


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Water makes us unique and gives life to Earth 25

Hydrological Cycle and Ground Water

Th H d l i l l


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The Hydrological cycle

W t di t ib ti i th h d l i l l


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Seas and Oceans 97.08 %

Snow and Ice 1.99 %

Ground water 0.62 %

Atmosphere 0.29 %Fresh water Lakes 0.01%

Salt water Lakes and Inland Seas 0.005%

Soil Moisture 0.004%

Rivers 0.001%

Water distribution in the hydrological cycle:


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Basin Hydrologic Cycle


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Water in Natural Formations

Aquifer: a rock unit that will yield water in a usable quantity

to a well or spring.(saturated geological formation, containing and transmitting significant quantitiesof water under normal field quantities)

Aquiclude: formation containing water do not transmit

significant quantities Aquifuge: formation does not contain nor transmit

Confining bed: rock unit with low hydraulic conductivity torestrict movement of GW either into or out of adjacentaquifers


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i d l i l / i


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Basin Hydrologic CycleGW/SW Interaction


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Unconfined Aquifers

Ground water occurring in aquifers: water fills

partly an aquifer: upper surface free to rise and decline:

unconfined or water-table aquifer: unsaturated orvadose zone

Near surface material not saturated

Water table: at zero gage pressure: separates saturated

and unsaturated zones: free surface rise of water in a

well


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Confined Aquifer

Artesian condition

Permeable material overlain by relatively

impermeable material

Piezometric or potentiometric surface

Water level in the piezometer is a measure of

water pressure in the aquifer


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Water storing and transmitting capability

Geologic

Formation

Water Storing

Capability

Water Transmitting

CapabilityAquifer Good Good

Aquiclude Fair None-Very Poor

Aquifuge None None


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G S f


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Groundwater Meets Surface Water

Stream and lake hydrodynamics is influenced by ground water.

How it is influenced depends on the water table level; hence, precipitation.

Wet seasons whentheres high water, groundwater may feed the stream in

addition to runoff events.

Dry seasons when the water table is low, sheet flow runoff may be the only

source of stream flow, in which case the stream feeds (recharges) the

groundwater.

Wet season Dry Season

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