Stress and strain: elastic and plastic deformation Notes

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Types of stress

Stress - stress in an internal force in opposition

to an external force

Force per unit

area normally measured in newton's (N) per square meter.

1 Nm-2

= 1 pascal Pa

For rocks

stresses are on the order of Mpa (106 Pa) or MN (106 Nm-2)

Stress is

generated through gravity, tectonics and earth surface processes (eg., waves)

Shear stress is

the most common erosional stress

Shear

Stress - force

Strain is the

deformation (change in dimension) resulting from an applied stress

The behavior of

natural materials under stress is defined by the relationship among applied

stress, strain and time.

There are 2

major models which describe this behavior

Elastic deformation

occurs when strain is proportional to applied

stress

And all strain

disappears when applied stress is removed (deformation is fully recoverable) -

like a spring balance.

However beyond

a critical stress level (elastic limit failure may occur)

Plastic deformation

Material shows no tendency to deform until a

critical level of stress (the yield point) is reached, then strain occurs

abruptly in the form of failure and is not reversible upon removal of load

(intermolecular or intergranular shear - causing flow similar to a fluid.

Slopes - land

of instability

The term slope

has 2 very different uses – the first is as a measurement of inclination of the

a surface and can be a slope angle in degrees, a slope gradient usually

expressed as the ratio of rise over run 1:7 or as a % which is rise/run X 100.

The second use of the term slope is to denote

land form elements and usually involves a description of slope shape (straight,

concave, convex), character, (steep, gentle) and terms like “break in slope” to

denote a change in slope shapes. Also, since all landforms are composed of

sloping surfaces the description of slope characteristics is an important part

of slope studies.

Slope Stability and Mass Wasting

An exposed ground surface that stands at an

angle with the horizontal is called an unrestrained slope.

The slope can

be natural or artificial.

If the ground

surface is not horizontal, a component of gravity will tend to pull the soil

down slope.

If the

component of gravity is large enough and the soils internal shear strength is

small enough, a slope failure can occur.

when failure

occurs this is mass wasting

Gravity

The main force responsible for mass wasting is

gravity, in fact “mass wasting is defined as the down slope movement of

material under the influence of gravity”.

Gravity is the force that acts everywhere on

the Earth's surface, pulling objects in a direction toward the center of the

Earth.

Slopes and gravity

On a flat surface gravitational force acts to

hold objects in place by pulling on them in a direction perpendicular to the

surface. So long as the material remains on the flat surface it will not move

under the force of gravity

On a slope the

tangential component of gravity acts down a slope: it causes objects to move

downhill.

Shear stress

is the

downslope component of the total stress involved

t (tau)

Normal stress is the

perpendicular component

s (sigma)

Factor

affecting slope stability

Increases in shear stress by

removal of lateral support (rivers, road cuts), steepening

addition of

mass,

earthquakes,

regional

tilting (tectonics)

removal of

underlying support (solution, mining)

lateral

pressure (swelling, freezing)

Decrease shear strength by

weathering,

increased pore

water pressure

Mass wasting

When the shear stress becomes greater than the

combination of forces holding the object on the slope, the object will move

down-slope - slope failure.

Mass Wasting is the downslope movement of material

under the influence of gravity

Slope failure

is the collapse of rock or sediment mass.

3 major modes

of slope failure or mass wasting:

Slides

Flows

Heaves.

Weathering – an important factor in slope stability

Most rocks

and minerals are formed deep within the

Earth's crust where temperatures and pressures differ greatly from the surface.

The physical

and chemical nature of materials formed in the Earth's interior are

characteristically in disequilibrium with conditions occurring on the surface.

Because of this

disequilibrium, these materials are easily attacked, decomposed, and eroded by

various chemical and physical surface processes.

Weathering is

the first step for mass wasting processes

At the Earth’s

surface, rocks are exposed to the effects of weathering: the chemical

alteration and mechanical breakdown of rock, when exposed to air, moisture, and

organic matter.

Weathering

converts rock to regolith.

Weathering Processes:

Weathering is

the collective term used to describe "the

physical, chemical and biological processes responsible for the in place (in

situ ) disintegration and decomposition of solid rock and other earth materials

reducing them to a physically and chemically altered state that is more readily

removed by the various agents of erosion".

Physical,

Chemical

Biological

in situ – means

in place,

without

transport, or

without the

influence of an erosive force

Volume change

in voids – an important cause of mass wasting processes

Hydro fracture and frost action

ice wedging

ice cracking

Salt crystals - salt

weathering

Plant roots and

stems

Mass wasting:

three modes of movement

Slides - rapid and

accelerating, dry, failure along a well-defined plane, constant velocity

profile moves as a cohesive mass

Flows - rapid and

accelerating, wet or dry, velocity profile decreases through the deforming

mass, moves by inter-particle shearing - like a fluid

Heaves and Creep - slow

imperceptible, involves up & down pattern of movement of individual

particles, up normal to surface, down is vertical, velocity decreases with

depth

Landslides

There are a

number of slide-based mass wasting features

The most basic

are simple translational slides where a block of soil or rock slides along an

inclined surface

Slumps (rotational) usually involve a mass of soil moving along a

curved failure plain

Fall and topples

Falls and

topples involve a block of soil or rock that initially moves along a failure

plane but eventually free falls.

Talus is the

accumulation of rock debris that collects at the base of a slope

Flows (mudflows and debris flows)

Flows involve

the downslope movement of material by internal

deformation much like a viscous fluid

Water is

usually the key component

But in some

cases dry flows involve fine particles moving on a layer/cushion of air

Creep

Gradual plastic

deformation of soil and rock materials under the influence of gravity

Carbon

dioxide-rich water attacks rocks, dissolves some minerals, and at some point,

new minerals precipitate – this case Calcite (calcium carbonate)

Chemical Weathering - Processes

Carbonation (solution)

dissolution of

calcium carbonate in acidic soil and groundwater

CaCO3 + H2CO3

——> Ca+2 + 2HCO3-

bicarbonate

represents the largest constituent of the dissolved load of most rivers

carbonation of

limestone results in karst topography

the insoluble

minerals form soil

Land Subsidence

Land subsidence

is most often caused by human activities, mainly from the removal of subsurface

water; also oil and gas extraction.

What is soil?

Soil is the

unconsolidated cover on the surface of the earth.

Soil is made up

of mineral particles, organic particles, air, and water.

Soil is capable

of supporting plant growth.

The basic unit of study: soil profiles

A soil profile

is a vertical cross- section of a soil. It is divided into a number of distinct

layers, referred to as horizons.

The horizons

are normally designated by symbols and letters.

The presence or

absence of particular horizons allows pedologists (soil scientists) to classify

the soil.

In addition,

the organic or O horizon can form above the mineral soil- commonly in forested

areas, resulting from the dead plant and animal remains.

Soil Horizons

The horizons

may be further subdivided.

For example, in

this soil profile the A horizon has been divided into 4 further pedological

horizons:

(L) Leaf litter

(F) Fermenting leaf litter

(H) Humus

(E) eluvial

These lie above

the (B) or illuvial horizon.

Parent Material

This is the material

from which the soil has developed and can vary from solid rock to deposits like

alluvium and clay.

It has been

defined as ‘the initial state of the soil system’.

The parent

material can influence the soil in a number of ways:

§ colour

§ texture

§ structure

§ mineral composition

§ permeability/drainage

Climate

This is

probably the most important factor (soils produced from the same parent

material under different climates contrast).

Climate governs

the rate and type of soil formation and is also the main determinant of

vegetation distribution.

Soil climate

has two major components; moisture (precipitation) and temperature, influencing

evaporation.

When

precipitation exceeds evaporation, leaching of the soil will occur.

Temperature

determines the rate of reactions; chemical and biological decay and so has an

influence on weathering and humification.

Organisms: vegetation, fauna and soil microbes

Organisms

influencing soil development range from microscopic bacteria to large animals

including man.

Micro-organisms

such as bacteria and fungi assist in the decomposition of plant litter.

This litter is

mixed into the soil by macro organisms (soil animals) such as worms and

beetles.

Soil horizons

are less distinct when there is much soil organism activity.

Higher plants

influence the soil in many ways.

The nature of

the soil humus is determined by the vegetation cover and resultant litter

inputs.

Roots

contribute dead roots to the soil, bind soil particles together and can

redistribute and compress soil.

Relief (Landforms and topography)

Relief is not

static; it is a dynamic system (its study is called geomorphology).

Relief

influences soil formation in several ways:

§ It influences

soil profile thickness i.e. as angle of slope increases so does the erosion hazard

§ it has an

effect on climate which is also a soil forming factor

§ gradient

affects run-off, percolation and mass movement

§ it influences

aspect which creates microclimatic conditions

In this

photograph soils are thin on the glacially eroded rock outcrops but are much

deeper on the raised beach deposits in the foreground.

Soil on a steep slope

As slopes

increase, soils become shallower and have thinner and fewer horizons.

This is because

the steeper the slope the greater the runoff and the greater the erosion.

Weathering

This refers to

the breakdown and decomposition of rocks and minerals by factors including air,

water, sun and frost.

Physical

weathering involves continual breakdown or rocks into smaller and smaller

particles.

Chemical

weathering involves alteration of the chemical composition of rock minerals.

Decomposition and Humification

Decomposition

is the breakdown of plant derived material into its simpler organic

constituents. This is accomplished by

enzymes, earthworms, mites and other organisms.

Humification is

the breakdown of plant remains- leading to the formation of different types of

humus. It is probably the most important

biological process taking place in soils.

MULL humus develops

under deciduous woodland, where base-rich plant remains are actively broken

down by a prolific soil biota.

MODER humus is

intermediate between mor and mull.

MOR humus

usually develops beneath coniferous woodland or heather moorland, under cool,

wet climatic conditions. Breakdown is slow due to the absence of soil biota.

Leaching

Wherever

rainfall exceeds evaporation and there is free downward movement of water

through the soil pore system, soluble minerals are leached or removed from the

soil profile.

Continual

leaching tends to impoverish the upper mineral horizons by removal of basic cations

(cations are ions having a positive electrical charge e.g. Ca2+).

Leaching is

most active in sandy soils with high porosity and is least in fine-textured

soils such as clays which have restricted pore spaces.

Capillary Action

Where

evaporation exceeds precipitation, moisture moves upwards within the soil

profile by capillary action. It is therefore in the reverse direction to

leaching.

In deserts

precipitation is generally lower than water loss by evaporation. As a result capillary action dominates.

Translocation

The movement of

material in solution or suspension from one horizon to another is referred to

as translocation.

The upper

mineral horizon losing the material is the ELUVIAL or E horizon. This is where

maximum leaching or eluviation (from the Latin word ex or e meaning out, and

luv meaning washed) takes place. The E horizon near the surface of a podzol is

a good example of an eluvial horizon.

The lower

horizon gaining the material is the ILLUVIAL horizon (often a subsoil or B

horizon) (from the latin words il, meaning in, and luv, meaning washed). This is the zone of maximum accumulation.

Soil structure

The arrangement

of sand, slit and clay particles to form larger aggregates.

Organic matter is the glue

that holds the aggregates together

Large pores

(spaces) between aggregates are filled with air in a moist

soil.

Small pores are filled

with water in a moist soil. Even smaller pores inside the aggregates (not

shown) are also filled with water.

People and soils

Erosion

Salination

Contamination

Desertification

Soil erosion - a global environmental problem

Covers land area

globally (following table) – 11 million km2 affected by water erosion and 5

million km2 by wind erosion.

Up to 38% of

world’s farmable land has been lost to erosion since 1950 – about 5 million ha

per year.

On-site losses

in crop productivity etc.

Estimated that

75 billion tons of sediment carried in rivers, much of this derived from

human-accelerated rates of erosion. Off-site damages in rivers, floodplains

etc.

Types of soil erosion

Soil creep: slow downslope

movement of soil particles in response to disturbances – expansion/contraction

with wetting/drying, soil faunal activity etc. Very slow rates (

Landslides and

earthflows: rapid mass movements when soil strength exceeded

by gravity, usually follows heavy rainfall.

Aeolian: wind removal

of surface layer; requires absence of protective vegetation cover, dry soils

and strong winds. Up to 200 tons ha-1 yr-1.

Fluvial: by running

water. By far the most important form of erosion.

Fluvial erosion of soil

Processes:

Raindrop

impact.

Creation of

overland flow.

Rainfall erosivity

Combination of:

Rainfall

intensity – affects creation of overland flow by exceeding

soil infiltration rate. Rainfall with intensity > 25 mm hr-1 likely to

generate overland flow.

Rainfall energy – ability to

splash soil particles downslope and destroy soil aggregates.

Soil Erodibility

Combination of:

Soil

infiltration rate –

sand > loam

> clay

Ease of

detachment of soil particles –

silt > clay

> sand

Ease of

transport of soil particles -

clay > silt

> sand > gravel

Environmental effects of accelerated rates

of soil erosion: off-site

Sedimentation of eroded soil

in river channels, dams, reservoirs: 880 million tons yr-1 in U.S.

Leads to

eutrophication of water bodies (rivers, lakes) – N and P transported

adsorbed to eroded soils. Also pesticide transfer from land surface to aquatic

system through erosion.

Deterioration

of water quality and higher water treatment costs.

Increased frequency

of flooding – greater surface runoff and channel volume may be reduced

by deposited soil.

Strategies to reduce soil erosion rates

Change crop

type or planting/harvesting schedules

Protect soil

surface and increase infiltration rate – add organic matter (mulch) or change

tillage practices (no-till or less intense ploughing).

Change length

and angle of field by creating terraces or ploughing parallel to slope.

Reduce grazing

densities to allow grass cover: >25% grass cover is critical in reducing

erosion rates.

Environmental effects of accelerated rates of soil

erosion: on-site

Reduced soil

fertility:

Leads to loss

of fertilizers, especially N and P, by overland flow.

Removes

smallest particles (clay, organic matter) which have highest available nutrient

content and cation exchange capacity.

Reduces rooting

depth for plants (shallower soils).

Gully creation loses

agricultural land and increases accessibility costs of farming.

Greater energy

costs for ploughing, tillage, seeding and fertilizer

needs.

Leads to

positive feed-back:

Exposure of

lower soil horizons by erosion

Þ decreased

infiltration rates

Þ increased

overland flow

Þ more erosion

Þ exposure of

lower soil horizons

This island earth - biosphere

About 4.5

billion years old

Geologically

dynamic internally and externally

Only planet

presently known to support life

As far as life

is concerned: “Goldilocks of the Solar System” (relative to Sun, not too far,

not too close, just right!?)

Earth as a closed system

Closed system:

exchange of energy but negligible exchange of mass with surroundings

Biosphere

Life evolved on earth during its early history

by at least 3.5 billion years ago and the biosphere readily distinguishes our

planet from all others in the solar system (as far as we know)

The chemical

reactions of life (e.g., photosynthesis-respiration, precipitation of minerals

in skeletons, etc.) have also imparted a strong signal on the chemical

composition of the atmosphere through time.

For example, our oxygen-rich atmosphere is largely a product of

photosynthesis.

Biosphere: Interactions with other Earth System

components

Atmosphere:

Life processes involve a many chemical

reactions which either extract or emit gases to and from the atmosphere (e.g.

photosynthesis consumes carbon dioxide and releases oxygen, whereas respiration

does the opposite).

Hydrosphere:

Evaporation of water from leaf surfaces

(transpiration) transfers water to the atmosphere.

Geosphere:

The biosphere is connected to the geosphere

through soils (mixtures of air, mineral matter, organic matter, and water).

Plant activity (e.g. root growth and organic acid production) are also for the

mechanical and chemical breakdown of the rocks.

Where do humans fit in?

As components

of the biosphere, humans are temporary receptacles of the matter and energy

that flows through the Earth System

“You are what

you eat, drink, and breathe”

Human health

is, to some degree, a function of how this flow of matter and energy flows

through, and interacts with, the human body

In many cases,

problems of human health are fundamentally linked to the natural distribution

of Earth materials

Some human effects on the earth's systems

Global warming - a warming

trend of global temperature in the past 140 years. It is predicted to continue

to rise.

It is partially

due to the industrial release of greenhouse gas such as CO2, by burning

petroleum and coal

Earths biosphere

Biosphere: the

thin layer enveloping the Earth where all living organisms reside, including

The oceans

The lowest few

kilometers of the troposphere

Organization of the biosphere

Life interacts

with its environment at several levels

Organism - Individual

organisms interacting with the environment

Population - Group of

individuals of the same species living in a particular geographic area

Community

All the

populations of different species that inhabit a particular area

Ecosystem

All the biotic

and abiotic components in a certain area

The biosphere

is the total of all of Earth's ecosystems

The biosphere

is the global ecosystem

The biosphere

is self-contained and characterized by patchiness

Connection to humans

Environmental

problems reveal the limits of the biosphere

No part of the

biosphere is untouched by human activities

Current

awareness of the biosphere's limits stems from past environmental practices

Physical and

chemical factors influence life in the biosphere

Major abiotic

factors determine the biosphere's structure and dynamics

Solar energy

Water

Temperature

Wind

Disturbances

such as fire, hurricanes

Organisms are

adapted to abiotic and biotic factors by natural selection

Species exist

in a given place because they evolve there or disperse there

Unique

adaptations that fit a particular environment allow organisms to survive there

Organisms vary

greatly in their ability to tolerate fluctuations and long-term changes in

their environment

Regional

climate influences the distribution of biological communities

Because of its

curvature, Earth receives an uneven distribution of solar energy

Uneven heating

of the Earth sets up patterns of precipitation and prevailing winds

Doldrums are

near the equator

Trade winds

dominate the tropics

Seasonal

climate variation and more moderate temperatures exist in the temperate zones

Prevailing

winds are major global air movements

Ocean currents

have a profound effect on regional climates

Generally

modify climate of nearby land

Landforms can

also affect local climate

Variations in

climate determine the character of the world's biomes

Aquatic biomes

Oceans occupy

most of Earth's surface

Abiotic factors

influencing the distribution of aquatic biomes

Light

Distance from

shore

Availability of

nutrients

Oceanic zones: Intertidal

zone: where land meets ocean

Continental shelf: submerged

part of a continent

Coral reef: found in warm

waters above continental shelf

Biologically

very diverse

Estuary: area where

freshwater stream or river merges with ocean

Among the most

productive biomes on Earth

Wetland: area that is

transitional between an aquatic and terrestrial ecosystem

Aquatic

communities are environmentally threatened

Freshwater biomes include lakes,

ponds, rivers, streams, and wetlands

Light has a

significant effect on freshwater biomes

All but the

most shallow ponds and lakes have photic and aphotic zones

Temperature

creates stratification of water

Availability of

nutrients and dissolved oxygen influence organic growth

Nitrogen and

phosphorus runoff create algal blooms

Terrestrial biomes

Terrestrial biomes reflect regional variations in

climate

Many biomes are

named for climatic features and predominant vegetation

Each biome is also

characterized by the organisms adapted to that environment

The

distribution of biomes depends largely on climate

Within each

biome there is local variation

Tropical forests cluster near

the equator

Rainfall

generally determines the type of vegetation in a tropical forest

Tropical rain

forests are the most complex of all biomes

§ Very high

diversity

§ Complex

structure

§ Human impact is

of great concern

Tropical Forest: Vertical stratification with trees in canopy

blocking light to bottom strata. Many

trees covered by epiphytes (plants that grow on other plants).

Savannas :are tropical

grasslands with scattered trees

Savannas have

two seasons (wet and dry)

Frequent fires

inhibit invasion by trees

Large grazing

animals migrate during seasonal droughts

Savannas are

naturally “engineered” landscape – tree-less conditions maintained by megafauna

and fire.

Evolution of ecosystems:

Ecological succession

The observed

process of change in the species structure of an ecological community over

time.

The community

begins with relatively few pioneering plants and animals and develops through

increasing complexity until it becomes stable or self-perpetuating as a climax

community.

Succession occurs in all

natural environments. Each environment has a particular name that expresses the

nature of their ecological succession: Primary and Secondary

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