GEO 12 Chapter 12 : Denudation: Weathering and Mass Wasting Notes and Chapter Review

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GEO 12Chapter 12 : Denudation: Weathering and Mass Wasting Notes and Chapter Review

Transcript of GEO 12 Chapter 12 : Denudation: Weathering and Mass Wasting Notes and Chapter Review

  • Geography 12 Chapter 12 Denudation: Weathering and Mass Wasting Page 1 of 7 (KWP)

    Chapter 12 : Denudation: Weathering

    and Mass Wasting

    Part A - Notes

    Weathering The earth is constantly trying to reduce itself down If there were no other processes at work, eventually the earth would become a

    featureless sphere

    This is called the base level That is the level at which gravity no longer has an effect. The process of denudation is the wearing down of the earth by erosive forces and

    the change of the landscape by deposition

    Denudation then is divided into two categories Degradation Aggradation Degradation includes all of the process that degrade or wear down the surface of

    the earth

    This includes weathering and erosion. Weathering and erosion slowly chisel, polish, and buff Earth's rock into ever evolving

    works of art - and then wash the remains into the sea.

    The processes are definitely independent, but not exclusive Weathering is the mechanical and chemical hammer that breaks down and sculpts

    the rocks

    Working together they create and reveal marvels of nature from tumbling boudlers high in the mountains to sandstone arches in the parched desert to polish cliffs

    braced against violent seas. The whole process is driven by wind and water Water is the most powerful force in weathering and erosion It is relentless Water's ability to seep into small crevices and cracks, then expand due to freezing,

    gives it the ability to cause great changes in solid rock Frost shearing Two types of weathering

    1) Mechanical and Physical weathering

    No change in the rock's chemical composition Agents of mechanical weathering include

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    Temperature: causes repeated expansion and contraction

    Ice wedging (frost action, freeze-thaw cycle): when

    liquid water goes into cracks and then freezes

    causing the cracks to get wider

    Erosion

    The power of water as an erosive force should not be understated

    Water is relentless It carries sediments in suspension These sediments can act as an abrasive as it continually hits the surface of the rock The stone forest of china

    Organic Activity

    We have all seen plants take root in strange places Plants and animals can speed up the erosion process Hundreds of years of animals following certain mountain trails can erode that trail

    and leave it exposed to the weathering process

    Rock fall

    Gravity: rocks falling and colliding with other rocks Abrasion: wearing away by solid particles Unloading: the expanding of high pressure when exposed to a lower pressure

    Pressure Release

    Unloading is also known as pressure release As the heavy land formations are slowly eroded away, they do not weigh as heavy

    on the underlying rock This release in pressure will cause

    the rock to "bounce" back

    This motion causes breaks and fissures

    Another example of pressure release is found on shorelines

    where the land "popped up" after

    the retreat of the great ice sheets

    in the last ice age

    Craggy cliffs along the sea with broken rock faces

    Exfoliation

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    Contraction due to crystallization

    Crystal growth

    Salts from crystals These crystals grow and if they are left in

    a rock, the expansion of the crystals can cause the rock to fracture

    This process is common in desert areas

    where evaporation is rapid.

    2) Chemical weathering

    The altering of the composition of minerals within a rock that results in a reduction in size

    Agents of chemical weathering include:

    Water: Dissolves minerals out of rocks making them weaker Solution - minerals breaks down in water

    Oxidation - minerals reacts with oxygen

    Hydrolysis - mineral reacts with water SOLUTION (or dissolution) refers to the process that places dissolved ions into water

    and usually carriers them away The loss of some ions, of course, necessarily carries with it a change in mineral content

    In some cases solution acts alone as a chemical weathering agent; more often it works

    in concert with other processes What typically happens to limestone in a wet climate is an excellent example of

    weathering purely by solution Remember that one way to identify calcite and other carbonate minerals is by placing

    a drop of acid on them

    Acid: Dissolves minerals in rocks (examples: carbonic acid, acid rain, and plant acid)

    Lichens such as these growing on

    the rocks in the picture can produce

    weak acids that react with the rocks Oxidation: Oxygen combines with

    iron minerals sulfur minerals

    changing the composition of the

    rock

    Oxidation is also called rusting

    Factors that determine the rate of weathering

    Composition

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    Physical conditions of rock o cracks, holes, crevices - eariers weathering

    o solid, unbroken - more weather resistant

    More surface area - weathering occurs faster Climate Cold climates - mechanical weathering breaks down rocks rapidly Warm, wet climates - chemical weathering breaks down rocks rapidly Topography - the position of the rock air pollution exposure time surface area exposed Mechanical and chemical weathering work together

    Part B Chapter Review (Textbook summary)

    12.1 Denudational Processes and Weathering P.241-242

    Landforms result from the uplift caused by tectonic forces and the process of

    denudation, or levelling that are constantly at work to lower the earths surface to a

    common level.

    Uplift powered largely by energy from below; Denudation powered by energy from

    above

    Denudation reduces the land surface to base level the lowest level to which a land

    surface can be eroded.

    Why dont we use sea level? 1) It fluctuates; 2) There are desert depressions that are

    below the sea level

    Denudation can be divided into two categories

    1) Degradation includes the processes of weathering, erosion, and transportation

    a) Weathering the first step disintegration or decomposition of rocks in places

    on or near the earths surface

    b) Erosion the removal and movement of rock debris and associated organic

    matter

    c) Transportation whether carried out by running water, ice, wind, or wave

    action, is an integral part of erosion

    2) Aggradation involves building up the land surface by the deposition of rock materials

    a) Deposition occurs when a drop in energy slows the transporting agent to the

    point where it deposits some of its rock materials.

    END RESULT of 1) & 2) levelling out of the land surface at, or close to BASE LEVEL.

    Denudation slow and subtle but rate varies from place to place/ time to time

    Weathering continues after rock fragment are carried by agents of erosion as rock

    particles being transported until they are finally deposited in oceans, seas or lakes.

    12.1 Weathering : A Key Process P.243-246

    Weathering reduces solid rocks to smaller and smaller particles. It is important as

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    o to make water, wind, waves and ice effective erosive agents

    o essential steps to provide parent materials needed for soil formation

    2 major sources of energy for the breakdown of rock materials are:

    o Solar energy powers chemical activity, the hydrologic cycle, and winds;

    o Gravitational energy brings down, moisture as rain and causes stream flow

    and other downhill movements

    Intensity of weathering DECRESES with depth of the surface

    A. Physical Weathering disintegration of fragmentation of rocks into smaller particles

    without chemical alteration of the minerals that make up the rock

    a) Frost Shattering Water freezes within rock and expands in volume

    Also called: ice wedging

    b) Thermal Expansion

    c) Pressure Release

    Also called exfoliation/ sheeting

    d) Animals and Plants

    e) Crystal Growth

    B. Chemical Weathering Rocks suffer decay or alteration because of a change in

    chemical composition of the minerals that make them up

    a) Solution

    rainwater falls, dissolves small amount of CO2 gas, turning droplets into very weak

    solution of carbonic acid (H2CO3) react with minerals, dissolving them and carrying

    them away in solution

    Limestone (CaCO3) is subject to solution weathering

    b) Oxidation combine oxygen with a mineral

    Often: Iron, combing with oxygen to form rust-coloured mineral called Iron oxide

    c) Hydrolysis ions of water replace ions of a mineral

    Produces a new clay mineral and soluble mineral compound; these compounds are

    dissolved and carries away

    Kaolinite weathered clay that is left behind after the minerals (mentioned above)

    are dissolved are carried away

    Identify Physical or Chemical Weathering

    a) Physical

    Angular, broken rock fragments

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    little or no change in colour or internal strength of consolidation

    b) Chemical

    produces rounded rocks

    altered surface colours

    less consolidated

    likely to crumble when put under pressure

    12.3 Weathering: Variations over time and Space

    Rate Variations

    a) temperature control the intensity of weathering activity

    b) precipitation provides water, an effective agent in breaking down rocks and

    minerals

    Weathering is greatest at high temperatures and high levels of precipitation

    Such conditions are found in tropical rainforests

    Lower temperatures/precipitation, will therefore indicate decreases in rate and depth

    of weathering

    Number of joints and bedding planes in rocks also influence rate of weathering; More

    joints and bedding planes -> higher rate of weathering

    Almost all rocks at the earths surface have fractures known as joints produced by

    stresses and strained caused by tectonic processes

    Bedding planes are parallel surfaces separating different layers of sedimentary rocks

    Chemical vs. Physical Weathering

    Chem. Weathering favours warm, moist climates

    Phys. Wea is most effective in polar and high mountain regions favours cold, dry

    climates

    Most environments have a combination of chemical and physical weathering

    12.4 Mass Wasting: Weathering and Gravity

    Mass Wasting downhill movement of weathered materials resulting from pull of

    gravity

    Common forms: Creep, flows, slides and falls

    a) Creep

    especially on slopes in humid areas

    Loose rock and soil slowly slide downslope

    b) Flows

    in humid climates

    flows move like a thick liquid

    solifluction describe a slow, downhill flow of water-saturated rock and soil

    materials; common in cold climates where soil ice-saturated and ground is

    permanently frozen

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    Rock glaciers are formed where large volumes of rock fragments accumulate on

    mountain slopes and begin to move downhill

    occurs in high mountains of humid regions

    Avalanches rapid flows of snow, ice, earth and/or rock masses

    c) Slides

    1. Rock Slide: Simplest form of slide

    Occur in mountainous areas when large sections of rock are weathered along bedding

    planes

    2. Slump: a block of soil and rock that makes a rotational slip along

    a concave surface

    o this form of slide is common where thick clay soils make up a cliff face

    3. Debris slides are caused when loose rock and soil are shaken

    loose by an earthquake, or are undercut by river or glacial

    erosion. They move rapidly.

    d) Falls

    Individual rocks fall regularly in high mountains, where frost shattering is very effective

    As they tumble down the steep slopes, the accumulate in fan-shaped piles of rubble

    known as talus and scree

    Factors affective mass wasting

    1. Friction

    an internal resistance to movement that rocks have

    Water and ice have lubricating effect

    2. Gravity

    If resistance to movement is smaller then pull of gravity, mass of weathered

    material will begin to move downslope

    Effectiveness of gravity is determined by the steepness of slope and the thickness of

    loose materials

    o Steeper the land surface, greater the force available to move objects downslope

    o greater the thickness of the moving mass of rock or debris, the greater the speed

    at which it is likely to move