Download 3.1 Gradation Complete

Gradation
What is gradation?
Gradation refers to the processes which bring a land surface to a state
of uniform grade by eroding the land. Another word, denudation also
refers to the levelling processes which erode land features.
Degradation
Degradation refers to the processes of weathering, erosion and
transportation.
Weathering
Weathering is the disintegration or decomposition of rocks on or near the
earth’s surface. There are two types of weathering – Physical weathering in
which rock materials are eroded by pressure applied to them by heat, wind,
water, ice etc and Chemical weathering in which the compounds of rock
materials are chemically changed by contact with water or air. Physical
weathering leaves angular broken fragments while chemical weathering
leaves rounded surfaces.
Kinds of Physical Weathering
Frost shattering is when water seeps into rock fractures, freezes,
expands and splits a rock apart. If this happens regularly due to
fluctuating temperatures above and below zero, this is referred to as
ice wedging.
Thermal Expansion
When rocks composed of different minerals (like granite) are heated
causing thermal expansion and then cooled (as in a desert), the
minerals expand and contract at different rates which eventually cause
the rock to fragment apart.
Pressure Release
When rocks that have been under pressure have overlying layers
eroded away, the lower pressure can cause cracks and fractures to
develop. Exfoliation (the flaking off of the outer parts of a rock) and
sheeting (the splitting off of sheets of rock from its outside) are two
terms used to describe rocks that are breaking up due to pressure
release.
The Physical Activity of Animals and Plants
Burrowing animals expose rock materials to water and air while plant
roots probing rock cracks and growing in width may split rock
materials. Growing plant roots can exert phenomenal pressure to
crack rock materials.
Crystal Growth
When water seeps into rock pores, dissolving minerals in the rocks and
then evaporates, it often leaves behind salt crystals which expand in
the pores causing pressure which splits the rock apart. This is a
common process in desert areas leading to caves at the base of
sandstone formations like Mesa Verde.
Kinds of Chemical Weathering - Solution
Solution weathering is a kind of chemical weathering in which some chemical
is dissolved in water and this solution acts on rocks, dissolving them, causing
pores and cracks. A very common kind of solution weathering is when rain
passing through the air or water passing through soil reacts with carbon
dioxide gas to form hydrogen ions (H+ - acid) and bicarbonate ions (HCO3- ).
Magnesium carbonate (MgCO3) and calcium carbonate (limestone - CaCO3)
rocks dissolve with constant exposure to hydrogen ions.
Chemical Weathering - Oxidation
Oxygen in air reacts with many compounds in rocks, making oxides.
Iron oxide forms reddish brown stains where oxygen from the air has
combined with iron in rocks. Iron oxide has a tendency to expand and
crack rocks apart.
Chemical Weathering - Hydrolysis
Water ionizes as follows: H2O <-> H+ + OH- . When these ions of
water replace ions of a mineral, clay materials are formed, a substance
called kaolinite. This kind of chemical weathering occurs commonly in
warm moist climates.
Erosion
Erosion is the removal and movement of rock debris and associated
organic matter.
Transportation
Transportation is the movement of eroded debris by running water, ice
wind or wave action.
Aggradation
Aggradation is the building up of a land surface by the deposition
(depositing) of rock materials.
Rate and Depth of Weathering: Temperature and Moisture
At colder temperatures, physical weathering is most common. The
greatest and deepest weathering takes place in forest biomes. The
greater the moisture and temperature, the greater and deeper the
chemical weathering of rock substrates.
Rate of Weathering: Joints and Planes
Joints are cracks in rocks caused by tectonic stresses. Bedding planes
are parallel surfaces to rocks layers. The greater the number of joints
and bedding planes, the faster rock materials will weather because the
cracks in rocks create more surface area for water, solutions, and gases
to pass in and through the rocks.
Weathering Affected by Temperature and Moisture
High temperatures and moisture
conditions favour strong
chemical weathering while colder
temperatures and less moisture
favours strong physical
weathering.
Mass Wasting
Mass wasting is the downhill movement of weathered materials
resulting from the pull of gravity. The more water content a
weathered slope contains, the more likely mass wasting will be.
Soil Creep
Soil Creep is the slow movement of loose rock and soil downward. It
causes a downslope tilt on fences, stone walls, tree trunks,
gravestones.
Soil Flow or Solifluction
Solifluction is a downhill flow of water-saturated rock and soil
materials. Solifluction is common in colder climates during periods
when the soil melts and flows.
Rock Glaciers and Avalanches
Rock glaciers are large volumes of rock fragments that accumulate and
move downhill, flowing slowly like ice glaciers. Rock glaciers are
common in high mountains of humid environments. Rapid flows of
snow, ice, earth and/or rock masses are called avalanches.
Rock Slides
A rock slide is a rapid movement of
weathered rocks splitting off along
bedding planes in a mountainous
region.
A Slump
A slump is when a block of clay-rich soil
and rock slips along a concave surface.
Debris Slide
A debris slide is a rapid movement of
loose soil and rock in a steeply sloping
terrain that is shaken loose by an
earthquake or is undercut by a river or
glacial erosion.
Talus and Scree
Talus and scree refer to the rubble that accumulates in fan-shaped
piles from physical weathering of uplifted rocks or mountains. Talus
technically refers more to the sloping debris shape while scree refers
to the rock debris itself.
Factors Affecting Mass Wasting
Both friction and gravity affect how much mass wasting a terrain will
experience. Friction is the internal resistance to movement and is
reduced by the weigh and lubricating effect of water. Gravity is the
earth’s force that pulls on slopes causing them to move. The steeper a
land surface the greater the moving force of gravity becomes.
The Effect of Climate on Mass Wasting
Warm, moist climates foster more mass wasting while cooler and drier
climates produce less mass wasting.
End of Presentation