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Grade 8
Science
Unit 4: Landforms and Topography
NOTES
Weathering and Erosion
Weathering- the breakdown of the materials of Earth’s crust into smaller pieces.
Physical Weathering- Process by which rocks are broken down into smaller pieces by external conditions.
Types of Physical weathering
– Frost wedging
– Plant roots
– Friction and impact
– Burrowing of animals
– Temperature changes
Chemical Weathering- The process that breaks down rock through chemical changes.
The agents of chemical weathering:
Water - weathers rock by dissolving it
Oxygen - Iron combines with oxygen in the presence of water in a process called oxidation
- the product of oxidation is rust
Carbon dioxide - CO2 dissolves in rain water and creates carbonic acid
- Carbonic acid easily weathers limestone and marble
Living organisms - Lichens that grow on rocks produce weak acids that chemically weather rock
Acid rain - Compounds from burning coal, oil and gas react chemically with water forming acids.
- Acid rain causes very rapid chemical weathering
Erosion- the process by which water, ice, wind or gravity moves fragments of rock and soil.
Water Erosion - rivers, streams, and runoff
Ice Erosion - glaciers
Wind Erosion
Mass Movements - landslides, mudslides, slump and creep
Soil Formation
Soil is a mixture of weathered rock & organic matter that usually covers bedrock (solid rock that underlies
all soil). Both chemical & mechanical processes are involved in the development of soils.
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Chemical weathering turns hard minerals into soft ones
Mechanical weathering breaks solid rock into smaller pieces
Plant & animals add organic materials in the form of waste products & dead organisms
The decay of organic matter produces acids which accelerate chemical weathering
Burrowing Animals, such as earthworms, insects, & rodents, help circulate air and water through the
soil & mix mineral & organic remains
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The material from which soil forms is called its parent material.
A cross section of soil exposed by digging is called the soil profile. The
weathering of soil produces layers known as soil horizons. The topsoil
or A horizon is usually rich in dark-colored organic remains called
humus (labeled O
horizon below). The
subsoil or B horizon
contains minerals that
have been transported
deeper by groundwater.
Most of the clay in soil has also been washed down to this
layer. The partially weathered bedrock or C horizon is
composed of broken up bedrock on top of the solid bedrock
(parent material).
Soil erosion is the removal of topsoil by the action of running
water or wind. It takes between 100 & 400 years for one
centimeter of topsoil to form.
Topographic Maps
A map is a representation of Earth’s surface
Topographic maps show the physical features of a small section of Earth’s surface.
Elevation – the height of a location above a fixed reference point, usually sea level.
Contour lines are lines on a map that connect points of equal elevation
Every 5th contour line is called an index contour line. It is bold and is labeled with an elevation
A profile is a side view of a landform projected from a straight line on a topographic map.
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Topographic map – a map that uses contour lines to depict variations in elevation across a
landscape.
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Contours that form closed curves usually indicate hills
or mountains but can also indicate depressions.
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Contours that are close together indicate steep slopes,
while contour lines that are far apart indicate flatter
terrain.
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Contour lines in a river valley or stream are often Vshaped, with the point of the V pointing uphill.
Depression contour – a contour line with hachure marks to indicate a
depression or hollow.
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Depression contours form closed curves.
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The hachure marks distinguish depression contours from contour
lines that indicate a hill or mountain.