Name: __________________________________
Date: ___________
CHAPTER 12: WEATHERING & EROSION
MODERN EARTH SCIENCE P. 219 - 239
SECTION 12.1: WEATHERING PROCESSES
A. Most rocks deep within earth’s crust were formed under high pressure and
temperatures, but when they are uplifted to the surface of the earth they are subjected to
lower temperatures and pressures as well as gases and water present in the atmosphere
B. These changes result in changes in the physical appearance and composition of rocks.
C. A change in the chemical composition or physical form of a rock is called weathering.
a. There are two types of weathering:
i. _____________________ Weathering: occurs when processes
physically break rock into smaller pieces, but do not change the
chemical composition of the rock
ii. _____________________ Weathering: occurs when processes break
down a rock by changing its chemical ___________________
MECHANICAL WEATHERING
A. Mechanical weathering is strictly a physical process.
a. Common agents of mechanical weathering are ice, plants, animals, gravity,
running water and wind
b. Joint – long, curved crack parallel to the surface that forms when a rock formed
deep within earth (granite) is uplifted towards the surface and overlying
materials are removed, which allows the rock to expand and crack.
c. ________________________ – process by which curved sheets, created by
fractures at joints, are peeled away from the underlying rock
B. Ice Wedging
i.
process that occurs when water seeps into the cracks of rocks and expands
by about 10% when it freezes
ii. each time the ice thaws and freezes it wedges further into the
rock
and the crack widens and deepens and eventually splits the rock into pieces
iii. typically occurs in areas of high latitude where the temperature fluctuates
above and below the freezing point of water (Northern United States)
C. Organic Activity
a. Plants – the roots of plants can work their way into cracks of rock, as the root
grows and expands it increases the pressure it exerts on the rock
b. Animals – the digging activities of animals expose new rock surfaces to
weathering
c. These activities can be effective weathering agents over a long period of time
D. ___________________________ – collision of rocks with one another, resulting the
breaking and wearing away of rocks
a. Agents of abrasion are gravity, running water and wind
i. gravity – causes loose soil and rocks to move down the slope of a hill or
mountain; rocks break into smaller pieces as they fall and collide
ii. running water or wind – also carry particles of sand and rock that
scrape against each other and against other stationary rocks
CHEMICAL WEATHERING
A. Chemical weathering or decomposition takes place when chemical reactions take place
between the minerals in the rock, water, carbon dioxide, oxygen and acids, which alters
the structure of the original mineral and leads to the formation of new minerals.
B. As a result of chemical weathering both the physical appearance and chemical
composition are changed.
C. Hydrolysis – change in the composition of minerals when they react chemically with
water
a. Leaching – minerals affected by hydrolysis may ______________________ in
water and are carried by water to lower layers of rock
b. Mineral ore deposits may occur as a result of leaching
D. __________________________ – when carbon dioxide from the air dissolves in the
water, a weak carbonic acid solution is produced:
H2O + CO2  H2CO3
Some minerals come into contact with carbonic acid and form new products
E. Oxidation – when metallic elements combine with oxygen, often attacks rocks with iron
bearing minerals
4 Fe + 3 O2  2 Fe2O3
F. Acid Precipitation
a. Rainwater itself is naturally slightly acidic because it contains dissolved carbon
dioxide
b. Industrialized areas emit waste gases that contain nitrogen and sulfur, which
creates stronger acids and give acid precipitation a greater ability to weather
rock
c. An increased ability to weather rock greatly accelerates the weathering process
G. Plant Acids – acids produced naturally by certain living things.
a. Lichens and mosses grow on rocks and produce weak acids that can dissolve the
surface of rocks and produce cracks that eventually cause the rock to split apart
SECTION 12.2: RATES OF WEATHERING
A. The processes of weathering generally work very slowly and rocks do not weather at
the same rate; different rates of weathering produces different formations
B. Rock Composition
a. Composition of a rock is a major factor in the rate at which it is weathered
i. _________________ is least affected by the effects of chemical
weathering because it is one of the hardest minerals; resists mechanical
weathering and retains its structure in tiny grains of sand
ii. Limestone and other rocks containing calcite are most rapidly
weathered; resistant to mechanical weathering by easily weathered by
carbonation
iii. Rate of weathering is also dependent on the material that holds the
sediments/particles together
C. Amounts of _______________________ – the more exposure a rock receives to
weathering processes the faster it will weather
a. The amount of time a rock is exposed and the amount of surface area that is
exposed to weathering are important factors
b. Fractures and joints increase the surface area, which increases the rate of
weathering
D. _____________________ – rainfall and freezing/thawing produced by alternating hot
and cold temperatures have the greatest effect on the rate of weathering
a. Weathering occurs slowest in hot dry climates and very cold climates
E. _____________________ – the topography of land, elevation and slope of the surface,
affect the rate of weathering
a. Higher elevations are more rapidly weathered by decreased temperatures and
increased ice wedging
b. Steep slopes are also weathered more quickly because gravity pulls the
fragments towards the earth and new surfaces are introduced to weathering
quickly
SECTION 12.3: WEATHERING AND SOIL
A. Weathering process break and alter all rocks exposed at the surface of the earth
a. ________________________ – layer of weathered rock fragments covering
much of the earth’s surfaced
b. Bedrock – solid, _________________________ rock that lies beneath the
regolith
B. Soil Composition
a. Rock material in soil consists of three types of particles:
i. Clay - < .002 mm diameter
ii. Sand - .06 to 2 mm diameter
iii. Silt - .002 to .06 mm diameter
b. Proportion of these types of particles is largely dependent on the parent
material
i. Large amounts of clay comes from parent material rich in feldspar
ii. Large amounts of sand comes from weathered granite and other rocks
with large amounts of quartz
iii. Silt particles are too small to be seen easily, but are usually found in
soils near riverbanks
c. Transported soil – weathered mineral grains that are carried away and form a
soil in a new location
i. May have a different composition than the bedrock on which it lies
C. Soil Profile – cross section in which layers of soil and the bedrock beneath the soil can
be seen
a. Residual soil – soil that rests on top of parent material
b. Horizons – layers of soil in the soil profile, soil profile generally consist of three
principal horizons
i. ______________________________ – top soil, mixture of organic
matter and rock material, almost all living things that live in the soil
inhabit the A horizon, contains the humus and other organic material
ii. ________________________________ – immediately below the A
horizon, subsoil, contains the minerals leached from the A horizon, may
be considered a zone of accumulation in dry areas
iii. _______________________________ – bottom layer of the soil,
consists of bedrock that has been partially weathered, in the first stages
of mechanical and chemical weathering
D. Soil and Climate
a. Climate is one of the most important factors influencing soil formation, climate
determines the weathering processes that occur in a region
b. Humid tropical climates with a lot of rain and high temperatures allows
chemical weathering to produce thick soils quickly
i. These soils are called laterites – contains iron and aluminum and
doesn’t dissolve easily in water
ii. Constant addition of organic matter sometimes tends to have a layer of
humus on the B horizon
c. Two main types of soil are found in temperate climates
i. Pedalfer – receives more than 65 cm of rain yearly
ii. Pedocal – receives less than 65 cm of rain yearly
1. Very acidic and fertile soil, southeast US
E. Soil and Topography
a. Soils at the top and bottom of the slope tend to be thicker than the soil on the
slope itself
SECTION 12.4: EROSION
A. Erosion – the process by which the products of weathering are transported
B. Most of the eroded soil is deposited elsewhere as transported soil
C. Accelerated Soil Erosion – unwise farming and ranching increase soil erosion
a. _________________________________________________________________
D. Gullying – process of rainwater eroding soil to produce gullies
E. Sheet Erosion – stripping away of parallel layers of topsoil to expose the surface of
subsoil or bedrock
F. Soil Conservation
a. Terracing – creation of _____________________ ridges that follow the
contours of the land; prevent rapid erosion
b. Crop rotation – rotating of crops planted on a yearly basis; stops erosion in its
early stages and allows gullies formed during one season to be filled with soil in
the next
G. Gravity and Erosion
a. Gravity causes rock fragments to move down inclines
b. Movement of fragments down an incline is called mass movement; can either be
rapid or slow
c. Talus – accumulation of fragments at the base of a slope
i. May be reduced to smaller fragments through mechanical and chemical
weathering
H. Rapid Mass Movements
a. Rockfall – fall of rock from a steep cliff
b. Landslide – sudden movement of masses of loose rock and soil down the slope of
a hill, mountain or cliff
c. Mudflow – movement of a large mass of mud
d. __________________________ – movement of a large piece of soil or rock
down a slope in a single piece
I.
Slow Mass Movements
a. Solifluction – occurs in the mountain and arctic climates where there is
permafrost and soil that has thawed above the permafrost and flows down the
slope
b. Solifluction means “soil flow”
c. ___________________________ – extremely slow downhill movement of rock
fragments
J.
Erosion and Landforms
a. Landform – physical features on the Earth’s surface
b. Three major landforms
i. Mountains
ii. Plains
iii. Plateaus
c. Erosion of Mountains
i. Mountains are generally uplifted faster than they are eroded; mountains
being uplifted are rugged with sharp peaks
ii. Peneplain – low almost featureless surface near sea level caused as a
result of erosion
1. New England
iii. Monadnocks – knobs of hard rock (granite) that are resistant to erosion
and protrude above the peneplain
d. Erosion of _____________________ and ______________________
i. Plain – flat landform that is generally not high above sea level
ii. Plateau – flat landform, high elevation; subject to more erosion than a
plain
1. Mesa – smaller fragments of a plateau created by erosion
2. Buttes – smaller fragments of a mesa created by erosion