Transcript Chapter 14

Chapter 14
Weathering and Erosion
Section 1: Weathering Processes
A. Weathering
1. weathering the natural process by which
atmospheric and environmental agents, such
as wind, rain, and temperature changes,
disintegrate and decompose
2. 2 types
a. mechanical
b. chemical
B. mechanical weathering the process by which
rocks break down into smaller pieces by
physical means
1. Common agents of mechanical weathering
are ice, plants and animals, gravity, running
water, and wind.
2. Ice Wedging
a. occurs in cold climates
b. occurs when water seeps into the cracks in
rock and freezes.
c. When the water freezes, its volume
increases by about 10% and creates
pressure on the surrounding rock.
d. eventually splits the rock apart.
3. abrasion the grinding and wearing away of
rock surfaces through the mechanical
action of other rock or sand particles
a. caused by gravity, running water, and
wind.
C. Organic Activity
1. As plants grow, the roots grow and expand
to create pressure that wedge rock apart.
2. Earthworms and other animals that move
soil expose new rock surfaces to both
mechanical and chemical weathering.
D. Chemical Weathering
1. Chemical weathering changes both the
composition and physical appearance of
the rock
2. oxidation the process by which an element
combines with oxygen
a. commonly occurs in rock that has ironbearing minerals, such as hematite
and magnetite.
b. 4Fe + 3O2  2Fe2O3
3. hydrolysis a chemical reaction between
water and another substance to form two
or more new substances
4. carbonation the conversion of a compound
into a carbonate
5. acid precipitation precipitation, such as
rain, sleet, or snow, that contains a high
concentration of acids, often because of
the pollution of the atmosphere
6. Acid precipitation weathers rock faster than
ordinary precipitation does.
Section 2: Rates of Weathering
A. Rates of Weathering depends on
1. rock composition,
2. climate
3. topography.
B. differential weathering - the process by
which softer, less weather resistant rocks
wear away at a faster rate than harder, more
weather resistant rocks
C. Rock Composition
1. Limestone and other sedimentary rocks that
contain calcite are weathered most rapidly.
2. shales and sandstones that are not firmly
cemented together gradually break up to
become clay and sand particles
D. Amount of Exposure
1. Surface Area
a. The part of a rock that is exposed to air, water,
and other agents of weathering is called
the rock’s surface area.
b. As a rock breaks into smaller pieces, the
surface area that is exposed increases.
2. Fractures and Joints
a. These structures are natural zones of
weakness within the rock.
b. increase the surface area of a rock and allow
weathering to take place more rapidly.
c. form natural channels through which
water flows.
E. Climate
1. climates that have alternating periods of
hot and cold weather allow the fastest
rates of weathering.
2. slowest rates of weathering occur in hot,
dry climates.
F. Topography
Because temperatures are generally cold at high
elevations, ice wedging is more common at high
elevations than at low elevations
G. Human Activities
1. Mining and construction often expose rock
surfaces to agents of weathering.
2. exposes rock to strong acids and other chemical
compounds that are used in mining processes.
3. Recreational activities such as hiking or riding allterrain vehicles can also speed up weathering by
exposing new rock surfaces.
H. Plant and Animal Activities
1. Rock that is disturbed or broken by plants
or animals also weathers more rapidly
than undisturbed rock does.
2. The roots of plants and trees often break
apart rock.
3. Burrowing animals dig holes into rock and
soil.
Section 3: Soil
A. Soil
1. soil a loose mixture of rock fragments and
organic material that can support the
growth of vegetation
2. Bedrock is the solid, unweathered rock that
lies beneath the soil.
B. Soil Composition
1. refers to the materials of which it is made.
2. The color of soil is related to the
composition of the soil.
3. Soil moisture can also affect color.
C. Soil Texture
1. Rock material in soil consists of three main
types:
a. Clay particles have a diameter of less
than 0.004 mm.
b. Silt particles have a diameter from 0.004
to 0.06 mm.
c. Sand particles have diameters from 0.06
to 2mm.
2. The proportion of clay, silt, and sand in soil
depends on the soil’s parent rock.
D. Soil Profile
1. soil profile a vertical section of soil that
shows the layers of horizons
2. humus dark, organic material formed in soil
from the decayed remains of plants and
animals
3. The A horizon, or topsoil, is a mixture of
organic materials and small rock particles.
4. The B horizon or subsoil, contains the
minerals leached from the topsoil, clay,
and sometimes, humus.
5. The C horizon consists of partiallyweathered bedrock.
E. Soil and Climate
1. Climate is one of the most important factors
that influences soil formation.
2. Climate determines the weathering
processes that occur in a region
F. Soil Erosion
1. Soil erosion is considered by some scientists
to be the greatest environmental problem
that faces the world today.
2. This erosion prevents some countries from
growing the crops needed to prevent
widespread famine.
3. One farming technique that can accelerate
soil erosion is the plowing of furrows, or
long, narrow rows -> these become gullies
4. sheet erosion the process by which water
flows over a layer of soil and removes the
tops