Download Weathering and Erosion Section 1

Weathering and Erosion
Section 1
Identify three agents of mechanical
weathering.
 Compare mechanical and chemical
weathering processes.
 Describe four chemical reactions that
decompose rock.

Weathering and Erosion
Section 1
Surface rocks undergo changes in their
appearance & composition.
Weathering - natural process by which
atmospheric & environmental agents (wind,
rain, & temperature changes) disintegrate
& decompose rocks
 There are two main types of weathering
processes:

› mechanical weathering
› chemical weathering
Weathering and Erosion
Section 1
Mechanical weathering - the process by
which rocks break down into smaller pieces
by physical means
 Mechanical weathering is strictly a physical
process & does not change the
composition of the rock.
 Common agents of mechanical
weathering are:
› ice, plants and animals, gravity, running water,
and wind
Weathering and Erosion
Section 1
A type of mechanical weathering that
occurs in cold climates is called ice
wedging.
 Ice wedging occurs when water seeps
into the cracks in rock and freezes.
 Its volume increases by about 10% &
creates pressure on the rock.
 It eventually splits the rock apart.

Weathering and Erosion
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Abrasion - the grinding and wearing away of rock
surfaces through the mechanical action of other
rock or sand particles
 Gravity causes loose soil & rocks to move down the
slope of a hill.
 As rocks fall and collide, they break into smaller
pieces.
 Water can carry sand or rock particles that scrape
against each other.
 Wind lifts & carries small particles, it can hurl them
against surfaces, such as rock.
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
Plants & animals are important agents of
mechanical weathering.

As plants grow, their roots grow & create
pressure that wedge rock apart.

The digging of animals affect the rate of
weathering.
› Earthworms move soil & expose new rock
surfaces to both mechanical & chemical
weathering.
Weathering and Erosion
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Chemical weathering - the process by
which rocks break down as a result of
chemical reactions
 Commonly occur between rock, water,
carbon dioxide, oxygen, & acids.
 Chemical weathering changes both the
composition and physical appearance
of the rock.
Weathering and Erosion
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2.
3.
4.
Oxidation
Hydroylis
Carbonation
Acids
Section 1
Weathering and Erosion
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Oxidation - the process by which a metallic element
combines with oxygen
 Commonly occurs in rock that has iron-bearing
minerals.
 Iron in rocks and soil combine with oxygen that is
dissolved in water to form rust, or iron oxide.
 The red color of much of the soil in the southeastern
United States is due to mainly the presence of iron
oxide produced by oxidation.
Weathering and Erosion
Section 1
Hydrolysis - a chemical reaction between
water & another substance to form two
or more new substances
 Minerals that are affected by hydrolysis
often dissolve in water.
 Water can then carry the dissolved
minerals to lower layers of rock in a
process called leaching.
Weathering and Erosion
Section 1
The image below shows how water plays a
crucial role in chemical weathering.
Weathering and Erosion
Section 1
Carbonation
 When carbon dioxide from the air
dissolves in water a weak acid called
carbonic acid forms.
 It speeds up the process of hydrolysis.
Carbonation - the conversion of a
compound into a carbonate
Weathering and Erosion
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Organic Acids
 Acids are produced naturally by certain
living organisms.
 Lichens & mosses grow on rocks and
produce weak acids that can weather
the surface of the rock.
Weathering and Erosion
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Rainwater is slightly acidic because of small
amounts of carbon dioxide.
 When fossil fuels are burned, nitrogen oxides
and sulfur dioxides are released into the air.
 These combine with water to produce nitric
acid, nitrous acid, or sulfuric acid.
Acid precipitation - precipitation, such as rain,
sleet, or snow, that contains a high
concentration of acids, often because of
the pollution of the atmosphere

Weathering and Erosion
Section 1
Explain how rock composition affects the
rate of weathering.
 Discuss how surface area affects the
rate at which rock weathers.
 Describe the effects of climate and
topography on the rate of weathering.

Weathering and Erosion
Section 1
Mechanical & chemical weathering work very
slowly.
 The rate at which rock weathers depends on:
› rock composition, climate, & topography
Differential weathering - process by which softer, less
weather resistant rocks wear away at a faster rate
than harder, more weather resistant rocks do.

Ex. The chemical composition & crystal structure of
quartz makes it resistant to chemical & mechanical
weathering.
 Ex. Rocks that contain calcite are weathered
rapidly because they commonly undergo
carbonation.

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Surface Area
 The part of a rock that is exposed to air,
water, and other agents of weathering is
called the rock’s surface area.
 As a rock breaks into smaller pieces, the
surface area that is exposed increases.
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The image below shows the ratio of total
surface area to volume.
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Most rocks on Earth’s surface contain
natural fractures & joints.
› natural zones of weakness within the rock
› increase the surface area of a rock
› allow weathering to take place more rapidly
› form natural channels through which water
flows
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Climates that have alternating periods of hot & cold
allow fastest rates of weathering.
 In warm, humid climates, chemical weathering is
also rapid.
› Constant moisture is highly destructive to
exposed surfaces
 Slowest rates of weathering occur in hot, dry
climates.
› lack of water limits processes like carbonation &
ice wedging.
 Weathering is also slow in very cold climates.

Weathering and Erosion
Section 1
Topography is the elevation & slope of the
land
› influences the rate of weathering
 Temperatures are generally cold at high
elevations, so ice wedging is more
common.
 On steep slopes,(mountainsides), rocks are
pulled downhill by gravity & washed out by
rains.
› So new surfaces of the mountain are
continually exposed to weathering.

Weathering and Erosion
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Mining & construction often expose rock
surfaces to weathering.
 Mining can expose rock to strong acids &
other chemical compounds used in mining
processes.
 Recreational activities (hiking or riding allterrain vehicles) can also speed up
weathering.
 Rock that is disturbed or broken by human
activities weathers more rapidly than
undisturbed rock does.

Weathering and Erosion
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The roots of plants & trees often break
apart rock.
 Burrowing animals dig holes, exposing
new rock surfaces.
 Biological wastes of animals can cause
chemical weathering.
 Insects also can speed up mechanical
weathering.

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
Summarize how soil forms.

Explain how the composition of parent
rock affects soil composition.

Describe the characteristic layers of
mature residual soils.

Predict the type of soil that will form in
arctic and tropical climates.
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Regolith - a layer of weathered rock
fragments that covers much of Earth’s
surface.
Bedrock -is the solid, unweathered rock that
lies beneath the regolith.
Soil - a loose mixture of rock fragments &
organic material that can support the
growth of vegetation
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
The characteristics of soil depend on the
rock from which the soil was weathered,
the soil’s parent rock.

Soil that forms & stays directly over its
parent rock is called residual soil.
› Kids that never leave home are residual. LOL

Soil that results from the deposition of
other materials is called transported soil.
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
Soil composition refers to the materials from which
the soil is made.

The color of soil is related to the composition of the
soil.

Rock material in soil consists of 3 types:
› Clay - diameter of less than 0.002 mm
› Silt - diameter from 0.002 to 0.05 mm
› Sand - diameters from 0.05 to 2 mm

The proportion of clay, silt, & sand in soil depends
on the soil’s parent rock.
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Transported soils are commonly deposited
in unsorted masses.
 Residual soils commonly develop distinct
layers over time.

Soil profile - vertical section of soil that shows the
layers, or horizons
Horizon - horizontal layer of soil that can be
distinguished from the layers above and below it
humus -dark, organic material formed in soil from the
decayed remains of plants and animals
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Residual soils consist of three main
horizons:
1. The A horizon, or topsoil, is a mixture of
organic materials & small rock particles.
2. The B horizon or subsoil, contains the
minerals leached from the topsoil, clay,
& sometimes, humus.
3. The C horizon consists of partiallyweathered bedrock.
Weathering and Erosion
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The image below shows the soil horizons
of residual soils.
Weathering and Erosion
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
Climate is one of the most important factors that
influences soil formation because it determines the
weathering processess

In humid tropical climates, where much rain & high
temperatures causes thick soils to develop rapidly.
› Heavy rains cause a lot of leaching of the topsoil, & keep the A
horizon thin.
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
In temperate climates, temperatures range
between cool & warm, and rainfall is not excessive
› both mechanical & chemical weathering occur

Temperate soils have the thickest A horizon.
Pedalfer soil is a type of temperate soil that
contains clay, quartz, & iron compounds.
 Pedocal soil contains large amounts of calcium
carbonate, which makes it very fertile & less acidic
than pedalfer soil.

Weathering and Erosion
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
In desert and arctic climates, rainfall is
minimal & chemical weathering occurs
slowly.

The soil is thin & mostly regolith

Desert and arctic climates are also often
too warm or too cold to sustain life, so
their soils have little humus.
Weathering and Erosion
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
Because rainwater runs down slopes, much of the
topsoil washes away.

So. . . the soil at the top & bottom of a slope tends
to be thicker than the soil on the slope.

Topsoil that remains on a slope is often too thin to
support dense plant growth.

A fairly flat area that has good drainage provides
the best surface for the formation of thick, fertile
layers of residual soil.
Weathering and Erosion
Section 1
Define erosion, and list four agents of
erosion.
 Identify four farming methods that
conserve soil.
 Discuss two ways that gravity contributes
to erosion.
 Describe the three major landforms
shaped by weathering and erosion.

Weathering and Erosion
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Erosion - process in which the materials of
Earth’s surface are loosened, dissolved,
or worn away & transported from one
place to another by a natural agent,
such as wind, water, ice, or gravity
Weathering and Erosion
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Usually, new soil forms about as fast as
existing soil erodes.
 Land use & unusual climactic conditions
can upset this natural balance.
 Soil erosion is considered by some scientists
to be the greatest environmental problem
that faces the world today.
 This erosion prevents some countries from
growing the crops needed to prevent
widespread famine.

Weathering and Erosion
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The image below shows a map of soil
vulnerability worldwide to erosion by water.
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Sheet erosion the process by which water
flows over a layer of soil and removes the
topsoil
› occurs where continuous rainfall washes
away layers of the topsoil.
› Wind also can cause sheet erosion during
unusually dry periods.
Weathering and Erosion
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Reduces the fertility of the soil by
removing the A horizon, which contains
the fertile humus.
 Without plants, the B horizon has nothing
to protect it from further erosion.
 So, within a few years, all the soil layers
could be removed by continuous erosion.

Weathering and Erosion
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Human activities, including certain
farming and grazing techniques and
construction projects, can also increase
the rate of erosion.
 This land clearing removes protective
ground cover plants and accelerates
topsoil erosions.
 Rapid, destructive soil erosion can be
prevented by soil conservation methods.

Weathering and Erosion
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Contour Plowing
 Farmers in countries around the world
use planting techniques to reduce soil
erosion.
 In one method, called contour plowing,
soil is plowed in curved bands that follow
the contour, or shape of the land.
 This method of planting prevents water
from flowing directly down slopes, so the
method prevents gullying.
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Strip-Cropping
 In strip-cropping, crops are planted in
alternating bands.
 A crop planted in rows may be planted
in one band, and another crop that fully
covers the surface of the land will be
planted next to it.
 The cover crop protects the soil by
slowing the runoff of rainwater.
 Strip-cropping is often combined with
contour plowing. The combination of
these two methods can reduce soil
Weathering and Erosion
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Terracing
 The construction of steplike ridges that
follow the contours of a sloped field is
called terracing.
 Terraces, especially those used for
growing rice in Asia, prevent or slow the
downslope movement of water and thus
prevent rapid erosion.
Weathering and Erosion
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In crop rotation, farmers plant one type
of crop one year & a different type of
crop the next.
 The main purpose of other types of crop
rotation is to help maintain soil fertility.

› For example, a farmer might plant a crop
that will be harvested one year, and then
plant a cover crop the next year. The cover
crop does not get harvested. It helps to slow
runoff and hold the soil in place.
Weathering and Erosion
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Gravity causes rock fragments to move
down a slope.
Mass movement -the movement of a large
mass of sediment or a section of land
down a slope
 Some occur rapidly, & others occur very
slowly.

Weathering and Erosion
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The most dramatic & destructive mass
movements occur rapidly.
 The fall of rock from a steep cliff is called
a rockfall.

› fastest kind of mass movement
Weathering and Erosion
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When masses of loose rock combined
with soil suddenly fall down a slope, the
event is called a landslide.
› Heavy rainfall, spring thaws, volcanic
eruptions, & earthquakes can trigger
landslides.
Weathering and Erosion
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The rapid movement of a large amount
of mud creates a mudflow.
› occur in dry, mountainous regions during
sudden, heavy rainfall or as a result of
volcanic eruptions.
› frequently spreads out in a large fan shape
at the base of the slope.
Weathering and Erosion
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Sometimes, a large block of soil & rock
becomes unstable and moves downhill
in one piece.
 The block of soil then slides along the
curved slope of the surface.
 This type of movement is called a slump.

› occurs along very steep slopes
› Saturation by water & loss of friction within
underlying rock causes loose soil to slip
downhill over the solid rock.
Weathering and Erosion
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Solifluction -the slow, downslope flow of soil
saturated with water in areas
surrounding glaciers at high elevations
› in arctic & mountainous climates where the
subsoil is permanently frozen.
› When top layers thaw, it becomes muddy
and slowly flows down.
Weathering and Erosion
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Creep - the slow downhill movement of
weathered rock material
› moves the most soil of all types of mass
movements
› may go unnoticed unless buildings, fences,
or other surface objects move along with the
soil.
Weathering and Erosion
Section 1
Landforms – a physical feature of Earth’s
surface.
 Three major landforms shaped by
weathering & erosion:
› mountains, plains, & plateaus

Minor landforms include:
› hills, valleys, & dunes