Download Weathering

Weathering
Objectives
• Distinguish between weathering and erosion.
• Identify variables that affect the rate of weathering.
Vocabulary
– weathering
– chemical weathering
– erosion
– hydrolysis
– mechanical weathering
– oxidation
– frost wedging
– exfoliation
Weathering
Weathering
• Changes occur every day to Earth’s rocks
and surface features.
• Weathering is the chemical and physical
processes by which rocks on or near Earth’s
surface break down and change.
• Erosion is the removal and transport of
weathered material from one location to another.
Weathering
Mechanical Weathering
• Mechanical and chemical weathering are the
two processes that can wear down rocks
and minerals.
• Both types of weathering occur at the same time
on Earth’s landforms.
• Mechanical weathering, or physical weathering,
is the process by which rocks and minerals break
down into smaller pieces without changing their
composition.
• Mechanical weathering does not involve any
change in a rock’s composition.
Weathering
Mechanical Weathering
Temperature
– In many places on Earth’s surface, water collects in the
cracks of rocks and rock layers.
– If the temperature drops to the freezing point of water,
it freezes, expands, exerts pressure on the rocks, and
may cause them to split.
– When the temperature then increases, the ice in the
cracks of rocks and rock layers melts.
– Frost wedging is the repeated thawing and freezing
of water in the cracks of rocks.
Weathering
Mechanical Weathering
Pressure
– Bedrock at great depths is under pressure from the
overlying rock layers.
– When the overlying rock layers are removed, the
pressure on the bedrock below is reduced.
– The bedrock surface, formerly buried, is then able to
expand, and long, curved cracks known as joints
can form that lead to exfoliation.
– Exfoliation is the process by which outer rock layers
are stripped away over time.
– The roots of plants can also exert pressure causing
rocks to split.
Weathering
Chemical Weathering
• Chemical weathering is the process by which
rocks and minerals undergo changes in their
composition as the result of chemical reactions.
• Significant agents of chemical weathering include
water, oxygen, carbon dioxide, and acids.
• Chemical reactions between rocks and water
result in the formation of new minerals and the
release of dissolved substances.
• Some minerals, such as calcite, may
dissolve completely.
Weathering
Chemical Weathering
• Temperature influences the rate at which
chemical reactions occur.
• Generally, chemical reaction rates increase as
temperature increases.
Weathering
Chemical Weathering
Water
– Water is an important agent in chemical weathering
because it can dissolve many kinds of minerals
and rocks.
– Hydrolysis is the chemical reaction of water with other
substances.
Weathering
Chemical Weathering
Oxygen
– Oxidation is the chemical reaction of oxygen with
other substances.
– Iron in rocks and minerals readily combines with this
atmospheric oxygen to form minerals with the oxidized
form of iron as shown in the following reaction.
2Fe3O4 + ½ O2  3Fe2O3
Weathering
Chemical Weathering
Carbon Dioxide
– Carbon dioxide, which is produced by living organisms
during the process of respiration, contributes to the
chemical weathering process.
– When carbon dioxide combines with water in the
atmosphere, it forms a weak carbonic acid that falls to
Earth’s surface as precipitation.
H2O + CO2  H2CO3
– Carbonic acid reacts with minerals such as calcite in
limestone and marble to dissolve rocks and can also
affect silicate minerals such as mica and feldspar.
Weathering
Chemical Weathering
Acid Precipitation
– Acid precipitation is caused mainly by the oxidation of
sulfur dioxide and nitrogen oxides that are released into
the atmosphere by human activities.
– These two gases combine with oxygen and water in the
atmosphere to form sulfuric and nitric acids.
– Acid precipitation
is precipitation
that has a pH
value below
5.6, the pH of
normal rainfall.
Weathering
Chemical Weathering
Acid Precipitation
– Acid precipitation adversely affects fish and aquatic
plant populations in lakes.
– When lake water becomes too acidic, the species
diversity decreases.
Weathering
Chemical Weathering
Acid Precipitation
Weathering
What Affects the Rate of Weathering?
• The natural weathering of Earth materials occurs
very slowly.
• Certain conditions and interactions can
accelerate or slow the weathering process.
Weathering
What Affects the Rate of Weathering?
Climate
– The climate of an area—including precipitation,
temperature, and evaporation—is a major influence
on the rate of chemical weathering.
– The interaction between temperature and precipitation
has the greatest effect on a region’s rate of weathering.
– Chemical weathering occurs readily in climates with
warm temperatures, abundant rainfall, and lush
vegetation.
Weathering
What Affects the Rate of Weathering?
Climate
Weathering
What Affects the Rate of Weathering?
Climate
– Physical weathering occurs readily in cool, dry climates.
– Physical weathering rates are highest in areas where
water undergoes repeated freezing and thawing.
– Because of these differences in their climates, rocks
and minerals in Asheville experience a higher rate of
mechanical and chemical weathering than those in
Phoenix do.
Weathering
What Affects the Rate of Weathering?
Climate
– There is a higher rate of mechanical and chemical
weathering in Asheville than in Phoenix.
Weathering
What Affects the Rate of Weathering?
Rock Type and Composition
– The characteristics of rocks, including how hard or
resistant they are to being broken down, depend on
their type and composition.
– In general, sedimentary rocks are more easily weathered
than harder igneous and metamorphic rocks.
Weathering
What Affects the Rate of Weathering?
Surface Area
– Mechanical weathering breaks up rocks into
smaller pieces.
– As the pieces get smaller, their surface area increases.
– The greater the total surface area, the more
weathering that occurs.
Weathering
What Affects the Rate of Weathering?
Surface Area
Weathering
What Affects the Rate of Weathering?
Topography and Other Variables
– Earth materials on level areas are likely to remain in
place as they undergo changes.
– Materials on slopes have a greater tendency to move
as a result of gravity, thereby exposing underlying rock
surfaces and thus providing more opportunities for
weathering to occur.
– Decaying organic matter and living plant roots release
carbon dioxide, which combines with water to produce
acid, which in turn increases the weathering rate.
Erosion and Deposition
Objectives
• Analyze the impact of living and nonliving things on
the processes of weathering and erosion.
• Describe the relationship of gravity to all agents
of erosion.
Vocabulary
– deposition
– rill erosion
– gully erosion
Erosion and Deposition
Erosion and Deposition
• Erosion is the process that transports Earth
materials from one place to another.
• A number of different agents transport weathered
materials on Earth including water and wind.
• Erosion can result from the loss of plant cover,
which increases the amount of soil lost to wind
and water erosion.
• Deposition is the process of dropping materials
in another location when the movement of
transported materials slows down.
Erosion and Deposition
Gravity’s Role in Erosion
• Gravity is associated with many erosional
agents, because the force of gravity tends to
pull all materials downslope.
• Without gravity, glaciers would not move
downslope and streams would not flow.
Erosion and Deposition
Erosion by Running Water
• With few exceptions, water has more power to
move large particles of weathered material than
wind does.
• Stream erosion is greatest when a large volume
of water is moving rapidly.
• Swiftly flowing water can carry material over a
greater distance.
• Small streams at high elevations flow down to
join larger streams at lower elevations draining
an area called a watershed.
Erosion and Deposition
Erosion by Running Water
• Rill erosion is the erosion by running water in
small channels, on the side of a slope.
• Rills commonly form on a slope.
• Gully erosion is when a rill channel evolves to
become deep and wide.
• Gullies, which can be more than 3 m deep, can be
a major problem in farming and grazing areas.
Erosion and Deposition
Erosion by Running Water
Coastal Deposition and Erosion
– Each year, streams and rivers carry billions of
metric tons of sediments to coastal areas.
– When a river enters a large body of water, the water
slows down and deposits large amounts of sediments
which form deltas.
– The volume of river flow and the action of tides
determine the shapes of deltas, most of which
contain fertile soil.
Erosion and Deposition
Erosion by Running Water
Coastal Deposition and Erosion
– Ocean currents, waves, and tides carve out cliffs,
arches, and other features along the continents’ edges.
– The constant movement of water and the availability of
accumulated weathered material result in a continuous
erosional process, especially along ocean shorelines.
– Sand along a shoreline is repeatedly picked up, moved,
and deposited by ocean currents.
– Sandbars form from offshore sand deposits and can
become barrier islands.
Erosion and Deposition
Erosion by Running Water
Coastal Deposition and Erosion
– Changing tides and conditions associated with coastal
storms can have a great impact on coastal erosion.
– Human development and population growth along
shorelines have led to attempts to control the ocean’s
movements of sand.
– Efforts to keep the sand on one beachfront disrupt the
natural migration of sand along the shore, thereby
depleting sand from another area.
Erosion and Deposition
Glacial Erosion
• Although glaciers currently cover less than ten
percent of Earth’s surface, their erosional effects
are large-scale and dramatic.
• Because they are so dense, glaciers have the
capacity to carry huge rocks and piles of debris
over great distances.
• The erosional effects of glaciers also include
deposition.
Erosion and Deposition
Wind Erosion
• Wind is a major erosional agent in areas on
Earth that experience both limited precipitation
and high temperatures.
• The abrasive action of wind-blown particles can
damage both natural features and human-made
structures.
• Shore areas also experience wind erosion.
• Wind erosion is relatively insignificant when
compared to the erosion accomplished by
running water and glacial activity.
Erosion and Deposition
Wind Erosion
Wind Barriers
– Planting wind barriers, or windbreaks, is one farming
method that reduces the effects of wind erosion.
– Wind barriers are trees or other vegetation planted
perpendicular to the direction of the wind.
– In addition to reducing soil erosion, wind barriers can
trap blowing snow, conserve moisture, and protect
crops from the effects of the wind.
Erosion and Deposition
Erosion by Plants, Animals, And Humans
• As plants and animals carry on their life
processes, they move Earth’s surface materials
from one place to another.
• The effects of erosion by the activities of
plants, animals, and humans are minimal in
comparison to the erosional effects of water,
wind, and glaciers.
Formation of Soil
Objectives
• Describe how soil forms.
• Explain the relationship between the organic and
inorganic components of soil.
• Identify soil characteristics.
• Recognize soil horizons in a soil profile.
Vocabulary
– soil
– soil profile
– residual soil
– soil horizon
– transported soil
Formation of Soil
Formation of Soil
• Soil is essential to life on Earth.
• Humans and other organisms are dependent on
plants, which grow in soil, for food and other
basic needs.
Formation of Soil
Development of Soil
• Except for some steep mountain slopes and
extremely cold regions, soil is found almost
everywhere on Earth’s surface.
• Soil is the loose covering of broken rock
particles and decaying organic matter, called
humus, overlying the bedrock of Earth’s surface.
• Soil is the result of chemical and mechanical
weathering and biological activity over long
periods of time.
• While all soils contain some organic matter, the
amount varies widely among different types of soil.
Formation of Soil
Soil Composition
• Soil forms in layers during the process of its
development.
• The parent rock is the solid bedrock from which
weathered pieces of rock first break off.
• The smallest pieces of weathered rock, along
with living and dead organisms, remain in the
very top layer.
• Rainwater seeps through this top layer of
materials, dissolves soluble minerals, and
carries them into the lower layers of the soil.
Formation of Soil
Soil Composition
• Residual soil is soil located above its
parent bedrock.
• Transported soil is soil that has been moved
to a location away from its parent bedrock by
agents of erosion, such as running water, wind,
and glaciers.
• The parent bedrock determines what kinds of
minerals a soil contains.
• The parent rock and climatic conditions of an
area determine the length of time it takes for
soil to form.
Formation of Soil
Soil Profiles
• A soil profile is the vertical sequence of
soil layers.
• A soil horizon is a distinct layer, or zone, within
a soil profile.
• There are three major soil horizons: A, B, and C.
– Horizon A contains high concentrations of organic
matter and humus.
– Horizon B contains subsoils that are enriched with
clay minerals.
– Horizon C, below horizon B and directly above solid
bedrock, contains weathered parent material.
Formation of Soil
Soil Profiles
Topography
– The topography of a region affects the thickness of
developing soil.
– Soils on slopes tend to be thin, coarse, and infertile.
– Soils formed in lower areas, such as in valleys, are
thick and fertile.
Formation of Soil
Soil Types
• Because climatic conditions are the main influence
on soil development, soils are often classified
based on the climates in which they form.
• The four major types of soil are polar, temperate,
desert, and tropical.
Formation of Soil
Soil Types
Formation of Soil
Soil Types
Polar Soils
– Polar soils form at high latitudes and high elevations in
places such as Greenland, Canada, and Antarctica.
– These soils have good drainage but no distinct
horizons because they are very shallow, sometimes
only a few centimeters deep.
– Permanently frozen ground, called permafrost, is
often present under thin polar soils.
Formation of Soil
Soil Types
Temperate Soils
– Temperate soils vary greatly and are able to support
such diverse environments as forests, grasslands,
and prairies.
– The specific amount of rainfall in an area determines
the type of vegetation that will grow in temperate soils.
– Grasslands, which have an abundance of humus, are
characterized by rich, fertile, soils.
– Forest soils are characterized by less deep and less
fertile soils that contain aluminum-rich clays and
iron oxides.
Formation of Soil
Soil Types
Desert Soils
– Deserts receive low levels of precipitation.
– Desert soils often have a high level of accumulated salts
and can support only a limited amount of vegetation.
– Desert soils have little or no organic matter and a very
thin A horizon, but they often have abundant nutrients.
– Desert soils are also light-colored, coarse, and may
contain salts and gypsum.
Formation of Soil
Soil Types
Tropical Soils
– Tropical areas experience high temperatures and
heavy rainfall, leading to the development of intensely
weathered and often infertile soil.
– The intense weathering combined with a high degree
of bacterial activity leave tropical soils with very little
humus and very few nutrients.
– These soils experience much leaching of soluble
materials, such as calcite and silica, but they have
high concentrations of iron and aluminum.
Formation of Soil
Soil Textures
• Particles of soil are classified
according to size as being
clay, silt, or sand, with clay
being the smallest and sand
being the largest.
• The relative proportions of
these particle sizes determine
a soil’s texture.
• The texture of a soil affects its capacity to
retain moisture and therefore its ability to
support plant growth.
Formation of Soil
Soil Textures
Formation of Soil
Soil Fertility
• Soil fertility is the measure of how well a soil can
support the growth of plants.
• Factors that affect soil fertility include:
– Availability of minerals and nutrients
– Number of microorganisms present
– Amount of precipitation available
– Topography
– Level of acidity
Formation of Soil
Soil Fertility
Soil Color
– A soil’s composition and the climate in which it develops
are the main factors that determine a soil’s color.
– Topsoil is usually dark-colored because it is rich
in humus.
– Red and yellow soils may be the result of oxidation of
iron minerals.
– Yellow soils are usually poorly drained and are often
associated with environmental problems.
– Grayish or bluish soils are common in poorly drained
regions where soils are constantly wet and lack oxygen.