Chapter Five

Weathering, Soil, and Mass

Movements

Weathering

• Weathering is a basic part of the rock cycle and a key process in the Earth system.

– Weathering: The breaking down and changing of rocks at or near Earth’s surface.

• There are two types of weathering:

1. Mechanical Weathering

2. Chemical Weathering

• Even though they are different, they at are work at the same time.

WeatheringMechanical Weathering

• Occurs when physical forces break rock into smaller and smaller pieces without changing the rock’s mineral composition.

– Each piece has the same composition.– Breaking a rock into smaller pieces increases the

surface area of the rock.• In nature, three physical processes are especially

important causes of mechanical weathering: 1. Frost wedging2. Unloading3. Biological activity

Weathering1. Frost Wedging:• When liquid freezes, it expands by about 9%,

exerting a tremendous outward force.• When water freezes in the cracks of rocks, it

enlarges the cracks.• This process is known as frost wedging.• It is most common in mountainous regions in the

middle latitudes.• Sections of rock that are wedged loose may

tumble into large piles called talus, which typically form at the base of steep, rocky cliffs.

Weathering

Weathering2. Unloading:• Large masses of igneous rocks may be exposed through

uplift and erosion of overlying rock. • When this happens, the pressure exerted on the igneous

rock is reduced.• This is known as unloading.• Unloading causes the outer layers of the rock to expand

more than the rock below.• Slabs of outer rock separate like layers of an onion and break

loose in a process called exfoliation.– Especially common in granite.

• It often produces large, domed shaped rock formations.– Ex: Stone Mountain in Georgia and Liberty Cap in Yosemite National

Park.

Weathering

Weathering

3. Biological Activity:• The activities of organisms, including plants,

burrowing animals, and humans can also cause mechanical weathering.

• Examples:– Tree roots.– Burrowing animals move rocks to the surface.– Decaying organisms produce acids which cause chemical

weathering.– Humans accelerate weathering through deforestation and

blasting.

Weathering

A. Differential Weathering

B. Chemical Weathering

C. Mechanical Weathering

D. Erosion

The process that occurs when physical forces break rock into

smaller pieces without changing the rock’s chemical

composition is called

A. Unloading

B. Frost Wedging

C. Exfoliation

D. Spheroidal Weathering

Which of the following weathering processes involves the constant

freezing and thawing of water?

Which of the following is not associated with

mechanical weathering?

A. Frost Wedging

B. Unloading

C. Biological Activity

D. Reactions with Oxygen

What is responsible for the formation of exfoliation

domes?

A. Frost Wedging

B. Biological Activity

C. Reactions with Oxygen

D. Unloading

What type of mechanical weathering is most

common in mountainous regions in the middle

latitudes?A. Frost Wedging

B. Biological Activity

C. Oxidation

D. Unloading

When water freezes, its volume

A. Decreases slightly.

B. Increases.

C. Stays the same.

D. Decreases greatly.

WeatheringChemical Weathering

• Chemical weathering is the transformation of rock into one or more new compounds.

• The new compounds remain mostly unchanged as long as the environment in which they formed does not change.

• Water is the most important agent of chemical weathering.• Water promotes chemical weathering by absorbing gases

from the atmosphere and the ground.• These dissolved substances then chemically react with

various minerals.• Oxygen dissolved in water reacts easily with certain

minerals , forming oxides.– Ex: Iron-rich minerals get a yellow to reddish-brown coating of iron

oxide (rust) when they react with oxygen.

Weathering

• Water also absorbs carbon dioxide when rain falls through the atmosphere.

• Water that seeps into the ground also picks up carbon dioxide.

• The dissolved carbon dioxide forms carbonic acid, which reacts with many common minerals.– Carbonic acid is a weak acid found in carbonated

drinks.

Weathering

• Water can also absorb the sulfur oxides in the atmosphere produced by the burning of fossil fuels (coal and petroleum).

• Through a series of chemical reactions, these pollutants are changed into acids that cause acid precipitation.

• Acid precipitation accelerates the chemical weathering of stone monuments and structures.

Weathering

Weathering

• When granite (feldspar/quartz) is exposed to water containing carbonic acid, the feldspar is converted mostly into clay minerals.

• As the feldspar is converted into clay, the quartz grains are released from the granite.

• Sometimes it is then transported by rivers to the ocean where the tiny particles are carried far from shore and the quartz crystals are deposited near the shore where they become the main component of beaches and sand dunes.

Weathering

• When silicate minerals undergo chemical weathering, the sodium, calcium, potassium, and magnesium they contain dissolve and are carried away by groundwater.

• The three remaining elements are aluminum, silicon, and oxygen which usually combine with water and produce clay minerals.

Weathering

Weathering• Chemical weathering can change the physical

shape of rock as well as its chemical composition.– Ex: When water enters along the joints in a rock, it

weather the corners and edges most rapidly. As a result, the corners and edges become more rounded.

• This process is called spheroidal weathering.– The rock takes on a spherical shape.

• Spheriodal weathering sometimes causes the outer layers of a rock to separate from the rock’s main body.

• Similar to exfoliation except that they are chemically weathered.

Weathering

Which of the following is not related to chemical

weathering?

A. Frost wedging.

B. Dissolution

C. Reactions with oxygen

D. Reactions with water

The gradual rounding of the corners and edges of

angular blocks of rock is called

A. Exfoliation

B. Unloading

C. Spheroidal Weathering

D. Mechanical Weathering

Which of the following is the result of chemical

weathering?A. A rock that has been changed into one or

more new compounds.

B. A rock that has been broken into tiny pieces.

C. A rock that has been split in two.

D. A rock that has lost its outer layers.

The chemical weathering of feldspar produces

A. Quartz.

B. Iron oxide

C. Clay minerals

D. Calcium carbonate

Whenever the characteristics and

chemical composition of weathered materials have been altered, they have

undergoneA. Chemical weathering.

B. Mechanical weathering.

C. Mass movement.

D. Frost wedging.

The atmospheric gas that forms a mild acid when

dissolved in water is

A. Carbon Dioxide

B. Oxygen

C. Aluminum

D. Sulfur

Weathering

• Mechanical weathering affects the rate of chemical weathering.

• By breaking rocks into smaller pieces, chemical weathering is increased due to the increased amount of surface area of the rock.

• Two other factors that affect the rate of weathering are:

1. Rock characteristics2. Climate

Weathering

1. Rock Characteristics:• Physical characteristics of rock (cracks) are

important in weathering because they influence the ability of water to penetrate rock.

• Mineral composition also affects the rate of weathering.

– Ex: Granite vs. Marble; granite is relatively resistant to chemical weathering while marble is not very resistant to chemical weathering (reacts with weak acids).

Weathering

2. Climate:

• Climatic factors, especially temperature and moisture, have a strong effect on the rate of weathering (mechanical and chemical).

• The climate most favorable for chemical weathering has high temperature and abundant moisture.– Slow in arid and polar regions.

Weathering

• Different parts of a rock mass often weather at different rates.

• This process, called differential weathering, has several causes.

• Differences in mineral composition in one cause.– More resistant rock protrudes as pinnacles, or high peaks.

• Variations in the number and spacing of cracks in different parts of a rock mass is another cause of differential weathering.

Weathering

What would cause the inscription on a marble gravestone to become

harder and harder to read over time?

A. Frost wedging

B. Mechanical weathering

C. Exfoliation

D. Chemical weathering

Which of the following is not a factor that affects the rate of weathering in

rocks?

A. Biological evolution

B. Climate

C. Rock characteristics

D. Amount of exposed surface area

Which of these factors affects the rate of

weathering?A. Climate

B. Chemical composition of the exposed rock.

C. Surface area of the exposed rock.

D. All of the above.

Rock features such as the sculpted pinnacles seen in

Bryce Canyon National Park are the result of

A. Exfoliation.

B. Differential Weathering.

C. Unloading.

D. Frost Wedging.

Chemical weathering would be

A. Most effective in a warm, dry climate.

B. Most effective in a cold, dry climate.

C. Most effective in a warm, humid climate.

D. Equally effective in any climate.

If granite and marble were exposed in an area with a hot and humid climate,

A. The granite would weather most rapidly.

B. The marble would weather most rapidly.

C. Both rocks would weather at the same rate.

D. Neither rock would become weathered.

Weathering

Soil• Soil is an important product of weathering.• All life depends on a dozen or so elements that

come from Earth’s crust.• Weathering produces a layer of rock and mineral

fragments called regolith.• Soil is the part of the regolith that supports the

growth of plants.• Three important characteristics of soil are its:1. Composition2. Texture3. Structure

Soil

1. Soil Composition:• Soil has four major components:

– Mineral Matter (Broken-down rock)– Organic Matter (Humus – decayed organisms)– Water– Air

• The percentages of the four major components varies greatly.

• In most soils, organic matter (humus) is an essential component for plants nutrients and the soil’s ability to retain water.

Soil

Soil

2. Soil Texture:• Most soils contain particles of different sizes.• Soil texture refers to the proportions of

different particles sizes.• To classify soil texture, the U.S. Department

of Agriculture has established categories based on the percentages of clay, silt, and sand in soil.

• Texture strongly influences a soil’s ability to support plant life.

Soil

Soil

3. Soil Structure:• Soil particles usually form clumps that give

soils a particular structure.• Soil structure determines how easily a soil

can be cultivated and how susceptible it is to erosion.

• Soil structure also affects the ease with which water can penetrate the soil and thus influences the movement of nutrients to plant roots.

Which of the following is not a major component of

soil?A. Mineral matter

B. Air

C. Humus

D. Earthworms

A soil’s texture is determined by

A. Mineral composition

B. Type of humus

C. Water content

D. Particle size

The main source of organic matter in soil is

A. Water

B. Plants

C. Fungi

D. Bacteria

Soil

• Soil forms through the complex interaction of several factors.

• The most important factors in soil are:– Parent material– Time– Climate– Organisms– Slope

Soil1. Parent Material:• The source of the mineral matter in soil.

– May be either bedrock (residual soil) or unconsolidated deposits such as those in a river valley (transported soil).

• The nature of the parent material influences soils in two ways:

– It affects the rate of weathering and the rate of soil formation.

– The chemical makeup of the parents material affects the soil’s fertility.

• Fertility influences the types of plants the soil can support.

Soil

Soil

2. Time:

• The longer a soil has been forming, the thicker it becomes.

• The parent material largely determines the characteristics of young soils.

• As weathering continues, the influence of the parent material can be overshadowed by the other factors, especially climate.

Soil

3. Climate:• Climate has the greatest effect on soil formation.• Variations in temperature and precipitation influence

the rate, depth, and type of weathering.• The influence of climate is so great that soil

scientists have found that similar soils can be produced from different parent materials in the same climate.

• Dissimilar soils can be produced from the same parent material in different climates.

Soil4. Organisms:• The types of organisms and how many there are in a soil

have a major impact on its physical and chemical properties.• Scientists name some soils – such as prairie soil, forest soil,

and tundra soil – based on the soil’s natural vegetation.• Plants are the main source of organic matter in soil

(animals/microorganisms are other sources).• Microorganisms (fungi, bacteria, and single-celled

protozoans) play an active role in decomposing dead plants and animals (nitrogen gas into nitrogen compounds).

• Burrowing animals mix the mineral and organic matter in soil.– Example: Earthworms – can mix thousands of kilograms of soil each

year in a single hectare (10,000 square meters).

Soil5. Slope:• The slope of the land can vary greatly over short

distances, which results in different soil types.• Many of the differences are related to the amount of

erosion and the water content of the soil.• On steep slopes, erosion is accelerated.

– Little water soaks in, little to no plants, which results in thin or nonexistent soils.

• In flat areas, there is little erosion and poor drainage.– Resulting in waterlogged soils that are typically thick and

dark (large amounts of organic matter).

Soil

• The direction the slope faces also affects soil formation.

• In the temperate zone of the Northern Hemisphere, south-facing slopes receive much more sunlight than do north-facing slopes.

• Soils on south-facing slopes are usually warmer and drier, which influences the types of plants that grow in the soil.

Soil

The factor that has the greatest effect on soil

formation isA. Climate

B. Parent material

C. Time

D. Slope orientation

In which of the following areas will soil formation be

greatest?A. A steep slope in a warm, wet climate.

B. A flat area in a cold, wet climate.

C. A flat area in a warm, wet climate.

D. A north-facing area on a steep slope.

Soil that forms on unconsolidated deposits is

calledA. Transported soil

B. Humus

C. Residual soil

D. Bedrock

Soil• The processes that form soil operate from the

surface downward.• Soil varies in composition, texture, structure, and

color at different depths.• These variations divide the soil into zones known as

soil horizons.• A vertical section through all of the soil horizons is

called a soil profile.• In some soil profiles, the soil horizons blend

gradually from one to another, in others, they are quite distinct.

• Mature soils usually have three distinct soil horizons (A, B, and C Horizons).

Soil1. A Horizon:• Commonly known as topsoil.• Upper part consists mostly of organic matter.• Full of insects, fungi, and microorganisms.• Lower part is a mixture of mineral matter and organic matter.2. B Horizon:• Commonly known as subsoil.• Contains fine clay particles washed out of the A Horizon.• In some soils, the clay that accumulates in the B Horizon forms a

compact, impenetrable layer called hardpan.• Is the lower limit of most plant roots and burrowing animals.3. C Horizon:• Between the B Horizon and the unaltered parent material.• Contains partially weathered parent material.• Resembles parent material.

Soil

In a well-developed soil profile, which horizon is the

uppermost layer?A. The C Horizon.

B. The B Horizon.

C. The A Horizon.

D. The Parent Horizon.

What kind of material is found in the C horizon of a

soil profile?A. Partially weathered parent material.

B. Clay Particles.

C. Hardpan.

D. Mineral and organic matter.

How are soil horizons ordered from the top of the profile to

the bottom?A. A, C, B

B. A, B, C

C. C, B, A

D. B, A, C

The B horizon is also called the

A. Topsoil

B. Unaltered parent material

C. Partially altered parent material

D. Subsoil

Soil• Climate has a major effect on the type of soil that forms.• Three common types of soil are:

– Pedalfer– Pedocal– Laterite

1. Pedalfer:• Usually forms in temperate areas that receive more than

63-cm (25 inches) of rain each year.• Present in much of the eastern half of the U.S., mostly in

forested regions.• The B Horizons in pedalfers contain large amounts of iron

oxide and aluminum-rich clays, giving it a brown to red-brown color.

Soil2. Pedocal:• Found in the drier western U.S. in areas that have grasses and brush

vegetation.• Generally contains less clay than pedalfers.• Contain abundant calcite, or calcium carbonates, and are typically a light

gray-brown.3. Laterite:• Form in hot, wet tropical areas.• Are usually deeper than soils that develop over a similar period in

temperate areas.• The large quantity of water that filters through these soils removes most

of the calcite and silica.• Iron oxide and aluminum oxide are left behind.• The iron oxide gives laterite a distinctive orange to red color.• When dried, laterite becomes very hard and practically waterproof

(bricks).

Soil

Soil

• Laterite contains almost no organic material.

• With the lack of organic material, the soil cannot support agriculture for more than a few years.

• The nutrients that the soil does have are quickly washed out by the plentiful rainwater that filters through the soil.

A soil that is characteristic of the humid eastern U.S. is

A. Laterite.

B. Pedalfer.

C. Pedocal.

D. Humus.

The soil associated with the hot and wet tropics is

A. Laterite

B. Pedocal

C. Pedalfer

D. Bedrock soil

Laterite soils contain high amounts of

A. Organic material

B. Iron oxide

C. Calcite

D. Calcium carbonate

Pedalfer soils would most likely be found

A. On an island close to the equator.

B. In a tropical rainforest.

C. In the dry areas of the western U.S.

D. In the eastern half of the U.S.

Which of the following is not true of laterite soils?

A. They form in the wet tropics.

B. They are red in color.

C. They are enriched in iron oxide.

D. They are very productive agriculturally.

Soil

• Soils are among our most abused resources.

• The loss of fertile topsoil is a growing problem as human activities disturb more of Earth’s surface.

• Water erodes soil every time it rains (tiny bombs).

• When water flows across the surface it then carries away dislodged particles, which is called sheet erosion.

Soil

Soil

• After flowing as a thin sheet for a short distance, the water forms tiny streams called rills.

• As more water enters the rills, they erode the soil further, creating trenches known as gullies.

Soil

Soil• Human activities that remove natural vegetation,

such as farming, logging, and construction, have greatly accelerated erosion.

• Without plants, soil is more easily carried away by wind and water.

• Scientists can estimate the rate of erosion due to water by measuring the amount of sediment in rivers.

• These estimates indicate that before humans appeared, rivers carries about 9 trillion kilograms of sediment to the oceans each year.

• The amount of sediment currently transported to the sea by rivers is about 24 trillion kilograms per year.

Soil• Wind generally erodes soil much more slowly than

water does.• During a long drought, strong winds can remove

large quantities of soil from unprotected fields.– Example: 1930’s Great Plains Dust Bowl.

• The rate of erosion depends on soil characteristics and on factors such as climate, slope, and type of vegetation.

• In many regions, including about one-third of the world’s croplands, soil is eroding faster than it is being formed.

• This results in lower productivity, poorer crop quality, and a threatened world food supply.

Soil• Another problem caused by erosion is the deposition of

sediment.• Rivers that accumulate sediment must be dredged to remain

open for shipping.• As sediment settles in reservoirs, they become less useful in

storing water, controlling floods, and generating electricity.• Some sediments are contaminated with agricultural

pesticides.• Sediments also contain soil nutrients, which may come from

natural processes and from added fertilizers.– Excessive nutrient levels in lakes stimulate the growth of algae and

plants, which accelerates a process that eventually leads to the early death of the lake.

Soil

• We can significantly slow erosion by using soil conservation measures:– Preserve environments– Protect the land.

• These measures include planting rows of trees (windbreaks), plowing along the contours of hills, and rotating crops.

• Preserving fertile soil is essential to feeding the world’s rapidly growing population.

Compared to the past, rates of soil erosion are

A. About the same.

B. Faster.

C. Slower.

D. More unpredictable.

Which of the following human activities has

caused an increase in soil erosion?

A. Clear-cut logging.

B. Clearing land for construction.

C. Plowing land for farming.

D. All of the above.

The rate of soil erosion depends on

A. Climate.

B. Slope steepness.

C. The type of vegetation.

D. All of the above.

Since humans have appeared, the amount of

sediments carried by rivers has

A. Increased dramatically.

B. Increased slightly.

C. Stayed about the same.

D. Decreased by about half.

What is the correct order for water eroding soil?

A. Gullies, rills, sheet erosion.

B. Sheet erosion, rills, gullies.

C. Sheet erosion, gullies, rills.

D. Rills, sheet erosion, gullies.

Mass Movements

• The transfer of rock and soil downslope due to gravity is called mass movement.– Ex: Landslides

• The combination of weathering and mass movement produce most landforms.

• Once weathering weakens and breaks rock apart, mass movement moves the debris downslope.

• There a stream usually carries is away.• Stream valleys are the most common of Earth’s

landforms.

Mass Movements• Several factors make slopes more susceptible to

the pull of gravity.– Saturation of surface materials with water.– Oversteepening of slopes.– Removal of vegetation.– Earthquakes.

1. Water:• Heavy rains and rapid melting of snow can trigger

mass movement by saturating surface materials with water.

• When the pores in sediment become filled with water, the particles slides past one another easily.

Mass Movements2. Oversteepened Slopes:• Loose soil particles can maintain a relatively stable

slope up to a certain angle (25 to 40°), depending on the size and shape of the particles.

• If the steepness of the slope exceeds the stable angle, mass movement is likely.

• Such slopes are said to be oversteepened.• This can result when:A. Streams undercut a valley wall.B. Waves pound against the base of a cliff.C. People, through excavation during construction of

roads/buildings.

Mass Movements3. Removal of Vegetation:• Plants make slopes more stable because of their

root systems.• When plants are removed, mass movements are

likely.4. Earthquakes:• Earthquakes are one of the most dramatic triggers

of mass movements.• They can dislodge rock and unconsolidated

material, which can cause more damage than the earthquake itself.

The process responsible for moving material downslope

under the influence of gravity is called

A. Erosion

B. Weathering

C. Mass movement

D. Soil formation

What is the force behind mass movements?

A. The Sun’s energy

B. Flowing water

C. Gravity

D. Moving ice

Which of the following is not true about mass

movements?A. Some mass movements are too slow to

be seen.

B. Mass movements always lead to landslides.

C. Gravity is the driving force behind all mass movements.

D. Mass movements are always downslope.

What factor commonly triggers mass movements?

A. Saturation of surface materials with water.

B. Earthquakes

C. Removal of vegetation

D. All of the above

Why can the removal of vegetation trigger mass

movements?A. The soil loses nutrients and begins the

crumble.

B. The plant roots bind the soil and regolith together.

C. The shaking triggers mass movements.

D. The plant roots lubricate the loose sediment.

Oversteepened slopes often lead to mass movements

becauseA. Plants cannot grow on them.

B. The angle of their slope is between 10 and 20 degrees.

C. The angle of their slope is less than 20 degrees.

D. The angle of their slope is greater than 40 degrees.

During what season would you expect mass

movements to be a greater threat?

A. A dry summer.

B. A wet spring before vegetation is growing.

C. A wet spring with lots of growing vegetation.

D. A dry autumn after the leaves have turned.

Mass Movements• Geologists classify mass movements based on the

kind of material that moves, how is moves, and the speed of the movement.

1. Rockfalls:• Occurs when rocks or rock fragments fall freely

through the air.• Common of slopes that are too steep for loose

material to remain on the surface.• Result from the mechanical weathering of rock

caused by freeze-thaw cycles or plant roots.• Sometimes trigger other mass movements.

Mass Movements

2. Slides:• A block of material moves suddenly along a flat,

inclined surface.,• Slides that include segments of bedrock are called

rockslides.• Often occur in high mountain areas.

– i.e. Andes, Alps, Rockies.

• Rockslides are among the fastest mass movements (speeds over 200 kmph ≈ 125 mph).

• Triggered by rain or melting snow.

Mass Movements

3. Slumps:

• Is the downward movement of a block of material along a curved surface.

• Usually does not travel very fast or very far.

• Slumps leave a crescent-shaped cliff just above the slump.

• Common on oversteepened slopes where the soil contains thick accumulations of clay.

Mass Movements

4. Flows:• Mass movements of material containing a

large amount of water, which move downslope as a thick liquid.

• Flows that move quickly, called mudflows, are common in semiarid mountainous regions.– i.e. Southern California

• Follows the contours of the canyon, taking trees and boulders with it.

Mass Movements

• Earthflows are flows that move relatively slow – from about a millimeter per day to several meters per day, and may continue for years.

• Occurs most often on hillsides in wet regions.• When water saturates the soil and regolith on a

hillside, the material breaks away, forming a tongue-shaped mass.

• They range in size from a few meters long and less than 1 m deep to over 1 km long and more than 10 m deep.

Mass Movements5. Creep:• The slowest type of mass movement.• Usually only travels a few millimeters or centimeters

per year.– Because it is slow, you cannot directly observe it.

• Alternating between freezing and thawing contributes to creep.

• Effects are easy to recognize:– Structures once vertical tilt downhill.– Displacement of fences.– Cracks in walls and underground pipes.

A mass movement that involves the sudden

movement of a block of material along a flat, inclined

surface is called aA. Slide

B. Rockfall

C. Slump

D. Flow

When a block of material moves downslope along a curved surface, the type of mass movement is called

A. A rockfall

B. A rockslide

C. A slump

D. Creep

What is the slowest type of mass movement?

A. A slump

B. A rockfall

C. An earthflow

D. Creep

A relatively rapid form of mass movement that is

most common in dry mountainous regions is

A. Creep

B. A mudflow

C. A slump

D. An earthflow

Which of the following statements best describes a

slump?A. Slippage of a block of material moving

along a curved surface.

B. Blocks of rock sliding down a slope.

C. Rapid flow of water-saturated debris, most common in mountainous regions.

D. Slow downhill movement of soil and regolith.

Alternate freezing and thawing often leads to

A. Creep

B. Slumps

C. Mudflows

D. Earthflows