Download Mass Movements

Mass Movements, Wind, and Glaciers
Objectives
• Identify factors that affect mass movements.
• Relate how mass movements affect people.
• Analyze the relationship between gravity and
mass movements
Vocabulary
– mass movement
– slump
– creep
– avalanche
– mudflow
– landslide
Mass Movements at Earth’s Surface
Mass Movements
• Mass movement is the downslope movement of
loose sediments and weathered rock resulting
from the force of gravity.
• Climatic conditions determine which materials
and how much of each will be made available for
mass movement.
• All mass movements occur on slopes and range
from extremely slow motions to sudden slides,
falls, and flows.
Mass Movements at Earth’s Surface
Variables That Influence Mass Movements
• Variables that influence the mass movements
of Earth’s material include:
– The material’s weight resulting from gravity, which
works to pull the material down a slope
– The material’s resistance to sliding or flowing
– A trigger, such as an earthquake, that works to
shake material loose from a slope
• Mass movement occurs when the forces
working to pull material down a slope are
stronger than the material’s resistance to
sliding, flowing, or falling.
Mass Movements at Earth’s Surface
Variables That Influence Mass Movements
• Some common types of mass
movement include:
Mass Movements at Earth’s Surface
Variables That Influence Mass Movements
Water
– On a slope, too little water may prevent sediment
grains from holding together at all, thereby increasing
the material’s potential for movement.
– The addition of water to sediments on a slope helps
to hold the grains together and makes the material
more stable.
– Too much water can make a slope unstable.
– Water is very important to the process of mass
movement, but it is not involved as a transport agent.
Mass Movements at Earth’s Surface
Variables That Influence Mass Movements
Water
Mass Movements at Earth’s Surface
Types of Mass Movements
Creep
– Creep is the slow, steady, downhill flow of loose,
weathered Earth materials, especially soils.
– The effects of creep usually are noticeable only over
long periods of time.
– Loose materials on almost
all slopes undergo creep.
– Soil creep moves huge
amounts of surface material
each year.
Mass Movements at Earth’s Surface
Types of Mass Movements
Flows
– In some mass movements, Earth materials flow as if
they were a thick liquid.
– Earth flows are moderately slow movements of soils.
– Mudflows are swiftly moving mixtures of mud
and water.
– Mudflows can be triggered by earthquakes or similar
vibrations and are common in volcanic regions.
– Mudflows are also common in sloped, semi-arid
regions that experience intense, short-lived rainstorms.
Mass Movements at Earth’s Surface
Types of Mass Movements
Slides
– A landslide is a rapid, downslope movement of Earth
materials that occurs when a relatively thin block of
loose soil, rock, and debris separates from the
underlying bedrock.
– The material rapidly slides
downslope as one block, with
little internal mixing.
– Landslides are common on
steep slopes, especially when
soils and weathered bedrock
are fully saturated by water.
Mass Movements at Earth’s Surface
Types of Mass Movements
Slides
– A rock slide is a type of landslide that occurs when a
sheet of rock moves downhill on a sliding surface.
– During a rock slide, relatively thin blocks of rock are
broken into smaller blocks as they move downslope.
Mass Movements at Earth’s Surface
Types of Mass Movements
Slumps
– A slump results when the mass of material in a
landslide rotates and slides along a curved surface.
– Slumps may occur in areas that
have thick soils on moderate-tosteep slopes.
– Slumps are common
after rains and leave
crescent-shaped scars
on slopes.
Mass Movements at Earth’s Surface
Types of Mass Movements
Avalanches
– Avalanches are landslides that occur in mountainous
areas, usually on slopes of at least 35°, with thick
accumulations of snow.
– Avalanches usually occur when snow has melted,
either due to radiation from the Sun or warmth from the
ground, and then refrozen into an icy layer.
– Snow that falls on top of this crust can eventually build
up, become heavy, slip off, and slide down a slope as
an avalanche.
– A vibrating trigger, even from a single skier, can send
such an unstable layer sliding down a mountainside.
Mass Movements at Earth’s Surface
Types of Mass Movements
Rock Falls
– Rock falls commonly occur at high elevations, in steep
road cuts, and on rocky shorelines.
– Rocks loosened by physical weathering processes
may fall directly downward or bounce and roll,
ultimately producing a cone-shaped pile of coarse
debris, called talus, at the base of the slope.
Mass Movements at Earth’s Surface
Mass Movements Affect People
• Human activities often contribute to the factors
that cause mass movements.
• Activities such as constructing heavy buildings,
roads, and other structures can make slope
materials unstable.
Mass Movements at Earth’s Surface
Mass Movements Affect People
Dangerous Mudflows
– Human lives are in danger when people live on steep
terrain or in the path of unstable slope materials.
– In December of 1999, northern Venezuela experienced
severe mudflows and landslides in which tens of
thousands of people died and more than 114 000
people were left homeless.
Mass Movements at Earth’s Surface
Reducing the Risks
• Catastrophic mass movements are most
common on slopes greater than 25° that
experience annual rainfall of over 90 cm.
• The best way to minimize the destruction caused
by mass movements is to avoid building
structures on such steep and unstable slopes.
Mass Movements at Earth’s Surface
Reducing the Risks
Preventative Actions
– Some actions can help to avoid the potential hazards
of landslides.
– Most of the efforts at slope stabilization and prevention
of mass movements, however, are generally successful
only in the short run.
Mass Movements at Earth’s Surface
Section Assessment
1. Match the following terms with their definitions.
___
C creep
___
A landslide
___
D slump
___
B avalanche
A. a rapid downslope movement
of Earth materials
B. landslides that occur in
mountainous areas with thick
accumulations of snow
C. the slow, steady, downhill
flow of loose, weathered
Earth materials
D. when the mass of material in
a landslide rotates and slides
along a curved surface
Mass Movements at Earth’s Surface
Section Assessment
2. What are four variables that influence mass
movements?
Four variables that influence mass movements
are the material’s weight resulting from gravity,
the material’s resistance to sliding or flowing, a
trigger, and quantity of water in the material.
Mass Movements at Earth’s Surface
Section Assessment
3. Identify whether the following statements are
true or false.
________
true
All mass movements occur on slopes.
________
false
Creep is a rare occurrence, limited to areas of steep
slope.
________
true
Mudflows can travel at speeds greater than 30 km/h.
________
false
About 2,000 avalanches occur each year in the
western United States.
________
true
Landslides cause almost $2 billion in damages per
year in the United States.
Wind
Objectives
• Describe conditions that contribute to the likelihood that
an area will experience wind erosion.
• Identify wind-formed landscape features.
• Describe how dunes form and migrate.
• Explain the effects of wind erosion on human activities.
Vocabulary
– deflation
– dune
– abrasion
– loess
– ventifact
Wind
Wind
• Moving air can pick up and transport Earth
materials in the process of erosion.
• Unlike water, wind can transport sediments uphill
as well as downhill.
• As an erosional agent, wind can modify and
change landscapes in arid and coastal areas.
Wind
Wind Erosion and Transport
Wind
Wind Erosion and Transport
• Wind transport and erosion primarily occur in
areas with little vegetative cover, such as deserts,
semi-arid areas, seashores, and some lakeshores.
• Wind erosion is a problem in many parts of the
United States.
Wind
Wind Erosion and Transport
Wind
Wind Erosion and Transport
• Deflation is the lowering of the land surface
that results from the wind’s removal of
surface particles.
• In areas of intense wind erosion, coarse gravel
and pebbles are usually left behind as the finer
surface material is
removed by winds.
• The coarse surface
left behind is called
desert pavement.
Wind
Wind Erosion and Transport
• Abrasion is a process of erosion that occurs
when particles, such as sand, rub against the
surface of rocks or other materials.
• In wind abrasion, wind picks up materials such
as sand particles and blows them against rocks
and other objects.
• Because sand is often made of quartz, a very
hard mineral, wind abrasion can be a very
effective agent of erosion.
Wind
Wind Erosion and Transport
• Windblown sand causes rocks to become pitted
and grooved.
• Ventifacts are rocks shaped by wind-blown
sediments.
Wind
Wind Deposition
• Wind deposition occurs in areas where there are
changes in wind velocity.
• As wind velocity decreases, some of the windblown sand and other materials can no longer
stay airborne, and they drop out of the airstream
to form a deposit on the ground.
Wind
Wind Deposition
Formation of Dunes
– In wind-blown environments, sand particles tend to
accumulate where an object blocks the particles’
forward movement.
– A dune is a pile of wind-blown sand that develops
over time.
– Conditions, including the availability of sand, wind
velocity, wind direction, and the amount of
vegetation present under which a dune forms,
determine its particular shape.
Wind
Wind Deposition
Formation of Dunes
– The velocity of the wind above the ground surface
determines the height of a dune.
– Although quartz sand is the most common component
of dunes, any dry, granular material can be formed
into a dune.
Wind
Wind Deposition
Types of Dunes
– Barchan dunes are solitary, crescent-shaped dunes that
form in flat areas where there is little sand or vegetation.
– Transverse dunes form where there is plenty of sand,
little or no vegetation, and steady, prevailing winds.
– Transverse dunes form in a series of long ridges that
are perpendicular to the direction of the wind.
– U-shaped parabolic dunes form between clumps
of plants.
– Longitudinal dunes form where there is limited sand
available and strong prevailing winds.
Wind
Wind Deposition
Wind
Wind Deposition
Types of Dunes
– On offshore islands and on lakeshores, dunes are
formed by winds blowing off the water toward the shore.
– Coastal dunes protect against beach and coastal
erosion by reducing the direct action of wind on
beach sand.
– They also act as buffers against the action of waves
and provide shelter for vegetation.
– Dune migration is caused when prevailing winds
continue to move sand from the windward side of a
dune to its leeward side.
Wind
Wind Deposition
Types of Dunes
Wind
Wind Deposition
Loess
– Wind can carry fine, lightweight particles such as silt
and clay in great quantities and for long distances.
– Many parts of Earth’s surface are covered by thick
layers of windblown silt known as loess.
– Where precipitation is adequate, loess soils are some of
the most fertile soils on Earth because they contain
abundant minerals and nutrients.
Wind
Wind Deposition
Wind
Section Assessment
1. Match the following terms with their definitions.
___
B deflation
___
A abrasion
___
C ventifact
___
D loess
A. a process in which particles
such as sand rub against the
surface of rocks or other
materials
B. the lowering of land surface
that results from the wind’s
removal of surface particles
C. rocks shaped by wind-blown
sediments
D. thick, wind-blown
silt deposits
Wind
Section Assessment
2. Match the description of how the dune
forms to its type.
A Most common dune,
___
generally forms in areas
of constant wind direction
D Form in areas with high,
___
somewhat variable winds
and little sand
C Form in areas with moderate
___
winds and some vegetation
B Form in areas with strong
___
winds and abundant sand
A. Barchan
B. Transverse
C. Parabolic
D. Longitudinal
Wind
Section Assessment
3. How do dunes migrate?
Wind blows sand particles up the windward side
of the dune, depositing it on the leeward side.
Over time, as sand is moved from the windward
side to the leeward side, the dune shifts toward
the leeward side.
Glaciers
Objectives
• Explain how glaciers form.
• Compare and contrast the conditions that produce valley
glaciers and those that produce continental glaciers.
• Describe how glaciers modify the landscape.
• Recognize glacial landscape features.
Vocabulary
– glacier
– moraine
– valley glacier
– outwash plain
– continental glacier
– drumlin
– cirque
– esker
Glaciers
Glaciers
• Glaciers formed much of the landscape that
exists presently in the northern United States
and elsewhere in the world.
• Today, scientists measure the movements of
glaciers and changes in their sizes to track
climatic changes.
• Air bubbles trapped deep in glacial ice can
provide data about the composition of Earth’s
atmosphere at the time when the ice layers
were formed.
Glaciers
Moving Masses of Ice
• A glacier is a large, moving mass of ice.
• Glaciers form near Earth’s poles and in
mountainous areas at high elevations.
• Cold temperatures year-round keep fallen snow
from completely melting, and allow it to
accumulate in an area called a snowfield.
• The weight of the top layers exerts downward
pressure that forces the accumulated snow below
to recrystallize into ice.
• Glaciers currently cover only about 10 percent
of Earth’s surface.
Glaciers
Moving Masses of Ice
Glaciers
Moving Masses of Ice
Valley Glaciers
– Glaciers can be classified as one of two types: valley
glaciers or continental glaciers.
– Valley glaciers are glaciers that form in valleys in high,
mountainous areas.
– As a valley glacier moves down the valley, deep cracks
in the surface of the ice, called crevasses, can form.
– As valley glaciers flow downslope, their powerful
carving action widens V-shaped stream valleys into Ushaped glacial valleys.
Glaciers
Moving Masses of Ice
Continental Glaciers
– Continental glaciers, also called ice sheets, are
glaciers that cover broad, continent-sized areas.
– A continental glacier is thickest at its center.
– The weight of this thicker central region forces the rest
of the glacier to flatten out in all directions.
– Continental glaciers are confined to Greenland,
northern Canada, and Antarctica.
Glaciers
Glacial Erosion
• Glaciers are the most powerful erosional agent
because of their great size, weight, and density.
• When glaciers with embedded rocks move over
bedrock valley walls, they grind out parallel
scratches into the bedrock.
• Small scratches are called striations, and the
larger ones are called grooves.
• Scratches and grooves provide evidence of a
glacier’s history and establish its direction of
movement.
Glaciers
Glacial Erosion
• Glacial erosion can create certain features.
– Cirques are deep depressions scooped out by
valley glaciers.
– An arete is a sharp, steep ridge where two cirques on
opposite sides of a valley meet.
– A horn is a steep, pyramid-shaped peak formed by
glaciers on three or more sides of a mountaintop.
– A hanging valley is a tributary valley that enters a
U-shaped valley from high up a mountain side.
Glaciers
Glacial Erosion
Glaciers
Glacial Deposition
• Glacial till is the mixed debris that glaciers carry
embedded in their ice and on their tops, sides,
and front edges.
• Moraines are ridges consisting of till deposited
by glaciers.
– Those at the foot of a large glacier are called
terminal moraines and those at its sides are called
lateral moraines.
– Where two glaciers join together, their lateral moraines
combine to form a medial moraine.
Glaciers
Glacial Deposition
Outwash
– When a glacier melts and begins to recede, meltwater
floods the valley below.
– Outwash is gravel, sand, and fine silt formed from the
grinding action of the glacier on underlying rock that is
deposited by meltwater.
– An outwash plain is the area at the leading edge of
the glacier, where the meltwater streams flow and
deposit outwash.
Glaciers
Glacial Deposition
Drumlins and Eskers
– Drumlins are elongated landforms that are formed
when glaciers move over older moraines.
– Eskers are long, winding ridges of layered sediments
that are deposited by streams flowing under a
melting glacier.
Glaciers
Glacial Deposition
Glacial Lakes
– Sometimes, a large block of ice breaks off a glacier
and is later covered by sediment.
– When the ice block melts, it leaves behind a
depression called a kettle hole.
– After the ice block melts, the kettle hole fills with water
from precipitation and runoff to form a kettle lake.
– Cirques also can fill with water, becoming cirque lakes.
– When a terminal moraine blocks off a valley, the valley
fills with water to form a lake.
Glaciers
Section Assessment
1. Match the following terms with their definitions.
___
D cirques
___
B moraines
___
C drumlins
___
A eskers
A. long, winding ridges of layered
sediments that are deposited
by streams flowing under a
melting glacier
B. deep depressions scooped
out by valley glaciers
C. elongated landforms that are
formed when glaciers move over
older moraines
D. ridges consisting of till deposited
by glaciers
Glaciers
Section Assessment
2. What causes a continental glacier to move?
A continental glacier is thickest at its center. The
weight of this thicker central region forces the
rest of the glacier to flatten out in all directions.
Glaciers
Section Assessment
3. What are striations and why are they significant?
Striations are small scratches left in bedrock
from rocks embedded in a glacier as they
passed. They provide evidence of a glacier’s
history and establish its direction of movement.
Chapter Resources Menu
Study Guide
Section 8.1
Section 8.2
Section 8.3
Chapter Assessment
Image Bank
Section 8.1 Study Guide
Section 8.1 Main Ideas
• Mass movement is the movement of Earth materials
downslope as the result of the force of gravity. Almost all of
Earth’s surface undergoes mass movement.
• Mass movements may occur very slowly and become
noticeable only over long periods of time. Creep is a form of
slow mass movement. Rapid mass movements are
noticeable.
• Variables involved in the mass movement of Earth materials
include the material’s weight, its resistance to sliding, and
sometimes a trigger such as an earthquake. Water is
important to the process of mass movement.
• Mass movements can cause great damage and loss of lives.
Human activities may increase the potential for the
occurrence of mass movements.
Section 8.2 Study Guide
Section 8.2 Main Ideas
• Arid, semi-arid, and seashore environments are likely to
experience wind erosion. Limited amounts of precipitation
and protective vegetation commonly contribute to wind
erosion in an area.
• Wind-carried sediments can cause abrasive action. Rocks
exposed to continual wind abrasion often exhibit angular shapes
with polished, smooth sides on the windward side. Features
formed in wind-affected areas include deflation blowouts, dunes,
and desert pavement. Dunes are classified by shape.
• The transport of Earth materials by wind can create problems
for humans. Migrating dunes can block highways and cover
structures.
• Loess soils deposited by wind are fertile soils because they
contain minerals and nutrients.
Section 8.3 Study Guide
Section 8.3 Main Ideas
• Glaciers are large, moving masses of ice that form near
Earth’s poles and in mountainous areas at high elevations.
• Valley glaciers are formed in mountains, and continental glaciers
are formed over broad regions of land. Valley glaciers move
down mountainsides and form unique glacial features.
Continental glaciers usually spread out from their centers.
• Features formed by glaciers include U-shaped valleys, hanging
valleys and waterfalls in the mountains, moraines, drumlins,
kettle holes along outwash plains, and several types of lakes.
Chapter Assessment
Multiple Choice
1. Catastrophic mass movements are most
common on slopes greater than ____.
a. 10º
c. 20º
b. 15º
d. 25º
A slope that is high risk for a catastrophic mass
movement also experiences over 90 cm of
annual rainfall.
Chapter Assessment
Multiple Choice
2. What accounts for most sand transport by wind?
a. suspension
c. deflation
b. saltation
d. abrasion
More material is moved by saltation than by
suspension. Deflation and abrasion are possible
results of wind erosion or transport, but not methods
of transport themselves.
Chapter Assessment
Multiple Choice
3. Which of the following features provides
evidence of a glacier and establish its
direction of movement?
a. grooves
c. kettle holes
b. aretes
d. eskers
Grooves and striations are scratches in bedrock
resulting from glacial movement dragging rocks across
its surface. All of the other answers are evidence of a
glacier, but they do not establish direction of movement
by themselves.
Chapter Assessment
Multiple Choice
4. Which type of dune would you expect to see in a
coastal area with some vegetation?
a. barchan
c. parabolic
b. transverse
d. longitudinal
The key is vegetation. Parabolic dunes form in U-shapes
between clumps of plants. The other three dune types are
more common in areas with little or no vegetation.
Chapter Assessment
Multiple Choice
5. In which state are you most likely to find
a loess deposit?
a. North Dakota
c. Illinois
b. Texas
d. Florida
More than half of the state of Illinois has loess soil. With
adequate precipitation, such as in Illinois, loess soils are
some of the most fertile soils on Earth because they
contain abundant minerals and nutrients.
Chapter Assessment
Short Answer
6. What differentiates a slump from a landslide?
In a landslide, a thin block of loose soil, rock,
and debris separates from the underlying
bedrock. The material rapidly slides downslope
as one block, with little internal mixing. When a
slump occurs, the material in a landslide
rotates and slides along a curved surface.
Chapter Assessment
Short Answer
7. How does the distribution of valley and
continental glaciers on Earth’s surface differ?
Valley and continental glaciers need the same
environmental conditions to exist. Perpetually
cold conditions only exist on a large scale near
the polar regions, limiting continental glaciers to
Antarctica, Greenland, and northern Canada.
Valley glaciers, being much smaller, can exist
anywhere in the world that has the cold
conditions. They are found high in mountainous
areas from the arctic to the equator.
Chapter Assessment
True or False
8. Identify whether the following statements are
true or false.
______
true More material is moved by creep than by other
means of mass movements.
______
false Rock falls are less likely to occur in
dry regions.
______
true The Great Plains are characterized by the
effects of deflation.
______
false Moraines are the effect of glacial straition.
______
false Glaciers cover about 17 percent of
Earth’s surface.
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