Download External dynamics of the Earth 8

921115 _ 0086-0097.qxd
29/5/08
13:27
Página 86
UNIT
8
External dynamics of the Earth
What do you remember?
• Can you name the solids in the photograph? And the liquid?
• Is the liquid moving? How does it move?
• What is the name of this formation of water?
• How do you think it was formed?
• How do you think this formation will change in the future?
Content objectives
Key language
In this unit you will …
• Identify different types of weathering
Defining
• Learn how erosion, transportation and
deposition change the landscape
Expressing possibility
• Understand the ways the landscape
is shaped by the action of different
agents
• Learn how sedimentary rocks are formed
• Make an experimental model of a cliff
recession
86
Glaciers are masses of moving ice.
The pressure of the roots may cause the rocks to break.
Wind may erode, transport and deposit rock materials.
Expressing cause and result
Weathering is mainly due to the action of atmospheric
agents.
Mechanical weathering is mainly caused by changes
in temperature.
921115 _ 0086-0097.qxd
29/5/08
13:27
Página 87
1.What is weathering?
Weathering is the process which causes rocks and minerals to break down
and disintegrate into smaller pieces. It is mainly due to the action of
atmospheric agents, such as water, wind, ice and ocean movements.
There are two climatic factors which also affect the processes of weathering.
These are:
• Temperature. High temperatures may facilitate chemical reactions which
help to disintegrate some types of rock.
• Humidity. Humidity may also facilitate chemical reactions, for example,
in forest areas. In dry desert areas these types of reaction cannot take place.
There are three types of weathering:
• Chemical weathering is the decomposition of rocks through chemical
reactions. This process changes the mineral composition of rocks.
It is mainly caused by water.
• Biological weathering is the breakdown of rocks through the activity
of living things, for example, plants.
• Mechanical weathering is the breakdown of rocks and minerals into
small-sized particles through physical forces. Mechanical weathering is
mainly caused by changes in temperature, such as freezing and thawing,
and the effects of wind, rain and waves.
Accumulation of rock fragments produced by weathering.
Biological weathering. Plants extend their roots into cracks
in rocks. The pressure of the roots may cause the rocks to
break. The roots may also produce organic acids which help
to break down rocks chemically.
87
921115 _ 0086-0097.qxd
29/5/08
13:27
Página 88
Example of mechanical weathering
Example of chemical weathering
Freeze-thaw weathering. This is an example
of mechanical weathering, mainly found in
mountainous regions. The main cause is a
temperature fluctuation from below freezing
point to above it.
Rainwater is slightly acidic. When it comes into
contact with rocks, such as sandstone or limestone,
a chemical reaction takes place, causing the rock to
break down. Water and warm temperatures can
speed up this process, so it mainly occurs in warm,
wet places.
water
Water flows into cracks in a rock during the day.
ice
Chemical weathering has shaped this landscape.
Activities
1. Copy and complete.
At night, when the water freezes, it expands.
The ice acts as a wedge.
Weathering
Process
involves
Consequences
Chemical
chemical
reactions
changes in mineral
composition of rocks
Biological
Mechanical
2. How do temperature and water affect each type
of weathering? Explain.
3. Where would chemical weathering be more active,
in the rainforest or in the desert? Explain.
4. Describe the process of freeze-thaw weathering.
The freeze-thaw cycle is repeated, and eventually
the rock cracks.
88
First, water flows into cracks in a rock. Then, at night …
921115 _ 0086-0097.qxd
29/5/08
13:27
Página 89
2. Which agents shape landscape?
The agents which shape landscape are water,
wind, ice and ocean movement. Gravity is the
main driving force behind all these agents.
These agents are responsible for the following
processes:
• Erosion. Erosion moves small pieces of
weathered rocks to another place. For millions
of years, erosion has shaped the landscape.
• Transportation. This process moves rock
material which has eroded. This process is
usually carried out by the same agent which
causes the erosion.
• Deposition. Moving rock materials are
deposited. This process occurs when there is a
decrease in the energy of the transporting agent.
Activities
5. Copy and complete the definitions.
The agents which shape the landscape are:
1 Erosion: ______________________________
2 Transportation: ______________________
3 ______________________________________
6. How are weathering and erosion different?
7. Explain how beaches may have sand originating
from weathered rocks from a far away place.
The amount of energy of the agent, in this case, water, affects
the intensity of erosion.
Different landscapes show how rocks can be more resistant or
less resistant to erosion.
Water in rivers can break off fragments of rock. Then, the
current transports them to another place. Particles can be
transported along surfaces or in suspension.
Formation of dunes. Sand is deposited by the wind. The wind
collects the sand from a beach where it has been deposited
by waves.
89
921115 _ 0086-0097.qxd
4/6/08
11:59
Página 90
3. How does wind shape landscape?
Wind may erode, transport and deposit rock materials to change the landscape.
All the processes involving the wind are known as eolian processes.
Wind is only an effective agent in places where it is strong, usually
in dry regions with not much vegetation.
A
B
sand columns
barchan
erg
mushroom shaped
rocks
D
C
A. The wind picks up small particles and leaves the larger ones. This eolian process is called deflation.
B. The accumulation of sand produces dunes. Ergs are areas of sandy desert. Barchans are crescent-shaped dunes.
C. Rock particles (sand) carried by the wind can erode solid rock.
D. Large grains of sand carried by the wind are blown at low heights. These erode the base of a rock. The result is
mushroom-shaped rocks.This eolian process is called abrasion.
Activity
8. Describe how a mushroom-shaped rock is formed. First, large grains of sand are carried by the wind ...
90
921115 _ 0086-0097.qxd
29/5/08
13:27
Página 91
4. How do glaciers shape landscape?
Glaciers are masses of moving ice. They are only found in alpine and polar
regions. Ice moves very slowly from high areas to low areas by the force of
gravity. As a glacier moves, it drags rock materials and erodes the surfaces in
its path. Glaciers cut U-shaped valleys which are called glacial valleys.
A
B
steep relief
glacial cirque
glacial
tongue
U-shaped
valley
moraine
D
C
A. Ice causes intense mechanical weathering
of rocks, forming very steep relief.
B. Glacial cirques are rounded depressions
where snow accumulates and changes to ice.
C. Ice forms glacial tongues which carry and
deposit rock material known as moraine.
D. Glacial tongues cut U-shaped valleys.
Activities
Did you know that...?
9. Write the meaning of the following words. Then make
a drawing to illustrate each one.
– barchan
– U-shaped valley
– erg
– moraine
– glacial tongue
10. How is moraine formed? In which part of the world can you
find moraine?
The word glacier comes from
French. In the Savoy dialect,
glacière means "moving mass
of ice." In Old French glace
means "ice”, from Classical
Latin, glacies.
91
921115 _ 0086-0097.qxd
17/11/09
12:31
Página 92
5. How do rivers and rain shape landscape?
Rivers shape landscape
source
Rivers also carry out erosion,
transportation and deposition to change
the landscape. The dominant process
depends on the velocity of the water:
• In the upper course, rivers flow fast.
Erosion and transportation are the
dominant processes. As a result,
valleys are narrow and deep.
• In the middle course, the velocity of
the water decreases. Transportation is
dominant. As a result, valleys are
wide. The river has less capacity of
erosion, so the water flows to form
deep bends called meanders.
• In the lower course, rivers are slow.
Deposition is dominant. As a result,
the valley flattens out.
upper course
wide
river
valley
middle course
meanders
lower course
Rainwater shapes landscape
Rainwater is a very effective agent for
eroding and shaping landscape,
especially in areas with dry climates
and torrential rains.
A
gullies
B
alluvial cone
Erosion by water forms gullies in the
land. Gradually, gullies get deeper and
become ravines and gorges.
ravine
C
Activities
alluvial fan
11. Match each statement to the upper,
middle or lower course of a river.
A. In the alluvial cone, erosion is very intense and gullies form.
a. Transportation is dominant here.
fairy chimneys
B. Fairy chimneys are tall, conical rock formations, caused by erosion.
C. Alluvial fans are areas which are at risk from floods.
b. The valley flattens out.
c. Erosion and transportation are
dominant.
d. Meanders are frequent here.
12. Gullies are found in the upper
course of a stream, where the
landscape is very steep. Can you
find gullies in the middle course or
lower course of a river?
alluvial fan
92
fairy chimneys
921115 _ 0086-0097.qxd
29/5/08
13:27
Página 93
6. How does groundwater shape landscape?
Landscapes produced by the
dissolving action of water on
rock, on or under the surface
of the Earth, are known as karst
landscapes.
sinkhole (doline)
A
B
cave
Water containing dissolved
carbon dioxide is acidic. Acidic
water can dissolve calcium
carbonate (calcite) found in
limestone. Most caves are
formed by this process.
When water drips inside caves,
the dissolved calcium carbonate
forms deposits.
• Stalactites are deposits which
grow downward from the
cave ceiling.
• Stalagmites are deposits
which grow upward from the
cave floor.
B
A
A. Sinkholes or dolines are depressions
which are produced by the collapse of
underground cavities.
B. Caves are large underground
cavities where stalactites and
stalagmites form.
7. How do oceans shape landscape?
beach
cliff
There are three types of ocean water motions
which affect coastal landscapes:
A
• Waves are effective agents of erosion,
transportation and deposition of rock particles
and sand. Wave action shapes sea cliffs which
form different coastal landforms.
arch
spit
A
sand bar
Waves undercut
the base of sea
cliffs. This causes
blocks of rock to
fall, resulting in cliff
recession. The
accumulation of
sand from erosion
forms beaches.
• Tides are effective agents of transportation.
During high tide, waves pick up and carry sand
and other materials from beaches. The waves
deposit this material when the tide is lower.
• Currents are effective agents of transportation.
Activities
13. What is the main agent which forms karst
landscapes?
14. What are the three main water motions which
affect coastal landscapes?
93
921115 _ 0086-0097.qxd
29/5/08
13:27
Página 94
8. How are sedimentary rocks formed?
Sedimentary rocks are generally formed through the following processes:
1 Sedimentation. Layers of sediment
are deposited over millions of years.
2 Compaction. The weight of the
sediments on top compresses the
sediments at the bottom. The water is
squeezed out from between the pieces
of rock. As a result, crystals
of different salts form.
3 Cementation. The crystals form a sort
of glue which sticks or cements the
pieces of rock together.
Sedimentation
1
water
2
Compaction
3
Cementation
A sedimentary rock
limestone
sandstone
conglomerate
gypsum
Types of sedimentary rocks
Activities
15. Copy and complete the definitions.
The formation of sedimentary rocks:
1 Sedimentation: _______________
2 _______________
3 _______________
94
16. Why do sedimentary rocks form in layers?
17. If you look at a large piece of sedimentary rock,
which layer was the earliest one formed?
Which was the most recent layer formed?
18. Explain the difference between compaction
and cementation. Compaction occurs when the
weight of the sediments on top ...
921115 _ 0086-0097.qxd
29/5/08
13:27
Página 95
Hands on
Experimental models: Cliff recession
Materials
– large, rectangular tray
– sand
– water
– bucket – spade
1. Prepare the model. First, pack wet sand in
the bucket until it is full. Next,
turn the bucket upside down in the tray.
The sandcastle represents the cliff.
Then, fill the tray half way up with water.
Finally, use the spade to make gentle waves.
The waves should hit the base of the sandcastle.
2. Observe the process. Continue to make waves
for about ten minutes. During this time, you can
observe these processes:
• The waves will undercut the base of the
sandcastle. Small blocks or pieces of sand
will fall off.
• The fallen sand will accumulate at the
bottom or form little islands.
• As the fallen sand is moved by the waves, it
will form a submerged, sandy surface.
3. Compare the model to the real process.
On a real cliff, fragments of rock accumulate
as they are broken off by the action of the
waves. In the model, this process is
represented by blocks or pieces of fallen sand
broken off by the “waves”. A part of the
sandcastle is left projecting over the water.
In the case of a real cliff, a beach could form
under this projection.
Activities
19. Record the stages of recession in the sandcastle cliff. Add drawings or photos of the experiment.
Label the changes which take place in the sandcastle cliff.
20. Do you think cliff recession could be a problem for people living in coastal areas? Explain your answer.
95
29/5/08
13:27
Página 96
21. Copy the following drawings in your notebook.
ies
+
Activities
tivit
Ac
921115 _ 0086-0097.qxd
28. What is the difference between stalactites and
stalagmites? Where do you find them?
29. Copy and complete.
How rivers shape landscape
1
2
3
Course
Velocity
Processes
Results
Upper course
Add labels to explain the process, and indicate the
type of weathering.
22. Find examples of chemical, mechanical or
biological weathering near where you live.
Which type of weathering is most dominant?
Why is this explained by the climate?
Middle course
Lower course
30. Copy the following drawing of a glacier in your
notebook.
23. On the Iberian Peninsula, many cliffs are eroding
because of wave motion. However, not all of them
are eroding at the same rate. Why?
24. Can chemical weathering continue during
transportation? Explain how.
25. Copy the drawing in your notebook.
Label the cirque and the glacial tongue, and
indicate where the moraine has been deposited.
31. Copy this diagram of a cliff.
Include an explanation of the process it shows.
Indicate whether it shows erosion or
sedimentation, and what the result will be.
26. What type of weathering would dominate in a
region with a cold, dry climate? Explain.
27. The Gran Canyon was formed by the Colorado
River over millions of years. Find out how long it is,
and how wide it is at its narrowest and widest
points.
96
Add labels to explain the erosion process
represented. Describe the formations which
erosion is causing.
32. Can a beach be formed in a coastal area with strong
currents? Explain why or why not.
921115 _ 0086-0097.qxd
29/5/08
13:27
Página 97
EXTERNAL DYNAMICS OF THE EARTH
What should you know?
Weathering
• Mechanical weathering breaks rocks by physical forces.
• Chemical weathering decomposes rocks by chemical
reactions.
• Biological weathering breaks down rocks by the action
of living things.
Erosion,
transportation
and
deposition
• Erosion moves weathered pieces of rocks to a different
place.
• Transportation moves eroded rock material along
surfaces or in suspension.
• Deposition deposits moving rock materials. This
occurs when there is a decrease in energy in the
transporting process.
Shaping of
the landscape
• Wind may erode, transport and deposit rock materials
to change the landscape. It is only an effective agent in
places where it is strong, usually in dry regions with
scant vegetation.
• Glaciers are masses of moving ice. They cut U-shaped
valleys known as glacial valleys.
• Rivers also carry out erosion, transportation and
deposition which change the landscape. The dominant
process depends on the velocity of the water.
• Rainwater is a very effective agent for eroding and
shaping relief features in areas with dry climates and
torrential rain.
• Groundwater causes the dissolution of limestone rock
and forms karst landscapes.
• Ocean waves, tides and currents shape coastal
landscapes.
Formation of
sedimentary
rock
Sedimentary rock is formed from sediments through the
following processes:
• Sedimentation
• Compaction
• Cementation
8
Projects
INVESTIGATE: Write a report on ways of preventing damage from rainwater, groundwater and wind.
INVESTIGATE: What relationship is there between weathering, acid rain, and the deterioration of monuments?
WEB TASK: Describe a trip to a glacier or to a desert.
97