Tab: Exercise 1.3


Lesson 1.3: Incremental Changes: Wind, Water, and Ice


Slow Changes that Reshape the Earth

Required Readings

Science in Action 7
pages 363 to 364
or
Science Focus 7
pages 373 to 374


Well, we made it through the lesson on fast changes without being smashed, crunched or burned up, so we can now examine some slower processes that can radically change the face of the Earth. There are some terms we must know before we can begin.

It is important to note that the material being broken down is moved from place to place but is never lost to the global system. In the next module you learn about the 'rock cycle' and that even though one type of rock is weathered away, a new rock will eventually be made from the deposited material.

Mechanical Weathering

There are a number of ways that material can be broken down. We use the term mechanical weathering when rock is broken down by physical forces.

Rocks that undergo changes in temperature expand and contract. As they get warmer, they expand and contract as they cool. But this warm and cooling, expanding and contracting does not occur evenly throughout the rock. This uneven expansion creates stresses in the rock that eventually cause it to crack. It is at this point that a second type of mechanical weathering begins. Water falling as rain or from melting snow seeps into the newly formed cracks in the rock. If that rock is in a climate where the temperature falls below freezing then the water in the crack will freeze. In an earlier lesson you learned that water expands as it freezes. This expansion is so powerful that it enlarges the crack in the rock and, eventually the rock will fall part. This process is called frost wedging.

Activity: The Power of Freezing Water

Parental permission and assistance is required to do this lab.

Materials:

1 small glass jar with a tight fitting lid
(this jar will be destroyed so don't use one you want to keep!)
water
freezer
3 plastic bags large enough to fit around the jar.
1 twist tie

Procedure:

1. Fill the jar with water until it is completely full. To ensure the jar is totally full with no air pockets, fill a large bowl with water. Completely submerge the jar and put the lid on while it is underwater.
2. Screw the cap on the jar tightly.
3. Wrap the jar in the plastic bags and secure with a twist tie.
4. Place the jar in the freezer overnight.
5. Carefully remove the bag and unwrap (Careful! Watch for broken glass).
6. Record your observations.


ACTIVITY A: Glaciers


Valley glacier carve out mountain valleys and leave steep-sided 'U' shaped valleys with their passing.



Required Readings

Science in Action 7
page 366
or
Science Focus 7
pages 376 to 377


Glaciers are huge rivers of ice. They are created when long winters deposit lots of snow and the summers are too cool to melt all the snow. If this happens over many years a permanent layer of ice begins to accumulate on the ground. Once the become a certain thickness, their own weight causes them to begin to move down hill. This tremendous mass of ice grinds up rocks, hills and mountains and carries the pieces away. There are two major types of glaciers, valley glaciers and continental glaciers.

Valley Glaciers

Valley glaciers can still be found in mountainous regions around the world. Even after they melt, there is still evidence of their weathering. Normal V-shaped valleys have been ground down into a U- shape, and striations (scratches on the rocks) tell us the direction they were travelling.

Continental Glaciers



In the past the Earth has gone through many periods of long cooling. These are called ice ages. The last ice age ended and the glaciers that covered all of Canada and much of the United States began to retreat northwards. Like the valley glaciers, the continental ground down all it came across. The rock, gravel and mud that they eroded became captured in the glaciers.

When the glaciers retreated, these materials were deposited as funny shaped geography that go by names such as: drumlins, eskers, and kettle lakes. These deposits can be found throughout Alberta and the rest of the continent as well. Sometimes huge boulders are carried hundreds of miles before being deposited by melting glaciers. These huge stones are called erratics.

Watch this animation of the receding Laurentide ice sheet that covered much of North America.
Exercise 1.3A: Glaciers




ACTIVITY B: Mechanical Weathering


These landscapes have been sandblasted by fine particles carried on the wind.



Required Readings

Science in Action 7
pages 364 to 365
or
Science Focus 7
pages 376 to 378


Another type of mechanical weathering is caused by small particles being blasted onto objects by the force of the wind. This is called Aeolian transport. Little by little these small particles blast away the rock face and turn it to fine particles of sand. The hoodoos in the Alberta badlands were created by this process. Examine the picture and try to answer the questions in the exercise below.

Weathering Caused by Moving Water



Swift moving water reshapes the landscape everywhere we find rivers. The most dramatic example of this is found in the Grand Canyon, but even in you local area you can find evidence of water erosion. The Colorado river has been grinding its way through the rock of the Grand Canyon for an extremely long time. In doing so, it has given us an interesting fossil record! The Colorado river, seen far below in the Grand Canyon.
It has taken millions of years for the river to weather
and erode the canyon walls away.



Moving water has worn pathways into solid rock.		 But smaller rivers and streams also change the face of the landscape. In many streams and rivers, the path the water follows changes into a series of bends called meanders.

A Meander in Your Mind

Lets do a thought experiment. Pretend that today is track and field day. You and your best friend are going to run a race. You friend is running on the inside of the track while you will be running in the far outside lane. How do you feel about that? OK, OK, I realize that its not fair but that's the way it is. So if you are to win this race you will have to run much faster than your friend even if we want to have a tie race. Well, it works the same way in a river. The water on the outside of the bend has to travel much faster than on the inside of the bend. In fact, the water on the inside of the bend will slow down! Scientist's who have studied weathering and erosion by moving water have found that the faster the flow of water the faster it wears the river bank away.




So let's look at my really bad diagram. The water is flowing from left to right. As the water enters the curve it is moving much faster at point A and has slowed down at point B. Because of its increased speed at the outside turn the water erodes the land away much faster than normal. But the very slow water on the inside can no longer hold the sediments its carrying so they are deposited on the inside curve so that bank grows into the river. So over a period of years the meanders become larger and larger. Eventually these large bends get cut off from the river as the river straightens out during spring flooding.
 
ACTIVITY C: Chemical & Biological Weathering

The gently falling rain is slightly acidic and will over hundreds of years dissolve some forms of rock away.



Required Readings

Science in Action 7
page 364
or
Science Focus 7
page 374


Chemical weathering is the second major type of weathering. Certain chemicals such as hydrogen, oxygen, sulphur, nitrogen and carbon can combine with water to produce weak acids that can eat away certain kinds of rocks.

Over the history of life on Earth, many organisms have used shells of one kind
or another to protect themselves. One such organism is coral. Over many, many years coral communities have created huge structures in the ocean. These limestone structures are so large they can be seen from space. These limestone deposits now cover large areas of the Earth, both above and below the ocean.

Limestone reacts with weak acids and dissolves away. This may leave huge holes in the ground called caves or caverns. Let's do some chemistry to see how it works. The carbon dioxide in the air combines with water in the atmosphere to create carbonic acid. Carbonic acid is a natural product and has been slowly dissolving limestone away for millions of years


Carbonic acid is the same acid that gives pop its zing.

The gas fizzing out of the pop is carbon dioxide!

Acid Rain


Earlier on in this science course, we learned about acid rain. Acid rain is caused by the burning of fossil fuels which include coal, oil and gas. These extra acids in the atmosphere have, and are, destroying lakes and forest and creating health problems for many people. But the extra acid in the air causes many beautify buildings and statues to be eaten away. This is also a form of chemical weathering.

Limestone Formations

Are you a spelunker? Would you like to be? Well, if you don't mind closed in spaces and tight squeezes you just might like to be a spelunker. Spelunker's explore the vast cave systems throughout the world. It also helps if your half fish and half mountain goat. There is also a branch of biology called biospeleology. These biologists study the forms of life that have evolved in caves. Take a virtual tour of cave systems. Caves have formed in the Earth's crust over millions of years.

But some of the dissolved limestone is re-deposited in the cave in the form of stalactites and stalagmites. Both are seen in the photographs. But which one is which?



These pictures represent limestone deposits called stalactites and stalagmite. But, which one is which? How do you think they we formed?



ACTIVITY E: Biological Weathering

Animals, like the gopher, bring rocks to the surface where other forces begin to break them down.




Required Readings

Science in Action 7
page 364
or
Science Focus 7
page 374


Biological weathering is defined as the weathering of parts of the Earth's crust by living organisms.

It is not uncommon to see trees growing in cracks of rocks as one hikes in the mountains. Over the years soil has accumulated in these cracks and the tree's roots seek out these areas for water and minerals. As the tree grows the cracks in the rocks are forced open even farther by the living tree. In your neighbourhood you might see plants growing out of the cracks in the sidewalk or brick wall. Many animals dig burrows in the ground, forcing rocks to the surface where they can be weathered by heat and cold and freezing water. In this case the weathering is actually a combination of biological and mechanical weathering. There is another organism that also weathers rock. Lichens slowly dissolve the rock they are living on for its minerals.

 




Conclusion

The weathering of the Earth's crust is usually a combination of all three of the major forms of weathering: mechanical, chemical and biological. Some rocks have all three forces working on them at the same time. But everything in nature works in cycles, and as you will see in the next lesson, new rocks will form from the remains of old ones.
Exercise 1.3B: Weathering