Unit E Lesson E11 Plate Tectonics
Completion requirements
Lesson E11: Plate Tectonics
Video Lesson
Alfred Wegener’s ideas about continental drift have proven to be correct, but it took more than 30 years for science to determine why he was right. The idea that Earth’s crust is divided into several tectonic plates that are moving slowly around the globe
explains much of the evidence that we see. However, what could be causing massive pieces of rock to change their position so drastically over long periods?
Lesson E11: Plate Tectonics
How do Tectonic Plates Move?
Learning what tectonic plates are and how they behave has been challenging for geologists. Scientific equipment and methods are providing new seismic information about Earth’s depths every day. For example, scientists recently discovered an old tectonic plate (the Farallon Plate) that is buried deep below the tectonic plate you are sitting on!
Read more about the Farallon Plate here .
Learning what tectonic plates are and how they behave has been challenging for geologists. Scientific equipment and methods are providing new seismic information about Earth’s depths every day. For example, scientists recently discovered an old tectonic plate (the Farallon Plate) that is buried deep below the tectonic plate you are sitting on!
Read more about the Farallon Plate here .
Reading and Materials for This Lesson
Science in Action 7
Materials:
Science in Action 7
Reading: Pages 395–400
Materials:
waxed paper, graham wafers, fruit strips, plastic spoon and/or knife, frosting (icing) or whipping cream, water
Perhaps the easiest way to understand plate tectonics is to think of ice on a lake. Perhaps you have skated or walked on a pond and noticed (or even been frightened by) cracks in the ice. Imagine a sheet of ice a few centimetres thick being broken into
several large chunks and many more smaller pieces.
All the ice floats, but sometimes the pieces collide. One piece might slide under another, or they might smash together and push smaller chunks of ice upward. Some pieces might be thick, and others thin. Some float high in the water; others float just below the surface. (You may have seen something like this on a river during spring break-up.) This situation is very like the plates of Earth!
But why do tectonic plates move at all? If they were like pieces of ice, wouldn’t they just jostle around in place, or maybe even lock into one position? Geologists have discovered that parts of the plates are changing constantly, being enlarged or destroyed where they meet other plates. It is a series of amazing processes happening in slow motion all around the world!
In addition, you might be wonder about how to differentiate plate boundaries and faults. A fault is anywhere that two pieces of rock are moving in different directions; thus, all plate boundaries are faults. However, plate boundaries are found only where tectonic plates meet, and all over the world many faults exist that are not near plate boundaries.
Figure E.3.11.1 – Earth’s tectonic plates are moving and interacting with each other in many ways.
Figure E.3.11.2 – Not all earthquakes occur along plate boundaries, but many do as you can see on this map.
Tectonic Plates
If you are asked to imagine Earth’s surface, probably you picture the land and the mountains. After you learn about Earth’s crust, you can add the ocean floor to your image of Earth. Of course, the land, mountains, and ocean floors are all part of some bigger structures, and these immense parts of Earth’s crust are called tectonic plates.
Tectonic plates are more than just the continents. These plates are huge sections of Earth’s lithosphere (the crust and upper mantle) that are moving slowly on the convection currents in the hot, fluid lower mantle below. In fact, some of these plates are not continents at all. The Pacific Plate is the largest of all tectonic plates, and it is almost entirely the bottom of the Pacific Ocean. Another plate has disappeared almost completely. The North American plate rode up over the Farallon Plate, driving most it under and down into the mantle. Only small pieces of the Farallon Plate remain at the surface, such as the Juan de Fuca Plate off the coast of British Columbia.
Each tectonic plate is up to about 100 km thick and is made of one of two types of crust. Oceanic crust is heavy, dense, and thin when compared to lighter, less dense continental crust. As the tectonic plates ‘float’ around on the mantle, the oceanic plates ‘sink’ deeper than the lighter continental plates do. The fact that the oceanic plates are thinner and sink deeply into the mantle helps to explain why the oceans fill with water that runs from the land.
If you are asked to imagine Earth’s surface, probably you picture the land and the mountains. After you learn about Earth’s crust, you can add the ocean floor to your image of Earth. Of course, the land, mountains, and ocean floors are all part of some bigger structures, and these immense parts of Earth’s crust are called tectonic plates.
Tectonic plates are more than just the continents. These plates are huge sections of Earth’s lithosphere (the crust and upper mantle) that are moving slowly on the convection currents in the hot, fluid lower mantle below. In fact, some of these plates are not continents at all. The Pacific Plate is the largest of all tectonic plates, and it is almost entirely the bottom of the Pacific Ocean. Another plate has disappeared almost completely. The North American plate rode up over the Farallon Plate, driving most it under and down into the mantle. Only small pieces of the Farallon Plate remain at the surface, such as the Juan de Fuca Plate off the coast of British Columbia.
Each tectonic plate is up to about 100 km thick and is made of one of two types of crust. Oceanic crust is heavy, dense, and thin when compared to lighter, less dense continental crust. As the tectonic plates ‘float’ around on the mantle, the oceanic plates ‘sink’ deeper than the lighter continental plates do. The fact that the oceanic plates are thinner and sink deeply into the mantle helps to explain why the oceans fill with water that runs from the land.
Figure E.3.11.3 – The thin crust under the ocean and the thick crust of the continents float on top of the mantle.
Even the continents that you know are only part of the tectonic plates on which they are located. Every continent has some land that is above sea level and some that is below sea level. The area below sea level, usually covered by an ocean or a sea, is
called the continental shelf. Therefore, if we think about Earth’s crust as similar to a cracked shell of an egg in water, the continental plates are the portions of shell that have dipped below the surface but are still a part of the larger piece
of shell you can see.
The continental plates meeting at plate boundaries are where earthquakes rumble and volcanoes erupt. Also, these locations are where new plates are made and where mountains are built. The plates are all moving in their own directions, jostling and clunking together in various ways. The three types of these boundaries are the following:
The continental plates meeting at plate boundaries are where earthquakes rumble and volcanoes erupt. Also, these locations are where new plates are made and where mountains are built. The plates are all moving in their own directions, jostling and clunking together in various ways. The three types of these boundaries are the following:
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Diverging boundaries (moving apart)
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Converging boundaries (coming together)
- Transform boundaries (moving past each other)
Figure E.3.11.4 – Magma rises from below to form new plate material, pushing the plates apart and producing what is called a ridge.
Figure E.3.11.5 – Ridges are found at divergent plate boundaries. New material is being added to plates on either side of the ridge, pushing the rocks apart at the surface.
Diverging Boundaries
Diverge means “to move apart”. At a diverging plate boundary, two tectonic plates are forced away from each other. How is this possible? New crust forms from mantle that rises into the gap between the two plates – oozing upward along the crack to form ridges. This type of plate boundary is a constructive boundary because material is being added to the plates – they are growing!
A convenient model used to understand the plate boundaries is the floating conveyer belt or moving sidewalk model. The tectonic plates are moving slowly around Earth’s surface, but each plate is being enlarged slowly at one end and disappearing on the other. The edge being extended is where magma in the mantle is rising slowly into the space that forms at the diverging plate boundary. Diverging plate boundaries can occur on land or under the ocean.
Look carefully at the map of Earth’s tectonic plates and find a location that has arrows going in opposite directions. These are diverging plate boundaries.
The Mid-Atlantic Ridge is probably the most famous diverging boundary on Earth. It runs almost the entire length of our planet. It is considered to be the longest mountain range in the world, and almost all of it lies at the bottom of the Atlantic Ocean.
Diverge means “to move apart”. At a diverging plate boundary, two tectonic plates are forced away from each other. How is this possible? New crust forms from mantle that rises into the gap between the two plates – oozing upward along the crack to form ridges. This type of plate boundary is a constructive boundary because material is being added to the plates – they are growing!
A convenient model used to understand the plate boundaries is the floating conveyer belt or moving sidewalk model. The tectonic plates are moving slowly around Earth’s surface, but each plate is being enlarged slowly at one end and disappearing on the other. The edge being extended is where magma in the mantle is rising slowly into the space that forms at the diverging plate boundary. Diverging plate boundaries can occur on land or under the ocean.
Look carefully at the map of Earth’s tectonic plates and find a location that has arrows going in opposite directions. These are diverging plate boundaries.
The Mid-Atlantic Ridge is probably the most famous diverging boundary on Earth. It runs almost the entire length of our planet. It is considered to be the longest mountain range in the world, and almost all of it lies at the bottom of the Atlantic Ocean.
Figure E.3.11.6 – This map shows how Iceland is being pushed apart (and, therefore, becoming larger) by the divergent boundary across it.
Figure E.3.11.7 – Notice how the spreading ridge resembles a conveyor belt of new material being added to each plate.
Each side of the Mid-Atlantic Ridge diverging boundary is pushed high with newly formed rock filling the middle. The actual boundary between the plates is a deep rift valley. This makes the boundary appear to be two small mountain ranges on each side
with a flat-bottomed valley in the middle
Seafloor spreading occurs at divergent boundaries under the ocean. A similar event occurs below the continental crust, but we cannot see it because it lies deep underground. This means the crust is thin, the plate nearby is moving, and hot magma is near the surface. Therefore, earthquakes and volcanoes occur often near these areas.
Seafloor spreading occurs at divergent boundaries under the ocean. A similar event occurs below the continental crust, but we cannot see it because it lies deep underground. This means the crust is thin, the plate nearby is moving, and hot magma is near the surface. Therefore, earthquakes and volcanoes occur often near these areas.
Figure E.3.11.8 – The Mid-Atlantic Ridge looks like a long zipper or scar that runs from near Antarctica all the way to and through Iceland. There is also a long diverging boundary that runs across much of the South Pacific Ocean. In this image
the youngest oceanic crust is coloured red, and the oldest is coloured blue.
Converging Boundaries
Converging boundaries occur where tectonic plates move towards each other and meet. This is a destructive plate boundary because one of the plates in the collision is being melted in the mantle, or both plates are being crushed together on the crust. In the case of one plate being forced under the other, the plate that is destroyed is more dense. The process of a more dense plate being destroyed by being pushed down into the hot mantle is called subduction.
Based on the types of crust involved, three types of converging boundaries are continental-continental, oceanic-oceanic, and continental-oceanic.
Continental-Continental convergent boundaries occur where two continental plates meet. Continental crust is less dense than oceanic crust is, so continental crust sits above oceanic crust. When two pieces of continental crust collide, the plates can fold and buckle, and mountains are formed as one plate is pushed below the other. This is how the Rocky Mountains (75 million years ago), Himalayas (45 million years ago), and Appalachian Mountains (480 million years ago) all were constructed.
Converging boundaries occur where tectonic plates move towards each other and meet. This is a destructive plate boundary because one of the plates in the collision is being melted in the mantle, or both plates are being crushed together on the crust. In the case of one plate being forced under the other, the plate that is destroyed is more dense. The process of a more dense plate being destroyed by being pushed down into the hot mantle is called subduction.
Based on the types of crust involved, three types of converging boundaries are continental-continental, oceanic-oceanic, and continental-oceanic.
Continental-Continental convergent boundaries occur where two continental plates meet. Continental crust is less dense than oceanic crust is, so continental crust sits above oceanic crust. When two pieces of continental crust collide, the plates can fold and buckle, and mountains are formed as one plate is pushed below the other. This is how the Rocky Mountains (75 million years ago), Himalayas (45 million years ago), and Appalachian Mountains (480 million years ago) all were constructed.
Figure E.3.11.9 – Ridges are found at divergent plate boundaries. New material is being added to plates on either side of the ridge, pushing the rocks apart at the surface.
Oceanic-Oceanic convergent boundaries occur where two oceanic plates meet. Oceanic crust is very dense and floats low on the mantle below. Where the plates meet, one of the plates is subducted. This means that one plate goes under the other, and
the area is called a subduction zone. The plate melts as it encounters the magma below, and earthquakes and volcanoes occur.
Oceanic-Continental convergent boundariesoccur where a continental plate and an oceanic plate meet. Because oceanic crust is made from rock that is more dense than continental crust is, the oceanic plate is subducted under the continental plate where they meet. This forms a trench as the oceanic plate goes under the continental plate. Mountains can form as the continental crust is pushed up where the collision occurs. The subduction zone is where earthquakes and volcanoes occur. The volcanoes of the Pacific Northwest, as well as the Cascadia Mountain range, were formed by oceanic crust of the small Juan De Fuca plate subducting under continental crust of the North American plate along the Pacific Coast. This is the source of the Mount St. Helens volcanic eruption and the Haida Gwaii earthquakes.
Oceanic-Continental convergent boundariesoccur where a continental plate and an oceanic plate meet. Because oceanic crust is made from rock that is more dense than continental crust is, the oceanic plate is subducted under the continental plate where they meet. This forms a trench as the oceanic plate goes under the continental plate. Mountains can form as the continental crust is pushed up where the collision occurs. The subduction zone is where earthquakes and volcanoes occur. The volcanoes of the Pacific Northwest, as well as the Cascadia Mountain range, were formed by oceanic crust of the small Juan De Fuca plate subducting under continental crust of the North American plate along the Pacific Coast. This is the source of the Mount St. Helens volcanic eruption and the Haida Gwaii earthquakes.
Figure E.3.11.10 – High mountain ranges can form as one continental plate is subducted under another.
Figure E.3.11.11 – Trenches and volcanic arcs can form as one oceanic plate is subducted under another.
Figure E.3.11.12 – Volcanic arcs can form where an oceanic crust subducts under a continental plate.
Figure E.3.11.13 – Diverging boundaries also have transform boundaries, or faults.
Transform Boundaries
A transform boundary occurs when two plates grind past each other moving in opposite directions, but neither plate moves up or down. The two main locations of transform boundaries are along ocean ridges and along plate boundaries.
The Mid-Atlantic Ridge and other oceanic ridges are not actually a straight line of expanding ocean crust. They are a zigzag of diverging plate materials and short transform boundaries.
The other kind of transform boundary associated with plate tectonics involves the plates meeting. If two plates are sliding past each other in opposite directions, a transform boundary occurs. This is the site of major earthquakes, such as occur along the San Andreas Fault section of California. Here, the massive Pacific oceanic plate is moving north and the North American continental plate is moving south.
Figure E.3.11.14 – San Andreas Fault is actually three tectonic plates interacting.
Try It!
Yummy Plate Boundaries
This experiment explores the boundaries formed in plate tectonics by using graham wafers, fruit strips, and frosting as a model of parts of Earth. Using this model, you can simulate diverging, converging, and transform boundaries between tectonic plates.
Materials:
This experiment explores the boundaries formed in plate tectonics by using graham wafers, fruit strips, and frosting as a model of parts of Earth. Using this model, you can simulate diverging, converging, and transform boundaries between tectonic plates.
Materials:
- wax paper
- graham wafers
- fruit strips
- plastic spoon and/or knife
- frosting or whipping cream
-
water
Download:
DOWNLOAD this document. It provides the instructions for this activity, and a space for you to answers to questions later in this activity.
DOWNLOAD this document. It provides the instructions for this activity, and a space for you to answers to questions later in this activity.
Instructions:
Divergent Boundary
1. Place about two tablespoons of frosting on a piece of wax paper. Spread it a bit to make a thick layer.
2. Place two fruit strips side by side on the layer of frosting. Leave a very small gap between the two pieces of fruit strip.
3. Press down gently on both fruit strips and move them slowly apart. Observe what happens.
Convergent Boundary: Oceanic-Oceanic
4. Lift the two fruit strips gently from the frosting. Smooth the frosting and place the two fruit strips back on the frosting with a small gap between them.
5. Push the two fruit strips gently towards each other. Observe what happens.
Convergent Boundary: Oceanic-Continental
6. Remove the fruit strips gently from the frosting. You are finished with the subducted fruit strip (the one that went under the other). Keep the top fruit strip, or get a new strip if necessary.
7. Place a fruit strip and a graham wafer on the frosting with a small gap between them. The graham wafer should be sitting higher on the frosting than the fruit strip is.
8. Push the graham wafer gently towards the fruit strip. Observe what happens.
Convergent Boundary: Continental-Continental
9. Remove the fruit strip from the frosting. You do not need the fruit strip anymore. Smooth the frosting and add more if needed.
10. Split a graham wafer, forming two identical pieces.
11. Dip 1 cm of one end of each wafer into a glass of water for 2 seconds -- just count “one-and-two” and remove! (A soggy wafer is useless!)
12. Place the two wafer pieces side-by-side on the frosting layer with a small gap between them. Be sure the wet ends are closest together.
13. Push the wafers gently together so the wet ends meet. Continue pressing the wafers together gently. Observe what happens.
Transform Boundary
14. Remove the graham wafers gently from the frosting. You can use them again if they most of each is solid. Obtain two new pieces if they are not. Smooth the frosting and add more if needed.
15. Place the two wafer pieces side-by-side on the frosting layer so they look like one large piece. Be sure the ends that are closest together are dry.
16. Push the wafers gently past each other in opposite directions while also pushing them together gently. Observe what happens.
Questions:
Think about the following questions very carefully. Then, type or write your answers. After you have your answers, click the questions for feedback.
Think about the following questions very carefully. Then, type or write your answers. After you have your answers, click the questions for feedback.
The frosting is the lower mantle, the fruit strip is an ocean plate, and the wafer is a continental plate. These items work as simple models because the frosting is a fluid that allows the plates to move on top, and the fruit strip and wafer are rigid,
similar to tectonic plates. However, to be more accurate the frosting should be hot with convection currents, the fruit strip should be more rigid, and the fruit strip and graham wafer more complex than just a single type of material to represent
so much lithosphere.
Pressing down on the fruit strip simulates a divergent plate boundary. The model works because the frosting moves up and out through a break in the oceanic crust. However, in reality the magma rises up from below, not from pressure from above such as
you had to use. If this model were more accurate, the magma (frosting) would be producing new oceanic plate material (enlarging the fruit strip).
When one plate goes under another plate, subduction forms a trench. This model forms a trench as long as the wafer continues to slide over the fruit strip and not get caught on it.
In this model, the wet wafers bend similar to the way rocks do when continental plates collide. If the wafers were not soft (wet), no mountain building or subduction could occur because dry wafers are too rigid.
The wafers are rigid and thick enough that you can feel how the two plates would grind past each other along a transform boundary. The fruit strips are very thin and they bend. Therefore, to get them to line up well to model a transform boundary is difficult.
Make sure you have understood everything in this lesson. Use the Self-Check below, and the Self-Check & Lesson Review Tips to
guide your learning.
Unit E Lesson 11 Self-Check
Instructions
Complete the following 6 steps.
Don't skip steps – if you do them in order, you will confirm your
understanding of this lesson and create a study bank for the future.
- DOWNLOAD the self-check quiz by clicking here.
- ANSWER all the questions on the downloaded quiz in the spaces provided. Think carefully before typing your answers. Review this lesson if you need to. Save your quiz when you are done.
- COMPARE your answers with the suggested "Self-Check Quiz Answers" below. WAIT! You didn't skip step 2, did you? It's very important to carefully write out your own answers before checking the suggested answers.
-
REVISE your quiz answers if you need to. If you answered all the questions correctly, you can skip this step. Revise means to change, fix, and add extra notes if you need to. This quiz is NOT FOR MARKS, so it is perfectly OK to correct
any mistakes you made. This will make your self-check quiz an excellent study tool you can use later.
- SAVE your quiz to a folder on your computer, or to your Private Files. That way you will know where it is for later studying.
- CHECK with your teacher if you need to. If after completing all these steps you are still not sure about the questions or your answers, you should ask for more feedback from your teacher. To do this, post in the Course Questions Forum, or send your teacher an email. In either case, attach your completed quiz and ask; "Can you look at this quiz and give me some feedback please?" They will be happy to help you!
Self-Check Time!
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Self-Check Quiz Answers
Click each of the suggested answers below, and carefully compare your answers to the suggested answers.
If you have not done the quiz yet – STOP – and go back to step 1 above. Do not look at the answers without first trying the questions.
Any answer that you give should explain that the tectonic plates ‘float’ on top of the mantle and that the plates are enlarged (at divergent boundaries) and destroyed (at convergent boundaries). Because the mantle is hot, simple models usually are not very accurate, but you could use flat pieces of cookie, for example, pushed across a layer of peanut butter to describe the movement of tectonic plates on the mantle.
The Mid-Atlantic Ridge is adding to its plates at 3.5 cm per year (2 cm/year for Africa, 1.5 cm/year for South America). The rate that the continents are moving is equal to how fast new plate material is being added along the ridge.
Convergent boundaries result in one plate being subducted under another plate. In this case, one oceanic plate is being pushed and/or pulled under another. Oceanic plates are already deeply underwater, so the subduction of the Pacific Plate results in a very deep trench.
The Pacific Plate and the North American Plate are two huge plates pushing past each other, causing massive earthquakes in Alaska. In fact, some of the largest earthquakes ever measured have occurred along the Queen Charlotte transform boundary.
The hard cookies of the Oreo could be used as continental plates, and the inner soft filling would represent the magma on which the plates float. Return to Lesson E11 and review how graham crackers and frosting were used to model plate boundaries. Unfortunately, the Oreo does not work well for boundaries involving oceanic plates. You would have to use fruit strips or some similar material to simulate the thinner, more dense oceanic plates if you chose a boundary that models an oceanic plate.