GEOL 1110, Chapter 10, Plate Tectonics PDF

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Chapter 10. Plate Tectonics Plate Tectonics: The Unifying Theory of Geology Plate tectonics is the theory that the Earth’s crust and upper mantle is made up of pieces or plates that move. This movement of plates is responsible for all sorts of geological processes and features such as volcanoes, mountain building and earthquakes. The movement of plates is slow, 1 to 10 cm per year on average, which is about the same rate that your finger nails grow. The direction a plate moves depends on the plate itself and the behaviour of the plates around it. The city of Vancouver sits on the western edge of the North America Plate (Chapter 10, Figure 10.16, page 279) which is moving to the west at about 6 cm per year.

C

B

Vancouver

A

D

E F

Figure 10.16, page 279 in the course textbook. The Earth’s tectonic plates. 1

Until the theory of plate tectonics was understood and accepted in the early 1960s there was no generally accepted idea or unifying theory to explain how the Earth worked. With this theory we have a context within which to explain many other geological processes and features. Mountains, such as those along the south coast of British Columbia, were originally thought to be the result of the Earth cooling and contracting with the out layer or crust folding to form mountains as it shrank in size. We now explain mountain building as the result of a collision between two plates with one plate being folding and pushed up to form a mountain chain along the plate boundary.

Study Point #10 - 1. History of the Theory of Plate Tectonics A detailed history of the theory of plate tectonics is in sections 10.1 to 10.3, pages 260 to 278, in the course textbook. Make sure you read these sections and understand the development of this theory.

On page 259 in the course textbook there is a discussion about a number of reasons why Wegeners theory of “Continental Drift”, an early version of the theory of plate tectonics, was not accepted. While textbook talks about politics and competing ideologies Wegner published his theory of Continental Drift in 1915 in the book “The Origin of Continents and Oceans” during World War 1 - there is a more practical side to this discussion. In 1915 there was very little known about the oceans - how deep they were, what was the nature of the crust beneath them, etc - a vital aspect of any explanation of the Earth and no explanation at all of a force that could move large pieces of the crust. It wasn’t until the 1960s that technology and more research lead to the theory of plate tectonics and a better interpretation of how the Earth worked. Why is it still a theory? Despite all the research and all the information about the Earth, plate tectonics remains a theory. Why don’t we accept it as an absolute, as a law? The way we treat plate tectonics as a theory shows science at its best.

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Remember what a theory is “a supposition or a system of ideas intended to explain something, especially one based on general principles independent of the thing to be explained.” Look at all the vague or not absolute words in this definition - “supposition, ideas, intended” - all used to emphasize the point that we really don’t know all the details and that our explanation, our theory, is not absolute and could change. There is conclusive evidence that the Earth has a crust that’s separated into pieces or plates that move. We can map plates and see them move, especially during an earthquake. So, what is it that makes plate tectonics still theoretical? One aspect of plate tectonics we’re more uncertain about is the driving force behind plate motion. We cannot answer the question ‘what causes plate to move?’ with any certainty. Section 10.5, pages 291 to 292, in the course textbook provides a brief summary of the mechanisms of plate motion. The following is a more detailed explanation of plate motion and driving forces. 10.5. Mechanisms of Plate Motion Types of Crust. As we’ve seen in figure 1 - A in the study guide for chapter 1 and figure 1.7, page 12, in the course textbook, the interior of the Earth is made up of a core, mantle and crust. As far as plate tectonics goes, we are primarily interested in the crust, the thin outer later we live on and where most plate tectonic processes occur. There are two types of plate in the Earth’s crust: Oceanic and Continental. Table 10 - 1 summarizes all their key points.

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Table 10 - 1. A comparison of key features of oceanic and continental plates

Feature

Oceanic Plate

Continental Plate

Location

Ocean basins

Continental land masses

Thickness

7 to 10 kms

25 to 70 km

Rock Type

Mafic

Felsic

Rock Colour

Black

White

Density

Heavier (3.0 g/cm3)

Lighter (2.7 g/cm3)

A really important difference between oceanic and continental crust is density. Oceanic crust is slightly more dense than continental crust which is a key factor at plate boundaries. Convection Currents, Ridge Push and Slab Pull. The most important factor when it comes to what causes plate motion is the Earths internal heat. The inside of the Earth is hot, it is very hot. How hot and why is explained in chapter 3. With all this heat, rocks inside the Earth, specifically in the mantle, may be partially melted. This means that typically only about 10% of the rock is actually in a liquid state, but it’s enough to allow the entire mass of rock, liquid and solid, to flow or move like a fluid. So where does a rock like this in the mantle go when it starts to move? Up toward the surface. After all a partially melted rock is less dense than the solid rock next to it and so it rises. As this ‘hot’ rock rises it starts to cool. As it cools it begins to solidify, get heavier and slow down. Eventually it reaches a point closer to the surface where it stops rising. 4

Because there is more hotter, less dense rock rising behind it, this cooler, denser rock is pushed sideways, often in opposite directions, where it eventually sinks back down into the mantle as its density and weight increases. This completes a convection current, the cyclical movement of rock in the Earth’s mantle due to differences in temperature inside the Earth (Chapter 10, Figure 10.28, page 291). Convection Current

COLD

HOT Figure 10.28, page 291. Models for plate motion mechanisms.

What does a convection current have to do with plate tectonics? As rock in the mantle moves sideways or laterally under the crust it drags the crust along with it because of friction between the mantle and crust. The end result is the crust is broken apart and the pieces moved in different directions at different rates depending upon the nature of the convection current. But wait, there’s more. There are two other driving mechanisms behind plate motion: Ridge Push and Slab Pull (Chapter 10, Figure 10.28, page 291). Where a convection current is found beneath the crust it will often break the crust apart and move it in opposite directions as the currents stops rising and moves laterally under the crust. Most often the crust that is moving is oceanic crust because it’s more common and thinner which makes it easier to break. At the same time the two pieces of crust are pushed upward by the convection current to form an elevated ridge. Add to this molten rock extruded into the ridge from the mantle and you have what can amount to a mountain range at this break in the crust. 5

As this ridge rises gravity works at pulling it back down. However, gravity can’t simply pull the ridge back down. Instead it’s pulled down and out and this outward motion pushes the crust next to it forcing it to move. This is ‘ridge push’ and it causes the entire plate to move not just the elevated ridge that is one part of the plate. As the piece of crustal plate moves it gets heavier. It’s density increases as it moves away from the heat of the convection current and it’s covered with more and more sediment adding the weight pushing down on it. Eventually this plate sinks back into the mantle as it is too heavy to remain at the surface. As this heavier leading edge of the plate descends into the mantle it pulls the rest of the plate along with it. As a result the entire plate is moving. Which of these processes is most important? Who knows, it all depends. Suffice it to say they’re all important and it’s this uncertainty above something that we think takes place deep inside the Earth that is one reason why plate tectonics remains very much a theory. 10.4. Plates, Plate Motions, and Plate Boundary Processes Plate boundaries are where the pieces of the Earth’s crust meet. This is where many geological processes and features are found. Take the west coast of British Columbia. Here we have mountains, earthquakes and volcanoes, major geological processes and landforms that define the place where we live. Now consider Winnipeg, Manitoba. There are no mountains, very little in the way of earthquake activity and no volcanoes. What’s the different between these two locations? The west coast of British Columba, which includes the city of Vancouver, is located on a plate boundary. Southern Manitoba, including Winnipeg, is not on a plate boundary. Plate boundaries are very important. They help explain many aspects of geology - thank you Alfred Wegener and there are three types. 1. DIVERGENT PLATE BOUNDARIES Here is where two plates pull apart because a convection current is pulling the crust in opposite directions (Chapter 10, Figure 10.18, page 281). Most often this happens with oceanic crust because it is thinner and more common. As the crust is pulled apart a rift opens between the two plates into which molten rock from the Earth’s interior is intruded. This may result in volcanism on the surface. As 6

New, Hot, Thin, Less Dense Oceanic Crust

Elevated Ridge

Rift

with Volcanism New, Hot, Thin, Less Dense Oceanic Crust

Old, Cold, Thick,

Old, Cold, Thick,

More Dense

More Dense Oceanic

Oceanic Crust

Crust

Figure 10.18, page 281. A divergent plate boundary.

this molten rock cools and solidifies new crust if formed. Once formed, this crust is carried away by plate movement and more new crust is created in its place. As result the crust near the rift is new, hot, thin and less dense. Crust that was once at the rift and has been carried away is by comparison old, cold, thick and more dense. It’s this old heavy crust that contributes to slab pull. A good example of a divergent boundary is the Mid-Atlantic Ridge (labelled A in figure 10.16 on page 1 of this study guide) that extends down the center of the Atlantic Ocean. The best example where continental plate is rifting apart is the East African Rift in East Africa.

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2. CONVERGENT PLATE BOUNDARIES Where two plates push against one another there is a convergent plate boundary. There are three different types of convergent plate boundary depending upon the types of plate involved. 2 - A. OCEANIC VS CONTINENTAL CONVERGENT PLATE BOUNDARY Here is where an oceanic plate is colliding with a continental plate. When these two plates collide the oceanic plate, because it is more dense and therefore heavier, is subducted beneath or pushed under the less dense, lighter continental plate (Chapter 10, Figure 10.22, page 283). There’s that important difference talked about on pages 3 and 4 of this study guide. The subducted oceanic plate eventually starts to melt as it is pushed deeper into the Earth’s hot interior. As molten rock forms it rises upward adding more rock to the overlying continental plate and eventually resulting in volcanism on the continental plate. As the oceanic late descends past the continental plate friction develops between them and they get stuck together. The plate continues to converge and stress builds up. At this contact where the two plates are trying to move past one another is where many earthquakes originate. Eventually there is more stress then the rock can handle and the rock breaks. The frictional lock that was keeping the plates in place is lost, the plates move suddenly and energy is released that results in an earthquake. On the surface where the two plates meet there is often a trench because the two plates have been pulled down. This trench is sometimes partially or completely filled by an accretionary wedge, a big pile of sediment created by sediment carried to the trench from the adjacent continental plate. The continental plate is obducted or pushed up and over the oceanic plate because it is less dense. It is bent, broken and pushed upward to thicken the continental plate and create mountains. A good example of an oceanic vs continental convergent plate margin is the west coast of British Columbia where the continental North America Plate is colliding with the oceanic Juan de Fuca Plate (labelled B in figure 10.16 on page 1 of this study guide). 8

Mountain Trench

Building Volcanism

Continental

Oceanic Plate Earthquake

Melting

Zone

Plate

Figure 10.22, page 283. An ocean vs continental convergent plate boundary.

2 - B. OCEANIC VS OCEANIC CONVERGENT PLATE BOUNDARY Where an oceanic plate collides with another ocean plate there is an oceanic vs oceanic convergent plate boundary (Chapter 10, Figure 10.21, page 283). As these two similar plates push against each other one of them is pushed under or subducted. Which plate is subducted depends upon the thickness of the plates, the speeds at which they are moving and the angle of the collision. Once one plate is pushed under and the other goes over (obducted) then many of the same processes and features of an oceanic vs continental plate boundary occur here. The subducted plate is eventually melted and molten rock is created which results in volcanism on the non-subucted or obducted plate. The chain of volcanic islands that often appear above sea-level along this plate boundary is called an island arc. The obducted plate is also bent, broken and pushed upward to form mountains. There is a zone of earthquake activity where the two plates are trying to get past one another and 9

a deep sea trench forms where the two adjoining plates are pulled downward. The deepest point in the ocean, Marianna’s Trench is formed at an oceanic vs oceanic plate boundary. A good example of an oceanic vs oceanic convergent plate margin is Japan, a chain of volcanic islands where the oceanic Filipino and Pacific plates are colliding with the oceanic part of the Eurasian Plate (labelled C in figure 10.16 on page 1 of this study guide). Volcanism, a volcanic island that is part of an island arc

Oceanic Plate

Oceanic Plate

Earthquake Zone

Melting

Figure 10.21, page 283. An ocean vs oceanic convergent plate boundary.

2 - C. CONTINENTAL VS CONTINENTAL CONVERGENT PLATE BOUNDARY Where two continental plates collide there is a continental vs continental convergent plate margin (Chapter 10, Figure 10.23, page 284). These two plates are less dense than the underlying mantle and therefore float on top of it. As a result there is no subduction as neither plate is pushed into the Earth’s interior where it melts. Because there is no subduction of continental crust there is no melting and no volcanism.

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Instead both plates are bent, broken and pushed upward to form mountains. Where the two plates are trying to get past one another there is a zone of earthquakes. It may be that a piece of heavier oceanic crust was between the two continental plates as the pushed toward each other. This piece of oceanic crust will be subducted beneath the continental plates, it is after all heavier and can sink. Because it’s subducted and does eventually melt there could initially be volcanism on some continental plates at this kind of convergent plate margin. However, this is a rare occurrence because of the smaller size the slab of oceanic crust and the thickness of the overlying continental plate. A good example of a continental vs continental convergent plate margin is in Asia where India has collided with Asia to create the Himalaya mountain range (labelled D in figure 10.16 on page 1 of this study guide). Mountain Building

Continental Plate Continental Plate

Earthquake Zone Remnant of Oceanic Plate

Figure 10.23, page 284. A continental vs continental convergent plate boundary.

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3. TRANSFORM PLATE BOUNDARY A transform plate boundary is where two plates, either oceanic, continental or a mix of both, are trying to move past one another in a side-by-side motion (Chapter 10, Figure 10.24, page 285). The plates are moving horizontally across the Earth’s surface and there is no subduction. As a result there is no melting and no volcanism at this plate boundary. There is a zone along the plate boundary where earthquakes can occur because the two plates get stuck together as they try and push past one another. The exact location of a transform boundary needs a little more explanation. This type of boundary is only where the two plates are moving past one another in a side by side motion. If you look at figure 10.24 from page 285 in the course textbook, and on page 13 of this study guide, there is the San Andreas Fault, a well know transform boundary along the western edge of North America. The San Andrea Fault is also labelled E in figure 10.16 on page 1 of this study guide. The San Andreas Fault is the red line in the image on the next page. It is bounded by two divergent plate margins shown in the image by the thick black lines. The yellow arrows show the direction of plate motion along this fault or boundary. But where exactly is the San Andreas Fault a transform boundary? It is only moving in a side-byside motion between points 1 and 2, so it’s only between points 1 and 2 that it is a transform boundary. It’s important to be able to recognize where exactly these types of boundaries are and why. 4. PASSIVE PLATE BOUNDARY Another type of plate boundary is a passive boundary where two plates are connected and moving together as one single piece of crust. Here there is no rifting apart, no collision, no transform motion and no subduction. The east coast of North America is a passive margin where the western part of the Atlantic Ocean, a piece of oceanic plate, is joined to and moving with the North America continent which are both part of the North America Plate. Figure 10.26 from page 288 in the course textbook shows this passive margin and speculates on how it may change in future. This passive margin is labelled F in figure 10.16 on page 1 of this study guide.

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Vancouver

Divergent Plate Boundary

1 2

Earthquake Zone is all along the transform boundary

Divergent Plate Boundary

Figure 10.24, page 285. A transform plate boundary. 13

Study Point #10 - 2. Passive Margins Make sure you’re familiar with the reasons for the change to a convergent plate margin along the east coast of North America.

Figure 10.26, page 288. A passive plate boundary and the possible evolution of this plate margin along the east coast of North America. 14

The Lithosphere and Asthenosphere. We already know from chapter 1 and the chapter 1 study guide that the interior of the Earth is made up of four concentric layers (Chapter 1, Figure 1 - 7, page 12). From the center of the Earth outward to the surface they are the inner core, outer core - this is where the Earth’s magnetic field is generated - the mantle and finally the crust. The crust is primarily what we’re interested in for this course. These four layers are defined by composition. The inner and outer core are made up ...