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Chapter 8
Plate Tectonics
In this chapter, you will learn about one of the most important discoveries of the 20th century—plate tectonics. You have already learned about Earth’s surface and that it is covered with a lithosphere that is broken into pieces called “plates.” Plate tectonics is the study of the movement of these plates. It is a relatively new field of study. Scientists have only arrived at our current understanding of plate tectonics over the past 40 years. This is a very short time in science years!
1. How is the surface of Earth like a giant jigsaw **_puzzle?
- Why are magnetic patterns important?
- How do rocks change?_**
UNIT 3 PLATE TECTONICS AND EARTH ’S STRUCTURE
Figure 8.1: The continents on either side of the Atlantic Ocean fit together like puzzle pieces.
continental drift - the idea that continents move around on Earth’s surface. Pangaea - an ancient, huge landmass composed of earlier forms of today’s continents; an ancient supercontinent. plate tectonics - a theory explaining how the pieces of Earth’s surface (the plates) move.
8.1 Alfred Wegener’s Supercontinent
While looking at a map of the world, have you ever noticed that the continents look like pieces of a puzzle? If they are moved closer together across the Atlantic Ocean, they would fit neatly to form a giant landmass (Figure 8.1). In this section, you will learn about Alfred Wegener and his idea that a “supercontinent” once existed on Earth.
Movement of continents
Continental drift Alfred Wegener was a German climatologist and arctic explorer
who suggested the concept of continental drift_._ Continental drift is the idea that the continents move around on Earth’s surface.
Wegener’s
hypothesis
In the early 1900s, Wegener hypothesized that the continents were once connected. Today, after a lot of scientific research and collected evidence, we know that Wegener was right.
Pangaea —
a supercontinent
In 1915, Wegener published his ideas in a book, Origins of the Continents and Oceans. Wegener thought that the continents we know today had once been part of an earlier supercontinent. He called this great landmass Pangaea (Greek for “all land”). According to continental drift, Pangaea broke apart and the pieces moved to their present places, becoming today’s continents.
What is plate
tectonics?
In Chapter 1, you were introduced to plate tectonics, the study of lithospheric plates. You learned that the surface of Earth is broken into many pieces like a giant jigsaw puzzle. Plate tectonics describes how these pieces move on Earth’s surface. By the time you finish this chapter, you will know more about this theory than any scientist knew only forty years ago. Wow! Additionally, you will learn that the development of this theory is an excellent example of how the scientific process works. Now, let’s return to Wegener and his idea of continental drift.
UNIT 3 PLATE TECTONICS AND EARTH ’S STRUCTURE
Figure 8.2: Fossils that are found on modern continents. In the ancient past, the modern continents were connected as a supercontinent.
8.1 Section Review
- Who was Alfred Wegener?
- Alfred Wegener thought that all continents were once connected. Explain one observation that led to this belief.
- Why did scientists reject Wegener’s idea of continental drift?
- In this section, you read that the development of the theory of plate tectonics is a good example of the scientific process. a. How did Wegener follow the scientific process? b. When scientists rejected continental drift, were they using the scientific process? Why or why not?
- Answer these challenge questions. a. Name the seven modern continents. b. Make a table that lists the modern continents and describes the animal fossils that are found on each, according to the graphic below (Figure 8.2). c. A long time ago, glaciers covered parts of some the continents. Why aren’t glaciers on these continents today?
8.2 SEA- FLOOR SPREADING
CHAPTER 8: PLATE TECTONICS
8.2 Sea-floor Spreading
In Wegener’s time, the world’s ocean floors were largely unexplored. Mapping the sea floor provided more important evidence for the theory of plate tectonics.
Undersea mountains discovered
A map of the
ocean floor
During World War II, the United States Navy needed to locate enemy submarines hiding on the bottom of shallow seas. Therefore, large areas of the ocean floor were mapped for the first time. American geophysicist and Naval officer Harry Hess did some of the mapping. His work helped develop the theory of plate tectonics.
Mid-ocean ridges The naval maps showed huge mountain ranges that formed a
continuous chain down the centers of the ocean floors. These mountain ranges are called mid-ocean ridges. Hess was intrigued by their shape and location. He wondered if it was possible that new ocean floor was created at the mid-ocean ridges. If new ocean floor formed at the ridges, then continents on either side would get pushed apart during the process (Figure 8.3).
Figure 8.3: Harry Hess wondered if it was possible that new ocean floor was created at the mid-ocean ridges.
mid-ocean ridges - long chains of undersea mountains.
8.2 SEA- FLOOR SPREADING
CHAPTER 8: PLATE TECTONICS
Moving pieces of the lithosphere
After a
breakthrough
After the breakthrough discovery of magnetic patterns was understood, there was great scientific interest in the idea of sea-floor spreading. Scientists realized that large pieces of Earth’s surface moved about like rafts on a river.
Today we know these “rafts” are pieces of lithosphere called lithospheric plates that move over the aesthenosphere (review the interior of Earth in Chapter 7). Plate tectonics is the study of these lithospheric plates.
There are two kinds of lithospheric plates: oceanic plates and continental plates. Oceanic plates form the floor of the ocean. They are thin and made of dense basalt. Continental plates are thick and made of less-dense granite. Often a lithospheric plate is a mix of both kinds of plates.
Plate tectonics
answers other
questions
Science is a process that builds on itself. Early discoveries provide a better understanding that leads to more discoveries. The evidence that Alfred Wegener collected to support an ancient supercontinent is valid today. And our understanding of plate tectonics has allowed us to answer other questions such as:
- Why are volcanoes and earthquakes located where they are?
- Where can we find oil, gas, gold, and other important resources?
lithospheric plates - large pieces of Earth’s lithosphere that move over the aesthenosphere. oceanic plates - thin, dense lithospheric plates that are made of basalt and form the ocean floor. continental plates - thick, less- dense lithospheric plates that are made of granite and form the continents.
How is plate tectonics related to earthquakes and volcanoes? Write an answer based on what you know. Then, check your answer by doing research to answer this question. Use research resources in your classroom and school library.
UNIT 3 PLATE TECTONICS AND EARTH ’S STRUCTURE
Figure 8.5: A convection cell in the lower mantle.
subduction - a process that involves a lithospheric plate sinking into the mantle.
What drives lithospheric plates?
Convection cells Convection cells in Earth’s lower mantle drive the lithospheric
plates on the surface. Here again we see the effect of heat on materials. The rocks of the lower mantle are not brittle like the rocks of the lithosphere. They are hot enough so that they flow very slowly. The core heats the rock material of the lower mantle. As it is heated, it expands and becomes less dense.
The lower mantle rock material rises toward Earth’s surface and may divide the lithosphere above and form a mid- ocean ridge. The pieces of separated lithospheric plate will move away from each other on either side of the new mid-ocean ridge.
Subduction The far edge of a
lithospheric plate is much older than the edge close to the mid-ocean ridge that formed it. Over time, the far edge cools and becomes denser (Figure 8.5). Eventually, it may sink below another lithospheric plate and enter the lower mantle. This sinking process is called subduction. As the now-cool subducting plate enters the lower mantle, it cools the nearby lower mantle material in turn. Cooling makes the nearby material denser and it sinks deeper into the lower mantle. This sinking completes the lower mantle convection cell.
UNIT 3 PLATE TECTONICS AND EARTH ’S STRUCTURE
Create a table to compare and contrast continental drift and plate tectonics. Include the answers to the following questions. Which is a hypothesis and which is a theory? What is the difference between these two ideas when explaining why Africa and South America seem to fit together like two puzzle pieces?
Drummond Matthews and Fred Vine, British geologists from Cambridge University, England, are credited with recognizing the significance of magnetic patterns. Research these magnetic patterns so that you understand what they are and how they are caused. Make a poster to display your findings. Although the contributions of Matthews and Vine are considered huge among all earth scientists, they never received any special recognition for their work.
8.2 Section Review
- Explain why magnetic patterns are important evidence for plate tectonics?
- How were mid-ocean ridges discovered?
- What was Harry Hess’ hypothesis regarding the ocean floor and how it was made?
- What two discoveries supported Hess’ hypothesis?
- What is the study of lithospheric plates called?
- Over what surface do lithospheric plates move?
- Name the two types of lithospheric plates and describe them.
- What are some questions that are answered by plate tectonics?
- What is the source of energy that drives the movement of the lithospheric plates?
- Do lithospheric plates move quickly or slowly? Explain your answer.
- Describe the process of subduction in your own words. What causes subduction to happen?
- Name an example of an island chain formed by a mantle plume hot spot. Describe/draw the process of how it forms.
- Research question: The Mid-Atlantic Ridge (see page 16) goes through the country of Iceland. What effect is it having on this country?
- Research question: It is thought that when Pangaea broke apart, it first split into two large landmasses, each of which were given names. What are the names? What do these names mean?
a. lower mantle b. outer core c. inner core d. aesthenosphere
