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Earth 3: Plate Tectonics
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Printed: June 20, 2015
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Contents www.ck12.org
Contents
1 Inside the Earth 1
2 Earth’s Continental Drift 10
3 Seafloor Spreading on Earth 17
4 The Theory of Plate Tectonics 24
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www.ck12.org Concept 1. Inside the Earth
CONCEPT 1 Inside the Earth
Points to Consider
• If sediments fall onto the seafloor over time, what can sediment thickness tell scientists about the age of theseafloor in different regions?
Lesson Objectives
• Compare and describe each of Earth’s layers.• Compare some of the ways geologists learn about Earth’s interior.• Define oceanic and continental crust and the lithosphere.• Describe how heat moves, particularly how convection takes place in the mantle.• Compare the two parts of the core and describe why they are different from each other.
Vocabulary
• asthenosphere: part of the upper mantle below that lithosphere that can flow and bend• convection cell: circular current of warm material rising and cool material sinking that transfers heat• continental crust: thicker, denser part of Earth’s crust that makes up the continents• core: dense, metallic center of Earth, consisting of the inner and outer core crust thin, brittle outer shell of
Earth, consisting of continental and oceanic crust
• lithosphere: rigid part of Earth that consists of the crust and upper mantle, lying above the asthenosphere• mantle: middle layer of Earth, consisting of hot, solid rock• meteorite: metallic object from the early solar system that strikes Earth’s surface from space• oceanic crust: thinner, less dense part of Earth’s crust that makes up the ocean basins• plate tectonics: theory that Earth’s lithosphere is divided into plates that move over the asthenosphere• seismic wave: earthquake wave that transports energy from an earthquake through the ground in all directions
Fill in the blank with the appropriate term.
1. Scientists learn about Earth’s interior by studying __________ caused by earthquakes.2. Scientist’s think that Earth’s core is similar in composition to a(n) __________.3. The part of the crust that is made only of basalt is the __________ crust.4. The part of the crust that is made of many types of rock is the __________ crust.5. Continental crust is __________ dense than oceanic crust.6. The lower mantle is heated directly by the core through the process of __________.7. Heat moves throughout the mantle by the process of __________.
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Introduction
From outside to inside, Earth is divided into crust, mantle, and core. Each has a different chemical makeup. Earthcan also be divided into layers with different properties. The two most important are lithosphere and asthenosphere.
How Do We Know About Earth’s Interior?
If someone told you to figure out what is inside Earth, what would you do? How could you figure out what is insideour planet? How do scientists figure it out?
Seismic Waves
Geologists study earthquake waves to “see” Earth’s interior. Waves of energy radiate out from an earthquake’sfocus. These are called seismic waves ( Figure 1.1). Seismic waves change speed as they move through differentmaterials. This causes them to bend. Some seismic waves do not travel through liquids or gases. Scientists use allof this information to understand what makes up the Earth’s interior.
FIGURE 1.1The properties of seismic waves allowscientists to understand the compositionof Earth’s interior.
Meteorites
Scientists study meteorites to learn about Earth’s interior. Meteorites formed in the early solar system. These objectsrepresent early solar system materials. Some meteorites are made of iron and nickel. They are thought to be verysimilar to Earth’s core ( Figure 1.2). An iron meteorite is the closest thing to a sample of the core that scientists canhold in their hands!
Pause to Reflect
1. List two ways that scientists learn about what makes up the Earth’s interior.
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FIGURE 1.2The Willamette Meteorite is a metallic meteorite that was found in Ore-gon.
Crust
Crust, mantle, and core differ from each other in chemical composition. It’s understandable that scientists know themost about the crust, and less about deeper layers ( Figure 1.3). Earth’s crust is a thin, brittle outer shell. The crustis made of rock. This layer is thinner under the oceans and much thicker in mountain ranges.
Oceanic Crust
There are two kinds of crust. Oceanic crust is made of basalt lavas that flow onto the seafloor. It is relatively thin,between 5 to 12 kilometers thick (3 - 8 miles). The rocks of the oceanic crust are denser (3.0 g/cm3) than the rocksthat make up the continents. Thick layers of mud cover much of the ocean floor.
Continental Crust
Continental crust is much thicker than oceanic crust. It is 35 kilometers (22 miles) thick on average, but it varies alot. Continental crust is made up of many different rocks. All three major rock types — igneous, metamorphic, andsedimentary — are found in the crust. On average, continental crust is much less dense (2.7 g/cm3) than oceaniccrust. Since it is less dense, it rises higher above the mantle than oceanic crust.
Pause to Reflect
1. What type of rock makes up the oceanic crust?
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FIGURE 1.3A cross-section of Earth showing the fol-lowing layers: (1) continental crust, (2)oceanic crust, (3) upper mantle, (4) lowermantle, (5) outer core, (6) inner core.
2. What type of rock makes up the continental crust?
Mantle
Beneath the crust is the mantle. The mantle is made of hot, solid rock. Through the process of conduction, heatflows from warmer objects to cooler objects ( Figure 1.4). The lower mantle is heated directly by conduction fromthe core.
Hot lower mantle material rises upwards ( Figure 1.5). As it rises, it cools. At the top of the mantle it moveshorizontally. Over time it becomes cool and dense enough that it sinks. Back at the bottom of the mantle, it travelshorizontally. Eventually the material gets to the location where warm mantle material is rising. The rising andsinking of warm and cooler material is convection. The motion described creates a convection cell.
Core
The dense, iron core forms the center of the Earth. Scientists know that the core is metal from studying metallicmeteorites and the Earth’s density. Seismic waves show that the outer core is liquid, while the inner core is solid.
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FIGURE 1.4In the process of conduction, heat flowsfrom warmer objects to cooler objects.
FIGURE 1.5The rising and sinking of mantle materialof different temperatures and densitiescreates a convection cell.
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Movement within Earth’s outer liquid iron core creates Earth’s magnetic field. These convection currents form inthe outer core because the base of the outer core is heated by the even hotter inner core.
Pause to Reflect
1. When you put your hand near a pan above a pan filled with boiling water, does your hand warm up because ofconvection or conduction? If you touch the pan, does your hand warm up because of convection or conduction?
2. List two reasons that scientists know that the outer core is liquid.
3. Suppose that Earth’s interior contains a large amount of lead. Lead is very dense: 11.34 g/cm3. Would the lead bemore likely to be found in the crust, mantle, or core?
4. The oceanic crust is thinner and denser than continental crust. All crust sits atop the mantle. What might ourplanet be like if this were not true?
5. How might convection cells in the mantle affect the movement of plates of lithosphere on the planet’s surface?
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Lithosphere and Asthenosphere
Lithosphere and asthenosphere are layers based on physical properties. The outermost layer is the lithosphere. Thelithosphere is the crust and the uppermost mantle. In terms of physical properties, this layer is rigid, solid, and brittle.It is easily cracked or broken.
Below the lithosphere is the asthenosphere. The asthenosphere is also in the upper mantle. This layer is solid, butit can flow and bend. A solid that can flow is like silly putty.
Pause to Reflect
1. Describe the properties of the lithosphere and asthenosphere. What parts of the Earth do these layers include?
Lesson Summary
• The Earth is made of three layers with different composition: the crust, mantle, and core.• The lithosphere is made of the rigid, brittle, solid crust and uppermost mantle.• Beneath the lithosphere, the asthenosphere is solid rock that can flow.• The hot core warms the base of the mantle, which creates convection currents in the mantle.
Lesson Review Questions
True or False
Write true if the statement is true or false if the statement is false.
_____ 1. The mantle is divided into the inner mantle and outer mantle.
_____ 2. Earthquakes send waves of energy through rocks inside Earth.
_____ 3. Meteorites formed a long time ago in the early solar system.
_____ 4. Earth’s crust is made of solid rock.
_____ 5. Lava flows formed the oceanic crust.
_____ 6. The continental crust contains only igneous rock.
_____ 7. Heat travels from the top to the bottom of the mantle.
_____ 8. Earth’s core is very dense.
_____ 9. Convection currents occur in the inner core.
_____ 10. Plate tectonics is the theory that the lithosphere is divided into plates that move over Earth’s surface.
Multiple Choice
Circle the letter of the correct choice.
11. Compared with the other layers of Earth, the crust is very
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1. a. thick.b. warm.c. brittle.d. two of the above
12. Seismic waves reveal information about Earth’s interior because they travel
1. a. at different speeds through different materials.b. only through liquids and gases.c. at the same speed as sound.d. only in straight lines.
13. Earth’s layers differ from one another in
1. a. chemical makeup.b. temperature.c. state of matter.d. all of the above
14. Compared to oceanic crust, continental crust is
1. a. denser.b. thicker.c. less variable.d. all of the above
15. Earth’s magnetic field is created by movements in Earth’s
1. a. inner core.b. outer core.c. mantle.d. crust.
16. The lithosphere is
1. a. solid.b. rigid.c. able to flow.d. two of the above
17. The consistency of the asthenosphere is most like
1. a. hard plastic.b. frozen water.c. silly putty.d. solid metal.
References
1. Christopher Auyeung and Laura Guerin. CK-12 Foundation . CC BY-NC 3.0
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2. Flickr:herval. http://commons.wikimedia.org/wiki/File:Willamette_meteorite_AMNH.jpg . CC BY 2.03. Laura Guerin. CK-12 Foundation . CC BY-NC 3.04. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.05. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.0
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CONCEPT 2 Earth’s Continental Drift
Points to Consider
1. What is the difference between a hypothesis and a theory?
2. Why do you think continental drift referred to as a hypothesis and not a theory?
Lesson Objectives
• Be able to explain the continental drift hypothesis.• Describe the evidence Wegener used to support his continental drift idea.• Describe how the north magnetic pole appeared to move, and how that is evidence for continental drift.
Vocabulary
• continental drift: early 20th century hypothesis that the continents move over Earth’s surface• magnetic field: area surrounding a magnet over which it exerts magnetic force
Fill in the blank with the appropriate term.
1. The meteorologist who first proposed the idea of continental drift was __________.2. Continental drift is supported by glacial evidence that has been found near the __________.3. Wegener found coral reef fossils in areas that are too __________ today for corals to live.4. As lava cools, tiny iron-rich crystals line up with Earth’s __________.5. The theory of plate tectonics explains how Earth’s continents are able to __________.6. Evidence for continental drift includes rocks that were once side-by-side and are now located on separate
__________.
Introduction
To develop plate tectonics, first scientists had to accept that continents could move. Today they do. But it took along time for scientists to accept that this could happen ( Figure 2.1). This idea is called continental drift.
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FIGURE 2.1The supercontinent Pangaea containedall of the modern day continents.
The Continental Drift Idea
Alfred Wegener was an early 20th century German meteorologist. Wegener believed that the continents were onceall joined together. He named the supercontinent Pangaea, meaning “all earth.“ Wegener suggested that Pangaeabroke up long ago. Since then, the continents have been moving to their current positions. He called his hypothesiscontinental drift.
Evidence for Continental Drift
Wegener and his supporters collected a great deal of evidence for the continental drift hypothesis. Wegener foundthat this evidence was best explained if the continents had at one time been joined together.
Rocks and Geologic Structures
Wegener found rocks of the same type and age on both sides of the Atlantic Ocean. He thought that the rocks formedside by side. These rocks then drifted apart on separate continents.
Wegener also matched up mountain ranges across the Atlantic Ocean. The Appalachian Mountains were just likemountain ranges in eastern Greenland, Ireland, Great Britain, and Norway. Wegener concluded that they formed asa single mountain range. This mountain range broke apart as the continents split up. The mountain range separatedas the continents drifted.
Fossil Plants and Animals
Wegener also found evidence for continental drift from fossils ( Figure 2.2). The same type of plant and animalfossils are found on continents that are now widely separated. These organisms would not have been able to travel
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across the oceans.
Fossils of the seed fern Glossopteris are found across all of the southern continents. These seeds are too heavy tobe carried across the ocean by wind. Mesosaurus fossils are found in South America and South Africa. Mesosauruscould swim, but only in fresh water. Cynognathus and Lystrosaurus were reptiles that lived on land. Both ofthese animals were unable to swim at all. Their fossils have been found across South America, Africa, India andAntarctica.
Wegener thought that all of these organisms lived side by side. The lands later moved apart so that the fossils areseparated.
FIGURE 2.2Wegener used fossil evidence to sup-port his continental drift hypothesis. Thefossils of these organisms are found onlands that are now far apart. Wegenersuggested that when the organisms werealive, the lands were joined and the or-ganisms were living side-by-side.
Glaciation
Wegener also looked at evidence from ancient glaciers. Glaciers are found in very cold climates near the poles. Theevidence left by some ancient glaciers is very close to the equator. Wegener knew that this was impossible! However,if the continents had moved, the glaciers would have been centered close to the South Pole.
Climate
Coral reefs are found only in warm water. Coal swamps are also found in tropical and subtropical environments.Wegener discovered ancient coal seams and coral reef fossils in areas that are much too cold today. Wegener thoughtthat the continents have moved since the time of Pangaea.
Pause to Reflect
1.How do the continents resemble puzzle pieces?
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2. What other regions fit together besides South America and Africa?
3. List the evidence Wegener had for continental drift.
Magnetic Evidence
Some important evidence for continental drift came after Wegener’s death. This is the magnetic evidence. Earth’smagnetic field surrounds the planet from pole to pole. If you have ever been hiking or camping, you may have useda compass to help you find your way. A compass points to the magnetic North Pole. The compass needle aligns withEarth’s magnetic field ( Figure 2.3).
FIGURE 2.3Earth’s magnetic field is like a magnetwith its north pole near the geographicnorth pole and the south pole near thegeographic south pole.
Some rocks contain little compasses too! As lava cools, tiny iron-rich crystals line up with Earth’s magnetic field.Anywhere lavas have cooled, these magnetite crystals point to the magnetic poles. The little magnets point to where
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the north pole was when the lava cooled. Scientists can use this to figure out where the continents were at that time.This evidence clearly shows that the continents have moved.
During Wegener’s life, scientists did not know how the continents could move. Wegener’s idea was nearly forgotten.But as more evidence mounted, new ideas came about.
Pause to Reflect
1. Make a case before a scientific jury to convince them that continental drift is real. Line up all your evidence. Doesthe lack of a mechanism for continents to move destroy your case?
2. What ideas can you come up with for what could drive continental motions?
3. Why is Wegener’s continental drift idea accepted today?
Lesson Summary
• Alfred Wegener gathered evidence that the continents had moved around on Earth’s surface.• The evidence for continental drift included the fit of the continents; the distribution of ancient fossils, rocks,
and mountain ranges; and the locations of ancient climate zones.• Although the evidence was extremely strong, scientists did not yet know how continents could move, so most
rejected the idea.
Lesson Review Questions
True or False
Write true if the statement is true or false if the statement is false.
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_____ 1. The theory of plate tectonics was developed before the idea of continental drift.
_____ 2. Wegener believed that all the continents were once joined together.
_____ 3. Wegener’s hypothesis of continental drift was widely accepted as soon as it was introduced.
_____ 4. Wegener and his supporters provided a lot of evidence for continental drift.
_____ 5. Wegener found rocks of the same type and age on both sides of the Atlantic Ocean.
_____ 6. Wegener suggested that Pangaea broke up a short time ago.
_____ 7. Wegener developed a theory to explain how continents can drift.
Multiple Choice
Circle the letter of the correct choice.
8. To develop the theory of plate tectonics, scientists first had to accept the idea that
1. a. Earth’s core consists of molten metals.b. some organisms can cross the oceans.c. Earth’s continents are able to move.d. all of the above
9. The idea of continental drift was first proposed in the early
1. a. 1700s.b. 1800s.c. 1900s.d. 2000s.
10. Evidence for continental drift comes from ancient
1. a. magnetic compasses.b. maps of Pangaea.c. coal seams.d. all of the above
11. Wegener observed that the Appalachian Mountains in eastern North America matched mountain ranges in
1. a. western North America.b. South America.c. Greenland.d. Africa.
12. What was Alfred Wegener’s role in the development of the theory of plate tectonics?
1. a. He proposed the hypothesis of continental drift.b. He provided evidence that continents have moved.c. He identified magnetic evidence for plate tectonics.d. two of the above
13. In Pangaea, the present continent of South America was attached to present-day
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1. a. Australia.b. Eurasia.c. Africa.d. India.
References
1. Laura Guerin. CK-12 Foundation . CC BY-NC 3.02. Courtesy of the US Geological Survey, User:Osvaldocangaspadilla/Wikimedia Commons. http://commons.w
ikimedia.org/wiki/File:Snider-Pellegrini_Wegener_fossil_map.svg . Public Domain3. Courtesy of the US Geological Survey, User:Heron/Wikipedia, and User:Kevin Saff/Wikipedia. http://commo
ns.wikimedia.org/wiki/File:Dipole_field.jpg . Public Domain
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CONCEPT 3 Seafloor Spreading onEarth
Points to Consider
1. Describe what you think happens when the sea floor spreads.
Lesson Objectives
• List the main features of the seafloor: mid-ocean ridges, deep sea trenches, and abyssal plains.• Describe what seafloor magnetism tells scientists about the seafloor.• Describe the process of seafloor spreading.
Vocabulary
• echo sounder: device that uses sound waves to calculate distances to underwater objects and the seafloor• seafloor spreading: hypothesis explaining how the ocean floor forms and how continents can drift• trench: deep crack in the ocean floor
Fill in the blank with the appropriate term.
1. Echo sounders produce __________ that travel outward in all directions.2. Deep-sea trenches are found near chains of active __________.3. The deepest trench is the Mariana Trench in the __________ Ocean.4. The present locations of Earth’s magnetic poles is called __________ polarity.5. The continents are drifting apart because of seafloor __________.6. The youngest rocks on the seafloor are found at __________.7. Oceanic crust sinks into the mantle at __________.
Introduction
Ocean research during World War II gave scientists the tools to find out how the continents move. The evidence allpointed to seafloor spreading.
Seafloor Bathymetry
Before World War II, people thought the seafloor was completely flat and featureless. There was no reason to thinkotherwise.
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Echo Sounders
But during the war, battleships and submarines carried echo sounders. Their goal was to locate enemy submarines( Figure 3.1). Echo sounders produce sound waves that travel outward in all directions. The sound waves bounceoff the nearest object, and then return to the ship. Scientists know the speed of sound in seawater. They then cancalculate the distance to the object that the sound wave hit. Most of these sound waves did not hit submarines. Theyinstead were used to map the ocean floor.
FIGURE 3.1A ship sends out sound waves to createa picture of the seafloor below it. Theecho sounder pictured has many beamsand as a result it creates a three dimen-sional map of the seafloor beneath theship. Early echo sounders had only asingle beam and created a line of depthmeasurements.
Features of the Seafloor
Scientists were surprised to find huge mountains and deep trenches when they mapped the seafloor. The mid-oceanridges form majestic mountain ranges through the deep oceans ( Figure 3.2).
FIGURE 3.2A modern map of the eastern Pacific and Atlantic Oceans. Darker blueindicates deeper seas. A mid-ocean ridge can be seen running throughthe center of the Atlantic Ocean. Deep sea trenches are found along thewest coast of Central and South America and in the mid-Atlantic, east ofthe southern tip of South America. Isolated mountains and flat, featurelessregions can also be spotted.
Deep sea trenches are found near chains of active volcanoes. These volcanoes can be at the edges of continents or inthe oceans. Trenches are the deepest places on Earth. The deepest trench is the Mariana Trench in the southwesternPacific Ocean. This trench plunges about 11 kilometers (35,840 feet) beneath sea level. The ocean floor does havelots of flat areas. These abyssal plains are like the scientists had predicted.
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Seafloor Magnetism
Warships also carried magnetometers. They were also used to search for submarines. The magnetometers alsorevealed a lot about the magnetic properties of the seafloor.
Polar Reversals
Indeed, scientists discovered something astonishing. Many times in Earth’s history, the magnetic poles have switchedpositions. North becomes south and south becomes north! When the north and south poles are aligned as they arenow, geologists say it is normal polarity. When they are in the opposite position, they say that it is reversed polarity.
Magnetic Stripes
Scientists were also surprised to discover a pattern of stripes of normal and reversed polarity. These stripes surroundthe mid-ocean ridges. There is one long stripe with normal magnetism at the top of the ridge. Next to that stripe aretwo long stripes with reversed magnetism. One is on either side of the normal stripe. Next come two normal stripesand then two reversed stripes, and so on across the ocean floor. The magnetic stripes end abruptly at the edges ofcontinents. Sometimes the stripes end at a deep sea trench ( Figure 3.3).
FIGURE 3.3Scientists found that magnetic polarity inthe seafloor was normal at mid-oceanridges but reversed in symmetrical pat-terns away from the ridge center. Thisnormal and reversed pattern continuesacross the seafloor.
Pause to Reflect
1. How were the technologies that were developed during World War II used by scientists for the development of theseafloor spreading hypothesis?
2. Describe deep sea trenches and abyssal plains and their relative ages.
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Seafloor Ages
The scientists used geologic dating techniques on seafloor rocks. They found that the youngest rocks on the seafloorwere at the mid-ocean ridges. The rocks get older with distance from the ridge crest. The scientists were surprisedto find that the oldest seafloor is less than 180 million years old. This may seem old, but the oldest continental crustis around 4 billion years old.
Scientists also discovered that the mid-ocean ridge crest is nearly sediment free. The crust is also very thin there.With distance from the ridge crest, the sediments and crust get thicker. This also supports the idea that the youngestrocks are on the ridge axis and that the rocks get older with distance away from the ridge ( Figure 3.4). Somethingcauses the seafloor to be created at the ridge crest. The seafloor is also destroyed in a relatively short time.
FIGURE 3.4Seafloor is youngest near the mid-oceanridges and gets progressively older withdistance from the ridge. Orange areasshow the youngest seafloor. The oldestseafloor is near the edges of continentsor deep sea trenches.
Pause to Reflect
1. Describe a mid-ocean ridge. What geological processes are happening there?
2. Why is the oceanic crust so young? Why is the continental crust so old?
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The Seafloor Spreading Hypothesis
The seafloor spreading hypothesis brought all of these observations together in the early 1960s. Hot mantle materialrises up at mid-ocean ridges. The hot magma erupts as lava. The lava cools to form new seafloor. Later, more lavaerupts at the ridge. The new lava pushes the seafloor that is at the ridge horizontally away from ridge axis. Theseafloor moves!
In some places, the oceanic crust comes up to a continent. The moving crust pushes that continent away from theridge axis as well. If the moving oceanic crust reaches a deep sea trench, the crust sinks into the mantle. Thecreation and destruction of oceanic crust is the reason that continents move. Seafloor spreading is the mechanismthat Wegener was looking for!
Pause to Reflect
1. How does seafloor spreading provide a mechanism for continental drift?
Lesson Summary
• Using technologies developed during World War II, scientists were able to gather data that allowed them torecognize that seafloor spreading is the mechanism for Wegener’s drifting continents.
• Maps of the ocean floor showed high mountain ranges and deep trenches.• Changes in Earth’s magnetic field give clues as to how seafloor forms and the importance of mid-ocean ridges
in the creation of oceanic crust.• Seafloor spreading processes create new oceanic crust at mid-ocean ridges and destroy older crust at deep sea
trenches.
Lesson Review Questions
True or False
Write true if the statement is true or false if the statement is false.
_____ 1. An echo sounder with just one beam can create a three-dimensional map of the ocean floor.
_____ 2. A mid-ocean ridge runs from east to west through the center of the Atlantic Ocean.
_____ 3. Deep-sea trenches are found near the west coast of Central and South America.
_____ 4. The only mountains on the ocean floor are part of mid-ocean ridges.
_____ 5. Magnetometers were first used on ships to search for submarines.
_____ 6. Polar reversals have occurred only twice in Earth’s history.
_____ 7. Magnetic stripes on the ocean floor end abruptly at the edges of continents.
_____ 8. The rocks currently found at mid-ocean ridges have reversed polarity.
_____ 9. The seafloor is older than the continents.
_____ 10. The seafloor is spreading away from mid-ocean ridges.
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Multiple Choice
Circle the letter of the correct choice.
11. Before World War II, people thought the seafloor
1. a. had huge mountain ranges.b. contained deep trenches.c. was flat and featureless.d. had active volcanoes.
12. Echo sounders were first developed to
1. a. map the ocean floor.b. locate enemy submarines.c. determine the depth of the ocean.d. find evidence for seafloor spreading.
13. The deepest place on Earth is
1. a. 11 km below sea level.b. 110 km below sea level.c. 1100 km below sea level.d. none of the above
14. Reversed polarity means that the north and south magnetic poles are
1. a. located in the same positions as they are right now.b. located opposite their present positions.c. both in the same location.d. no longer magnetic.
15. The alternating magnetic stripes on the ocean floor show
1. a. how Earth first formed.b. why the seafloor spreads.c. when polar reversals occurred.d. where sediments were deposited.
16. New seafloor forms at
1. a. deep-sea trenches.b. mid-ocean ridges.c. continental edges.d. two of the above
17. Old seafloor sinks into the mantle at
1. a. deep-sea trenches.b. mid-ocean ridges.c. continental edges.d. two of the above
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References
1. Courtesy of United States Navy. http://www.navy.mil/view_image.asp?id=2767 . Public Domain2. Courtesy of NOAA. http://www.ngdc.noaa.gov/mgg/image/2minrelief.html . Public Domain3. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.04. Courtesy of NOAA. http://commons.wikimedia.org/wiki/File:Earth_seafloor_crust_age_1996.gif . Public
Domain
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CONCEPT 4 The Theory of PlateTectonics
Points to Consider
1. Why do you think earthquakes occur?
2. Why do you think volcanoes occur?
Lesson Objectives
• Describe what a plate is and how scientists can recognize its edges.• Explain how the plates move by convection in the mantle.• Describe the three types of plate boundaries and the features of each type of boundary.• Describe how plate tectonics processes lead to changes in Earth’s surface features.
Vocabulary
• continental rifting: splitting of a continent at a divergent plate boundary• convergent plate boundary: location where two lithospheric plates come together• divergent plate boundary: location where two lithospheric plates move apart• intraplate activity: geologic activity that takes plates within a plate away from plate boundaries• island arc: line of island volcanoes resulting from subduction of one oceanic plate beneath another oceanic
plate• plate: slab of lithosphere that can move over Earth’s surface• plate boundary: location where two lithospheric plates meet• plate tectonics: theory that Earth’s surface is divided into lithospheric plates that move over the planet’s
surface• subduction: sinking of one lithospheric plate beneath another• subduction zone: area where two lithospheric plates come together and one sinks beneath the other• transform fault: fracture in rock where one plate slides past another• transform plate boundary: location where two lithospheric plates slide past one another in opposite direc-
tions
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www.ck12.org Concept 4. The Theory of Plate Tectonics
Fill in the blank with the appropriate term.
1. Earth’s lithospheric plates interact at __________.2. A rift valley forms at a(n) __________ plate boundary.3. Subduction occurs at a(n) __________ plate boundary.4. The Himalayan Mountains formed where two continental plates __________.5. Crust is neither created nor destroyed at a(n) __________ plate boundary.6. A volcanic mountain range that forms where oceanic crust collides with a continent is called a(n) __________.7. The San Andreas fault in California occurs at a(n) __________ plate boundary.
Introduction
The theory of plate tectonics explains most of the features of Earth’s surface. Plate tectonics helps us to understandwhere and why mountains form. Using the theory, we know where new ocean floor will be created and where itwill be destroyed. We know why earthquakes and volcanic eruptions happen where they do. We even can search formineral resources using information about past plate motions. Plate tectonics is the key that unlocks many of themysteries of our amazing planet.
Earth’s Tectonic Plates
The Cold War helped scientists to learn more about our planet. They set up seismograph networks during the 1950sand early 1960s. The purpose was to see if other nations were testing atomic bombs. Of course, at the same time,the seismographs were recording earthquakes.
Earthquake Locations
The scientists realized that the earthquakes were most common in certain areas. In the oceans, they were found alongmid-ocean ridges and deep sea trenches. Earthquakes and volcanoes were common all around the Pacific Ocean.They named this region the Pacific Ring of Fire ( Figure 4.1). Earthquakes are also common in the world’s highestmountains, the Himalaya Mountains of Asia. The Mediterranean Sea also has many earthquakes.
FIGURE 4.1The Ring of Fire that circles the PacificOcean is where the most earthquakesand volcanic eruptions take place.
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Earthquakes and Plate Boundaries
Earthquakes are used to identify plate boundaries ( Figure 4.2). When earthquake locations are put on a map, theyoutline the plates. The movements of the plates are called plate tectonics.
FIGURE 4.2A map of earthquake epicenters showsthat earthquakes are found primarily inlines that run up the edges of somecontinents, through the centers of someoceans, and in patches in some land ar-eas.
The lithosphere is divided into a dozen major and several minor plates. Each plate is named for the continent orocean basin it contains. Some plates are made of all oceanic lithosphere. A few are all continental lithosphere. Butmost plates are made of a combination of both.
Scientists have determined the direction that each plate is moving ( Figure 4.3). Plates move around the Earth’ssurface at a rate of a few centimeters a year. This is about the same rate fingernails grow.
FIGURE 4.3Earth’s plates are shown in different col-ors. Arrows show the direction the plateis moving.
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How Plates Move
Convection within the Earth’s mantle causes the plates to move. Mantle material is heated above the core. The hotmantle rises up towards the surface ( Figure 4.4). As the mantle rises it cools. At the surface the material moveshorizontally away from a mid-ocean ridge crest. The material continues to cool. It sinks back down into the mantleat a deep sea trench. The material sinks back down to the core. It moves horizontally again, completing a convectioncell.
FIGURE 4.4Plates move for two reasons. Upwellingmantle at the mid-ocean ridge pushesplates outward. Cold lithosphere sinkinginto the mantle at a subduction zone pullsthe rest of the plate down with it.
Plate Boundaries
Plate boundaries are where two plates meet. Most geologic activity takes place at plate boundaries. This activityincludes volcanoes, earthquakes, and mountain building. The activity occurs as plates interact. How can platesinteract? Plates can move away from each other. They can move toward each other. Finally, they can slide past eachother.
These are the three types of plate boundaries:
• Divergent plate boundaries: the two plates move away from each other.• Convergent plate boundaries: the two plates move towards each other.• Transform plate boundaries: the two plates slip past each other.
The features that form at a plate boundary are determined by the direction of plate motion and by the type of crustat the boundary.
Divergent Plate Boundaries
Plates move apart at divergent plate boundaries. This can occur in the oceans or on land.
Mid-ocean Ridges
Plates move apart at mid-ocean ridges. Lava rises upward, erupts, and cools. Later, more lava erupts and pushesthe original seafloor outward. This is seafloor spreading. Seafloor spreading forms new oceanic crust. The risingmagma causes earthquakes. Most mid-ocean ridges are located deep below the sea. The island of Iceland sits righton the Mid-Atlantic ridge ( Figure 4.5).
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FIGURE 4.5The rift valley in Iceland that is part of theMid-Atlantic Ridge is seen in this photo.
Continental Rifting
A divergent plate boundary can also occur within a continent. This is called continental rifting ( Figure 4.6).Magma rises beneath the continent. The crust thins, breaks, and then splits apart. This first produces a rift valley.The East African Rift is a rift valley. Eastern Africa is splitting away from the African continent. Eventually, as thecontinental crust breaks apart, oceanic crust will form. This is how the Atlantic Ocean formed when Pangaea brokeup.
FIGURE 4.6The Arabian, Indian, and African plates are rifting apart, forming the GreatRift Valley in Africa. The Dead Sea fills the rift with seawater.
Convergent Plate Boundaries
A convergent plate boundary forms where two plates collide. That collision can happen between a continent andoceanic crust, between two oceanic plates, or between two continents. Oceanic crust is always destroyed in thesecollisions.
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Ocean-Continent Convergence
Oceanic crust may collide with a continent. The oceanic plate is denser, so it undergoes subduction. This meansthat the oceanic plate sinks beneath the continent. This occurs at an ocean trench ( Figure 4.7). Subduction zonesare where subduction takes place.
As you would expect, where plates collide there are lots of intense earthquakes and volcanic eruptions. Thesubducting oceanic plate melts as it reenters the mantle. The magma rises and erupts. This creates a volcanicmountain range near the coast of the continent. This range is called a volcanic arc. The Andes Mountains, alongthe western edge of South America, are a volcanic arc ( Figure 4.8).
FIGURE 4.7Subduction of an oceanic plate beneath acontinental plate forms a line of volcanoesknown as a continental arc and causesearthquakes.
Ocean-Ocean Convergence
Two oceanic plates may collide. In this case, the older plate is denser. This plate subducts beneath the younger plate.As the subducting plate is pushed deeper into the mantle, it melts. The magma this creates rises and erupts. Thisforms a line of volcanoes, known as an island arc ( Figure 4.9). Japan, Indonesia, the Philippine Islands, and theAleutian Islands of Alaska are examples of island arcs ( Figure 4.10).
Continent-Continent Convergence
Continental lithosphere is low in density and very thick. Continental lithosphere cannot subduct. So when twocontinental plates collide, they just smash together, just like if you put your hands on two sides of a sheet of paperand bring your hands together. The material has nowhere to go but up ( Figure 4.11)! Earthquakes and metamorphicrocks result from the tremendous forces of the collision. But the crust is too thick for magma to get through, so thereare no volcanoes.
Mountain Building
Continent-continent convergence creates some of the world’s largest mountains ranges. The Himalayas ( Figure4.12) are the world’s tallest mountains. They are forming as two continents collide. The Appalachian Mountainsare the remnants of a larger mountain range. This range formed from continent-continent collisions in the time ofPangaea.
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FIGURE 4.8A relief map of South America shows the trench west of the continent. TheAndes Mountains line the western edge of South America.
FIGURE 4.9A convergent plate boundary subduc-tion zone between two plates of oceaniclithosphere. Melting of the subductingplate causes volcanic activity and earth-quakes.
Transform Plate Boundaries
Two plates may slide past each other in opposite directions. This is called a transform plate boundary. These plateboundaries experience massive earthquakes. The world’s best known transform fault is the San Andreas Fault inCalifornia ( Figure 4.13). At this fault, the Pacific and North American plates grind past each other. Transform plateboundaries are most common as offsets along mid-ocean ridges.
Transform plate boundaries are different from the other two types. At divergent plate boundaries, new oceanic crust
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FIGURE 4.10The Aleutian Islands that border southernAlaska are an island arc. In this winterimage from space, the volcanoes are cov-ered with snow.
FIGURE 4.11When two plates of continental crust col-lide, the material pushes upward, forminga high mountain range. The remnants ofsubducted oceanic crust remain beneaththe continental convergence zone.
is formed. At convergent boundaries, old oceanic crust is destroyed. But at transform plate boundaries, crust is notcreated or destroyed.
Earth’s Changing Surface
Knowing where plate boundaries are helps explain the locations of landforms and types of geologic activity. Theactivity can be current or old.
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FIGURE 4.12The Karakoram Range is part of the Hi-malaya Mountains. K2, pictured here, isthe second highest mountain the world atover 28,000 feet. The number and heightof mountains is impressive.
FIGURE 4.13The red line is the San Andreas Fault. On the left is the Pacific Plate, whichis moving northeast. On the right is the North American Plate, which ismoving southwest. The movement of the plates is relative to each other.
Active Plate Margins
Western North America has volcanoes and earthquakes. Mountains line the region. California, with its volcanoesand earthquakes, is an important part of the Pacific Ring of Fire. This is the boundary between the North Americanand Pacific Plates.
Passive Plate Margins
Mountain ranges also line the eastern edge of North America. But there are no active volcanoes or earthquakes.Where did those mountains come from? These mountains formed at a convergent plate boundary when Pangaeacame together. About 200 million years ago these mountains were similar to the Himalayas today ( Figure 4.14)!There were also earthquakes.
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FIGURE 4.14The White Mountains in New Hampshireare part of the Appalachian province. Themountains are only around 6,000 feethigh.
The Supercontinent Cycle
Scientists think that Pangaea was not the first supercontinent. There were others before it. The continents are nowmoving together. This is because of subduction around the Pacific Ocean. Eventually, the Pacific will disappear anda new supercontinent will form. This won’t be for hundreds of millions of years. The creation and breakup of asupercontinent takes place about every 500 million years.
Intraplate Activity
Most geological activity takes place at plate boundaries. But some activity does not. Much of this intraplate activityis found at hot spots. Hotspot volcanoes form as plumes of hot magma rise from deep in the mantle.
Hotspots in the Oceans
A chain of volcanoes forms as an oceanic plate moves over a hot spot. This is how it happens. A volcano forms overthe hotspot. Since the plate is moving, the volcano moves off of the hotspot. When the hotspot erupts again, a newvolcano forms over it. This volcano is in line with the first. Over time, there is a line of volcanoes. The youngest isdirectly above the hot spot. The oldest is the furthest away ( Figure 4.15).
The Hawaii-Emperor chain of volcanoes formed over the Hawaiian Hotspot. The Hawaiian Islands formed mostrecently. Kilauea volcano is currently erupting. It is over the hotspot. The Emperor Seamounts are so old they nolonger reach above sea level. The oldest of the Emperor Seamounts is about to subduct into the Aleutian trench offof Alaska. Geologists use hotspot chains to tell the direction and the speed a plate is moving.
Hotspots Beneath Continents
Hot spots are also found under the continental crust. Since it is more difficult for magma to make it through the thickcrust, they are much less common. One exception is the Yellowstone hotspot ( Figure 4.16). This hotspot is veryactive. In the past, the hotspot produced enormous volcanic eruptions. Now its activity is best seen in the region’sfamous geysers.
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FIGURE 4.15This view of the Hawaiian islands showsthe youngest islands in the southeast andthe oldest in the northwest. Kilauea vol-cano, which makes up the southeasternside of the Big Island of Hawaii, is locatedabove the Hawaiian hotspot.
FIGURE 4.16Yellowstone Lake lies at the center of agiant caldera. This hole in the groundwas created by enormous eruptions atthe Yellowstone hotspot. The hotspot liesbeneath Yellowstone National Park.
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Pause to Reflect
1. Name the three types of plate boundaries? Which has volcanoes? Which has earthquakes? Which has mountainbuilding?
2. Describe convection. How does this work to create plate boundaries?
3. Make some generalizations about which types of plate boundaries have volcanoes and which have earthquakes.Could you look at a plate boundary and determine what geological activity there would be?
4. Why is continental crust thicker than oceanic crust? Why is oceanic crust relatively thin?
5. Now that you know about plate tectonics, where do you think would be a safe place to live if you wanted to avoidvolcanic eruptions and earthquakes?
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Lesson Summary
• Convection in the mantle drives the movement of the plates of lithosphere over the Earth’s surface. Newoceanic crust forms at the ridge and pushes the older seafloor away from the ridge horizontally.
• Plates interact at three different types of plate boundaries: divergent, convergent and transform fault bound-aries, where most of the Earth’s geologic activity takes place.
• These processes acting over long periods of time are responsible for the geographic features we see.
Lesson Review Questions
True or False
Write true if the statement is true or false if the statement is false.
_____ 1. The locations of earthquakes have been used to identify plate boundaries.
_____ 2. The movement of Earth’s plates is called plate tectonics.
_____ 3. The lithosphere is divided into just three major plates.
_____ 4. Most geologic activity takes place far from plate boundaries.
_____ 5. Mid-ocean ridges occur at convergent plate boundaries.
_____ 6. Many volcanoes occur along subduction zones.
_____ 7. The tallest mountains in the world formed at a transform plate boundary.
_____ 8. Geologic features called faults occur at divergent plate boundaries.
_____ 9. Scientists think that Pangaea was the first supercontinent.
_____ 10. The Aleutian Islands formed at a plate boundary.
Multiple Choice
Circle the letter of the correct choice.
11. Plate tectonics helps to explain
1. a. how mountains form.b. where new seafloor is created.c. why earthquakes occur where they do.d. all of the above
12. The Pacific Ring of Fire is a ring around the Pacific ocean where
1. a. volcanoes are common.b. tectonic plates interact.c. many hot spots occur.d. two of the above
13. Plates move over Earth’s surface at a rate of
1. a. 100 kilometers per year.b. a few kilometers per year.c. a few centimeters per year.
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d. a couple of millimeters per year.
14. Plates move over Earth’s surface because of
1. a. conduction within the crust.b. subduction in the outer core.c. radiation from the inner core.d. convection within the mantle.
15. Magma from the mantle rises up through Earth’s crust at
1. a. deep-sea trenches.b. mid-ocean ridges.c. hot spots.d. all of the above
16. The edge of a plate sinks into the mantle
1. a. where two plates diverge.b. at a subduction zone.c. at a transform boundary.d. none of the above
17. Continental plates do not subduct because they
1. a. are very thick and low in density.b. do not collide with other plates.c. have only intraplate activity.d. two of the above
References
1. Courtesy of the US Geological Survey. http://commons.wikimedia.org/wiki/File:Pacific_Ring_of_Fire.png .Public Domain
2. Courtesy of NASA. http://commons.wikimedia.org/wiki/Image:Quake_epicenters_1963-98.png . Public Do-main
3. Courtesy of US Geological Survey. http://commons.wikimedia.org/wiki/File:Plates_tect2_en.svg . PublicDomain
4. CK-12 Foundation. . CC BY-NC 3.05. User:Mangwanani/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Mid_Atlantic_Ridge.jpg
. Public Domain6. Courtesy of NASA. http://commons.wikimedia.org/wiki/File:Greatrift.jpg . Public Domain7. User:Booyabazooka/Wikipedia. http://commons.wikimedia.org/wiki/File:Active_Margin.svg . Public Do-
main8. Courtesy of NOAA. http://commons.wikimedia.org/wiki/File:Peru-Chile_trench.jpg . Public Domain9. User:Booyabazooka/Wikipedia. http://commons.wikimedia.org/wiki/File:Active_Margin.svg . Public Do-
main10. Courtesy of NASA. http://commons.wikimedia.org/wiki/File:Aleutians-space.jpg . Public Domain11. Courtesy of the US Geological Survey. http://commons.wikimedia.org/wiki/File:Continental-continental_c
onvergence_Fig21contcont.gif . Public Domain
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12. Maria Ly (Flickr:mariachily). http://www.flickr.com/photos/mariachily/3330744786/ . CC BY 2.013. Courtesy of Kate Barton, David Howell, Joe Vigil, US Geological Survey. http://commons.wikimedia.org/wik
i/File:Sanandreas.jpg . Public Domain14. Bryan Pocius. http://www.flickr.com/photos/pocius/5792332417/ . CC BY 2.015. Courtesy of the US Geological Survey. http://commons.wikimedia.org/wiki/File:Hawaii_hotspot_poster.jpg
. Public Domain16. Courtesy of NASA Earth Observatory. http://earthobservatory.nasa.gov/IOTD/view.php?id=5816 . Public
Domain
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