2 and Your Community Plate Tectonicsrteeter.tjspartans.org › ... › 2018 › 11 › ec_u1ch2act1g62.pdf · Plate Tectonics 2...and Your Community Getting Started Plate tectonics

Plate Tectonics...and Your Community2Getting Started

Plate tectonics is a scientific theory. A scientific theory is a well-tested concept that is supported by experimental or observationalevidence. It explains a wide range of observations. The theory ofplate tectonics explains the formation, movement, and changes ofthe outer, rigid layer of the Earth, called the lithosphere.

• How do you think the Earth’s lithosphere formed?• How do you know that the Earth’s lithosphere moves?• List some ways that the Earth’s lithosphere changes.

What do you think? Write down your ideas about thesequestions in your notebook. Be prepared to discuss your ideaswith your small group and the class.

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Ch 2 Plate Techtonics 9/17/04 11:00 AM Page 60

Chapter Challenge

Think about how you canuse the theory of platetectonics to help middle-school students understandscientific theories. Youmight want to prepare aPowerPointTM

presentation, a web page,or a three-panel poster display.Your project will need to address:• Evidence for movement

and changes in thegeosphere over time.

• The flow of matter andenergy in the Earth.

• The nature of the Earth’s interior.• How plate tectonics accounts for

the features and processes thatgeoscientists observe in the Earth.

• How the theory of plate tectonicsdeveloped.

Assessment Criteria

Think about what you have beenasked to do. Scan ahead through thechapter activities to see how theymight help you meet your challenge.Work with your classmates and your teachers to define the criteriafor judging your work. Record allthis information. Make sure thatyou understand the criteria as wellas you can before you begin. Your teacher may provide you with a sample rubric to help you get started.

Scenario

A middle-school science teacher inyour community has been asked toteach a unit on plate tectonics to herclass and has asked for your help.To her students, the fact that theyare riding on Earth on a floatingplate makes about as much sense asriding on a magic carpet. Thestudents feel that because platetectonics is “just a theory” thatscientists have “made up,” there isno need for them to learn about it,or try to understand it. Also, theybelieve it is totally irrelevant to their lives.

The teacher hopes that her studentsmight be more likely to listen tolocal high school students. Shewould like you to convince her classof the significance of plate tectonicsin their lives. She wants you toexplain to her class the key scienceconcepts behind the theory. Shethinks that it would help herstudents if they could see howresearch, evidence, modeling, andtechnology support plate tectonics.

Can the EarthComm students meet this challenge and help amiddle-school class to understandwhy the theory of plate tectonics is important to learn about and understand?

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GoalsIn this activity you will:

• Determine the direction and rate ofmovement of positions within theplate on which your community islocated, using data from the GlobalPositioning System and a computermodel.

• Predict the position of yourcommunity in the near future, and“retrodict” its position in the recentpast, by extrapolating from dataalready collected.

• Recognize that the rate anddirection of plate motion is notnecessarily constant.

• Describe several lines of evidencefor plate motion.

Think about It

The motion of anything (you, your automobile, a lithosphericplate, or the Milky Way Galaxy!) has to be described in relation tosomething else.

• How can you locate your position on the Earth’s surface?• How would you be able to determine whether your position on

the Earth has moved?

What do you think? Record your ideas about these questions inyour EarthComm notebook. Include sketches as necessary. Beprepared to discuss your responses with your small group and the class.

Activity 1 Taking a Ride on a Lithospheric Plate

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Earth’s Dynamic Geosphere Plate Tectonics

EarthComm


a) Find the WES2 station (in thenortheastern United States). Howdo you know that the WES2station has moved over time?

b) In what compass direction is theWES2 station moving? Be specific.

Investigate

Part A: Data from the GlobalPositioning System1. Data from Global Positioning System

(GPS) satellites will help you find outif the position of your community haschanged over time. The map showsmeasurements of movements at GPSrecording stations in North America.Each station has a four-character

symbol. Arrows show the rate anddirection of motion of the Earth’ssurface at that station. Longer arrowsindicate faster motion than shorterarrows. The motions shown arerelative to the GPS frame of reference,which you can think of as “attached”to the Earth’s axis of rotation.

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Measurements of movements at GPS recording stations in North America.


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c) The arrow in the lower leftcorner of the map is a scale. Itshows the length of a “20mm/yr” arrow. Is the WES2station moving more than orless than 20 mm/yr? Explain.

d) Are all stations on the mapmoving at the same speed?Explain.

e) Are all stations on the mapmoving in the same direction?Explain.

f) What is the general or averagedirection of movement ofNorth America?

2. A series of measurements of thelocation and elevation of a GPSstation over time is called a GPStime series. The graph shows theGPS time series for the WES2station. The solid sloping lines onthe three graphs are the “best-fit”lines through the data points.

Use the map and the time seriesto answer the followingquestions:

a) How many years of data doesthe time series show?

b) Were measurements recordedcontinuously or only at certaintimes? Explain your answer.

3. The top graph shows movementof the station to the north orsouth. Northward movement isindicated by positive values, andsouthward movement is shownby negative values. Find thecalculation above the top graph.How many millimeters per yeardid WES2 move? Convert


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96

10

0

-10

-20

97

WES2 North Offsetrate(mm/yr) = 4.3 ± 0.4

98 99

(mm

)

96

40

30

10

0

-10

-20

20

97 98 99

(mm

)

96

20

10

0

-10

-20

97

WES2 Up Offsetrate(mm/yr) = 0.5 ± 1.7

98 99

(mm

)

WES2 East Offsetrate(mm/yr)= -14.4 ± 0.5

The location and elevation of GPS station WES2 over time.Thevertical lines above and below each point are called “error bars”.They show the uncertainty in the measurement.They tell you thatthe real value might lie anywhere within the error bar.

this value to centimeters per year. Inwhich direction did it move?

a) Record the rate (in cm/yr) and thedirection of motion in yournotebook.


4. The middle graph shows movementto the east or west. Eastwardmovement is indicated by positivevalues, and westward movement isshown by negative values. Find thecalculation above the middle graph.How many millimeters per year didWES2 move? Convert this value tocentimeters per year. In whichdirection did it move?

a) Record the rate (in cm/yr) anddirection of motion in yournotebook.

5. The bottom graph shows themovement up or down.

a) Has the WES2 station alwaysstayed at the same elevation?Explain.

6. Do the speed and direction of motionof WES2 shown in the graphs matchthe direction and length of the arrowshown on the map?

7. Look at the “best-fit” line in the topand middle graphs.

a) Did the WES2 station move at aconstant speed since 1996?Explain your answer.

b) What additional data would youneed to decide whether thedifferences between the measureddata points and the best-fitstraight line are due to the overallmotion of the plate or are causedby processes in the local areaaround the WES2 station?

8. Obtain a GPS time series for astation nearest your community.

a) Record the directions and rates ofmotion in cm/yr for the stationnearest your community.

Part B. Data from a Computer Model1. Computer models that use geologic

data also provide information aboutthe changes in position of yourcommunity over time. To use thismodel, you will need to know thelatitude and longitude of your schoolin decimal format.

Find your school (or anotherfamiliar place) on your localtopographic map. Latitude andlongitude are used to identify aposition on the Earth’s surface.Latitude is a measure of location indegrees, minutes, and seconds northor south of the equator. Therefore, itis found on the left or right side ofthe map. Longitude is a measure oflocation in degrees, minutes, andseconds east or west of the PrimeMeridian, which passes throughGreenwich, England. Therefore, it is found on the top or bottom ofthe map.

a) Record the latitude and longitudeof the position you chose indegrees, minutes, and seconds.(These “minutes” and “seconds”are not the same as the familiarminutes and seconds of time! They describe positions on acircular arc.)

b) Convert the latitude and longitude values to a decimalformat. An example for you tofollow is provided on the following page.

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Example:

42° (degrees) 40' (minutes) 30"(seconds) north latitudeEach minute has 60". 30" divided by 60" equals 0.5'.This gives a latitude of 42° 40.5'north. Each degree has 60'.40.5' divided by 60' equals0.675°.The latitude in decimal format is42.675° north.

2. Obtain a world outline map showinglithospheric plates, similar to the oneshown.

a) Place a dot on the map torepresent the location of yourcommunity and label it with anabbreviation.

b) In your notebook, record thename of the plate your communitylies within. Record the name of aplate next to your community.

3. Visit the Relative Plate Motion(RPM) Calculator web site. (See theEarthComm web site for moreinformation.) The RPM calculatordetermines how fast your plate ismoving relative to another plate thatis assumed to be “fixed” (non-moving). At the web site, enter thefollowing information:• The latitude and longitude of your

community (decimal format).• The name of the plate on which

your community is located.• The name of the “fixed” reference

plate adjacent to your plate. Usethe African Plate as the referenceplate.


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Eurasianplate

0˚ 60˚

0˚ 60˚

Anatolianplate

Antarctic plate

Arabianplate

African plate

Eurasianplate

120˚ 180˚ 120˚ 60˚

120˚ 180˚ 120˚ 60˚

Pacific plate

Antarctic plateAntarctic plate

SouthAmerican

plate

Cocosplate

Nazcaplate

Philippine plate

North Americanplate

Caribbeanplate

120˚ 180˚ 120˚ 60˚

120˚ 180˚ 120˚ 60˚

60˚

30˚

0˚

30˚

60˚

60˚

30˚

0˚

30˚

60˚0 10,000 km5,000 km

Indian-Australianplate

African plateSouth

Americanplate

Caribbeanplate

Indian-Australianplate

World map of major lithospheric plates. Arrows show the relative motions of the plates relative to the African Plate,which happens to be moving slowest relative to the Earth’s axis of rotation.


Once you have entered the data, run the model. Print the results. (A sample printout for the location of station WES2 is provided for you.) Record the followinginformation in your notebook:

a) The rate of movement of the plateon which your community islocated (in centimeters per year).

b) Its direction of motion. (Note thatdirection is given in degrees,starting from 0°, clockwise fromnorth. For example, 90° is directlyeast, 180° is directly south and270° is directly west.)

c) In your own words, describe themotion of your plate over time.

d) How do the results from thecomputer model compare to thoseobtained from GPS data?

e) What data does GPS provide that the plate motion calculatordoes not?

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EarthComm

So far, you have learned that when youuse GPS data collected over time, youcan find the speed and direction ofmovement of your plate. You can alsouse data from a computer model toshow that your community is moving aspart of the movement of a large plate.You are riding on a piece of the Earth’slithosphere! Your maps show how yourcommunity, along with the plate onwhich it rides, has moved over time.You can also predict its future position.You have now gathered some evidenceto help you explain to the middle school students that their community isin fact moving.

Reflecting on the Activity and the Challenge

NUVEL-1A Calculation ResultsCalculation results are as follows:

Relatively fixed plate = AfricaRelatively moving plate = North AmericaLatitude of Euler pole = 78.8 degreeLongitude of Euler pole = 38.3 degreeAngular velocity = 0.24 degree/million yearsLatitude inputted = 42.364799 degreeLongitude inputted = –71.293503 degreeVelocity = 2.11 cmDirection = 283.39 degree


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Digging Deeper

MEASURING THE MOTION OF LITHOSPHERIC PLATESThe Interior Structure of the Earth

Refer to Figure 1 as you read this section.The thin, outermost layer of the Earthis called the crust.There are two kinds of crust: continental and oceanic.Thecontinental crust forms the Earth’s continents. It is generally 30–50 km thick,and most of it is very old. Some continental crust has been dated as old as fourbillion years! The geological structure of the continental crust is generally verycomplicated, as you will learn in a later chapter. In contrast, the oceanic crust isonly 5–10 km thick, and it is young in terms of geologic time.All of the oceaniccrust on the Earth is younger than about 200 million years.

Beneath the Earth’s crust is the mantle.The rocks of the mantle are verydifferent in composition from the crust, and the boundary between the crustand the mantle is sharp and well defined.The uppermost part of the mantle,which is cooler than below, moves as a rigid block, carrying the crust with it.The upper rigid part of the mantle, together with the crust, is called thelithosphere.The Earth’s plates are composed of the lithosphere.At greaterdepths, the mantle is hot enough that it can flow very slowly, just like a verystiff liquid.That part of the mantle is called the asthenosphere.

In terms of composition and origin, the crust and the mantle are very different,but in terms of how they move, they behave in the same way. On the otherhand, the lithosphere part of the mantle is the same in composition as theasthenosphere part of the mantle, but in terms of how they move, they behavevery differently.

mantle asthenosphere

ocean surface

continental crust

mantle lithosphere

oceanic crust

500 km

50 km

Figure 1 A schematic cross section through the outer part of the Earth. Note that the verticaland horizontal scales are very different.The diagram has a lot of “vertical exaggeration”. If thediagram had been drawn without distortion, all of the layers would look much thinner.Theboundary between continental crust and oceanic crust, shown by the shaded box, will bedescribed later in the chapter.

Geo Wordscrust: the thin outermost layerof the Earth. Continental crustis relatively thick and mostlyvery old. Oceanic crust isrelatively thin, and is alwaysgeologically very young.mantle: the zone of the Earthbelow the crust and above thecore. It is divided into theupper mantle and lowermantle with a transition zonebetween.lithosphere: the outermostlayer of the Earth, consistingof the Earth’s crust and part ofthe upper mantle. Thelithosphere behaves as a rigidlayer, in contrast to theunderlying asthenosphere.asthenosphere: the part ofthe mantle beneath thelithosphere. Theasthenosphere undergoes slow flow, rather thanbehaving as a rigid block, likethe overlying lithosphere.


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Measuring Plate Motions with GPS

The Global Positioning System (GPS) consists of 24 satellites that orbitthe Earth at a height of 20,200 km. Receivers at stations on Earth (such asWES2 in Westport, Massachusetts) use the signals from satellites to calculatethe location of the station. Geoscientists have set up a network of targets allover the world in order to monitor the movement of lithospheric plates.Steel spikes pounded into the ground (preferably embedded in solid rock)make up the targets, as shown in Figure 2.A high-precision GPS receiver isthen mounted on a tripod and positioned directly above the target.Thetargets are revisited over a period of months or years.The receiver measuresthe distance to four or more GPS satellites and then uses stored data onsatellite locations to compute the location of the target. Changes inhorizontal and vertical positions can be detected within several millimeters.

GPS data collected at stations all over the world confirm that the surface ofthe Earth is moving. However, GPS time series show data for only the lastseveral years. GPS is a new technology, and a global network of GPS stationshas existed for less than a decade. How do we know that the surface of theEarth has been moving for a longer period of time? The answer to thisquestion comes from the study of rocks.

Sea-Floor Spreading

The computer model at the Plate Motion Calculator web site uses severalsources of geologic data. One source comes from the study of themagnetism of rocks that make up the sea floor.All magnets and

Geo WordsGlobal Positioning System(GPS): a satellite-based systemfor accurate location of pointson the Earth.

Figure 2 A GPS receiver mounted in rock is used to measure changes in the elevationof this volcano.


EarthComm


materials that have magnetism have a north and south direction, or magneticpolarity. In the middle of the 20th century, geoscientists noted that theycould group rocks by their magnetic polarity. Rocks with normal magneticpolarity match that of the Earth’s magnetic field (the north end of the rock’s“compass needle” points toward magnetic north.) The other group hasmagnetic minerals with reversed polarity(the north end of the rock’scompass needle points south).

It was known that as lava cools to form basalt (an iron-rich volcanic rockthat makes up the ocean floor), its iron minerals (such as magnetite) becomemagnetized and “lock in” the polarity of the Earth’s magnetic field. Beginningin the 1950’s, scientists began noting patterns in the magnetism of rocks onthe ocean floor, as shown in Figure 3.The alternating belts of higher andlower than average magnetic field strength were of normal and reversepolarity, respectively.

In 1963, two scientists, F.J.Vine and D. H. Matthews, proposed therevolutionary theory of sea-floor spreading to explain this pattern.Accordingto their theory, the matching patterns on either side of the mid-oceanridge could be explained by new ocean crust forming at the ridge andspreading away from it.As ocean crust forms, it obtains the polarity of theEarth’s magnetic field at that time. Over time, the strength of the Earth’s

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Figure 3 The formation of magnetic striping. New oceanic crust forms continuously at thecrest of the mid-ocean ridge. It cools and becomes increasingly older as it moves away from theridge crest with sea-floor spreading: a. the spreading ridge about 5 million years ago; b. about 2 to 3 million years ago; and c. present day.


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magnetic field changes.When new ocean crust forms at the center of thespreading , it obtains a new kind of magnetic polarity. Over time, a series ofmagnetic “stripes” are formed.

Since the theory of sea-floor spreading was proposed, core samples ofvolcanic rock taken from the ocean floor have shown that the age of therock increases from the crest of the ridge, just as the theory predicts.What’smore, by measuring both the age and magnetic polarity of rocks on land,geologists have developed a time scale that shows when the magnetic fieldhas reversed its polarity. Because the magnetic striping on the ocean floorrecords the reversals of the Earth’s magnetic field, geoscientists can calculatethe average rate of plate movement during a given time span.These ratesrange widely.The Arctic Ridge has the slowest rate (less than 2.5 cm/yr),and the East Pacific Ridge has the fastest rate (more than 15 cm/yr).The computer model in the plate motion calculator uses spreading ratesfrom ocean ridges throughout the world to compute plate motion.

Geologic data is also used to find the direction of movement of the plate.Surveys of the depth of the ocean floor, mainly since the 1950s, revealed agreat mountain range on the ocean floor that encircles the Earth, as shownin Figure 4.This mid-ocean ridge zig-zags between the continents, winding itsway around the globe like the seams on a baseball.The mid-ocean ridge is

Geo Wordsbasalt: a kind of volcanicigneous rock, usually darkcolored, with a high contentof iron.mid-ocean ridge: a chain ofundersea ridges extendingthroughout all of the Earth’socean basins, and formed bysea-floor spreading.

Figure 4 Map of the world ocean floor.The crest of the mid-ocean ridge system is shown as a broad light blue line throughout the ocean floor.The flanks of the mid-ocean ridges slope gradually down to the deeper part of the oceans, nearer the continents.


EarthComm


not straight; it is offset in many places (Figure 5).The offsets areperpendicular to the axis of the ridge.When combined with knowledgethat the ocean floor is spreading apart at mid-ocean ridges, geologistsrealized that the offsets are parallel to the direction the plates aremoving. By mapping the orientations of these offsets, and entering thisdata into the computer model, the plate motion calculator is able togenerate the directions of plate motions. Comparisons between GPSmeasurements and results from geologic computer models show verygood agreement, within 4%.

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Figure 5 This map shows the network of fractures along the mid-ocean ridgein the eastern Pacific Ocean floor. Because the ocean floor spreads away fromboth sides of the ridge, the fractures indicate the direction of plate motion.

1. What is the differencebetween thelithosphere and theasthenosphere?

2. What does theabbreviation GPS stand for?

3. From where does aGPS receiver get itssignal?

4. Why are GPS data notenough to confirm that the Earth’s surfacehas been moving formany years?

5. What has caused the“zebra pattern” in therock of the ocean floor?

6. What is the significanceof the patterns ofoffsets along mid-ocean ridges?

Check YourUnderstanding

Understanding and Applying What You Have Learned

1. Describe the direction and the rateof movement for the plate onwhich you live.

2. Examine the scale of the USGStopographic map of yourcommunity. Given the rate of plate

motion in your community,estimate the minimum number ofyears that it would take for achange in the location of yourschool to be detected on atopographic map.


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3. How does GPS provide evidencethat the surface of the Earth movesover time?

4. Describe at least one advantage ofusing GPS technology to gatherevidence of plate motion.

5. How do studies of the magnetismof rocks on the sea floor provideevidence that the surface of theEarth moves over time?

6. What evidence examined in thisactivity suggests that the directionand rate of motion of plates is notconstant?

7. Look at the world map in theInvestigate section showing therelative motion of plates. This mapshows how plates move relative toeach other.

a) Look at the names of the plates.On what basis does it appearthat the plates were named?

b) Write down the name of yourplate and all the plates thatborder it. Describe the motionof your plate relative to all theplates that border it.

c) How might the differences inmotion of these plates affect theEarth’s lithosphere?


EarthComm

Preparing for the Chapter Challenge

In your notebook, write a paragraphto convince the middle-schoolstudents that your community ismoving as part of the movement of a

much larger segment of the Earth’slithosphere. Describe the evidenceused to make this determination.

Inquiring Further

1. Technology used to detect platemotions

Explore how GPS allows platemovement to be measured.Excellent web sites that describehow GPS works can be found onthe EarthComm web site.

2. Investigating scales of motion

Plate motion is extremely slow.Make a list of other things youknow about (or have heard about)that move or take place slowly.Possible examples include growth

of fingernails, grass, tree height,tree-trunk diameter, and so on.Find out how fast they move.Compare the rate of these motionsto the rate of movements of plates.

3. Study animations of plate motions

Visit the EarthComm web site forthe address of animated images ofthe motions of lithospheric plates.Describe how the motions shownin the animations match youranalysis from this activity.


Documents

2 and Your Community Plate Tectonicsrteeter.tjspartans.org › ... › 2018 › 11 › ec_u1ch2act1g62.pdf · Plate Tectonics 2...and Your Community Getting Started Plate tectonics