Activity 2 Plate Boundaries and Plate Interactions€¦ · of plate boundaries. a) On the map, color the boundary line that separates two converging plates. Do not outline both of

958EarthComm

Plate Tectonics

GoalsIn this activity you will:

• Classify and label the types ofmovement at plate boundaries,using a world map that showsrelative plate motion.

• Identify the distribution ofplates by means of the worldmap of relative plate motions.

• Describe the present plate-tectonic setting of yourcommunity, and infer possiblepast plate-tectonic activitybased on your knowledge.

Think about It

New plates are created at certain places on Earth, and existingplates are consumed at certain other places. The total surfacearea of the Earth stays the same, so the creation of new plateshas to be exactly equal to the consumption of existing plates.

• Where do you suppose you would have the most“interesting” ride on a plate? Would it be at the center, ona leading edge, on a trailing edge, or somewhere else onthe plate?

What do you think? Record your ideas about these questions in your EarthComm notebook. Include sketches as necessary. Be prepared to discuss your response with your small group and the class.

Activity 2 Plate Boundaries and Plate Interactions

CS_Ch15_PlateTechtonics 3/1/2005 6:36 PM Page 958

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Investigate

Part A: Observing Plate Motions andPlate Interactions1. Obtain the equipment shown in the

following diagram.

2. Use the equipment to model a steadysea-floor spreading and subduction,as follows: One student holds thetwo rolled-up dowels in one place,loosely, so that they can turn but notshift their position. Another studentholds the stapled piece of 2 x 4lumber “continent” and pulls it awayfrom the rolled-up dowels. A thirdstudent holds the dowel and piece of2 x 4 lumber “subduction zone” atthe other end loosely in place. Afourth student pulls the paper stripfrom under the piece of 2 x 4 lumber“subduction zone.” Be sure to unroll

the paper strips at the same rate, sothat the numbers of the stripes staymatched up as they appear.

a) What does the area at the rolledpaper strips on the dowelsrepresent?

b) What does the section of paperbetween the dowels and thecontinental lithosphere (the pieceof 2 x 4 lumber) represent?

c) What happens to the length of thissection of paper as the dowels areunrolled?

d) As the dowels are unrolled, whathappens to the width of thesection of paper between thedowels and the subduction zone(the other piece of 2 x 4 lumber)?

slide

rolled-updowels pull

2 X 4 2 X 4

spreading ridgecontinent subduction zone


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Plate Tectonics

3. Use the equipment to model acollision of a spreading ridge and asubduction zone, as follows: Beginwith the materials arranged in thesame way at the end of Step 1. Whiletwo students pull the paper strips tounwind the two rolled-up dowels, thestudent holding those dowels slidesthem slowly toward the subductionzone. The student operating thesubduction zone needs to make surethat the stripes appearing at thespreading ridge continue to havetheir numbers matched up.

a) What happens to the length of thestrip of paper between the dowelsand the “continent” side in thissituation?

b) What happens to the length of thestrip of paper between the dowelsand the “subduction” zone?

c) At what “place” does thespreading ridge eventually arrive?

4. Think about the following questions,and write a brief answer to each inyour notebook.

a) In the first part of the modeling(Step 2), how long will the oceanon the “subduction” side last?

b) In the second part of the modeling(Step 3), what do you think wouldhappen in real life when thespreading ridge arrives at thesubduction zone?

c) In the second part of the modeling(Step 3), how would the ocean onthe “continent” side change afterthe spreading ridge arrives at thesubduction zone?

d) In both cases, what do you thinkwould happen in real life if the

continent became blocked in itsmovement away from the spreadingridge by something happening on theother side of the continent?

Part B: How Transform Faults AreFormed1. When the theory of plate tectonics

was young, back in the 1970s, therewas an experiment to see howdivergent plate boundaries areformed. Scientists might describe this experiment as “beautiful” or“elegant.” It was small enough to bedone on a table top. You can run it asa “thought experiment.” A diagramof the setup is shown below. Use a shallow square pan. Fill it withmelted wax, and heat the bottom of the pan to keep the wax melted.Cool the surface of the wax with afan, so that it crusts over to form“lithosphere.” Install roller cylindersalong opposite edges of the top of thepan. These two rollers can be rotatedoutward in opposite directions to pullthe solid surface of the wax apart.

rollerroller

fan

pan

heat

"plate""plate"

"spreading ridge"




a) Predict what will happen to thecrust of wax when the rollers arerotated. Assume that everythingis adjusted right (composition ofthe wax; heating at the bottom;cooling at the top; speed of the rollers).

b) If a break were to occur in thesurface wax, what do you thinkwould happen to the liquid waxbelow?

2. In the actual experiment, the“lithosphere” broke along a crackacross the middle of the pan. As thetwo “plates” on either side of thecrack were formed, new“lithosphere” formed as the liquidwax welled up into the crack andsolidified. The amazing thing aboutthe experiment was that the originalcrack was formed with exactly thesame pattern of ridge-crest segmentsand transform faults that can be seenin the real mid-ocean ridges!

a) Were your predictions correct?How do you explain anydifferences between whathappened and your predictions?

b) What does the experiment suggestabout the age of the transformfaults compared to the mid-oceanridges?

Part C: Plate Boundaries on World Maps1. Look at the world map of Major

Lithospheric Plates in the Investigatesection of the previous activity, whichshows the relative motion of theplates. Observe what it shows abouthow plates move relative to eachother. Answer the following:

a) Name two plates that are movingtoward each other(colliding/converging).

b) Name two plates that are movingapart (diverging).

c) Name two plates that are slidingpast each other.

2. Use a blank world map to make amap that shows the three major typesof plate boundaries.

a) On the map, color the boundaryline that separates two convergingplates. Do not outline both of theplates completely; highlight onlythe boundary between the twoplates.

b) Using two other colors, highlightthe divergent plate boundaries,(where plates are moving awayfrom one another) and the plateboundaries, where plates slide pastone another (called transformboundaries). Make a key thatshows this color code.

Part D: The Plate-Tectonic Setting ofYour Community1. Describe the plate-tectonic setting of

your community. Refer to your worldmap and the map This DynamicPlanet (United States GeologicalSurvey) in your work.

a) How far is your community fromthe nearest plate boundary?

b) What type of plate boundary is it?

c) How might your communitychange its position relative toplate boundaries in the future?


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Digging Deeper

THE DYNAMICS OF PLATE BOUNDARIES

Types of Plate Boundaries

Plate boundaries are geologically interesting areas because they are wherethe action is! Geologists use three descriptive terms to classify theboundaries between plates: 1) divergent boundaries are where two platesmove away from each other; 2) convergent boundaries are where two platesmove toward each other; and 3) transform boundaries are where two platesslide parallel to each other.

Divergent Plate Boundaries

You have already learned some things about divergent plate boundariesin Activity 1, because mid-ocean ridges are divergent plate boundaries.Themid-ocean ridges are places where mantle asthenosphere rises slowlyupward.As it rises, some of the rock melts to form magma.Why doesmelting happen there? To understand that, you need to know that themelting temperature of rock decreases as the pressure on the rockdecreases.As the mantle rock rises, its temperature stays about the same,because cooling takes a long time. However, the pressure from the overlyingrock is less, so some of the rock melts.The magma then rises up, because itis less dense than the rock. It forms volcanoes in the central valley of themid-ocean ridge. Geoscientists in deep-diving submersibles can watch theseundersea volcanoes! Because of the great water pressure in the deep ocean,and also the cooling effect of the water, these volcanoes behave differentlyfrom volcanoes on land.The lava oozes out of cracks in the rocks, liketoothpaste out of a tube. Some of the magma stays below the sea floor andcrystallizes into rock there.All of these new igneous rocks, at the sea floor

You have seen that plates can interactin three different ways: they canconverge, diverge, or move parallel toeach other. You have gained someexperience in recognizing the threekinds of plate boundaries on worldmaps. You are better prepared for alater activity, on the different kinds oflandforms that develop at thedifferent kinds of plate boundaries,

and how earthquakes and volcanoesare caused at or near plateboundaries. You are now in a betterposition to help middle schoolstudents understand one of the keyconcepts of plate tectonics: thematerial of the ocean floors is alwaysmuch younger than the oceansthemselves!

Reflecting on the Activity and the Challenge

Geo Wordsdivergent plate boundary:a plate boundary where twoplates move away from oneanother.




Geo Wordsrift valley: a large, longvalley on a continent,formed where thecontinent is pulled apartby forces produced whenmantle material rises upbeneath the continent.

and below, make new oceanic crust, which then moves away from the ridgecrest.This would be a good time to go back to Activity 1 and review thematerial on sea-floor spreading.

In the investigation you modeled how the “continent” moved farther andfarther from the “spreading ridge.” Look back again at the world map oflithospheric plates in Activity 1. In both the North Atlantic Ocean and theSouth Atlantic Ocean, there is no plate boundary along the coastlines oneither side of the ocean.That tells you that the Atlantic Ocean is gettingwider as time goes on.Why? Because new lithosphere is being created allthe time at the mid-ocean ridge but is not being consumed at the edges ofthe continents. Does that make you wonder what would happen if you couldmake time run backward and watch the ocean shrink? At some time in thepast, there was no Atlantic Ocean!

A new ocean begins when hot mantle material begins to move upwardbeneath a continent. Geoscientists are still not certain about why thathappens.The lithosphere of the continent bulges upward and is stretchedsideways. Eventually it breaks along a long crack, called a rift. See Figure 1 forwhat a newly formed rift valley looks like. Magma rises up to feedvolcanoes in the rift.As the rift widens, the ocean invades the rift.A newocean basin has now been formed, and it gets wider as time goes on.

B.B.

A.A.

upwarpingupwarping

rift valleyrift valley

asthenosphereasthenosphere

lithospherelithosphere

Figure 1 The formation of a rift valley on a newly rifted continent.


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Plate Tectonics

Convergent Plate Boundaries

At a convergent plate boundary, two plates are moving toward eachother.Your common sense tells you that one of them has to go under theother. (Would it surprise you to hear that common sense is important to ascientist, even though sometimes common sense can fool you?) There arethree kinds of places where this happens.These are described below. In alater activity you will learn much more about the landforms that areproduced at convergent plates boundaries, and how earthquakes andvolcanoes are associated with convergent plate boundaries.

In some places, two oceanic lithospheric plates are converging.There aregood examples along the western edge of the Pacific Ocean. Look back at theworld map of lithospheric plates in Activity 1.The Pacific Plate and the Indo-Australian Plate are moving toward one another in the South Pacific, and thePacific Plate and the Philippine Plate are moving toward one another in theWestern Pacific. Look at the lower part of Figure 2. One plate stays at thesurface, and the other plate dives down beneath it at some angle.This processis called subduction, and so these plate boundaries are called subductionzones.Where the downgoing plate first bends downward, a deep trench isformed on the ocean floor.These trenches are where the very deepest oceandepths are found.As the plate goes down into the mantle asthenosphere,magma is produced at a certain depth.The magma rises up to the ocean floorto form a chain of volcanic islands, called a volcanic island arc.

Other subduction zones are located at the edges of continents. Look at theupper part of Figure 2.The west coast of South America, where the NazcaPlate and the South American Plate are converging, is a good example. Inplaces like this, the downgoing plate is always oceanic lithosphere and theplate that remains at the surface is always continental lithosphere.That’sbecause the continental lithosphere is less dense than the oceaniclithosphere. Ocean–continent subduction is similar in many ways toocean–ocean subduction, except that the volcanic arc is built at the edge ofthe continent, rather than in the ocean.The Andes mountain range inwestern South America is an example of a continental volcanic arc.

The third kind of convergent boundary is where two continental lithosphericplates have collided with each other.Think back to the investigation.Youwere asked what would happen when the “spreading ridge” arrives at the“subduction zone.” If you suspected that the spreading ridge would go downthe subduction zone, never to be seen again, you were right! If you hadcontinued the investigation after the spreading ridge had disappeared, whatwould have happened? As the plate that was on the other side of the

Geo Wordsconvergent plate boundary:a plate boundary where twoplates move toward oneanother.

subduction: the movement ofone plate downward into themantle beneath the edge ofthe other plate at aconvergent plate boundary.The downgoing plate always isoceanic lithosphere. The platethat stays at the surface canhave either oceanic lithosphereor continental lithosphere.




Geo Wordssuture zone: the zoneon the Earth’s surfacewhere two continentshave collided and havebeen welded together toform a single continent.

spreading ridge was consumed down the subduction zone, the continent onthe other side of the ocean would have moved closer and closer to thesubduction zone.This is how two continents can come together at asubduction zone. Remember that continental lithosphere is much less densethan the mantle, so continental lithosphere cannot be subducted.Thesubduction stops! The continent that was coming along toward thesubduction zone keeps working its way under the other continent, forhundreds of kilometers, until finally the friction between the two continentsis so great that plate movement stops.The zone where two continents havemet and become welded into a single continent is called a suture zone.There is only one good example on today’s Earth: the Indian Plate hascollided with the Eurasian Plate and is still working its way under it.

Figure 2 Cross sections through subduction zones.The upper crosssection shows subduction of an oceanic lithospheric plate beneath acontinental lithospheric plate (ocean–continent subduction).The lowercross section shows subduction of an oceanic lithospheric plate beneathanother oceanic lithospheric plate (ocean–ocean subduction).

ocean-continent subduction

ocean-ocean subduction


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Plate Tectonics

See Figure 3.There is a good reason why the Tibetan Plateau is the largestarea of very high elevations in the world: the continental lithosphere is muchthicker there because one continent has moved under another.

Transform Plate Boundaries

At transform boundaries, plates slide past one another.The surface alongwhich the plates slide is called a transform fault.As you saw in Activity 1in the material about mid-ocean ridges, transform faults connect the offsetsalong mid-ocean spreading ridges. Most are short, but a few are very long.The most famous transform fault forms the boundary between the NorthAmerican Plate and the Pacific Plate, in California. It is several hundredkilometers long.You can see from the map in Figure 4 that the San AndreasFault connects short segments of spreading ridges at its northern andsouthern ends.

The movement along the transform fault is limited to the distance betweenthe two segments of ridge crest. Figure 5 is a sketch map of a mid-oceanridge, showing segments of the ridge crest offset by transform faults.Between Points 1 and 2, Plates A and B are sliding past each other.

Geo Wordstransform plate boundary:a plate boundary where twoplates slide parallel to oneanother.

transform fault: a verticalsurface of slippage betweentwo lithospheric plates alongan offset between twosegments of a spreading ridge.

India

Tibetanplateau

Eurasian plateEurasian plate

Indo-Australian plateIndo-Australian plate

Himalayas

Figure 3 The Himalayas and the Tibetan Plateau wereformed by the collision of the Indian Plate with theEurasian Plate.The Indian Plate is being shovedhorizontally underneath the Eurasian Plate, so thecontinental crust in the Himalayas and the TibetanPlateau is much thicker than normal.




1. Name the three typesof boundaries betweenlithospheric plates.

2. How and where are riftvalleys formed?

3. How can ocean basinschange in size?

4. Convergent plateboundaries can be inthree different settings.What are they?

5. Describe subduction.

6. Why is it thattransform faults can beused to figure out thedirections of platemovements? Why can’tsubduction zones alsobe used for that?

7. What happens whentwo continents collidealong a convergentplate boundary?

Check YourUnderstanding

The transform fault extends only between Points 1 and 2.To the left of Point1, Plate A is in contact with itself along the east-west line, and to the right ofPoint 2, Plate B is in contact with itself along the east-west line. Since theplates are rigid, there is no slipping movement along the east-west lines,which are like “dead” transform faults!

subductionzone

Canada

United States

Mexico

relative motion ofNorth American

plate

relative motion ofPacific plate

Explorer ridge

Juan de Fucaridge

Blancofracture zone

Mendocinofracture zone

Molokaifracture

zone

Murrayfracture

zone

San Francisco

EastPacific rise

Los Angeles

Figure 4 Plates showing a transform boundary.

transform fault

ridge crest

ridge crest

fracture zone

plate A plate B

plate A plate B

1

2

Figure 5 Sketch map of a mid-ocean ridge, showing segments of the ridgecrest offset by transform faults.


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Plate Tectonics

Preparing for the Chapter Challenge

Inquiring Further

Reflect on your answers to the above.Write a short summary (one or twoparagraphs) describing what you havelearned so far about how platemovements and their interactions atplate boundaries can change thearrangement of continents and oceanbasins on the Earth. Be prepared toinclude this summary in your ChapterChallenge.

1. Evolution of the Biosphere at Mid-Ocean Ridges

Recently, new forms of life havebeen discovered at mid-oceanridges. They thrive in the presenceof superheated, mineral-rich water.This life does not depend upon theSun for energy, but instead uponthe energy and matter from Earth’sinterior. How has life evolved insuch environments? For furtherinformation check the EarthCommweb site.

Understanding and Applying What You Have Learned

1. Design an investigation, with thematerials like the ones you used inthis activity, to model the creationof a new ocean basin.

2. Identify on a world map oflithospheric plates one example ofeach of the following settings:

a) an established divergentboundary

b) a young divergent zone(continental rift zone)

c) an ocean–ocean convergentboundary

d) an ocean–continent convergentboundary

e) a continent–continent collisionzone

f) the interior of a plateg) a transform plate boundary

Black smokers form at the mid-ocean ridges.


Documents

Activity 2 Plate Boundaries and Plate Interactions€¦ · of plate boundaries. a) On the map, color the boundary line that separates two converging plates. Do not outline both of