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I DOCUMENT RESUME
ED 216 935 SE 038 139
AUTHOR Stoever, Edward, C., Jr. )TITLE Drifting Continents and Magnetic Fields. Crustal
Evolution Education Project. Teacher's Guide [and]Student Investigation.
INSTITUTION National Association:of Geology Teachers.SPONS AGENCY. National Science Foundation, Washington, D.C.REPORT NO CEEP-MOD1OH12-1-5; ISBN-0-89873-008-2;
- ISBN-0-89873-009-0'UB DATE 79GRANT SED-75-20151; SED-77-08539; SED-78-25104NOTE 30p.AVAILABLE FROM Ward's Natural Science Establishment, Inc., P.O. Box
1712, Rochester, NY 14603 (or P.O. Box 1749,Monterey, CA 93940.)
EDRS PRICE mFod. Plus 'Postage. PC Not Available from EDRS.DESCRIPTORS *Edrth Science; Geographic Location; Geology;
Instructional Materials;'*Science Activities;*Sci'ence Course Improvement Projects; ScienceCurriculum; Science Education; Science Instruction;Secondary Education; *Setondary School Science;Teaching Guides; Teaching Methods
IDENTIFIERS Continental Drift; 'Crustal Evolution EducationProject; National Science Foundation; *PlateTectonics
ABSTRACT 4
Crustal Evolution Education Project (CEEP) moduleswere designed to: (1) provide students with they methods and resultsof continuing investigations into the composition, history, andprocesses of the earth's crust and the application of this knowledgeto man's aCtiwities and (2) to be used by teachers with little, or noprevious background in the modern theories of sea-floor spreading,continental drift, and plate tectonics. Zach module consists of twobooklets: a teacher's guide and 'student investigation. The teacher'sguide contains all of the information present in the studentinvestigation booklet as well as: (1) a,general introduction; (2)prerequisite student background; (3) objectives; (4) list of requiredmaterials; (5) background information; (6) suggested approach; (7)procedure, recommending three.45-minute class periods for thismodule; (8) summary questions, (with answers); (9) extensionactivities; and (10) list of references-...Objectives includeexplainingAagnetic dip, using magnetic Aip preserved in rocks 'todetermine-11.;itude of India at certain timed in the past and itspresent ratekof movement, and identify collision of India and Asia asthe cause of the Himalayas. Familarity with continental drift,,paleomagnetism, latitude/longitude, and measuring angles witipaprotractgr are prerequisites for understandin e teacherdemonstration and completing the activities. uthor/JN)
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****A******************************************************************* Reproductions supplied by EDRS are the best that_ can be made *
* from the original document. . *
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US. DEPARTMENT OF EDUCATIONNATIONAL INSTITUTE OF EDUCATION
EDUCATIONAL RESOURCES INFORMATION
CENTER (ERIC)ItI/I'Tne document KaS been reproduced 'as
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"PERMISSION TO REPRODUCE THISc) MATERIAL IN MICROFICHE ONLY
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TO THE EDUCATIONAL RESOURCESINFORMATION CENTER (ERIC)::
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CRUSTALEVOLUTION
'EDUCATIONPROJECT
G.
DriftingConti ents
And agnelcFields
TEACHER'S GUIDECatalog No. 34W1005
For use with Student Investigation 34W1105Class time: three 45-minute periods,
Developed by
THE NATIONAL ASSOCIATION OF ,GEOLOGY TEACHERS
Produced and Distributed byWard's Natural Science Establishment, Inc. Rochester, NY Monterey+ CA 1
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NAGT Crustal Evolution Education ProjectEdward C. Stoever, Jr., Project Director
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Drifting- ContnentsAnd Magnetic Fields
INTRODUCTIONGeophysicists have been abtc to plot the pathof India, from its breakup with Antarctica, acrossthe equator to its present position; using theapparent magnetic dip preserved in the rocks ofIndia. In this module, students use paleotnagneticdating in the same way those geophysicistsdid, to determine the position of India at*varioustimes in the past. They also learn thatlhe,collision of India with Asia is the probable caus4\of the extensive earthquake activity in Tibetand southern China.
PREREQUISITE STUDENT BACKGROUNDStudents should be familiar with continentaldrift and paleomagnetism. They should beable to locate places on a world map usinglatitude and longitude. They should also be able tomeasure angles with a,protractor.
OBJECTIVESS
After you have completed these activities, youshould be able to1. Explain what is meant by magnetic dip2. Use the magnetic dip preserved in rocks todetermine the latitude of India 'at certaintimes in the past, and its rate of movement3. Identify the collision of India and Asia asthe cause of the Himalayai
MATERIALSOne set of each of the following is needed fordemonstration purpoSes:.Overhead projectorTransparency of the upper half of Worksheet 1Iron filings and a bar magnet of a size that willfit the transparencyA transparent lid or dish' to Support thetransparency above the magnetTransparent protractor, ruler and a marker penAt least one dip needle anda compass. If moreare availible, then students can determinethe answers to Question 1 themselves.
4rt One or more atlasesEach student should have a protractor, ruler,pencil and eraser
4
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Scientists have. determined that at one time Indiawas a part of Antarctica It broke off fromAntarctica and for 200 million years driftednorthward, isolated from any other landma sFinallyit collided with Asia, resulting in theuplift of thetHirpalaya Mo ems Perhaps y uhave heard about the earthq kes in that part ofthe world. These earthquake are the resultof this collision process of one, continent withanother They tell us that it is stiff going on.,
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BACKGROUND INFORMATIONThe difference in the magnetic dips of the rocksfound in India is not the only evidence usedby scientists to trace the northward movementof India. The magnetic anomalies of theOcean 'floor provide the major support. Studentsshould realize that the information th-ey dealwith in this activity is simplified to makeit readily understandable.
In the introduction of the student guide,students are told that the Himalaya Mountainsand the earthquakes in southern China are theresult of the collision of two continents.It is the movement of oceanic-crust into asubduction zone along the southern margin ofAsia that caused the collision: The two continentswere drawn together by this movement. Thecontinental crust of India, however, was toolow in density to be drawn down into thesubduction zone. Instead, it is colliding with thecontinental crust of Asia. In this way a great .
thickness of continental crust occurs at the zoneof collision. This greater-than-normal thicknessof lower density continental rock maybe largelyresponsible for the height of the Himalayas.There seems to be a shortening of crust caused bythis spilision. As the crust shatters, it slides alorigisthe resulting faults in a general compressionof the continental crusts of India and Asia.These movements result in the many shallowfocus earthquakes occurring in Tibet.See.Figure A.
SUGGESTED APPROACHPART A of the module is a teacher demonstration.Students, however, should respond to thequestions in the Student Investigation and onWorksheet 1 during and immediately after thedemonstration. The remainder of the activity canbe done in an individualized setting or In smallgroups with the demonstration as background.You should conduct a post-laboratorydiscussion at the end of the module usingthe SUMMARY QUESTIONS-as a guide. Thisdiscussion could also include informationrelating to the EXTENSION.
The Alps are another zone where two continentshave collided and produced mountains. TheAlps are older than the Himalayas and resultedwhen Africa collided with Europe. A secondperiod of collision may be occurring now. Africaseems to be moving into Europe. As a result,the Mediterranean Sea is closing. Millionsof years from now another mountain range,parallel to theAlps, may be formed bythis collision.
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Figure A. Schematic representation of thecollision of India and Asia, which prod cell theHimalayas and has resulted in continuingshallow earthquake activity.
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PROCEDURE acm=Tim_...*PART A. What is magnetic dip?In this part of the activity you conduct ademonstration and class discussion to reviewbasic concepts concerning the earths magneticfield and how it can be measured at variousplaces on the surface. .
Key words: dip needle, magnetic dip, horizon'lineTime required: one to two 45-minute pert/xisMaterials: protractbr, ruler and pencil
The earth is surrounded by a magnetic field,which is stkongest near the north and southmagnetic poles It can't be seen However, anyonewho has used a magnetic compass has observedit. The compass allows one to "tell directions"(north, south, east,4pr west). Telling directions:isusing only one part of the magnetic fieldthe horizontal part Since the field is three-dimensional, there is another part at right angle/to the hdrizontal part. We will call this the verticalpart To observe the vertical part of 'the field,you use a dip needle. It is similar to an ordinarycompass except that the needle is allowedto rotate vertically instead' of horizontallyThe angle formed by the dip needle and thehorizon is known as the magnetic dip.
Demonstrate d dip needle and a comp7ss. Youmay have to go outside the building to avoidmagnetic materials. Have students use a compass
'to determine the orientation of the building,,**)tor some othei feature, with reference &magnetic
north. Then ybu gan briefly discuss the three-dimensional nature of the magnetic fieldand introduce the dip needle as a compajs Inthe vertical plane. Then have them deterNne theiferticd1 component of the 'field at yourschool's location: .
If your students are familiar with the idea ofvectors, you should'discuss the magnetic fieldas'a,vector quantity having both magnitude;and directfon. There are two angles of interest:_.declInatioir and Inclinatioh. Declination is'the angle Of deviation qom the geographicnorth. Inclination is the angle that the dip needlemakes with the horizontal plane. In the activitythiS is called magnetic dip. See Figure B.
Although you marwant t6 disduss declination(or the difference between truenorthgeographicnorthand magnetic north) with yourstudents, this distinction is notfor understanding the basic OBJECTIVES ofthis module. In fact; it may confuse stude ,n ,js
,40 since later In the,aptivity they are told that overgeologies time, geographic north and `magneticmirth can be assumed to be essentially. "
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1, Your teacher has shown you a compass anda dip needle. Which would you use to measure the
. horizontal, part of the earth's magneticfield? compass Determine the direction,with a compass, to some object in your roomOf school yard..
,'ObjectDirectionWhat instrument would you use to measurethe vertical part of the earth's magnetic
-field? dip needle
In this activity you will be using the magneticdip recorded in rocks. When molten rocks reachthe surface of the earth and begin to cool, certainminerals within the rocks are magnetizedparallel with the magnetic field of the earthThese rocks will then have a weak magnetic fieldthemselveS. The positions of samples of therocks are recoYded befoire they are collectedScientists can then meWre the hor,j.26'rital andvertical parts of the magnetic fields of these'rocks. This will tell them about the magrietic fieldfn the area at the time the rocks cooled
This explanation of the paleomagnettsm ofrocks is highly simplified. The rocks becomemagnetized as they crystallize. When the Curietemperature is reached (about 500° C), thismagnetism will becorn4 permanent. If later therock is heated above the Curie point, it willlose its permanent magnetism. It willacquire a new fieldwhen, the temperaturefalls back down through the Curie point again.Therefore, the scientist must know the temperaturehistory of the rock.
Because of crustal disturbances, many rocks Lc.
are not found in their original positions. Magnetiadata from them can still be used, however,if the scientists are able to make correctionson the positions. The original positions can oftenbe determined from bedding planes andother'characteristics 4f the rocks.
CI
ivlagnetc Id, Geographic north
Figure B. The earth's magnetic field can bemeasured vertically and horizontally.
3
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ASet up the demonstration as follows:
1. Place the bar magnet on the'overheadprojector and cover it with the transparent lidor dish.2 Scatter the ironsfilings on the transparent ltdover the bar magnet.3. Place the transparency of the diagram ofthe earth's magnetic field (Worksheet 1) over .the iron filings so that its north and southpoles coincide with those of the bar magnet.Be careful not to disturb the iron filings.
Try out the demonstration at least a day or twoahead. Be sure the magnet and the diagram
tam of the correct size so that the iron filings willfollow closely the lines of magnetic forceon the diagram. You may.have to redo the t
diagram to make them fit correctly. This is animportant step in that,it allows students to "see"the earth's magnetic field as modeled by the fieldof the magnet. The student can then relatethe demonstration to orientation of the dipin diagrams representing rocks taken from variousplaces on the earth's crust..lf students don'tunderstand this; then the remaining part of themodule will have little meaning for them.
2. After you observe your teacher's dernao-
onthe north and south poles on the
diagram Worksheet 1
3. plotice that a horizon line has been drawn atloiation C. A horizon line is a straight line .
touching the cir=cle' but not intersecting it. Drawin the horizon line at the other four loCationsTo make a horizon line, first draw a straightfrom the center of the circle to the point on tcircle through which the horizon line must pass(This line is the radius of the circle.) Then drawealine 90° (perpendicular) to the radius at thepoint on the circle. This is the horizon line for thatpoint, Measure the angle of dip of the magneticfield at each location. It will be the angle formed bya ling of magnetic force and the horizon. Note thatthe radius and dip angle for C are already shown.A 90° B 60° C 350 D 60° 5°
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4. Worksheet 1 has drawings of rocks collectedat each of the five locations Draw arrows`on the rock drawings showing the direction ofdip recorded by the magnetic minerals inthese rocks. The rocks are all samples frofr) recentlava flows.
5. -Where on the earth is the dip the greatest?at the poles the least? at the equator
6. Samples A, 13, and C are from the nortlrnhemisphere. What is relationship bptween dipand distance from-the north tha,s-i)netic pole9
Dip becomes greater closer to the north ,magneticpole.
7. Samples D and,E are from the southernhemisphere..What happens to the dip inthe southern hemisphere9It is reversed. Thie north - seeking needle willpoint up rather than down.
Be certain that students understand therelationship of dip to latitude and position northor south of the equator. This understandingis essential for PART B..
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PROCEDUREPART B: What does magnetic dip tell aboutIndia?
In this part of the actiiitylstudents studyevidence from magnetic dip recorded,in certainrocks. This record has led geophysicists to believethat India has drifted northwardio collide withAsia. Students learn that changes in the rateof movement can be determined and that thecollision accounts for the Himalaya Mountainsand the extensive geological activityin that area.
Key words: radiometric dating, paleolafttpdeTime required: one 45-minute period(Materials: protractor, ruler and pencil, atlas
Geophysicists find that the ancient magneticdip may be different in rocks of different ages onthe same continent. This is particularlyt'cue of Indra.
Figure 1 shows drawings of six rocks. Each rockis from a different formation in India.Samples B and D areitipm lava flows. The othersare sedimentary rocks that contain pelliclesof iron. Iron particles will often act like tinymagnetic compasiNs as sediments are beingdeposited. They align themselves with themagnetic field of the earth. Sedimentary rocksand igneous rocks, then, can provide a'record ofthe earth's magnetic field.
Note that the age of each rock is given in years.Geologists can determine how long agoan igneous rock crystallized by a method knownas radiometric dating. They can also determinethe approximate time of deposition of sedimentsat a site where igneous rocks have cut ,
across the sedimentary rocks.
Figure 1 is based upon data from C.T. Klootwijk,1976. It has been translated from the formused by paleomagnetists into the form used Inthis activity to make it more understandableto students.
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The direction of magnetic dip is Indicafed bythe arrow on each sample.
1. Use the protractor to measure the angle ofmagnetic dip for each sample and place yourresults in the proper column of the Data Skeeton-Worksheet2.
What.could cause the magnetic dip of the rocksfound in India to change during the last 225million years? there are three possibilities.a. -The north magnetic pole has moved.b. India has moved. .
c. both the north magnetic pole and Indiahave moved
Most scientists now believe that the magneticpoles have always remained relatively dose to theearth's geographic poles and that the geographicpoles have always remained in an area neartheir present positions Therefore, a and c are notacceptable explanktions, arid the onlypossible explanatio1i is that India has moved.
Since the magnetic and geographic poles areclose together, you can assume for thisactivity that they are indeed the same. Therefore,the angle,of magnetic dip can give you latitudeand distance from a geographic pole ifyou use this graph (Figure 2).
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2. Using Figure 2, determine the paleolatitycie(ancient latitude) far each of your samplesfrom India. Record this on your Data Shipeton Worksheet 2.Paleolatilude is found by using the angle ofmagnetic dip the students recorded from Figure 1.For example, rock C has a 49° dip. UsingFigure 2, look for 49° ortthe bottom scale labeled"Angle of magnetic dip." Go up front that 4.
point to the curved line. From that point on thecurve read across to the Scale'of latitude tofind the "paleolatitude." For rock C it isapproximately 30° S. latitude Moteposition of arrow).
3. Plot the paleolatitudeand age Of each sampleon the graph on Worksheet 2.
4. From the graph you have Just plotted,determine the average rate of movement of Indiadurinlhe last 200 million years in cmiyear.Approximately 3.2 cm/pear
5. Has India always moved at the tame rate ofspeed during the least 200 million years'?No
0
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30 60
Angle of magnetic dip degreq N or S
90
0 km
5 000
10 000
Figure 2. Graph showing relationship between distance from magnetic poles and angle of magnetiC dip.
8 i
4 20
Data Street
Rock Sample A .8 C D E F
Age of Sample 225m y 200m y 80m y , 60m,y 20m y Present
Angle of Dip 56 66 49 37 17 37
Paleo latitude 37°S 48°S 30°S 19°S ,8°N 20°N
Or Graph
4.
20
30
4.40
50
60225
IP I
200
V
150
6000
100
Time in millions of years before present
Answer Sheet 2 (PART 13)
REFERENCES1Klootwijk, C T , 1976, The drift of the Indian
subcontinent, an interpretation of recentpaleomagnetic data. Geo/ogische Rundschau,v. 65, no. 3, P. 885-909.
Mead, D.F., 1976, How the Chinese predictearthquakes. Science Digest, v 79, no. 3 (Mar.),p. 57-61.
11 4.
50 0
Molnar, P. anq-TappoAnier, P , 1977, The collision.'between rndia and Eurasia. Scienti&American, v. 236, no 44/(Apr ), p 30-41,
Shaw, E., 1977, Can animals anticipate earth-quakes? Natural History, 486, no 9, (Nov ),p. 14.
China's quake forecasting: 50-50. Scierice News,1977, v. 112, no. 9 (Aug.), p. 133-134.
F.
6, Mark the Rosition of India for the varioug datesof your samples on the map on Worksheet 3.
''Scientists have discovered that India has nothad very much.east7west movement in its
-past. TherefOre, the path that you have plotted Isprobably pretty close to that actually takenby India.
The Himalayas are thought to be an area ,wheretwo continents are actually in the processof colliding. What would you expect some of theeffects orsubh a collision to be? Use anatlas. to answer'the next two questions.
X7. Where is the highest elevationl'in the world?Mt. Everest in the Himalayas .
, S. Locate the TibetariPlateau What are some'typical elevations?Mountains rise to 25;000 feet., Valley floors areat about 20,000 feet.
9. What might causes the elevations you foundfor the answers to the last two questions?The collision of two continents causes the highelevations. You may want to discuss themechanisms of this collision as described brieflyin BACKGROUND INFORMATION7,
SUMMARY QUESTIONS1. Describe the movements of India for thepast225 million yearsIndia has moved from the southern hemisphereto its present position. It wasonce part ofAntarctica. After its separation, it remainedisolateduntil its collision with Asia.
2. What types of evidence are used to determinethe movement of India.?In this activity', i fhy the evidence from magneticdip has been used to demons rate4the movementof the Indian continent. Other evidence,however, is also used by'scientists, includingmagnetic anomaly patterns observed in the oceanfloor, the correlation of rocks by using fossilsand radiometric dates, and the types pffossils found in rocks cif different ages.'
EXTUISIONSSom l devastating earthquakes have,occurredin China recently. See if you can find news storiesof these earthquakes in your library. What effectsdid they have upon people? Where were theylocated? Can they be explained by thecollision of the two plates?.The mo0 severe earthquakes in China have-occurred far from the mountains. Thkr cannotbe readily explained by the collision of thetwo continents.
10'
To answer the next question compare Figure,3with your atlas.
If a copy of either the 'World kelsmicilisMap(1974, U.S. Geological Survey) of Tergstrial HealFlqw Datia Map (1976, World Data Center Afor SolidlEarth Geophysics, Boulder, Colorado)is available, it may be used in place ofFigure 3.
10. Near what topographic, features do mostof the earthquakes in southeast Asia seem tooccur?In br near the Tibetan Plateau, in the HimalayaMountains, and in the mountains of Pakistan.
How can you explain theiroccurrence?The collision of India and Asia ban also a ountfor the manyearthquakes.that occur insoutheast Asia: Again, you may want to discusthe mechanisms as described in BACKGROUN#INFORMATION.
1
3. What is magnetic dip?This is the record of the inclination of the earth'smagnetic field preserved in a rock, at thetime and in the geographic location, where therock formed.
4. What types of.geological features and activitymay result where continents collide? '4Mountain ranges will be produced in the collisionof continents. Where this collision is stillactive, as in Ti t, there hay also beeahhquake activity.
7
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*4.
Chinese scientists have been recording theoccurrence of earthquakes for thousands of yearsRecently they have been attempting to predictep,ft-hquakes. Can yoU locate informationon their efforts to predict earthquakes? Whattypes of methods are they using? How successfulisay.ethey been?The articles by Mead and Shawand the onein Science News, discuss the wOrk of,Chineseseismologists who have been attempting topredict earthquakes. You might"rMer yourstudents to these afticles. It also would addinterest to present some of this information inyour poit-laboratory discussion of the activitiesin this module.
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-NAGT Crustal EvolutionEducattionProfect ModulesCEEP Modules are listed here in alphabeticalorder Each Module is,designed for use inthe number of class periods iridicated Forsuggested sequen-ces o CEEP Modules tocover specific topics and for correlationof CEEP Modules to standard earth sciencetextbooks, consult Ward's descriptiveliterature on CEEP TheCatalog Numbersshown here r4fer to the CLASS PACKof each Module consisting of a TeacbAr'sGuide and 30 copies of the StudentInvestigation Ward s descriptiveiterature for alternate order quantities
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Class CLASS PACKCEEP Module Periods Catalog No.
A.Sea-floor Mystery' Mapping 3
Polarity ReversalsContinents And 0j,,ean Basins. 3-5
Floaters And SYnkersCrustal Movement: A Major Force a 2-3
In Evolution,Deep Sea Trenches And Radioactive
WasteDrifting Continents And Magnetic
FieldsDrifting Continents And Wandering
Pbles' Earthquakes And Plate` Boundaries
Fossils As Clues To AncientContinents
Hot Spots In The Earth's CrustHow Do Continents Split Apart?How Do Scientists Decide Which Is
The Bette). Theory?How Does Heat Flow Vary In The
Ocean Floor?How Fast is The Ocean Floor
Moving/Iceland The Case Of The Splithng
PersonalityImaginary Continents: A Geological
Puzzle.Introduction To Lithospheric
Plate BoundariesLithospheric Plates And Ocean
Basin TopographyLocating Active Plate Boundaries
By Earthquake DataMeasuring Continentql Drift: The
Laser Ranging ExperimentMicrofossils, Sediments And
Sea-floor SpreadingMovement Of The Pacific Ocean
FloorPlate Boundaries And Earthquake
PredictionsPlotting The Shape Of The Ocean 2-3
FloorQuake Estate (board game)
1
Spreading Sea Floors And FracttlredRidges
The Rise And Fall Of The BeringLand Bridge
Tropics In Antarctica?Volcanoes: Where And Why?What Happens When Continents
Collide/When A Piece Of A Continent
Breaks OffWhich Way Is North? 3
Why Does Sea Level Change? 2-3
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34 W 1202
34 W 1203
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3 34 W 1205
4 34 W 1206
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2-3 34 W 1208
3 34 W 12092 34 W 12102 34 W 1211
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2-3 34 W 1213
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Ward's Natural Science Establishment, Inc.P.O. Box 1712, Rochester, New Yak 14603 P.O. Box 1749, Monterey, California 93940
DEEP MODULE NO. 0H12 1-5ISBN: 0-69513-005-2
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Student investigationCatalog No. 34W1105
Drifting Continents\ And Magnetic Fields
4 .1
4INTRODUCTIONScientists have determined that at one time Indiawas a part of Antarctica It broke off fromAntarctica and for 200 million years driftednorthward, isolated from any other landmassFinally it collided with Asia, resulting in theuplift of the Himalaya Mountains Perhaps youhave heard about the earthquakes in that part ofthe world. These earthquakes are the resultof this collision process of one continent withanother They tell us that it is still going onIOBJECTIVESAfter you have corApleted these activities, youshould be able to). Explain what is meant by magnetic dip2. Use the magnetic dip preserved in rocks todetermine the latitude of India at certaintimes in the past. anct,ts rate of movement3. Identify the collisilh of India and Asia asthe cause of the Himalayas
PROCEDUREPART A What is magnetic dip?Materials protractor, ruler and pencil
The earth is surrounded by a magnetic held,which is strongest near the north 'and southmagnetic poles. it can't be seen. However, anyonewho has used a magnetic compass has observedit. The compass allows one to "tell directions"(north, south, east, or west) Telling directions isusing only one part of the magnetic fieldthe horizontal part. Since the field is three-dimensional, there is another part at right anglesto the horizontal part. Vie will call this the verticalpart To observe the vertical part of the field,you use a dip needle. It is similar to an ordinarycompass except that the needle i allowedto rotate vertically instead of hori ntally.The angle formed by the dip need and t ehorizon is known as the magnetic dip. . .
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1. YoUr, teacher has shown you a compass anda dip nee Which would you use to measure thehorizontal art of the earth's magneticfield? . . Dete'rmine the direction,with a compass, to some object in your roomor schobf yard , ..
ObjectDirection
What instrument would you use to measure'the
v/er:tical part of the earth's magnetic
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field?
.,
In Ihis''activity you will be using the magneticdip recorded in rocks When molten rocks reachthe surface of the earth and begin to cool, certainminerals within the rocks are magnetizedparallel with the magnetic field of the earth.These rocks will then have a weak magnetic fieldthemselves The positions of samples of therocks are recorded before they are collectedScientists can then measure the horizontal andvertical parts of the magnetic fields of theserocks This will`tell them about the magnelic fieldin the area at the time the rocks cooled
2. After you observe your teacher's demo9-stration, label the north and south'poles on thediagram on Worksheet 1
3. Notice that a horizon line has been drawn atlocation C A horizon line is a strarght linetouching the circle but not intersecting it Drawin the horizon line at the other four locationsTo make a horizon line, first draw a straight linefrom the center of the circle to the point on thecircle through which the horizon line must pass.(This line is the radius of the circle.) Then draw aline 90° {`perpendicular} to the radius at the,point'on the circle. This is the horizon line for thatpoint Measure the angle of dip of the magneticfield at each location It will be the angle formed bya line of magnetic force and the horizon. Note thatthe radius and dip angle for C are already shown.A B C D E
4. Worksheet 1 has drawings of rocks collectedat each of the five Ickations. Draw arrowson the rock drawings showing the direction ofdip recorded by the magnetic mineraislinthese rocks. The rocks are all samples from recentlava flows.
j5. Where on the earth is the dip the greatest'?the least?
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6_ Samples A, B, and C are from the northernhemisphere. What is the,relationship between dipand distance from the north magnek pole?
.7. Samples D and E are from the southernhemisphere What happens to the dip inthe southern hemisphere'?
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PROCEDURE ---PART B What does magnetic dip tell about
Materials protractor, ruler and pencil, atlas
Geophysicists find that the ancient magneticdip may be different in rocks of different ages onthe same continent This is particularlytrue of India.
Figure 1 shows drawings of six rocks Each rockis from a different formation in India.Samples B and 0 are from lava flows. The otherSare sedimentary rocks that contain particlesof iron Iron particles will often act like tinymagne,tic compasses as sediments are beingdeposited They align themselves with themagnetic field of the earth. SVimentary rocksand igneous rooks, then, can provide a record ofthe earth's magnetic field
Note that the age of each rock is given in yearsGeologists can determine how long agoan igneous rock crystallized by a method knownas radiometric dating. They can also determine ,
the approximate time of deposition of sedimentsat a site where igrieous rocks have cutacross the sedimentary rocks
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The,direction of magnetic dip is indicated bythe arrow on each sample.
1. Use the protractor to measure the angle ofmagnetic dip for each sample and place yourresults in the proper column of the Data Sheeton Worksheet 2
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What could cause the magnetic dip of the rocksfound in India to change during the last 225million years? There are three possibilitiesa. The north magnetic pole has movedb India has moved.c. Both the north magnetic pole and Indiahave moved
Most scientists now believe that the magneticpoles have always remained relatively close to theearth's geographic poles and that the geographicpoles have, always remained in an area neartheir present positions. Therefore, a and c are notacceptable explanations, and the onlypossible explanation is that India has moved
Since the magnetic and geographic poles are- close together, you can assume for this
activity that they'are inVeed the same. Therefore,the angle of magnetic dl.:"-can give you latitudeand distance from a geogriphic pole ifyou use this graph (Figure 2)
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2. Using Figure 2, determine the paleolatitude(ancient latitude) for each Qj your samplesfrom India. Record this on your Data Sheeton Worksheet 2
3. PYot the paleolatitude and age of each sampleon the graph on Worksheet 2
4. From the graph you have just plotted,determine the average rate (-.0 movement of Indiaduring the last 200 million' years ir/ cm, year.
5.d-lAs India always moved at the same rate ofsp&d during the last 200 million years9
0 30 60
Angle of magnetic dip degrees N or S
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Figure 2. Graph showing relationship between distance from magnetic poles.and angle of magnetic dip,
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Data Sheet
Rock Sample A B 9 D E F
Age of Sample 225m y 200m y 80m y. 60m y 20m y Presen?
Angle of Dip
Paleo latitude --.,
Graph
Equator
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Worksheet 2 (Part B)
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Time in millions of years bafore present
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6000
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6. Mark the position of India for the various datesof your samples on the map on Worksheet 3.,
Scientists have.discoiered that India has nothad very much east-west movement in itspast. Therefore, The path that you have plotted isprobably pretty close to that actually takenby India
The Himalayas are thought to be an area wheretwo continents are actually in the process
, of colliding What would you expect some of theeffects of such a collision to be? Use anatlas to answer the next two questions
7. Where is the highest elevation in the world?
9. What might cause the elevations you foundfor the answers to the last tWo questions?
To answer the next question, compare Figure 3with your atlas
10. Near what topographic features do mostof the earthquakes in southeast Asia seem tooccur?
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8. Locate the Tibetan Plateau. What are some HoW can yod4explttypical elevations?
rSUMMARY QUESTIONS
1. Describe the movements of India for the past225 million years
2. What types of evidence*are used tb determinethe movement of India?
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3. What is magnetic dip?
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t"4. What types of geological features and Activitymay result where continents collide?
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EXTENSIONSSome devastating earthquakes have occurredin China_recently. See if you can find news storiesof these earthquakes in your library. What effectsdid they have upon people9 Where were theylocated? Can they be explained by thecollision of the two plates?
REFERENCES -Mead, D.F., 1976, How the Chinese preAt
earthquakes. Science Digest, v 79, no. 3 (Mar.),p.' 57761
Molnar, P. and Tapponnier, P., 1977, The collisionbetween India and Eurasia. ScientificAmerican, v 236, no. 4 (Apr.), p. 30-41.
China's.quake forecasting: 50-50. Science News,1977, v. 112, no 9 (Aug.), p. 133-134.
4
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--41
Chinese scientists have been recording theoccurrence of earthquakes foJhousands of yearsRecently they have been attempting to predictearthquakes. Can you locate informationon their efforts to predict earthquakes9 Whattypes of methods are they using9 How successfulhave they been?
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