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Plate tectonics & seafloor spreading
1) Seismic studies reveal that the Earth is composed of layers with different chemical compositions and physical properties, including a solid inner core, liquid outer core, soft asthenosphere, and rigid lithosphere. 2) Evidence from seafloor spreading and magnetic stripes on the ocean floor support the theory of plate tectonics, where new ocean crust forms at mid-ocean ridges and spreads outward as the plates move. 3) Paleomagnetic data from rocks show alternating magnetic polarities on either side of ridges, supporting the hypothesis that new crust forms at ridges and cools over time, recording reversals in Earth's magnetic field to reveal the history of plate movements.
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Plate tectonics & seafloor spreading
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- 2. Two Ways ofLooking at EarthTwo Ways of Looking at Earth SeismicSeismic discontinuitiesdiscontinuities reveal the crust,reveal the crust, mantle, and coremantle, and core and show that theyand show that they have differenthave different chemicalchemical compositionscompositions.. Seismic studiesSeismic studies reveal much aboutreveal much about thethe physicalphysical naturenature of theof the interior, revealing ainterior, revealing a solid inner core, asolid inner core, a liquid outer core, aliquid outer core, a soft asthenosphere,soft asthenosphere, and a rigidand a rigid lithosphere.lithosphere. Figure 01.05: The internal structure of Earth consists of layers of different composition and layers of different physical properties.
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- 16. Wegener, Continental DriftandWegener, Continental Drift and PangaeaPangaea
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- 19. Evidence for ContinentalDriftEvidence for Continental Drift Jigsaw Puzzle fit ofJigsaw Puzzle fit of continentscontinents Alfred Wegener during Greenland expedition
- 20. More evidenceMore evidence Matching fossils onMatching fossils on continents nowcontinents now located thousandslocated thousands of miles apart.of miles apart. Example =Example = Mesosaurus, aMesosaurus, a freshwater reptilefreshwater reptile Many othersMany others
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- 25. More evidenceMore evidence Matching geologicMatching geologic structuresstructures including:including: –Mountain chainsMountain chains –Ore depositsOre deposits –Same rocks ofSame rocks of same agesame age
- 26. More evidenceMore evidence Climate changeClimate change evidenceevidence – Glacial deposits atGlacial deposits at current equatorcurrent equator – Fossilized palmFossilized palm trees in Greenlandtrees in Greenland Map shows whyMap shows why according to theaccording to the placements ofplacements of current continentscurrent continents within Pangaeawithin Pangaea
- 27. Wegener not believedWegenernot believed Why? -Why? - –What could possibly force theWhat could possibly force the continents to move across thecontinents to move across the ocean floor in this way. Theyocean floor in this way. They would be crushed.would be crushed. –He was a meteorologist,He was a meteorologist, not a geologistnot a geologist
- 28. Developments 1950s and1960sDevelopments 1950s and 1960s World War II submarines using sonarWorld War II submarines using sonar found mountains under the oceans –found mountains under the oceans – the mid-ocean ridgesthe mid-ocean ridges Sea floor drilling showed rocksSea floor drilling showed rocks younger than expected and youngestyounger than expected and youngest towards the center of the mid-oceantowards the center of the mid-ocean ridgeridge Theory of seafloor spreadingTheory of seafloor spreading suggested by Princeton professor Dr.suggested by Princeton professor Dr. Harry HessHarry Hess
- 29. Indias role incontinental driftIndias role in continental drift
- 35. Seafloor spreadingSeafloor spreading Firstlook at the earth’s layers as shown here.
- 36. Structure of theEarthStructure of the Earth The Earth isThe Earth is made up of 3made up of 3 main layers:main layers: – CoreCore – MantleMantle – CrustCrust Inner core Outer core Mantle Crust
- 37. The CrustThe Crust This is where we live!This is where we live! The Earth’s crust isThe Earth’s crust is made of:made of: Continental Crust - thick (10-70km) - buoyant (less dense than oceanic crust) - mostly old Oceanic Crust - thin (~7 km) - dense (sinks under continental crust) - young
- 38. SonarSonar A devicethatA device that bounces soundbounces sound waves offwaves off underwater objectsunderwater objects and then recordsand then records the echoes of thesethe echoes of these sound waves.sound waves. Sonar mappedSonar mapped mid-ocean ridges.mid-ocean ridges.
- 39. Great Puzzles in1960Great Puzzles in 1960 Rock in oceans always basalt,Rock in oceans always basalt, younger than 225 million yearsyounger than 225 million years old.old. Ocean crust thin; continental crustOcean crust thin; continental crust thick & graniticthick & granitic Rocks on continent varied, oldestRocks on continent varied, oldest 3.8 billion years old3.8 billion years old Volcanoes & Earthquakes limited inVolcanoes & Earthquakes limited in locationslocations Atlantic & Pacific differentAtlantic & Pacific different topography from land and fromtopography from land and from each othereach other
- 40. 1963 – HarryHess1963 – Harry Hess Observed rocks near ridge youngerObserved rocks near ridge younger than rocks at edge of ocean basinthan rocks at edge of ocean basin Observed higher heat escaping fromObserved higher heat escaping from Earth’s interior at ridge crest (riftEarth’s interior at ridge crest (rift valley) than on sidesvalley) than on sides Ridge appeared to be under tensionRidge appeared to be under tension – like rift valley was two normal– like rift valley was two normal faultsfaults
- 41. Proposed answer: SeaFloorProposed answer: Sea Floor SpreadingSpreading New sea floor created by insertion of lava inNew sea floor created by insertion of lava in rift valleyrift valley Pushes ridge apart in centerPushes ridge apart in center Center is hot & expands; edges cold &Center is hot & expands; edges cold & contract, sinkscontract, sinks Old Cold Old Cold Young Hot Magma source Mantle, mafic
- 42. Vine & MatthewsTest & ConfirmVine & Matthews Test & Confirm Hypothesis:Hypothesis: –If rocks are being put into placeIf rocks are being put into place in rift valley and cooling throughin rift valley and cooling through Curie Point, then should seeCurie Point, then should see symmetrical magnetic polaritysymmetrical magnetic polarity anomalies on either side of theanomalies on either side of the ridgeridge ConclusionConclusion –Found symmetrical magneticFound symmetrical magnetic polarity anomalies on either sidepolarity anomalies on either side
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- 45. Calculate Spreading RatesCalculateSpreading Rates Age of point on sea floorAge of point on sea floor Distance from ridgeDistance from ridge Simply divide distance by date & getSimply divide distance by date & get spreading ratespreading rate Half rate – how fast crust is beingHalf rate – how fast crust is being formedformed Double half rate – how fastDouble half rate – how fast continents are moving apartcontinents are moving apart Typical half rates – 2 to 4 cm/yrTypical half rates – 2 to 4 cm/yr
- 46. Colors denote ageof rocks
- 47. Harry HessHarry Hess An AmericanAn American geologist whogeologist who studied mid-oceanstudied mid-ocean ridges.ridges. He suggested thatHe suggested that the ocean floorsthe ocean floors move like conveyormove like conveyor belts, carrying thebelts, carrying the continents alongcontinents along with them.with them.
- 48. At themid-ocean ridge, moltenAt the mid-ocean ridge, molten material rises from the mantle andmaterial rises from the mantle and erupts. The molten material thenerupts. The molten material then spreads out, pushing older rock tospreads out, pushing older rock to both sides of the ridge. As theboth sides of the ridge. As the molten material cools, it forms amolten material cools, it forms a strip of solid rock in the center of thestrip of solid rock in the center of the ridge. Then more molten materialridge. Then more molten material flows into the crack.flows into the crack.
- 49. Sea-Floor SpreadingSea-Floor Spreading HarryHess proposed that new ocean floor is formed at the rift of mid-ocean ridges. The ocean floor, and the rock beneath it, are produced by magma that rises from deeper levels. Hess suggested that the ocean floor moved laterally away from the ridge and plunged into an oceanic trench along the continental margin.
- 50. Evidence #1 -Molten MaterialEvidence #1 - Molten Material The submersible,The submersible, Alvin, foundAlvin, found strange rocksstrange rocks shaped like pillowsshaped like pillows or like toothpasteor like toothpaste squeezed from asqueezed from a tube. Such rockstube. Such rocks can form onlycan form only when moltenwhen molten material hardensmaterial hardens quickly afterquickly after erupting undererupting under
- 51. Evidence #2 -Magnetic StripesEvidence #2 - Magnetic Stripes ScientistsScientists discovered that thediscovered that the rock that makes uprock that makes up the ocean floor liesthe ocean floor lies in a pattern ofin a pattern of magnetizedmagnetized “stripes”. They“stripes”. They hold a record ofhold a record of reversals in Earth’sreversals in Earth’s magnetic field.magnetic field.
- 52. Evidence #3 -Drilling SamplesEvidence #3 - Drilling Samples The GlomarThe Glomar Challenger did aChallenger did a drilling sample anddrilling sample and found rocks thatfound rocks that the farther awaythe farther away from the ridge thefrom the ridge the older the rocksolder the rocks were. The youngerwere. The younger ones were in theones were in the center of the ridge.center of the ridge.
- 53. Evidence for seafloor spreadingEvidence for sea floor spreading Alvin and otherAlvin and other submersibles found crackssubmersibles found cracks and lava showingand lava showing spreading and volcanismspreading and volcanism at mid ocean ridges andat mid ocean ridges and odd life formsodd life forms Matthews and Vine’sMatthews and Vine’s survey of the Indiansurvey of the Indian Ocean sea floor showedOcean sea floor showed matching stripes ofmatching stripes of reversing polarities onreversing polarities on either side of ridge – whateither side of ridge – what caused these?caused these? – Lava spreading duringLava spreading during alternating magnetic periodsalternating magnetic periods
- 54. The Earth’s magneticfieldThe Earth’s magnetic field It is produced by theIt is produced by the outer core of the earthouter core of the earth which is made of liquidwhich is made of liquid iron and nickeliron and nickel This moving magneticThis moving magnetic material produces amaterial produces a moving magnetic field,moving magnetic field, which in turn produceswhich in turn produces a moving electric field.a moving electric field. It is a dynamo!It is a dynamo! Earth’s magnetic fieldEarth’s magnetic field varies over time and itvaries over time and it protects us fromprotects us from cosmic radiationcosmic radiation
- 55. Seafloor spreading andSeafloorspreading and paleomagnetismpaleomagnetism
- 58. Discovery of Sea-FloorSpreadingDiscovery of Sea-Floor Spreading 1. How we know plate tectonics1. How we know plate tectonics happenshappens
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- 63. Age of volcanoincreases linearly away from current hot spot!
- 64. Consistent with theidea that hotspot remains fixed relative to plate motion.
- 65. Hotspot tracks: Hawaii Linearincrease of ages with distance along the Hawaii-Emperor chain. Gradual decrease in elevation with increasing distance from the active volcano.
- 67. Timeline of HawaiianHistory NOT to scale! 85 mya: Earliest Emperor Seamounts visible today are formed 43 mya: Catastrophic event causes bend in Hawaiian chain 300-750 AD Polynesians arrive from Tahiti in outrigger canoes 1778: British explorer Captain James Cook lands on Hawaii 1900: Hawaii becomes a US territory 1959: Hawaii becomes a US state and the age of tourism begins 1983: Kilauea Volcano begins to erupt 65 mya: The dinosaurs (not native to Hawaii) become extinct 5 mya: Island of Kauai forms 1 mya: Island of Hawaii forms
- 68. The Hawaiian IslandsOverview: Quick Facts
- 70. Is the platetectonics concept universally accepted ? Not universally accepted by all geoscientists It does not provide satisfactory explanations ie 1)The total length of the subduction zones where crust is consumed is far shorter then the length of crust generating ridges. 2)While ocean floor spreading is recognised in all the ocean basins, Zones of subduction are almost entirely around the pacific. No explanation.. Some plates increased in area by 50% since the mesozoic but have no subduction zones 3) The drift of the spreading ridges is not inconsistent with concept of plate tectonics, but what is the motive force for the movement of the ridge marked by volcanicity 4) Evidences in some cases show that some plates move in two directions . 5) Benioff zone is not complete in some cases, in north America deep and intermediate earthquakes are missing. 6) North American plate moves in no regard to the spreading sites, plate margins and transform faults. Hard to explain… 7) In 1968, the concept was put forth , 6 major plates were recognised. Presently the number of minor plated is about 20… complications in te concept. 8) Mountain building not satisfactorily explained
- 71. Pangaea revisitedPangaea revisited By piecing togetherBy piecing together this information,this information, we can see howwe can see how the continentsthe continents have moved overhave moved over the past 200the past 200 million years, duemillion years, due to seafloorto seafloor spreadingspreading
- 72. Plate tectonicsPlate tectonics Sea floor spreading provides theSea floor spreading provides the driving mechanism for movementdriving mechanism for movement However, it is not the continents thatHowever, it is not the continents that are moving, but the “plates” ofare moving, but the “plates” of lithosphere “floating” in effect on thelithosphere “floating” in effect on the asthenosphereasthenosphere The lithosphere is made up of aboutThe lithosphere is made up of about 20 plates which move relative to20 plates which move relative to each other in several wayseach other in several ways Let’s look at a generalized sketchLet’s look at a generalized sketch
- 73. Evidence supporting the theoryof plate tectonics: Apparent Polar wandering: plate movement causes the apparent position of the magnetic poles to have shifted. The paleomagnetic fields in the rocks would indicate a single pole until the continents drift apart. Confirmation of the Theory ofConfirmation of the Theory of Plate TectonicsPlate Tectonics
- 74. Paleomagnetism: strips ofalternating magnetic polarity at spreading regions. Confirmation of Plate Tectonics TheoryConfirmation of Plate Tectonics Theory The patterns of paleomagnetism support plate tectonic theory. The molten rocks at the spreading center take on the polarity of the planet while they are cooling. When Earth’s polarity reverses, the polarity of newly formed
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- 77. ““Plates” of lithosphereare moved around by the underlying hot mantlePlates” of lithosphere are moved around by the underlying hot mantle convection cellsconvection cells
- 78. Plate Tectonics An overviewof the tectonic system.
- 79. Sea-Floor Spreading andPlateSea-Floor Spreading and Plate BoundariesBoundaries
- 80. 3 types ofboundaries3 types of boundaries Convergent –Convergent – where plates comewhere plates come together. Seetogether. See example next slideexample next slide
- 81. Age of OceanicCrustAge of Oceanic Crust Courtesy of www.ngdc.noaa.gov
- 82. Mid Atlantic Ridge MidAtlantic Ridge
- 83. Spreading ridgesSpreadingridges – As plates move apart new material isAs plates move apart new material is erupted to fill the gaperupted to fill the gap Divergent BoundariesDivergent Boundaries
- 85. Iceland hasa divergentIceland has a divergent plate boundary runningplate boundary running through its middlethrough its middle North American Plate andNorth American Plate and Eurasia Plate divergingEurasia Plate diverging Iceland:Iceland: An example of continental riftingAn example of continental rifting
- 86. East Africa RiftValleyEast Africa Rift Valley
- 87. East Africa RiftValleyEast Africa Rift Valley
- 91. Oceanic lithosphereOceaniclithosphere subducts underneath thesubducts underneath the continental lithospherecontinental lithosphere Oceanic lithosphereOceanic lithosphere heats and dehydrates asheats and dehydrates as it subsidesit subsides The melt rises formingThe melt rises forming volcanismvolcanism E.g. The AndesE.g. The Andes SubductionSubduction
- 92. Called SUBDUCTIONCalledSUBDUCTION Continent-Oceanic Crust CollisionContinent-Oceanic Crust Collision
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- 96. Andes Mountains inSouth AmericaAndes Mountains in South America
- 97. Andes MountainsAndes Mountains formedalong the coastformed along the coast of South Americaof South America Nazca Plate divesNazca Plate dives under the Southunder the South American PlateAmerican Plate SubductionSubduction
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- 101. Japan formed asan island chain asJapan formed as an island chain as the Pacific Plate dives beneath thethe Pacific Plate dives beneath the Eurasian PlateEurasian Plate Mt Fuji
- 102. Aleutian Islands ChainAleutianIslands Chain off Alaskaoff Alaska
- 103. Aleutian Islands formedAleutianIslands formed as an island chain asas an island chain as Pacific Plate divesPacific Plate dives beneath Northbeneath North American PlateAmerican Plate
- 106. Forms mountains,Formsmountains, e.g. European Alps,e.g. European Alps, HimalayasHimalayas Continent-Continent CollisionContinent-Continent Collision
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- 109. Converging Margins: India-AsiaConvergingMargins: India-Asia CollisionCollision
- 110. Alps inAlps in EuropeEurope Collisionof Africa and EurasiaCollision of Africa and Eurasia
- 111. Interesting plate collisionInterestingplate collision This picture shows aThis picture shows a place inplace in Newfoundland whereNewfoundland where a massive collisiona massive collision actually forcedactually forced mantle rock on topmantle rock on top of the crust, duringof the crust, during the collision thatthe collision that formed Pangaea andformed Pangaea and the Appalachianthe Appalachian mountains. Thismountains. This looks down the oldlooks down the old plate boundary.plate boundary.
- 112. Mantle rocks aretoxicMantle rocks are toxic These rocks have veryThese rocks have very different compositionsdifferent compositions than crustal rocks.than crustal rocks. They contain heavyThey contain heavy metals, which do notmetals, which do not support life forms onsupport life forms on the earth’s surface, sothe earth’s surface, so few organisms livefew organisms live there.there. However, in someHowever, in some places their heavyplaces their heavy metal concentrationsmetal concentrations produce rich metalproduce rich metal deposits and are mineddeposits and are mined
- 113. Divergent BoundariesDivergent Boundaries Found at spreadingFound at spreading centers – either midcenters – either mid ocean ridges or midocean ridges or mid continental rift zonescontinental rift zones
- 114. Transform fault boundaryTransformfault boundary This shows the SanThis shows the San Andreas Fault.Andreas Fault. It is a transform faultIt is a transform fault boundary, where theboundary, where the plates move sidewaysplates move sideways past each other, ratherpast each other, rather than away from eachthan away from each other (at divergentother (at divergent boundaries), orboundaries), or towards each other (attowards each other (at convergent boundaries)convergent boundaries)
- 115. Transform Fault BoundaryTransformFault Boundary
- 117. Transform Faults andSeafloorTransform Faults and Seafloor SpreadingSpreading
- 118. Review of differentboundariesReview of different boundaries Divergent –mid ocean ridge like Iceland orDivergent –mid ocean ridge like Iceland or continental rift zone like the African Riftcontinental rift zone like the African Rift ValleyValley ConvergentConvergent – Ocean/ocean like Japanese IslandsOcean/ocean like Japanese Islands – Continent/ocean like Andes and CascadesContinent/ocean like Andes and Cascades – Continent/continent like the HimalayasContinent/continent like the Himalayas Transform fault like the San Andreas faultTransform fault like the San Andreas fault Hot spots are not at plate boundaries, butHot spots are not at plate boundaries, but give us information about plate motiongive us information about plate motion ActivityActivity
- 119. What causes platetectonics?What causes plate tectonics? Convection in the mantle, asConvection in the mantle, as the plastic asthenospherethe plastic asthenosphere flows, carrying the plates withflows, carrying the plates with it.it. This is probably aided by slabThis is probably aided by slab pull at subduction zones andpull at subduction zones and ridge push at mid ocean ridgesridge push at mid ocean ridges and rising plumes in the mantleand rising plumes in the mantle This diagram shows severalThis diagram shows several different model hypothesesdifferent model hypotheses
- 120. Plate tectonics causesPlatetectonics causes volcanic activityvolcanic activity
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- 122. It causes rocksto be tiltedIt causes rocks to be tilted
- 123. Or even tofold or breakOr even to fold or break
- 124. Volcanoes and EarthquakesandVolcanoes and Earthquakes and Plate Boundaries – GIS activityPlate Boundaries – GIS activity
- 127. Structural GeologyStructural Geology IsostasyIsostasy MountainsMountains Stress and strainStress and strain FoldsFolds FaultsFaults
- 128. Formation of mountainsFormationof mountains Two forces areTwo forces are constantly atconstantly at work on thework on the earth.earth. – WeatheringWeathering and erosionand erosion tear structurestear structures down whiledown while – Plate tectonicsPlate tectonics builds them upbuilds them up
- 129. So we havemountains!So we have mountains! But they will not last forever.
- 130. Mountains form indifferent waysMountains form in different ways -Volcanic mountains--Volcanic mountains- Volcanoes form by subduction andVolcanoes form by subduction and melting of platesmelting of plates Volcanic mountains form over hotVolcanic mountains form over hot spotsspots Volcanic mountains form at rift zonesVolcanic mountains form at rift zones
- 131. Other mountain typesOthermountain types Folded mountains form From convergingFolded mountains form From converging continents like the Himalayascontinents like the Himalayas How do these look?How do these look? Direction?Direction? Fault block mountains form where blocks ofFault block mountains form where blocks of rock drop at faults – mostly near platerock drop at faults – mostly near plate boundaries, but not alwaysboundaries, but not always Uplifted mountains – where large sectionsUplifted mountains – where large sections of the crust are pushed up, perhaps byof the crust are pushed up, perhaps by magma, or other forcesmagma, or other forces
- 132. IsostasyIsostasy Just asa boat sinks or rises with changes inJust as a boat sinks or rises with changes in weight, so does the crust sink or rise withweight, so does the crust sink or rise with changes in weight. Plate tectonics buildschanges in weight. Plate tectonics builds mountains and the extra weight causes the crustmountains and the extra weight causes the crust to sink. As erosion occurs the weight of theto sink. As erosion occurs the weight of the mountains decreases and the crust rises again.mountains decreases and the crust rises again. This process is called isostasy or isostatic changeThis process is called isostasy or isostatic change
- 133. IsostasyIsostasy Isostasy is balance.A floating object is balanced in the water, like an iceberg. If some of the top melts, the iceberg rises in the water to stay in balance. If you get into a boat, the boat sinks to maintain balance. The same thing happens with mountains. As plate motions push it higher the mountain sinks into the mantle to stay in balance. On the other hand, when the mountain erodes, it will rise in the mantle as the top erodes. So, a mountain may lose 1 meter from erosion, but regain 0.8 meters from rising due to isostasy. It maintains isostatic equilibrium. •This is also a great example of feedback within a system. Is it positive or negative? Figure it out. Remember that maintaining equilibrium is __ feedback because it keeps the system in balance. So, what is it, positive or negative? •This is a good example of Newton’s 3rd law. For every force, there is an equal and opposing force. In this case the force of gravity is opposed by the buoyant force. Right! It’s negative.
- 134. StressStress Due mostly toplate movements, the earth’s crust is under a lot of stress. There are 3 types, shown at the right A occurs where plates pull apart, divergent boundaries, and is called tension B occurs where plates converge, and is called compression C occurs where plates move past each other, at transform fault boundaries and is called shearing
- 135. StrainStrain This stressleads to strainThis stress leads to strain on the crust which bends it.on the crust which bends it. –– If it is warm, underground,If it is warm, underground, it can bend. This calledit can bend. This called ductile deformation.ductile deformation. Features are called folds.Features are called folds. Upturned folds areUpturned folds are anticlines while downturnedanticlines while downturned folds are synclines.folds are synclines. Or, the rock may break, ifOr, the rock may break, if it is brittle. This causesit is brittle. This causes faults –breaks of the earth.faults –breaks of the earth. A fold above and a fault below anticline syncline Stress and strain video Economic value of folds/faults
- 136. Different faultsDifferent faults Faults move inFaults move in different ways,different ways, depending on thedepending on the type of stress ontype of stress on them. Here are 3them. Here are 3 types that form.types that form. 1 is a normal fault1 is a normal fault 2 is a strike slip2 is a strike slip faultfault 3 is a reverse fault3 is a reverse fault 2 3 1
- 137. MatchingMatching These formatThese form at different platedifferent plate boundaries. Canboundaries. Can you figure outyou figure out which formswhich forms where? Matchwhere? Match themthem A. Convergent C. Transform Fault B. Divergent 2 3 3 2 1 1
- 138. Can you matchthe stress and strain?Can you match the stress and strain? A is 1st , B is 3rd , C is 2nd 2 3 1
Editor's Notes
- #37 The interior of the Earth is divided into layers based on chemical and physical properties. The Earth has an outer silica-rich, solid crust, a highly viscous mantle, and a core comprising a liquid outer core that is much less viscous than the mantle, and a solid inner core. Working from the centre of the Earth out we have: The inner core is a primarily solid sphere about 1220 km in radius situated at Earth's center. Based on the abundance of chemical elements in the solar system, their physical properties, and other chemical constraints regarding the remainder of Earth's volume, the inner core is believed to be composed primarily of a nickel-iron alloy, with small amounts of some unknown elements. The temperature is estimated at 5,000-6,000 degrees Celsius and the pressure to be about 330 to 360 GPa (which is over 3,000,000 times that of the atmosphere!) The liquid outer core is 2300 km thick and like the inner core composed of a nickel-iron alloy (but with less iron than the solid inner core). Iseismic and other geophysical evidence indicates that the outer core is so hot that the metals are in a liquid state. The mantle is approximately 2,900 km thick and comprises 70% of Earth's volume. (the core makes up about 30% of Earth's volume, with the outer crust [where we live] <1%!!). The mantle is divided into sections based upon changes in its elastic properties with depth. In the mantle, temperatures range between 500-900 degrees Celsius at the upper boundary with the crust to over 4,000 degrees Celsius at the boundary with the core. Due to the temperature difference between the Earth's surface and outer core, and the ability of the crystalline rocks at high pressure and temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a convective material circulation in the mantle (mantle convection cells). Hot material rises up as mantle plumes (like a lava lamp!), while cooler (and heavier) material sinks downward to be reheated and rise up again. - We shall see that this process is very important for plate tectonic motion… The outer most layer is the crust - this is the most familiar to us as it is where we live. The distinction between crust and mantle is based on chemistry, rock types and seismic characteristics. Presenter: Ask the students to guess what the most abundant element in the earth’s crust is…..they may be surprised to learn that it is actually Oxygen (46.6% Oxygen; 27.7% Silica; 8.1% Aluminum; 5.0% Iron; 3.6% Calcium; 2.8% Sodium; 2.6% Potassium; 2.1% Magnesium; plus trace elements) Click to next slide for more on the Crust….
- #38 The Earth has two different types of crust: Continental crust and Oceanic crust. Each has different properties and therefore behaves in different ways. Continental crust: Continental crust forms the land (the continents, as the name suggests) that we see today. Continental crust averages about 35 km thick. Under some mountain chains, crustal thickness is approximately twice that thickness (about 70 km thick). - The mountains we see on earth have deep roots in the crust that we can’t see. The crust “floats” on the more dense mantle and, like how only the tip of an iceberg sticks up out of the water, we see only the tip of the continental crust - the mountain ranges. Continental crust is less dense and therefore more buoyant than oceanic crust Continental crust contains some of the oldest rocks on Earth. - Ancient rocks exceeding 3.5 billion years in age are found on all of Earth's continents. The oldest rocks on Earth found so far are the Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 Ga) [Ga = billion years ago] and the Isua Supracrustal rocks in West Greenland (3.7 to 3.8 Ga), but well-studied rocks nearly as old are also found in the Minnesota River Valley in the USA (3.5-3.7 billion years), in Swaziland (3.4-3.5 billion years), and in Western Australia (3.4-3.6 billion years). Oceanic crust: As the name already suggests, this crust is below the oceans. Compared to continental crust, Oceanic crust is thin (6-11 km). It is more dense than continental crust and therefore when the two types of crust meet, oceanic crust will sink underneath continental crust. The rocks of the oceanic crust are very young compared with most of the rocks of the continental crust. They are not older than 200 million years.
- #77 This diagram shows the major Tectonic Plates. Presenter: Point out the UK, sitting on the Eurasian Plate. Also the plate boundary between Africa and South America (note that it has the same shape as the coastlines in these countries).
- #78 How and Why do tectonic Plates move around? The question of how tectonic plates are moved around the globe is answered by understanding mantle convection cells. In the mantle hot material rises towards the lithosphere (like hot air rising out of an open oven - ever opened an oven door and felt the blast of hot air coming past your face?). The hot material reaches the base of the lithosphere where it cools and sinks back down through the mantle. The cool material is replaced by more hot material, and so on forming a large “convection cell” (as pictured in the diagram). This slow but incessant movement in the mantle causes the rigid tectonic plates to move (float) around the earth surface (at an equally slow rate).
- #80 To view this animation, click “View” and then “Slide Show” on the top navigation bar.
- #82 Ocean Ridges: This map shows the age of the oceanic crust. The red colouring shows the youngest ages, whilst the dark blue shows the oldest ages (around 200 million years old). Presenter ask: Where are the Ocean Ridges located? I.e. where are the divergent boundaries? Answer: The divergent boundaries are where the plates are pulling apart and new material is being produced. Therefore the Ocean ridges are in the middle of the red areas (the boundaries are in fact shown on the map). We can see a progression of the oceanic crust getting older away from the ocean ridges (like a conveyer belt). Presenter: Before moving on to the next slide, point out Iceland. The divergent boundary runs straight through Iceland….
- #84 In plate tectonics, a divergent boundary is a linear feature that exists between two tectonic plates that are moving away from each other. These areas can form in the middle of continents or on the ocean floor. As the plates pull apart, hot molten material can rise up this newly formed pathway to the surface - causing volcanic activity. Presenter: Reiterate the process by going through the diagram, including the presence of mantle convection cells causing the plates to break apart and also as a source for new molten material. Where a divergent boundary forms on a continent it is called a RIFT or CONTINENTAL RIFT, e.g. African Rift Valley. Where a divergent boundary forms under the ocean it is called an OCEAN RIDGE.
- #86 Iceland is located right on top of a divergent boundary. In fact, the island exists because of this feature. As the North American and Eurasian plates were pulled apart (see map) volcanic activity occurred along the cracks and fissures (see photographs). With many eruptions over time the island grew out of the sea! Question: Why don’t we have islands like Iceland where ever we get an Ocean Ridge? Answer: Scientists believe that there is a large mantle plume (an upwelling of hot mantle material) located right underneath where Iceland has formed. This would mean that more material would be erupted in the Iceland area compared with if there was just the divergent boundary without the plume underneath it.
- #92 The Andes mountain range along the western edge of the South American continent is an example of a mountain belt formed by subduction. The continental crust of the South American plate has buckled under the compressional strain of converging with the Nasca and Antarctic plates. Additionally there are many volcanoes, the result of melting of the subducting slab and the production of new material that has risen through the crust to the surface.
- #93 At a convergent boundary where continental crust pushes against oceanic crust, the oceanic crust which is thinner and more dense than the continental crust, sinks below the continental crust. This is called a Subduction Zone. The oceanic crust descends into the mantle at a rate of centimetres per year. This oceanic crust is called the “Subducting Slab” (see diagram). When the subducting slab reaches a depth of around 100 kilometres, it dehydrates and releases water into the overlying mantle wedge (Presenter: explain all of this using the diagram). The addition of water into the mantle wedge changes the melting point of the molten material there forming new melt which rises up into the overlying continental crust forming volcanoes. Subduction is a way of recycling the oceanic crust. Eventually the subducting slab sinks down into the mantle to be recycled. It is for this reason that the oceanic crust is much younger than the continental crust which is not recycled.
- #107 When continental crust pushes against continental crust both sides of the convergent boundary have the same properties (think back to the description of continental crust: thick and buoyant). Neither side of the boundary wants to sink beneath the other side, and as a result the two plates push against each other and the crust buckles and cracks, pushing up (and down into the mantle) high mountain ranges. For example, the European Alps and Himalayas formed this way.
- #109 Example: India used to be an island, but about 15 million years ago it crashed into Asia (see map). As continental crust was pushing against continental crust the Himalayan mountain belt was pushed up. “Mountains” were also pushed down into the mantle as the normally 35 km thick crust is approximately 70 km thick in this region. Mt Everest is the highest altitude mountain on our planet standing 8,840 metres high. This means that below the surface at the foot of the mountain the crust is a further 61 km deep!!
- #110 To view this animation, click “View” and then “Slide Show” on the top navigation bar.
- #118 To view this animation, click “View” and then “Slide Show” on the top navigation bar.
- #129 From here is ESCP only since I am combining plates and mountains chapters for them