Plate TectonicsPlate Tectonics

Text ref. Ch.17 (pg. 442)Text ref. Ch.17 (pg. 442)

ContentsContents

Continental DriftContinental Drift

Seafloor SpreadingSeafloor Spreading

Plate TectonicsPlate Tectonics

Continental DriftContinental Drift

Text ref. Ch.17 (442-447)Text ref. Ch.17 (442-447)

ObjectivesObjectives

Describe one piece of early evidence that Describe one piece of early evidence that led people to suggest the Earth’s led people to suggest the Earth’s continents may have once been joined.continents may have once been joined.

Discuss evidence of continental drift.Discuss evidence of continental drift.

Explain why continental drift was not Explain why continental drift was not accepted when it was first proposed.accepted when it was first proposed.

Drifting ContinentsDrifting Continents

Early observationsEarly observations

Map-makers noticed how continents fit on Map-makers noticed how continents fit on either side of the Atlantic.either side of the Atlantic. Thought continents had been separated by Thought continents had been separated by

floods and earthquakesfloods and earthquakes Edward Suess proposed continents had once Edward Suess proposed continents had once

been joined together as supercontinent known been joined together as supercontinent known as Gondwanaland.as Gondwanaland.

Continental DriftContinental Drift

Wegener proposes continental drift, calling Wegener proposes continental drift, calling his supercontinent Pangaea, which means his supercontinent Pangaea, which means “all Earth” in Greek“all Earth” in Greek

Proposes supercontinent began breaking Proposes supercontinent began breaking apart 200 million year ago.apart 200 million year ago.

Begins collection of scientific evidence to Begins collection of scientific evidence to support his theorysupport his theory

Evidence from Rock FormationsEvidence from Rock Formations

Wegener hypothesized that the same Wegener hypothesized that the same types of rock formations should exist on types of rock formations should exist on both sides of the Atlantic.both sides of the Atlantic.Noticed rocks in the Appalachian Noticed rocks in the Appalachian mountains shared features with rocks in mountains shared features with rocks in Greenland and Europe.Greenland and Europe.All rocks were dated older than 200 million All rocks were dated older than 200 million years; therefore found together before years; therefore found together before continental drift began.continental drift began.

Evidence from Fossil RecordsEvidence from Fossil Records

Fossils of the same animals and plants were can Fossils of the same animals and plants were can be found on separate continents.be found on separate continents.

Kannemeyerids Kannemeyerids andand Labyrinthodonts Labyrinthodonts fossils can fossils can be found in Africa, North America, South be found in Africa, North America, South America, Antarctica, Greenland, and Northern America, Antarctica, Greenland, and Northern Europe.Europe.

Glossopteris plants can also be found in many Glossopteris plants can also be found in many continents, strengthening the hypothesis that continents, strengthening the hypothesis that these continents shared the same climate.these continents shared the same climate.

Kannemeyerid

Glossopteris

Ancient Climatic EvidenceAncient Climatic Evidence

Wegener studied sedimentary rocks and Wegener studied sedimentary rocks and found evidence of widespread climate found evidence of widespread climate change. change.

Coal deposits in Antarctica suggest that Coal deposits in Antarctica suggest that plants once lived in Antarctica; therefore plants once lived in Antarctica; therefore Antarctica was once closer to the equator.Antarctica was once closer to the equator.

Glacial EvidenceGlacial Evidence

Glacier deposits can be found in Africa, Glacier deposits can be found in Africa, India, Australia, and South America. This India, Australia, and South America. This suggests that these areas were once cold suggests that these areas were once cold enough for glaciers to form.enough for glaciers to form.

Opposition to the HypothesisOpposition to the Hypothesis

Wegener could not explain why the Wegener could not explain why the continents had moved throughout geologic continents had moved throughout geologic time.time. Scientists doubted that such forces could Scientists doubted that such forces could

exist on Earth to move entire continents.exist on Earth to move entire continents. Wegener could explain how continents could Wegener could explain how continents could

move through the ocean floor to new locations move through the ocean floor to new locations on Earth.on Earth.

Wegener continued to collect evidence until Wegener continued to collect evidence until his death in 1930. his death in 1930.

Seafloor SpreadingSeafloor Spreading

Text ref. Ch.17.2 (pg.448-455)Text ref. Ch.17.2 (pg.448-455)

ObjectivesObjectives

Summarize the evidence that led to the Summarize the evidence that led to the discovery of seafloor spreading.discovery of seafloor spreading.

Explain the significance of magnetic Explain the significance of magnetic patterns on the seafloor.patterns on the seafloor.

Explain the process of seafloor spreading. Explain the process of seafloor spreading.

Technological AdvanceTechnological Advance

People before the 1900’s assumed the People before the 1900’s assumed the seafloor was essentially flat.seafloor was essentially flat.

Advances in technology in the 1940’s and Advances in technology in the 1940’s and ’50s led to new ideas.’50s led to new ideas.

Sonar can map surfaces by bouncing high Sonar can map surfaces by bouncing high frequency sound waves in order to frequency sound waves in order to calculate distances (elevation)calculate distances (elevation)

Ocean Floor TopographyOcean Floor Topography

Use of magnetometer to detect variations Use of magnetometer to detect variations in magnetic fields allow scientists to map in magnetic fields allow scientists to map the seafloor.the seafloor.

Images revealed underwater mountain Images revealed underwater mountain ranges, called ocean ridges.ranges, called ocean ridges.

Ridges form the largest continuous Ridges form the largest continuous mountain range on Earth.mountain range on Earth.

Sonar revealed ridges had corresponding Sonar revealed ridges had corresponding trenchestrenches Mariana Trench is over 11 km deep.Mariana Trench is over 11 km deep.

QuestionsQuestions

What could have formed the ridges and What could have formed the ridges and trenches found on the seafloor?trenches found on the seafloor?

What is the source of volcanism What is the source of volcanism associated with these locations?associated with these locations?

What forces could be at work to move the What forces could be at work to move the entire seafloor and create trenches 6 times entire seafloor and create trenches 6 times as deep as the Grand Canyon?as deep as the Grand Canyon?

Ocean Rocks and SedimentsOcean Rocks and Sediments

Scientists found predictable patterns in the ages Scientists found predictable patterns in the ages of rock on the seafloor.of rock on the seafloor.

Rock ages with distance from ocean ridgesRock ages with distance from ocean ridges

Ocean floor is dated at its oldest around 180 Ocean floor is dated at its oldest around 180 million years.million years.

Continental rocks are dated at 3.8 billion years.Continental rocks are dated at 3.8 billion years.

Why is the seafloor so young in comparison?Why is the seafloor so young in comparison?

Ocean Rocks and Sediments Ocean Rocks and Sediments cont…cont…

Thickness of ocean sediments is much thinner Thickness of ocean sediments is much thinner than expected.than expected.

Typically a few hundred meters thick.Typically a few hundred meters thick.

Continents may have sediments up to 20 km Continents may have sediments up to 20 km thick.thick.

Why are the sediments so thin?Why are the sediments so thin?

Why isn’t seafloor as thick as continental crust?Why isn’t seafloor as thick as continental crust?

Sediment is found to be thicker the further it is Sediment is found to be thicker the further it is from ridges.from ridges.

MagnetismMagnetism

Study of the Earth’s magnetic field in iron-Study of the Earth’s magnetic field in iron-bearing minerals is known as bearing minerals is known as paleomagnetism.paleomagnetism.Seafloor is composed of iron-rich basalt, in Seafloor is composed of iron-rich basalt, in which the iron becomes aligned with which the iron becomes aligned with Earth’s magnetic field as molten rock Earth’s magnetic field as molten rock cools.cools.When rock hardens, the iron orients itself When rock hardens, the iron orients itself to Earth’s magnetic field at that timeto Earth’s magnetic field at that time

The Geomagnetic Time ScaleThe Geomagnetic Time Scale

Studies in the 1960’s revealed pattern of Studies in the 1960’s revealed pattern of magnetic reversals over time.magnetic reversals over time.

Magnetic reversals are changes in the Magnetic reversals are changes in the Earth’s magnetic field.Earth’s magnetic field.

Data from these reversals allowed Data from these reversals allowed scientists to construct a geomagnetic time scientists to construct a geomagnetic time scale.scale.

The InvestigationThe Investigation

Scientists towed magnetometers behind Scientists towed magnetometers behind ships to measure the magnetic field of the ships to measure the magnetic field of the ocean floor.ocean floor.

Scientists noticed alternating + and – Scientists noticed alternating + and – values for the magnetic field readings.values for the magnetic field readings.

Magnetic SymmetryMagnetic Symmetry

Scientists noticed that the + and – Scientists noticed that the + and – magnetic bands were symmetrical, magnetic bands were symmetrical, originating from the ocean ridges.originating from the ocean ridges.

Scientists also discovered basaltic lava Scientists also discovered basaltic lava flows on land matching these magnetic flows on land matching these magnetic reversals.reversals.

Age of the ocean floor was ascertained Age of the ocean floor was ascertained from this data.from this data.

Isochronic MappingIsochronic Mapping

Isochron is a line on a map that connects Isochron is a line on a map that connects points that have the same age.points that have the same age.

Young rock is found at mid-ocean ridges, Young rock is found at mid-ocean ridges, while older rock is found further away near while older rock is found further away near deep sea trenches.deep sea trenches.

Seafloor SpreadingSeafloor Spreading

After all this data, Harry Hess proposes After all this data, Harry Hess proposes the theory of seafloor spreading.the theory of seafloor spreading.Theory proposes that new ocean floor is Theory proposes that new ocean floor is created at the ridges and destroyed at the created at the ridges and destroyed at the trenches.trenches.Magma rises to the surface and hardens Magma rises to the surface and hardens at the ocean ridges.at the ocean ridges.Consecutive bands of seafloor are created Consecutive bands of seafloor are created in this way.in this way.

Wegener’s Theory RevisitedWegener’s Theory Revisited

Wegener’s theory of continental drift did Wegener’s theory of continental drift did not account for the forces in action.not account for the forces in action.Seafloor spreading completes the picture Seafloor spreading completes the picture and describes the forces that shape the and describes the forces that shape the drifting of the continents.drifting of the continents.Continents are like groceries on the Continents are like groceries on the conveyer belt at the checkout line; they conveyer belt at the checkout line; they don’t push through the ocean floor, they don’t push through the ocean floor, they ride on top of it as the ocean floor moves.ride on top of it as the ocean floor moves.

Theory of Plate Theory of Plate TectonicsTectonics

Text ref. Ch.17.3 (pg. 455 – 459)Text ref. Ch.17.3 (pg. 455 – 459)

ObjectivesObjectives

Explain the theory of plate tectonics.Explain the theory of plate tectonics.

Compare and contrast the three types of Compare and contrast the three types of plate boundaries and the features plate boundaries and the features associated with each.associated with each.

The TheoryThe Theory

Why do earthquakes happen?Why do earthquakes happen?Why do volcanoes erupt?Why do volcanoes erupt?Why do mountains exist?Why do mountains exist?Plate TectonicsPlate Tectonics is a theory that states the is a theory that states the Earth’s crust and rigid upper mantle are Earth’s crust and rigid upper mantle are broken into enormous slabs called broken into enormous slabs called plates.plates.Each plate moves at a particular rate, and Each plate moves at a particular rate, and measured using a system of satellites and measured using a system of satellites and receiversreceivers

Plate BoundariesPlate Boundaries

Tectonic plates interact at places called Tectonic plates interact at places called plate boundaries.plate boundaries. DivergentDivergent ConvergentConvergent TransformTransform

Each boundary has certain geologic Each boundary has certain geologic characteristics.characteristics.

Divergent BoundariesDivergent Boundaries

Places where two tectonic plates are moving Places where two tectonic plates are moving apart.apart.

Most are found on the seafloor as ocean ridges.Most are found on the seafloor as ocean ridges.

Formation of new ocean crust accounts for Formation of new ocean crust accounts for volcanism along these locations.volcanism along these locations.

A A rift valley rift valley is formed when a divergent plate is formed when a divergent plate boundary is located on continental crust.boundary is located on continental crust. Ex. East African Rift ValleyEx. East African Rift Valley

Convergent BoundariesConvergent Boundaries

Places where two tectonic plates are Places where two tectonic plates are moving toward each other.moving toward each other.

Crust is destroyed at these boundaries.Crust is destroyed at these boundaries.

Three types:Three types: Oceanic converging with oceanic crustOceanic converging with oceanic crust Oceanic converging with continental crust Oceanic converging with continental crust Continental converging with continental crustContinental converging with continental crust

Subduction and ConvergenceSubduction and Convergence

Occurs when a cooler, denser plate Occurs when a cooler, denser plate descends below a less dense plate.descends below a less dense plate.In the event of oceanic and oceanic plate In the event of oceanic and oceanic plate convergence, a trench is formed.convergence, a trench is formed.Subducted plate melts and becomes Subducted plate melts and becomes magma, rises to the surface, and erupts as magma, rises to the surface, and erupts as a volcano.a volcano. Ex. Mariana Trench and islandsEx. Mariana Trench and islands Ex. Aleutian Trench and islandsEx. Aleutian Trench and islands

Oceanic-Continental ConvergenceOceanic-Continental Convergence

Denser oceanic plate subducts under less Denser oceanic plate subducts under less dense continental plate.dense continental plate.

Forms mountain ranges of volcanoes.Forms mountain ranges of volcanoes. Ex. Peru-Chile Trench and Andes MountainsEx. Peru-Chile Trench and Andes Mountains

Continental-Continental Plate Continental-Continental Plate ConvergenceConvergence

Occurs when two continental plates Occurs when two continental plates collide.collide.

Produces large mountain ranges with little Produces large mountain ranges with little volcanism.volcanism.

Since continental crust is buoyant, crust Since continental crust is buoyant, crust crumples rather than subducts.crumples rather than subducts. Ex. HimalayasEx. Himalayas

Transform BoundariesTransform Boundaries

Place where two plates slide horizontally past Place where two plates slide horizontally past each other.each other.Crust is deformed or fractured at these Crust is deformed or fractured at these boundaries, but not created or destroyed.boundaries, but not created or destroyed.Characterized by long Characterized by long faultsfaults, hundreds of km in , hundreds of km in length.length.Earthquakes.Earthquakes.Most transform boundaries are not found on Most transform boundaries are not found on continents.continents.Exception: San Andreas FaultException: San Andreas Fault