plate tectonics 1

8/3/2019 Plate Tectonics 1

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Plate TectonicsPlate Tectonics



Continental Drift

Continental Drift

Researchers noted geographic fit of

continents, e.g. Africa and S. America

Seuss, 1885, proposed super continent by

studying fossils, rocks, mountains

Alfred Wegener - First proposed his

continental drift hypothesis in 1915



Continental Drift

Continental Drift

Hypothesis:

Supercontinent called

Pangaea began

breaking apart about

200 million years ago



Break up of PangaeaB

reak up of Pangaea



Wegener¶s Evidence

Wegener¶s Evidence

Fit of the continents



Wegener¶s Evidence

Wegener¶s Evidence


Rock type & structural

similarities



Wegener¶s Evidence

Wegener¶s Evidence



similarities

Fossil evidence

Fossils of Mesosaurus have been found on both

sides of the South Atlantic and nowhere else in the

world. Fossil remains of this and other organisms

on the continents of Africa and South America

appear to link these landmasses during the late

Paleozoic and early Mesozoic eras.



Wegener¶s Evidence

Wegener¶s Evidence



similarities

Fossil evidence

Paleoclimatic evidence



Sea Floor SpreadingSea Floor Spreading

HypothesisHypothesis Proposed by Harry Hess in the late 1950s /

early 1960s

± New crust forms at ocean ridges

± Old crust consumed at trenches

± Developed theory of sea floor spreading driven

by convection cells



Sea Floor SpreadingSea Floor Spreading

HypothesisHypothesis Evidence

Earthquakes at ridges and subduction zones

Heat flow highest at ridges, decreases awayfrom ridges

Radiometric dating shows oldest rocks on seafloor of 200 m.y.

Sediments older away from ridges

Earth¶s magnetic field reverses, this changerecorded by iron minerals in rocks



SeaSea--Floor Spreading and PlateFloor Spreading and Plate

BoundariesBoundaries



The Earth¶s Magnetic FieldThe Earth¶s Magnetic Field Earth's magnetic

field resembles that

produced by a large

bar magnet.

Magnetic north is

not in the samelocation as true

north



PaleomagnetismPaleomagnetism Magnetic field of the Earth

is imprinted upon iron-

bearing rocks when theyform

Provides a record of magnetic reversals andoriginal latitude

Seafloor mapping in1950¶s and 1960¶srevealed magnetic stripingon the ocean floor.



The ocean floor as a magneticThe ocean floor as a magnetic

tape recorder.tape recorder.



MagneticMagnetic

patterns off patterns off

the Eastthe Eastcoast of coast of

NorthNorth

America.America.



PolarPolar

WanderWanderA. The more westerly

path determined from

North American data

is thought to have been caused by the

westward drift of

North America by

about 24 degrees

from Eurasia.



PolarPolar

WanderWander

B. The positions of

the wandering paths

when the landmasses

are reassembled in

their predrift

locations.



Putting it all together: ThePutting it all together: The

Theory of Plate TectonicsTheory of Plate Tectonics

Continental Drift

+ Sea-Floor Spreading

+ Paleomagnetism

= Plate Tectonics



Plate TectonicsPlate Tectonics Earth¶s lithosphere is broken up into plates

Hot, weak asthenosphere allows for plates

to move

Plates are in motion and continually

changing in shape and size

± Move very slowly ± 5 cm/yr



Because Earth¶s size is constant, expansionB

ecause Earth¶s size is constant, expansionof the crust in one area requiresof the crust in one area requires

destruction of the crust elsewhere.destruction of the crust elsewhere.

Currently, the Pacific Ocean basin is shrinking as other

ocean basins expand.

Destruction of sea floor occurs in subduction zones.

Seismicity is the frequency, magnitude and distribution

of earthquakes. Earthquakes are concentrated alongoceanic ridges, transform faults, trenches and island arcs.

Tectonism refers to the deformation of Earth¶s crust.

3-3Global Plate Tectonics



Benioff Zone is an area of increasingly deeper seismic activity, inclined from the trench

downward in the direction of the island arc.

Subduction is the process at a trench wherebyone part of the sea floor plunges below another

and down into the asthenosphere.

3-3 Global Plate Tectonics



Earth¶s surface is composed of a series of Earth¶s surface is composed of a series of

lithospheric plates. Plate edges extend throughlithospheric plates. Plate edges extend throughthe lithosphere and are defined by seismicity.the lithosphere and are defined by seismicity.

Plate edges are trenches, oceanic ridges and

transform faults. Seismicity and volcanism are concentrated

along plate boundaries.

Movement of plates is caused by thermalconvection of the ³plastic´ rocks of the

asthenosphere which drag along the

overlying lithospheric plates.

3-3 Global Plate Tectonics



Structure of the EarthS

tructure of the Earth Layers ± by composition

± Crust

± Mantle

± Core




tructure of the Earth Layers ± by physical properties

± Lithosphere

± Asthenosphere

± Mesosphere

± Outer Core

± Inner Core




tructure of the Earth Crust

± Continental crust

Average rock density about

2.7 g/cm3

Composition = the felsic

igneous rock granodiorite




tructure of the Earth Crust

± Oceanic crust

Density about 3.0 g/cm3

Composed mainly of the

igneous rock basalt




tructure of the Earth Mantle

± Contains 82% of Earth¶s

volume ± Solid, rocky layer

± Upper portion has thecomposition similar to

peridotite ± Two parts

Mesosphere (lower mantle)

Asthenosphere or upper

mantle




tructure of the Earth Core

± Larger than the planet

Mars ± Mostly iron with some

nickel

Average density is nearly

11 g/cm3 ± Two parts

Outer core - liquid

Inner core - solid




tructure of the Earth Core

± Responsible for

Earth¶s magneticfield

Made of material

that conducts

electricity

Core is mobile



S

tructure of the EarthS

tructure of the Earth Seismic waves

± P waves

Travels through liquids as

well as solids

In all materials, P waves

travel faster than do S waves

± S waves

Cannot travel through

liquids



S

tructure of the EarthS

tructure of the Earth Seismic waves

± Seismic waves refract as they pass from one

material to another

± P & S wave shadow zones



P wave shadow zoneP wave shadow zone



S

wave shadow zoneS

wave shadow zone



I

sostacyI

sostacy The balancing of

pressures exerted

by mass of continents andocean crust onmantle

Continents float onmantle likeicebergs in water



MantleC

onvectionMantleC

onvection



Distribution of earthquake fociDistribution of earthquake foci

at plate boundariesat plate boundaries



Lithospheric PlatesLithospheric Plates




Plate boundaries

± All major interactions among individual plates

occur along their boundaries




Plate boundaries

± Types of plate boundaries

Divergent plate boundaries

Convergent plate boundaries

Transform fault boundaries




Divergent Plate boundaries

± Most are located along the crests of oceanic

ridges

± Also continental rifts

± A few are on land

Quicktime Movie



Formation of Formation of Ocean BasinsOcean Basins



The East African riftThe East African rift

Spreading the continent

apart 100 times as slow as

a typical oceanic rift zone.

The rift forms one arm of a

triple junction, from which

the Red Sea and Gulf of Aden form somewhat more

rapidly spreading rifts.



ContinentalContinental

RiftingRifting Basin and Range

± Region is slowly

pulling apart

± No active

spreading center



This Basin and Range terrain is found southwest of Salt Lake City, Utah.




Convergent plate boundaries

± Where two platescollide

± Types

Oceanic-continental

convergence Oceanic-oceanic

convergence

Continental-continentalconvergence



OceanOcean--Continent ConvergenceContinent Convergence



Cascade Mountains, Pacific NWCascade Mountains, Pacific NW



OceanOcean--Ocean ConvergenceOcean Convergence



ContinentContinent--Continent ConvergenceContinent Convergence



Converging Margins: IndiaConverging Margins: India--

Asia CollisionAsia Collision



The collision of India and AsiaThe collision of India and Asia

produced the Himalayas (before)produced the Himalayas (before)



The collision of India and AsiaThe collision of India and Asia

produced the Himalayas (after)produced the Himalayas (after)




Transform fault boundaries

± Plates slide past one another

± No new lithosphere is created or destroyed

± Most join two segments of amid-ocean ridge as parts of

prominent linear breaks in theoceanic crust known as fracturezones



Transform FaultsTransform Faults



Terrane FormationTerrane Formation




Hot spots

± Caused by rising plumes of mantle material

± Volcanoes can form over them (HawaiianIsland chain)

± Most mantle plumes are long-lived structures

and at least some originate at great depth,

perhaps at the mantle-core boundary



Global Hot SpotsGlobal Hot Spots



Volcanism on a tectonicVolcanism on a tectonic

plate moving over a hot spotplate moving over a hot spot



The Hawaiian Islands haveThe Hawaiian Islands have

formed over a stationary hot spotformed over a stationary hot spot



Hawaiian IslandsHawaiian Islands--EmperorEmperor

Seamount ChainSeamount Chain



Wilson

Cycle

plate tectonics 1

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