Global Plate Tectonics n Plate Tectonics = Sea Floor Spreading + Continental drift n These two concepts came from separate sets of observations n The Plate

Global Plate TectonicsGlobal Plate Tectonics

Plate Tectonics = Sea Floor Spreading + Plate Tectonics = Sea Floor Spreading + Continental driftContinental drift

These two concepts came from separate sets of These two concepts came from separate sets of observationsobservations

The Plate Tectonic theory:The Plate Tectonic theory: combined these ideascombined these ideas answered “all” our questionsanswered “all” our questions provided incredible predictive POWER!provided incredible predictive POWER!

Global Plate Tectonics Global Plate Tectonics (Cont.)(Cont.)

To understand the Theory of Plate Tectonics we will To understand the Theory of Plate Tectonics we will considerconsider Lithospheric platesLithospheric plates Seismic activity / plate boundariesSeismic activity / plate boundaries The types of plate boundariesThe types of plate boundaries The opening and closing of ocean basinsThe opening and closing of ocean basins


Proposes that the earth’s surface is composed of a Proposes that the earth’s surface is composed of a series of series of lithospheric plateslithospheric plates the edges of which are the edges of which are defined by seismic activitydefined by seismic activity The plates consist of some combination of continental and The plates consist of some combination of continental and

oceanic crustoceanic crust The bottom boundaries extend downward through the entire The bottom boundaries extend downward through the entire

lithospherelithosphere


The rigid and brittle lithospheric plates overlie The rigid and brittle lithospheric plates overlie the molten and plastic asthenospherethe molten and plastic asthenosphere

Lithospheric platesLithospheric plates Lithosphere = rigid (won’t bend)Lithosphere = rigid (won’t bend)

Note that it can contain Either continental or oceanic crustNote that it can contain Either continental or oceanic crust OR BOTH!OR BOTH!

Asthenosphere = pliable (will bend or even flow)Asthenosphere = pliable (will bend or even flow)


Nearly all Nearly all seismic activityseismic activity is associated with is associated with plate plate boundariesboundaries..

Seismic activity gives rise to earthquakes and Seismic activity gives rise to earthquakes and volcanic activity, most of which occur in the Pacific volcanic activity, most of which occur in the Pacific Ocean.Ocean.

Plate boundaries and seismic activity:Plate boundaries and seismic activity: co-occurco-occur exist on continuous linesexist on continuous lines

Seismic activitySeismic activity

These occur together!These occur together!


Three flavors of plate boundaries:Three flavors of plate boundaries: 1) Midocean ridges1) Midocean ridges

Sites of tension, plates divergeSites of tension, plates diverge Addition of new crust to trailing plate edgeAddition of new crust to trailing plate edge

Global Plate Tectonics Global Plate Tectonics (Cont.)(Cont.)Midocean ridgesMidocean ridges are: are:

also called “spreading centers”also called “spreading centers” site of the generation of new crust / lithospheresite of the generation of new crust / lithosphere

Global Plate Tectonics Global Plate Tectonics (Cont.)(Cont.) Midocean ridgesMidocean ridges occur in the occur in the midmiddle of the dle of the ocean.ocean.

start off as a rift in a continentstart off as a rift in a continent the ocean forms on both sides of the ridgethe ocean forms on both sides of the ridge


Three flavors of plate boundaries:Three flavors of plate boundaries: 2) 2) Subduction zonesSubduction zones

Sites of compression, plates convergeSites of compression, plates converge Destruction of old lithosphere/crustDestruction of old lithosphere/crust


The most significant feature of subduction is the The most significant feature of subduction is the volcanic activity that goes with itvolcanic activity that goes with it

Global Plate Tectonics Global Plate Tectonics (Cont.)(Cont.) Subduction is ocurring Subduction is ocurring

under:under: JapanJapan Western South AmericaWestern South America Central AmericaCentral America AlaskaAlaska New ZealandNew Zealand CaribbeanCaribbean Northwest USA!Northwest USA!

Mt. St. HelensMt. St. Helens

Mt. RanierMt. Ranier


Three flavors of plate boundaries:Three flavors of plate boundaries: 3) Transform faults3) Transform faults

Sites of lateral movementSites of lateral movement Lithosphere/Crust is neither created or destroyedLithosphere/Crust is neither created or destroyed Conservative plate boundaryConservative plate boundary

Sea-Floor Spreading Sea-Floor Spreading (Cont.)(Cont.)

Transform Faults:Transform Faults: occur where plates are occur where plates are

sliding past each othersliding past each other California: San Andreas California: San Andreas

FaultFault No volcanoesNo volcanoes

In most casesIn most cases Some exceptionsSome exceptions

LOT of earthquakesLOT of earthquakes


The amount of old lithosphere destroyed by The amount of old lithosphere destroyed by subduction zones equals the amount of new subduction zones equals the amount of new lithosphere created by ridge systemslithosphere created by ridge systems


Assuming the the size of the Earth isn’t changing,Assuming the the size of the Earth isn’t changing, the net change in plate volume is zerothe net change in plate volume is zero the total RATE of production = the total RATE of destructionthe total RATE of production = the total RATE of destruction

The locations of the zones of sea-floor formation and The locations of the zones of sea-floor formation and destruction implies thatdestruction implies that the Atlantic Ocean is gradually expanding, the Atlantic Ocean is gradually expanding, the Pacific Ocean is gradually decreasing in size.the Pacific Ocean is gradually decreasing in size.

However, the rates are not identical because the However, the rates are not identical because the production of new crust at some ridge systems in the production of new crust at some ridge systems in the Pacific Ocean counteracts the destructive processesPacific Ocean counteracts the destructive processes

One idea of what is going on:One idea of what is going on:


Notice that most earthquakes occur on active plate boundariesNotice that most earthquakes occur on active plate boundaries Some occur where new boundaries are forming (rifting)Some occur where new boundaries are forming (rifting) Others occur where ancient continents have collided (Alps, Himalyas, Others occur where ancient continents have collided (Alps, Himalyas,

Rockies, New Madrid………)Rockies, New Madrid………)


Based upon the Theory of Plate Tectonics ocean Based upon the Theory of Plate Tectonics ocean basins open and close. basins open and close.

The The Wilson CycleWilson Cycle describes the stages in ocean describes the stages in ocean

basin development.basin development.

•Early rifting:Early rifting:•underwaterunderwater•produces “pillow lava”produces “pillow lava”

What causes the plates to move?What causes the plates to move? Thermal convectionThermal convection

Produces slow moving currents in the underlying asthenosphere Produces slow moving currents in the underlying asthenosphere which exert drag on the lithospheric plates setting them in motionwhich exert drag on the lithospheric plates setting them in motion

Plate Tectonic ForcesPlate Tectonic Forces These forces require an enormous amount of energyThese forces require an enormous amount of energy they may be combined on some platesthey may be combined on some plates

SummarySummary Plate Tectonics is a wonderful theoryPlate Tectonics is a wonderful theory

unifyingunifying understandingunderstanding predicitivepredicitive

Three kinds of plate boundaries:Three kinds of plate boundaries: constructive (spreading center)constructive (spreading center) destructive (subduction zone)destructive (subduction zone) conservative (transform fault)conservative (transform fault)

Plates are defined by their boundariesPlates are defined by their boundaries seismicityseismicity NOT the kind of crust!NOT the kind of crust!

Next we’ll look at:Next we’ll look at: details of each kind of boundarydetails of each kind of boundary predictive nature of the theorypredictive nature of the theory applicationsapplications

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Global Plate Tectonics n Plate Tectonics = Sea Floor Spreading + Continental drift n These two concepts came from separate sets of observations n The Plate