global plate tectonics (cont.) n three flavors of plate boundaries: n midocean ridges n sites of...

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

Three flavors of plate boundaries:Three flavors of plate boundaries: Midocean ridgesMidocean 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

Subduction zonesSubduction zones Sites of compression, plates convergeSites of compression, plates converge Destruction of old crustDestruction of old crust

Transform faultsTransform faults Sites of lateral movementSites of lateral movement Crust is neither created or destroyedCrust is neither created or destroyed

We will study each of these in limited We will study each of these in limited detaildetail


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

Early RiftingEarly Rifting

Oceans begin within continents by Oceans begin within continents by rifting them apartrifting them apart

We don’t fully understand how this We don’t fully understand how this startsstarts could be due to magma sourcescould be due to magma sources could be related to existing could be related to existing

weaknesses in the plateweaknesses in the plate

Best model is that mantle Best model is that mantle convection is not homogeneousconvection is not homogeneous results in concentrations of energyresults in concentrations of energy plumes of magmaplumes of magma These thin and weaken the plateThese thin and weaken the plate


This process can take a very long This process can take a very long timetime

During this time, other processes During this time, other processes continuecontinue lava flowslava flows faultingfaulting erosionerosion

The erosion which takes place The erosion which takes place during this time is significantduring this time is significant remember, it’s a continent at this pointremember, it’s a continent at this point substantial relief=major erosion and substantial relief=major erosion and

deposits.deposits.

Early RiftingEarly Rifting When water fills the new ocean basin, volcanism When water fills the new ocean basin, volcanism

produces “pillow lava”produces “pillow lava” very distinctivevery distinctive glassy surfaceglassy surface

RiftingRifting At active spreading centers:At active spreading centers:

water percolates down into crackswater percolates down into cracks becomes heatedbecomes heated dissolves ions from basaltdissolves ions from basalt exits circulation in “smokers”exits circulation in “smokers”

Black SmokerBlack Smoker

Anatomy of a SmokerAnatomy of a Smoker

These chimneys form from These chimneys form from precipitated metalsprecipitated metals

note scale: 20m!note scale: 20m! First discovered in early 70’s First discovered in early 70’s

by Ballard and othersby Ballard and others We still don’t understand We still don’t understand

detailsdetails This discovery helps explain This discovery helps explain

the origin of many important the origin of many important ore depositsore deposits


As rifting As rifting continues, specific continues, specific structures form:structures form:

Intrusive igneous Intrusive igneous rocksrocks sheeted dikessheeted dikes gabbrosgabbros

““Normal” faultingNormal” faulting Rift ValleyRift Valley Brand new Brand new

seafloor!seafloor! Oceanic crustOceanic crust continental crust continental crust

on each sideon each side


This is an ocean in the process of formationThis is an ocean in the process of formation Part has filled in with water alreadyPart has filled in with water already Here we are looking right down the rift valleyHere we are looking right down the rift valley Note the “normal” faultingNote the “normal” faulting

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

These seas are examples of early rifting which is tearing a continent These seas are examples of early rifting which is tearing a continent apart.apart.

Any wonder the Bible is full of seismic and volcanic events?Any wonder the Bible is full of seismic and volcanic events?

RiftsRifts

JerichoJericho

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

As the sea floor spreads,:As the sea floor spreads,: it gets older (doesn’t everything?)it gets older (doesn’t everything?) it cools and thickensit cools and thickens it accumulates sedimentit accumulates sediment


Subduction zonesSubduction zones are regions where plates are converging are regions where plates are converging (colliding)(colliding) oceanic crust is being destroyedoceanic crust is being destroyed

Subduction Zone FeaturesSubduction Zone Features 1) Mountan building volcanism:1) Mountan building volcanism:

melting of subducting plate causes formation of plutons of magmamelting of subducting plate causes formation of plutons of magma these rise and eventually erupt as volcanoesthese rise and eventually erupt as volcanoes if on a continent, make mountain chains (Andes)if on a continent, make mountain chains (Andes) if in the ocean, make island chains (Japan)if in the ocean, make island chains (Japan)

Subduction Subduction VolcanismVolcanism

Our northwest Our northwest coast is coast is strongly strongly influenced by influenced by thisthis

no trench no trench because ofbecause of sediment sediment

accumulationaccumulation proximity of proximity of

spreading spreading centercenter

Subduction Zone FeaturesSubduction Zone Features 2) Earthquakes:2) Earthquakes:

form along subducting plateform along subducting plate called the “Benioff Zone”called the “Benioff Zone” persists as deep and far as plate can support thempersists as deep and far as plate can support them results in wide zone of deep quakesresults in wide zone of deep quakes

Subduction Zone FeaturesSubduction Zone Features 3) Trench:3) Trench:

hardly shown in this diagramhardly shown in this diagram results from subduction of oceanic plateresults from subduction of oceanic plate can be very, VERY deep: up to 11km (7 miles)can be very, VERY deep: up to 11km (7 miles)

The Mariana TrenchThe Mariana Trench

Subduction Zone FeaturesSubduction Zone Features 4) Acretionary Wedge4) Acretionary Wedge

sediments are scraped offsediments are scraped off accumulate on landward side of trenchaccumulate on landward side of trench typical of Washington/Oregon coasttypical of Washington/Oregon coast

Accretionary PrismAccretionary Prism

Layers are Layers are invertedinverted

very messyvery messy OregonOregon

Subduction with GravitySubduction with Gravity

Ocean - Ocean ConvergenceOcean - Ocean Convergence Results in an Island ArcResults in an Island Arc

Japan, Marianas etc.Japan, Marianas etc.

A Massive QuakeA Massive Quake

Subduction zone:Subduction zone: Indian plate sliding Indian plate sliding

under Burma Plateunder Burma Plate

750 miles of fault 750 miles of fault slipped slipped simultaneouslysimultaneously

Slip was about 50 Slip was about 50 feet!feet!

Indonesian Earthquake of 12/26/04Indonesian Earthquake of 12/26/04 Resulted Resulted

from a from a quake in quake in IndonesiaIndonesia

Spread Spread across across entire Indian entire Indian OceanOcean

Images from Images from http://www.surfersvillage.com/news.asp?Id_news=15298

Water is drawn away Water is drawn away from beach before the from beach before the big wave hitsbig wave hits

Satellite images Satellite images show before and show before and afterafter

Note how the Note how the water is drawn water is drawn away from the away from the shoreshore

Massive quakeMassive quake 750 miles of 750 miles of

fault slipped fault slipped simultaneoussimultaneous

Slip was about Slip was about 50 feet!50 feet!

Photos from Photos from USGSUSGS

New Zealand Quake 2-22-2011New Zealand Quake 2-22-2011

Continental CollisionContinental Collision Eventually, all the oceanic plate (crust) is used up Eventually, all the oceanic plate (crust) is used up

and continents collideand continents collide Massive Mountain building takes placeMassive Mountain building takes place earthquakesearthquakes

IndiaIndiaChinaChina

Continental CollisionContinental Collision

IndiaIndia

ChinaChinaHimalayasHimalayas

Everest is still growingEverest is still growing

Transform FaultsTransform Faults Conservative plate boundaryConservative plate boundary strike-slip motionstrike-slip motion result in fracture zones as scars of fault activityresult in fracture zones as scars of fault activity

Transform FaultsTransform Faults

Connect segments of spreading Connect segments of spreading centerscenters

may even do a little spreading may even do a little spreading on their ownon their own

have crust of different ages on have crust of different ages on each sideeach side

Transform Faults / Transform Faults / Fracture ZonesFracture Zones

Note:Note: offset of spreading offset of spreading

centercenter ages of crust on ages of crust on

either side of either side of transform fault AND transform fault AND fracture zonefracture zone

effect this will have effect this will have on topographyon topography

location of active and location of active and inactive segments of inactive segments of the fracture zonethe fracture zone

Fracture ZoneFracture Zone

Here’s the effect of crossing a fracture zone while performing Here’s the effect of crossing a fracture zone while performing a magnetic surveya magnetic survey

VERY difficult to interpret results VERY difficult to interpret results

Transform Faults / Fracture ZonesTransform Faults / Fracture Zones Fracture zones Fracture zones

result from result from geometry of geometry of spreading on a spreading on a curved surfacecurved surface same angular same angular

velocityvelocity different different

separation separation ratesrates

fracture zones fracture zones take up the take up the differencedifference

A & B

Transform Faults / Transform Faults / Fracture ZonesFracture Zones

On our West Coast is the site of a major On our West Coast is the site of a major transform boundary cutting through a transform boundary cutting through a continentcontinent

not all that common; most are in oceanic not all that common; most are in oceanic crustcrust

resulted from subduction of a spreading resulted from subduction of a spreading centercenter

this transform fault connects the Juan de this transform fault connects the Juan de Fuca and East Pacific Rise spreading Fuca and East Pacific Rise spreading centerscenters

spreading rates are fastspreading rates are fast we KNOW that motion is constantly we KNOW that motion is constantly

occurring at depthoccurring at depth is expressed at the surface periodicallyis expressed at the surface periodically

Transform Transform FaultsFaults

The area around The area around San Francisco is San Francisco is very “striking”very “striking”

dominated by fault dominated by fault topographytopography

The most famous The most famous fault is the San fault is the San AndreasAndreas

The The Great Great

US US FaultsFaults

Easy to Easy to see from see from the airthe air

Note Note resevoirs resevoirs with with dams dams across across faultfault

HaywardHayward

San AndreasSan Andreas

Transform faultsTransform faults The same The same

topography you see topography you see near San Francisco near San Francisco exists in many, exists in many, many places on the many places on the sea floorsea floor

http://www.oceanexplorer.noaa.gov/explorations/05lostcity/logs/july23/media/karson1atf_600.jpghttp://www.oceanexplorer.noaa.gov/explorations/05lostcity/logs/july23/media/karson1atf_600.jpg

Plate Tectonics (con.t)Plate Tectonics (con.t)

These earthquakes:These earthquakes: are certain to continueare certain to continue vary in strength according to vary in strength according to

built-up stressbuilt-up stress

Continental Continental MarginsMargins

Any place continental Any place continental and oceanic crust and oceanic crust meet, a continental meet, a continental margin existsmargin exists

this may be plate this may be plate boundary:boundary: divergent=passive=nodivergent=passive=no translative=probably nottranslative=probably not convergent=subduction=convergent=subduction=

usuallyusually This diagram is This diagram is

confusing but the only confusing but the only one I haveone I have

Spreading Spreading centers are centers are bounded on bounded on each side by each side by Divergent Divergent Continental Continental MarginsMargins

Until Until subduction subduction beginsbegins

note the note the terminologyterminology

Divergent Continental MarginDivergent Continental Margin Continental and oceanic crust meet Continental and oceanic crust meet on the same plateon the same plate Faces a spreading center (Atlantic Ocean)Faces a spreading center (Atlantic Ocean) NOT a plate boundaryNOT a plate boundary Ages to form a “typical” continental shelfAges to form a “typical” continental shelf

sediment damming / accumulationsediment damming / accumulation very flat, depositional structurevery flat, depositional structure

Note FaultingNote Faulting

Divergent MarginDivergent Margin

Cape CodCape Cod Note gravity Note gravity

and magnetic and magnetic anomoliesanomolies

faultingfaulting reefreef formation of formation of

shelfshelf

The “Wilson Cycle” ReviewThe “Wilson Cycle” Review

Stage 1: splintering of continental crustStage 1: splintering of continental crust e.g., the East African rift valleyse.g., the East African rift valleys

Stage 2: expansion and formation of shallow basinStage 2: expansion and formation of shallow basin e.g., the Red Seae.g., the Red Sea


Stage 3: gradual separation into two distinct continentsStage 3: gradual separation into two distinct continents e.g., the Atlantic Oceane.g., the Atlantic Ocean

Stage 4: weakening of old crust from accumulated Stage 4: weakening of old crust from accumulated sediment and formation of a subduction zonesediment and formation of a subduction zone

e.g., the Pacific Oceane.g., the Pacific Ocean


Stage 5: gradual shrinking of the basinStage 5: gradual shrinking of the basin e.g., the Mediterraneane.g., the Mediterranean

Stage 6: colliding of the land massesStage 6: colliding of the land masses e.g., the Himalayase.g., the Himalayas

SummarySummary Features of the earth’s surface are determined by the Features of the earth’s surface are determined by the

interaction of lithospheric platesinteraction of lithospheric plates Seismic activity is intense at plate boundariesSeismic activity is intense at plate boundaries Crustal material is created at ridges and destroyed at Crustal material is created at ridges and destroyed at

subduction zonessubduction zones The movement of lithospheric plates disrupts The movement of lithospheric plates disrupts

continents leading to the expansion or contraction of continents leading to the expansion or contraction of ocean basinsocean basins

global plate tectonics (cont.) n three flavors of plate boundaries: n midocean ridges n sites of...

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