4 Plate Tectonics KD

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OceanographyGEO 9

Kirk Domke

Winter 2013

Lecture 4:Plate Tectonics

Earths interior

Surface observations

Formation of a theory

Types of plate boundariesPaleogeography

PLATE TECTONICS

(AKA: the theory of just about everything in geology)

Why is this a theory and not a fact in science?

Facts are observations, theories are attemptsto explain those observations.

Thetheory: The Earths surface is covered by a series ofrigid slabs (plates) that move in relation to one anotherand interact at the margins of the plates.

Lets look at some observations:

At the turn of the 20th Century, it was commonlythought that the Earth consisted of a rigid solid outercrust with a hot molten (liquid) interior. WHY?

1. Temperature increases 3.5C/100m depth in mines.

2. Rocks begin melting at about 500C,

so the depth to the fluid interior was about15 km. (Earths radius is 6371 km).

but the Earth did not deform as much as a ballof fluid should under the influence of solar and lunartides, so scientists were beginning to think this waswrong.


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Along come seismologists(people who studyearthquakes)

1) P waves (primary)

2) S waves (secondary)Particle motion is perpendicularto direction that wave is traveling.Sideways motion. or transverse

ONLY IN SOLIDS

IN SOLIDS, LIQUIDS, GASES

Using seismometers, they begin recording two differenttypes of energy waves from earthquakes:

Particle motion is in direction thatwave is traveling. Traveling patternof compression and rarefaction.In and out motion. compressionalwaves

S waveP wave

P waves travel about twice as fast as S waves

Seismographic recording on seismometer

(detector)

Earthquake Source

They found S waves traveling through the Earths interior, butnot through a really deep part they called the core.

S-wave shadow zone!


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Solid mantle (83% vol)Solid crust (1% vol)

The Earth is layered.

Solid

innercore

Moltenouter

core

(16% vol)

Alfred Wegener suggested

that continents wereoriginally all together in agiant landmass calledPangaea (1912)

Fit of Continents

ca. 300-210 Myr ago

ca. 300-210 Myr ago

He also used fossilevidence for his theory ofcontinental drift.

Fit of Continents

How can you explain where fossils of land reptiles are found?(They cannot swim!) and what about seed ferns in

Antarctica??

Fossil Evidence:


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Desert climates

w/ sand dunes,dry lakesin Greenland??

Tropical climates

w/ coral reefs,coalin Maine??

Arctic climatesw/ glacial deposits

in India??

Geologic deposits indicating

past climates make senseONLY IF continents arereassembled and movedfrom where they are today.

Past climates

Why do Africa, India, and Australia haveevidence of glaciers?

What climates do these places have now?

250 million years ago Present day

so geologists made a prediction based on their theoryof continental drift that similar rocks should be found in Africaand the corner of South America:

Rocks on continents on opposite sides of oceans match!

Matching Rocks Across Oceans

At the end of WWII, the geologic community is at adeadlock. There is evidence for continental drift, butit is largely ignored because there is no apparentmechanism to move continents.

Mid- Atlantic Ridge

but depth (echo) sounding begins to provide anocean basin profile that does not match with

expectations.


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Not only was the middle of the ocean high, but the thickestsediments were at the basin margins (not the middle)

Distributions of earthquakes seem to occur at plate boundaries

Note how shape of earthquakes in mid-Atlantic matches the coast of Africa?

(and these earthquakes fell on the mid-Atlantic ridge!)

Today: Major lithospheric plates and relative movement

7 major and many minor sized plates

and what about the volcanos?

80% of the earths volcanoes are found near plateboundaries of the Pacific Ocean


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Where would you find the youngest oceanic crust?

Where would you find the oldest oceanic crust?

This led in 1960 to the radical idea that thesea floor was spreading. New crust was addedat the mid Atlantic ridge.

Harry Hess American shipcaptain during the war

(Princeton professor afterward)

The oldest oceanic crust is often at the trenches!

Notice: oldest oceanic

crust is 161 Ma,

compared to 4600 Ma

for the Earth

(Note the time scales: the fastest plates move about as fast

as your fingernails grow (~8 cm/year))

from

paleomagnetic

patterns

Seafloor magnetism
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Seafloor magnetism Seafloor magnetism

Paleomagneticpropertiesof rocks of the same ageFrom different continents-- N Magnetic pole in Pacific?!

explanation

(continental configuration has changed over time)

But, if new crust was being added, then:A) the Earths surface has to expand ORB) we have to remove crust somewhere else.

Subduction zone -

where the crust isremoved

This led in 1960 to the radical idea that thesea floor was spreading. New ocean crust was addedat the ocean ridges e.g. mid-Atlantic ridge.


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Plate

Tectonics

ContinentalDrift

Sea FloorSpreading

Fossils

Past climates

Shapes of

coastlines

Similar

rock types

Age of sea floorRidges,

trenches

Finally, in 1965, John Tuzo Wilson puts continental drift and sea

floor spreading together into the theory ofplate tectonics

Paleomagnetism

patterns of sea floor

Finally, in 1965, John Tuzo Wilson puts continental drift and sea

floor spreading together into the theory ofplate tectonics

But, what was themechanism???

Mechanism for Plate Tectonics =

Mantle Convection

The tectonic system is powered by heat (convection currents)

[Gravity also helps this system along]
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Mid- Atlantic Ridge

The tectonic system creates the ocean floor!

Lets look at the ocean floor more carefully.

Blah blah blah tectonics.

blah blah blah convection.

.what does this stuff have to do withoceanography???

Why are the continents (and continental shelves) so

much higher than the ocean basin?

Shelf

What do we know that might help us?

1. Continentsare made of many rocks butcompositionally are very similar to granite (like yousee in the Box Springs Mountains).

These are light (less dense)

3. Light things float and heavy things sink. This is

buoyancy and is a consequence of GRAVITY!!!

Is it possible that the ocean floor is lower than the

continents because the basalt is denser and thusfloats lower than the continents?

2. Oceanic rocks beneath the sediments (the oceanic

crust) are made ofbasalt.Thisis heavy (more dense).

The Earths crust + uppper mantle (lithosphere)is in isostatic equilibrium - plates float on theunderlying mantle (asthenosphere)

Isostasy = buoyancyThe Iceberg principle - pressure forces from the displacedwater (or rock) balance the weight of the object


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The Iceberg principle - Objects higher above sea level musthave deeper roots.

Lets make a prediction: continents are higher and oceanfloors are lower. Which should have deeper roots?

Continental crustcan be up to 65 km

thick

Oceanic crustis about 8km

thick

These observations support our hypothesis that ocean floorsare lower because the oceanic crust is denser.

Mid- Atlantic Ridge

SUMMARY:

Continental shelves are higher because they are composedof lighter crust and thus float higher than the oceaniccrust. This is because of gravity creating buoyant forces(aka isostasy).

Hot magma rising there to form volcanoes alongocean ridge. Buoyancy of the magma pushesthe overlying crust upward.

Why might the ocean crust at the mid-Atlanticridge be higher than the surrounding ocean floor?

1. Move away from one another: divergent margin(aka ocean ridge, spreading center)

Three kinds of interactions:

2. Move toward another:

convergent margin

(aka subduction zone,collision zone, trench)

3. Slide past one another:transform margin(aka transform fault)


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Summary of Plate Interactions

2 kinds of p late divergences:

Divergent oceanic crust(e.g. Mid-Atlantic, ridge at spreading center)

Divergent continental crust(e.g. Rift Valley of East Africa, continents spread)



3 kinds of p late convergences: Oceanic crust toward continental crust

(e.g. west coast of South America-Andes, trenches) Oceanic crust toward oceanic crust

(e.g. northern Pacific, deep trenches and island arcs) Continental crust towards continental crust

(e.g. Himalayas, plates folded and uplifted)



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Transformboundaries mark locations at which crustal plates

move past one another (e.g. San Andreas Fault)

Lets look more closely at the different margins and howthey affect the oceans:

Divergent Margin

Magma (molten rock) rises to the surface here andencounters what?

1. New ocean crust is created (making the oceans bigger).

2. Outgassing - gases added to hydrosphere and atmosphere

3. Chemicals from mantle added to ocean - black smokers

4. Heat is added to the ocean.

WATER, that cools and solidifies it!

Convergent Margin

1. Ocean crust is consumed(oceans get smaller.)

2. Volcanic islands (like Japan)

form.3. Volcanos providesediment from erosion

4. Big earthquakescreate tsunami!

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Island arcs form, continents collide, and crust recycles atconvergent plate boundaries

A cross section through southern China, showing theconvergence of two continental plates.

Neither plate is dense enough to subduct; instead, theircompression and folding uplift the plate edges to form theHimalayas.

Transform Margin

1. Crust is neither added or removed at these margins.

2. No volcanos either, just earthquakes!

Transform margins form because the ocean crust breaks indifferent places as it is pulled apart by subducting plates.

MAGMA DOES NOT PUSH THE PLATES APART!

What makes the tectonic system run?

To answer this, we need to know a bit more aboutthe geology insidethe Earth.

HOW WILL WE FIND OUT?

1. Deepest drill hole = 10 km (or not even through crust!)

2. Seismic waves

3. Xenoliths - foreign rocks. Samples of lowercrust and mantle brought up in volcanic eruptions.

4. Experimental petrology - take probable types of rocksand put them under high P, T to see what forms.

Actually, 2 different setsof terms for layering:

Mechanical Composition:Lithosphere- rigid outer layer, plates

Asthenosphere - weak, flowing layer

Lower mantle - rigid lower layer

Outer core - dense liquid

Inner core - dense solid

(upper 100-200 km)Crust + upper mantle = lithosphere

Chemical Composition:Crust - granite, light (continental)

basalt, dense (oceanic)

Mantle - Fe, Mg silicates

Core - Fe, Ni

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and we know that subducting plates go quitedeep into the mantle, to the core-mantle boundary

Colder,dense

Vertical slice through the Earths mantle showing the

distribution of warmer (red) and colder (blue) material

WHY?

Buoyancy!!! (due to gravity)

Hot stuff (magma) rises at the ocean ridgesand cold (dense) ocean plates sink back into

the mantle at the trenches.

The tectonic system is powered by convectiveheatand platemovement is driven by gravitational forces causing hot, lessdense magma to rise at the ocean ridges and cold, denseroceanic lithosphere to sink at the trenches

Moon

NO MORE MOVING PLATES! (like the Moon, Mars, etc.)

This is due to the differentiationof the Earth that hascontinued since its formation. What do you think will happenwhen all of the heat is transferred from the core to the surface

of the Earth (most of this heat is lost to space, by the way)?

Good news:Radioactiveelements are still

making heat, so wehave a few billionyears left.

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What have you learned today about the

coupling between the lithosphere and hydrosphere?

Mid- Atlantic Ridge

2. Isostasy (buoyancy) explains why the continentalshelves are so much higher than the ocean basins andwhy the ridges are higher than the surrounding basins.

1. The shape of the ocean basins is controlled by thetectonic system shaping the lithosphere.

3. Volcanic processes at the divergent and convergentmargins of plates contribute chemicals, heat, andsediment to the ocean (more on these later).

4. The tectonic movement of plates results in transfer of

heat from the interior to the surface of the Earth. Whenthe interior has cooled, the system will likely stop.

5. Ocean floor is oldest at the trenches (or adjacent tocontinents) and youngest at the ridges.

6. Divergent margins create new ocean floor; convergentmargins consume it. Also get mountain building orvolcanic islands at convergent margins though assubduction is not 100% efficient.

and one last item concerning oceans in the geologicpast.

Do you think that the oceanbasins have always been the same shape?

Movement of continents has been significant over geologic time

600Ma



220Ma

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and one last item concerning oceans in the geologic past.


090Ma



Present

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4 Plate Tectonics KD