3- Plate Tectonics Presentaion - Assiut Univ

8/19/2019 3- Plate Tectonics Presentaion - Assiut Univ

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Plate tectonics


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The division of the Earth's interior into lithospheric and

asthenospheric components is based on their mechanical

differences.

The lithosphere is cooler and more rigid, whilst theasthenosphere is hotter and mechanically weaker.

This division should not be confused with the chemical

subdivision of the Earth into core, mantle, and crust.

The key principle of plate tectonics is the lithosphere existsas separate and distinct tectonic plates, which float on the

fluid-like asthenosphere.

The relative fluidity of the asthenosphere allows the tectonic

plates to undergo motion in different directions.

The plate tectonics model says that the outer layer of the

Earth consists of a number of plates that interact at their

boundaries, creating volcanoes, mountains, earthquakes, andsha e the face of the lanet.


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Types of plate

boundaries


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Three types of plate boundary

There are three types of plate boundaries, characterized by the

way the plates move relative to each other.

1-Convergent plate boundaryceanic!oceanic convergence

"ontinental!continental convergence

ceanic ! continental convergence

2-Divergence plate boundary

ceanic spreading ridge

"ontinental #ift zone

3-Transform plate boundary$ithout the activity arising from these plate motions

%deformation, volcanism and earthquakes& the earth world be

as dead as the moon.


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Convergent plate boundary

Divergence plate boundary

Transform plate boundary


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Transform-1boundaries


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1-Transform boundaries

ccur where plates slide past each other along transform

faults.

The relative motion of the two plates is either sinistral %left

side toward the observer& or detral %right side toward the

observer&.

!arth"uakes generally occur at these boundaries.

good example of this type of plate boundary is the (an

ndreas )ault complex, which is found in the western coast of

*orth merica. t this location, the +acific and *orthmerican plates move relative to each other such that the

+acific plate is moving northwest with respect to *. merica.


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Transform boundaries


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Transform #aults


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(an ndreas fault in

(an )rancisco, "alifornia,


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magine a road built sometime in the middle of the seismic cycle


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The road records deformation


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-ntil it is offset by the event


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2-Divergentboundaries


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2-Divergent boundaries

$%preading center or &ift 'one(

ccur where two plates slide apart from each other and thespace that this creates is filled with new crustal material

sourced from molten magma that forms below.

The origin of new divergent boundaries at triple unctions is to

be associated with the phenomenon known as hotspots.

The hot spot which may have initiated the )id-*tlantic

&idge system currently underlies iceland which is widening ata rate of a few centimetres per century.


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The mid-ocean ridge is an example of this type of boundary

because the sea floor is spreading apart at this boundary/

upwelling of magma %lava inside the Earth& occurs at these

divergent boundaries forming volcanoes,The lava flowing from these volcanoes is basalt, this is the

rock that makes up the entire ocean floor.

The type of lava that flows from the mid!ocean ridge is called

pillow lava, it flows out in melted blobs that resemble pillows

in the ocean.

!arth"uakes near the surface of the Earth occur at divergent

plate boundaries.


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nother example of a divergent boundary is the !ast Pacific

&ise+ this is the site of sea floor spreading in the +acific

cean, the +acific cean is spreading at a rate of 01.2 cm per

year, while the *tlantic ,cean is spreading approximately

34 cm per year, this spreading causes seismic activity to

occur

which is why there are so many more earthquakes, volcanoes,

and tsunamis in the +acific cean and its surrounding

continents.

n the other hand, a formation called a rift valley occurs in

the mid ocean ridge in the tlantic cean, this is the centralarea of the ridge characterized by deep fractures


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Divergent boundaries 5reakup of acontinent


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(eafloor (preading


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The East frican rift 6 a divergent boundary on land


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7ivergent and transform margins


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3-Convergent

boundaries

3 C t b d i


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3-Convergent boundaries

$or destructive or active margins( ccur where two plates slide towards each other commonly

forming either a subduction 'one %if one plate movesunderneath the other& or an orogenic belt %if the two simply

collide and compress&.

$hen this occurs, ocean crust is destroyed as one plate

plunges below another and is remelted deep within the Earth/

an example of this type is the deep ocean trenches.

Trenches are deep, linear scars where

subduction occurs.

olcanoes are also found at convergent plate boundaries, the

melting at the subduction zone causes volcanoes to form.

!arth"uakes deep within the Earth are found at convergent

boundaries


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Examples of convergent boundaries is, the ndes mountain.

The nature of a convergent boundary depends on the type of

the plates that are colliding.

1-,ceanic-contenental plates

$here a dense oceanic plate collides with a less-dense

continental plate, the oceanic plate is typically thrust

underneath, forming a subduction 'one.

t the surface, the topographic expression is commonly an

oceanic trench on the ocean side and a mountain range on the

continental side.

n example of a continental-oceanic subduction 'one is the

area along the western coast of (outh merica where the

oceanic *azca +late is being subducted beneath the

continental (outh merican +late.


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s the subducting plate descends, its temperature rises driving

off volatiles %most importantly water&.

s this water rises into the mantle of the overriding plate, it

lowers its melting temperature, resulting in the formation ofmagma with large amounts of dissolved gases.

This can erupt to the surface, forming long chains of

volcanoes inland from the continental shelf and parallel to it.


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Convergent boundaries


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cean!"ontinent

(ubduction 8one


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"onvergent plate margins


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2-Contenental-contenental plates

$here two continental plates collide the plates either

crumple and compress or one plate burrows under or overridesthe other.

Either action will create extensive mountain ranges.

The most dramatic effect seen is where the northern margin of

the ndian +late is being thrust under a portion of the Eurasian

plate, lifting it and creating the 9imalayas.

t has also caused parts of the sian continent to deform

westward and eastward on either side of the collision.


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"ontinent!"ontinent "ollision


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3-,ceanic-oceanic plates

$hen two plates with oceanic crust converge they typically

create an island arc as one plate is subducted below the other.The is formed from volcanics which erupt through the

overriding plate as the descending plate melts below it.

deep undersea trench is located in front of such arcs where

the descending slab dips downward.

:ood examples of this type of plate convergence would be

;apan and the leutian slands in laska.


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cean6cean (ubduction 8one


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%ources of plate

motion


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%ources of plate motionThe plates are able to move because of the relative weakness

of the asthenosphere.

7issipation of heat from the mantle is the source of energydriving plate tectonics.

This energy must be translated to the lithosphere in order for

tectonic plates to move.

There are essentially two forces that could be accomplishing

this< friction and gravity.

#riction

1-)antle drag"onvection currents in the mantle are transmitted through the

asthenosphere/ motion is driven by friction between the

asthenosphere and the lithosphere.

2 T h ti


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2-Trench suction

=ocal convection currents exert a downward frictional pull on

plates in subduction zones at ocean trenches.

. it


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.ravity

*-&idge-push

+late motion is driven by the higher elevation of plates at mid!

ocean ridges. Essentially stuff slides downhill.

The higher elevation is caused by the relatively low density of

hot material upwelling in the mantle.

The real motion producing force is the upwelling and the

energy source that runs it./-%lab-pull

s the lithosphere cools it becomes more dense than the

asthenosphere it rides above and it sinks.

t pulls the lithosphere behind it in towards the mantle.

+late motion is driven by the weight of cold, dense plates

sinking into the mantle at trenches.


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)ig. >b< 9ow plates move

)antle convection


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)antle convection

The mantle is semi!solid rock %except for small, partially

melted zones in the asthenosphere&, it is so hot that over

geologic time it flows slowly as a stiff fluid.9ot rock from deep in the mantle rises to the base of the

lithosphere.

t the same time, cooler upper!mantle rock sinks.

This circular flow of solid rock is called mantle convection.?antle convection is related to movement of lithospheric

plates.

)antle convection as the cause of plate movement


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)antle convection as the cause of plate movement

"onvection occurs when a fluid is heated from below, as in a

pot of soup on a stove.

The soup at the bottom of the pot expands as it is heated and becomes less dense than the soup at the top.

5ecause it is less dense, it rises.

$hen the hot soup reaches the top of the it flows along the

surface as cooler soup sinks to take its place.f the heat source persists, this cool, sinking soup is then

warmed.

t rises, and the convection continues.

?antle convection might occur in a manner similar to that in

the soup pot. n this model, the base of the mantle is heated

from below, perhaps by the hot core.


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(oup convects because it is heated from

the bottom of the pot.


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n turn, the heating causes mantle convection, imagine a

block of wood floating on a tub of honey.

f you heated the honey from below so that it started to

convect, the horizontal flow of honey along the surface

would drag the block of wood along with it.

(ome geologists suggest that lithospheric plates are dragged

along in a similar manner by a convecting mantle.


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=ithospheric plates are dragged along by mantle convection.

)easured plate motion


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)easured plate motion

Plate motion is measured directly with the :lobal +ositioning

(atellite system %:+(& that assumes no movement of the

lithosphere relative to the mantle.

$hen analysing plate motions relative to the mantle, plates

rather move @westward@.

Plate velocity"alculations based on several methods show that plates move

away from spreading centers at rates that vary from 0 to 0A

centimeters per year .

n recent years, plate motion has been measured directly bysurveying techniques that bounce laser beams off the ?oon

and satellites and by other methods that use radio waves

originating outside our :alaxy.


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+late velocities in centimeters per year.


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)a0or plates

)inor plates


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)inor platesThe minor plates are

?inor plates include

0!The ndian +late,3!The rabian +late,

4!The "aribbean +late, and

B!The (cotia +late.

The movement of plates has caused the formation and breakup

of continents over time, including occasional formation of a

supercontinent that contains most or all of the continents.

The eight continents later re!assembled into another

supercontinent called Pangaea/ Pangaea eventually broke up

into aurasia %which became *orth merica and Eurasia& and

.ondwana %which became the remaining continents&.

)inor plates


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)inor platesThe minor plates are

?inor plates include

0!The ndian +late,3!The rabian +late,

4!The "aribbean +late, and

B!The (cotia +late.


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$orld!wide active volcanes %red circles&, ma0or and minor tectonic

plates and the #ing of )ire.


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otspot

otspots


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otspots

?antle plumes are areas of hot molten rock that rise form deep

within the EarthCs mantle.

reas where the plumes come to the surface are calledhotspots. hotspot is a location on the Earth's surface that has activevolcanism for a long period of time.

9awaii is an example of a hot spot, the plate keeps movingand forming more islands from volcanic eruptions.

otspots are caused by a narrow stream of hot mantle

convecting up from the Earth's core!mantle boundary called a

mantle plume, although some geologists prefer upper!mantleconvection as a cause.

:eologists have identified some BD62D such hotspots around

the globe, with 9awaii, ellowstone, and celand overlying

the most currently active.

t t


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otspots

?ost hotspot volcanoes are basaltic because they erupt


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?ost hotspot volcanoes are basaltic because they erupt

through oceanic lithosphere & e.g.9awaii &

$here hotspots occur under continental crust, basaltic magma

is trapped in the less dense continental crust, which is heated

and melts to form rhyolites.

:eologists use hotspots to help track the movement of the

Earth's plates.(uch hotspots are so active that they often record step!by!step

changes in the direction of the Earth's magnetic poles.

http://en.wikipedia.org/wiki/File:Tectonic_plates_hotspots-en.svg


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$orld map showing the locations of selected prominent hotspots. 0&

7ivergent plate boundaries, 3& Transform plate boundaries, 4&

"onvergent plate boundaries, B& +late boundary zones, 2& (elected

prominent hotspots

Conse"uences of moving plates

http://en.wikipedia.org/wiki/File:Tectonic_plates_hotspots-en.svg


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Conse"uences of moving plates

0!Earthquakes

3!Folcanoes4!?ountain building

B!ceanic trench

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3- Plate Tectonics Presentaion - Assiut Univ