PLATE TECTONICS

Dr.D.Panda
• The concept of plate tectonics states that the outer solid lithosphere is made up of several
individual plates.

• These plates move in relation to one another upon a weaker zone in the upper mantle known as
the asthenosphere.

• The lowest part of the plates comprising the cooler solid part of the mantle.

• The Moho discontinuity marks the upper contact of this layer with the crust.

• The mobility of the plates are due to the plastic behaviour of the asthenosphere by the greater

• temperarure and pressure existed at greater depths.

• Larger plates with the exception of the Pacific plate,are composed of both continental and
oceanic crust.

• Under the oceans this crustal layer may be only a few kilometers thick capped by lava flows and
solidified magma extruded along the mid oceanic ridges,rises and other volcanic centers.

• Under the continents the crustal layer may be 30km. or more thick

• The lower crust may include a continuation of oceanic gabbroic layer.

• The upper crust is sialic includes crystalline granitic masses ,the roots of mountains covered by a
relatively thin layer of sedimentary and metamorphic rocks at the continental surface.

• In the oceanic regions the sediments of the sea floor consisting of inorganic debris plus the
remains of organisms that lived in the upper portion of the ocean are accumulated.

• The sedimentary layer is thin or non existent adjacent to the spreading centers due to the
outward movement of the crust away from the spreading center.

• The oldest crust receives sediments for periods of time longer than the younger crust.

• Each plate moves as a distinct unit.

• All major changes occur along plate boundaries.

• The origin,movement and interrelation of these plates can be associated with the sea floor
spreading ,eathquake belts,crustal movements and continental drift.
 • The world system of plates

• The whole earth’s surface is consisting of at least six major plates and a host of minor plates.

• The major plates are

• the Indian plate

• the Pacific plate

• the Antarctic plate

• the American plate

• the African plate

• the Eurasian plate

• The boundaries of these plates are the location of earthquakes and volcanic activities.

• Shallow quakes are along the oceanic ridge.

• Both shallow and deep earthquakes are found along arcs and trenches.

• The Pacific plate on its southern and south eastern side is bounded by an oceanic rise that
under goes active spreading or sea floor spreading.

• On the northern and north western side of the Pacific plate are the island arcs ,deep sea
trenches ,tectonic and volcanic activity.

• Its northern side along the western United States is marked by the San Andreas fault ,where
the Pacific plate moves in north west ward and hits the American plate.

• Classification of Plate boundaries

• The first plate boundaries are made on the basis of earthquake and volcanic activity.

• Later on the basis of motion of these lithospheric plates with respect to other adjacent plates
results in three principal kinds of boundaries.

( i) Divergent boundaries

( ii)Convergent boundaries

(iii ) Transform boundaries

( i) Divergent boundaries (Constructive)

• Along the divergent boundaries plates move apart,leaving a gap between them.
 • It occurs, where plate spreading occurs are situated at the mid oceanic ridges.

• As the plates separate ,the gap created is quickly filled with molten rock that oozes up from the
hot asthenosphere.

• These materials slowly cool to produce a new sea floor.

• Successive separation and fillings continue to add new oceanic crust between diverging plates.

• The mechanism which has produced the floor of the Atlantic ocean during the past 200million
years is called sea floor spreading.

• The rate of sea floor has been estimated to be 5 cms per year.

• It occurs more than ten times during the 5000million years history of our planet.

• The actual break up of a continent is thought to be initiated by upwelling of the hot rock from
below.

• The crustal stretching associated with the up warping generates numerous cracks.

• As the hot rock spreads laterally away from the region of upwelling ,the broken lithosphere is
pulled apart.

• Many of the broken slabs slide downward in to the gaps created by the separating plates,results
volcanic activity.

• The large down faulted valleys created by the process are called rift or rift valleys.

• As the spreading continues the rift valley will lengthen and deepen extending out to the ocean.

• It becomes zone of volcanic activity and would develop in to a new oceanic ridge.

• As the newly formed lithosphere travels away from the spreading center,this effect result
increase in ocean depth that exists away from the ridge.

• Divergent boundary results shallow focus earthquakes and volcanoes along the ridge crests of
ocean-ocean region and along the rift valleys of continent – continent zones.

( ii)Convergent boundaries(Destructive)

• Along the convergent boundaries the plates move towards each other.

• Causing one plate to go under the other as happens with the oceanic crust, where plates colloid
each other when leading edges are made of continental crust.

• The zone of plate convergence is the site of plate destruction to compensate the new
lithosphere as the total surface area of the earth remains constant.
• When a section of the continental crust converges with oceanic crust the less dense continental
material remains floating ,while more dense oceanic crust sink in to the asthenosphere.

• The sinking oceanic crust entering in to the hot region, the solid plate and any sediments carried
with it begun to melt.

• The plate melts in to molten magma which moves upward upon reaching the surface gives rise
to volcanic eruptions.

• The region where an oceanic plate is sliding in to the asthenosphere because of convergence is
called a subduction zone or benioff zone.

• As the oceanic plate is thrust beneath the overriding plate,it bends and produces a linear
oceanic trench adjacent to the zone of subduction.

• Some trenches like the Japanese and the Aleutian are hundreds of kilometers long 8-10 kms
deep.

• The volcanic Andes mountains of South America are believed to have been produced

• by such activity when the Nazca plate melted as it plunged beneath the continent of South
America.

• The numerous earthquakes generated within the Andes indicate the activity below it.

• When two oceanic plates converge and one is subducted ,volcanic activity is also initiated on
the ocean floor.

• The continuity of the volcanic activity emerges new land from the ocean depths.Japan is an
example is called an island arc.

• Island arcs are located adjacent to an ocean trench where active subduction and melting of the
lithosphere are taking place.

• In the early stage of its development an island arc is a linear belt of numerous small volcanic
islands .

• The Alutian,Mariana and Tonga islands are such features.

• The convergent plate boundaries results shallow ,intermediate and deepfocus earthquakes
along ocean-ocean contacts.

• The continental plates when move together ,they colloid rather than slide one beneath the
other,because of lighter composition and greater buoyancy of continental crust.

• During the collision the continental crust buckled ,fractured and generally shortened.
• The collision was accompanied by volcanic activity which welded together the once separated
land masses.

• When continental plates meet other continental plates young mountain ranges result from the
piling up or doubling of the higher continental masses.

• Volcanoes are rare or nonexistent ,but shallow focus quakes are common ,intermediate focus
earth quakes are rare and deep focus quakes are entirely absent.

• Such a collision occurred when the separated continent of India”rammed” in to Asia,producing

• ( iii) Transform boundaries(Conservation)

• Along the transform boundary plates slide paste each other ,scrapping and deforming as they
pass.

• The transform boundaries are located ,where plates slip past each other without production or
destruction of the crust along the faults.

• These faults form the direction of the plate movement and first discovered in the oceanic
ridges.

• Continent-continent interactions are characterized by fault zones and shallow focus earthquakes
over a broad zone.

• Fracture zones with shallow focus earthquakes in narrow belts make ocean-ocean transform
boundaries.

• Little volcanic activity occurs along any transform boundaries.

• Most transform faults are located in oceanic crust ,a few are situated within continents.

• The San Andres fault of California is a transform fault within the continent.

• Along this fault the Pacific plat is moving towards the north west ,paste the North American
plate.

• If this movement continues for millions of years that part of California west of the fault zone will
become an island off the west coast of the United States and Canada and could eventually reach
Alaska.

• Each plate is bounded by a combination of these zones.

• Movement along one boundary requires adjustment to be made at another boundary.

• The plates may be entirely dense lithosphere or may include the lighter rafts of continental
lithosphere.
• Movement of these plates may involve ocean-ocean,ocean-continent or continent-continent
interaction.

• Delineation of the lithospheric plates

• The mountain ranges mark boundaries ,where one or both plates consist of continental
lithosphere.

• The short boundaries are called transforms ,which are vertical faults along which plates move
past each other horizontally.

• Many transforms in the ocean are marked by scarp or deep gash like valleys.

• Transforms also determine the direction of plate movement ,because that direction must be
parallel to the trace of the transform on a map.

• The spreading rate increases away from the pinion.

• In the case of the Atlantic from about 0.3 inch per year near Iceland to nearly 2 inches per year
near the equator.

• Cooling and heating of plates

• The lithosphere is warmest at rises and volcanic chains as heating at rises is caused by the
intrusion of basaltic melt from the underlying mantle and by the recent crystallization of the
oceanic crust on both sides of the rise from melts .

• The lithosphere slowly cools as it moves away from the away from the ridge which results shrink
causing a reduction in volumes that accounts for the broad shape of the rise.

• The cool lithosphere slowly heated again as it sinks in subduction zones.

• The cooling may transform the asthenosphere to rigid lithosphere.

• Movement of plates

• Plate tectonics describe plate motion and the effects of this motion rather than its cause.

• The basic cause of plate motion is local heating deep in the mantle perhaps the heat escaping
from the core or by decay of radioactive elements.

• The heating expands and lightens the rocks,they are displaced upward by gravity flowing slowly
in the solid state.

• Divergent of the current near the top of the mantle pulls the lithosphere apart and melted hot
mantle flow in to the cracks formed.
• Lithospheric plates are carried laterally until the mantle cools,becomes denser and sinks,thus
dragging the plates downward.

• Circulation is completed by the mantle materials flowing laterally at depth,where they are
slowly heated and eventually replace the material rising toward the surface.

• Criticism-There is no upward thrust by the rising column.

• Measurement of gravity gives no suggestion that an excess material has been pressed to the
surface.

• The slightly arched shaped of the rise are explained by cooling of the lithosphere away from
the axes of the rise.

• Convection cells

• The convection current moving in the cells provide a mechanism for moving the ocean floor.

• The ocean ridges lay over a rising convection current.

• The heat flowing from the interior is higher at the ridge crest than any other places.

• The current is thought to lie in the upper mantle ,well below the Moho and represent ascending
limbs of adjacent convection cells.

• At some point below the Moho ,the rising current is pictured as splitting in to two currents
flowing laterally away from the mid oceanic ridge.

• Plum theory

• Movement of the plates suggested by Jason Morgan.

• He infers the existence of hot spots or plums of hot rising solid mantle in zones of few hundred
kilometers in diameter.

• The hot mantle rock rises and spreads out laterally in the asthenosphere in about 20 major
thermal plums around the world creating hot spots of volcanic activity.

• The rest of the mantle undergoes very slow downward movement to balance the upward flow in
the plums.

• The movement of any plate is caused by the combination of forces exerted by the upward flow
in the plume,the outward radial flow away from the plume and the return downward flow of
the mantle.

• Recycling the sea floor

• Obduction

• Some of the subducted plate may be broken off and pushed upon to the overriding plate
during the process of plate collision.

• The subducted plate may have igneous rocks and deep sea sediments attached to it .

• Formerly deep oceanic rocks can be raised in mountain masses exposed at the earth’s
surface.

• This process is called obduction.

• Reaction in the asthenosphere and lithosphere

• The main driving force is the eight of the sinking lithospheric plate,which has become heavy
because of cooling and contraction and formation of dense metamorphic minerals in it as it
sinks.

• The sinking part of the slab thus drags the rest of the plate behind it causing a simple pulling
apart at the rise.

• The upper part of the asthenosphere is dragged somewhat by the moving plates .

• It provides a decoupling between the rapid movement of the plates and the slow return in the
deeper part of the asthenosphere.

• Seismicity and earthquakes

• Distribution of subduction zones with respect to latitudes suggest another cause of plate
movements.

• The deeper sources of the earthquakes are concentrated within 30° of the equator and shallow
sources are scarce more than 60° from the equator.

• The subduction zones are concentrated within 30° of the equator and are absent at mre than
60° from the equator.

• The correlation between subduction and latitude suggests that plate movements are connected
with the earth’s rotation and the gravitatioal pull of the moon.

• These forces are called earth tides ,broad buldge on the solid earth of few inches height at the
equator and decrease symmetrically towards the poles.

• The plates shift westward near the equator.

• This is a mechanism that involves an active spreading of the sea floor outward away from the
crests of the main ocean ridges.

• As the materials move from the ridge,new materials replace it along the ridge crest by welling
upwards from the mantle.

• As the mantle materials cool,the magnetization of the earth’s field takes place.

• The new materials deposited along the ridge by the continued pulling apart of the crust and its
movement laterally away from the ridge.

• The change in the earth’s polarity is recorded by the added materials along the crest, spreading
laterally .

• T carries this record to be followed at a later time by the next polarity change.

• The rate at which the new sea floor forms at spreading centers varies.

• In the northern Atlantic ocean it is about 1cm/year in the south Atlantic it is about 2.3 cm/year
in the east Pacific rise it is about 4.6 cm/year reaching a rate of 9 cm/year in some sections.

• So the continents on either side of the mid oceanic ridge moves twice.

• Sea floor spreading indicates in terms of continental drift,thatexpansion of the ocean basins may
also affect the location and position of continents.

• The width of the magnetic strips vary according to the spreading rate.

• It becomes wide where the spreading rate is rapid as in the equatorial Pacific.

• The spreading rate is 1 to 1.5 cm/year in the Atlantic and Indian ocean and the most rapid 6
cm/year on the Pacific rise.

• If there were no sea floor spreading ,the layers of sediments would simply show alternating
polarity as the earth’s magnetic field change.

• With sea floor spreading each sedimentary layer begins at a ridge with the magnetization of that
particular time in the earth’s history.

• As the lithosphere migrates away from this spreading centre,the sediments are carried farther
and farther away.

• Sea floor spreading indicates that the ocean floor is a dynamic system.
• In terms of the continental drift ,it suggests that expansion of ocean basin may also afect the
location and position of continents.

• The sea floor spreading explains geologically young basin crust.

• The mid Atlantic ridge extends from the south Atlantic,turns east and enter the Indian ocean.

• Then one branch penetrates Africa while the other continues east between Australa and
Antarctica then swing across the south Pacific nearing south America,it turns north and
penetrates North America at the hed of the Gulf of California.

• The aerial rift of the mid-oceanic ridge is the origin of numerous earthquakes indicating
continued tectonic activity.

• The axial rift is broken across at right angles by numerous fractures a typicla fault.

• The sea floor spreading which produces the floor of the Atlantic ocean during the past
200million years,the slow rate is rapid enough to open and reclosed the Atlantic ocean more
than 10 times during the 5000 million years history of our planet.

• All spreading centers are not ancient.

• The red sea is thought to be the sea of the recently formed spreading centre.

• The Arabian peninsula separated from Africa and moved towards the north-west.

• The actual break up of a continent is thought to be initiated by upwelling crust in the region
directly above active zone.

• The crustal stretching associated with the upwarping generates numerous cracks.

• As the hot rock spreads laterally away from the region of upwelling the broken lithosphere is
pulled apart.

• Many of the broken slabs slide down ward in to the gaps created by the separating plates.

• During this phase of the continental breaker volcanic activity will be prevalent.

• The large down faulted valleys that are created by the process are called rift valleys or
rift.Ex.the Great Rift Valley of East Africa.

• As the spreading process continues,the rift valleys become elongated and deepen extending out
in to the ocean.

• The valley will become a linear sea with an outlet in to the ocean.

• The location of active spreading centre,the md oceanic ridges are characterised by an elevated
position and numerous volcanic structures.
• As the newly formed lithosphere travels away from the spreading center,it gradually cools and
contracts,which results increase in ocean depth away from the ridge.

• Mid-oceanic ridge

• i) One of the significant developments in earth sciences since world war-II has been the
recognition of continuous submerged ridge system over a total disatnce of about 60,000km.

• 2)The features frequently positioned asymmetrically within the ocean basin and its numerous
dimensions are scarcely converged by the world ridge.

• 3)The most distinctive topography is to be found near the ridge crest.

• Here the relief is at its most rugged with abrupt falls 1000m or more from the culminating peaks
to the highest of the flanking plateaus.

• The culminating peaks themselves are often cleft by a deep central fossa which may be as much
as 2000m deep and 25 to 50 km wide.

• Such an aerial trench is usually well developed in the Atlantic and Indian oceans,but much less
obvious in the Pacific where ridge relief is much subdued.