PLATE TECTONIC THEORY
PLATE TECTONIC THEORY
INTRODUCTION
Plate tectonic theory was a great scientific achievement of the decade of 1960s , was based on two major concepts
The concept of continental drift theory
the concept of sea floor spreading.
The rigid lithospheric slab tectonically called plates. It may be highlighted that tectonically plates are important because all tectonical activities such as seismic events , vulcanicity , mountain building and faulting.
NATURE OF MOVEMENT OF PLATES
- Getting information from GPS ,
- PACIFIC ocean moves 10sm per year in westward direction
- European plate moves 1 -1.5 cm per year in north and north-west direction
- American plates moves 2-3 cm per year in westward direction
- African plates moves slowly in accordance with others
Evaluation of Plate tectonic theory
It is stated that the term “ Plate” was used in MAGAZIN Named “ NATURE” by Canadian geophysicist J.T.WILSON in 1965.
Mckenzie and Parker discussed in detail the mechanism of plate motions on the basis of Euler’s geometrical theorem in 1967.they postulated “A PAVING STONE” hypothesis wherein new oceanic crust was considered to be newly formed .
ISACKS and SYKES confirmed the “ THE PAVING STONE” hypothesis in 1967.
W.J.MORGAN AND LE PICHON elaborated the various aspects of plate tectonic theory in 1968
Concept of Plate and main features of PLATES
The rigid lithospheric slab tectonically called plates
The entire mechanism of the evolution , nature and motion of the plates and resultant reaction is called plate tectonics .
The entire process of plate motion is called plate tectonic.
According to A.N Strahler and A.H.Strahler in 1978 “ Moving over the weak asthenosphere , individual lithospheric plates glide slowly over the surface of the globe ,much as a pack of ice of the arctic ocean drifts under the dragging force of current and winds”
MAIN FEATURES
Plates are consisted of continental crust and oceanic crust.
Plates movement are always dynamic in nature.
the size of the plates are varies i.e small , medium and large.
TYPE OF PLATES
Seven majors and eight medium and 20 minors plates have been identified so far.
MEDIUM PLATES
MINOR PLATES
Minors plates are Panama plate,somali plates, adriatic plate, anatolia plate, madagascar plate, iranian plate,burma plate,tonga palte,galapagos plate,greenland plate , srilanka plate, fiji plate etc.
CAUSES OF PLATE TECTONIC
- GRAVITATIONAL FORCE
- CONVENTIONAL FORCE
- UPWELLING OF MAGMA
- HOTSPOT
- SUBDUCTION ZONE
CONCEPT OF PLATE MARGINS AND PLATE BOUNDARIES
PLATE MARGINS
Plate Margins are the marginal part of a particular plate where as Plate boundary lies between two plates .it represents “surface trace of the zone of motion, It is very important to highlighted that all techtonically plate boundaries or margins are most significant because all tectonically activities occurs along the plate margin such as seismic events, vulcanicity, mountainbuilding and faulting etc.
TYPE OF PLATE MARGINS
Generally Plate Margin are divided into three type
Constructive Plate Margin
Destructive Plate Margin
Transform fault Margin
Divergent Boundaries
Another name of divergent boundaries are constructive boundaries , accereting plate boundaries
two plates move in the opposite direction
there is continous upwelling of magma and molten materials.
new oceanic crust is formed due to upwelling of magma
due to movement of opposite direction , plates splits apart along the mid oceanic ridges .
they are also called contructive boundary because of continous upwelling of molten magma , after cooling and solidifying of lava new oceanic crust formed
Divergent movement of plates results in seismic events , vulcanic activities, new oceanic crust form, transform fault, and drifting of oceanic plates.
LAND FORMS ON DIVERGENT BOUNDARY
Beneath the ocean , when two plates split apart , it forms mid Atlantic ridge.
MID ATLANTIC RIDGE
when two plates moves in opposites direction , the hot , molten magma moves upward, forming a new oceanic crust. This process is going on in continuous way and spreading the sea floor.
For example, the north american plate moves in west direction and eurasian plate moves in east direction spilit apart on the other hand the south america and africa moves in opposite direction thus the mid atlantic ridge forms.
Horst and Graben
Divergent boundaries can also develop on land, as did those that broke up Pangaea about 200 million years ago.
Horst and Graben
Horst and Graben (valley and range) refers to a type of topography created when the earth's crust is pulled apart. This process, called extension, can stretch the crust up to 100% of its original size. As the crust is strained in this way, normal faults develop and blocks of the crust drop down to form grabens, or valleys
Fault-block Mountains
When divergent plate motion occurs beneath a continental crust, rift structures and normal fault form. This continental rifting causes valleys floors to drop down along fault lines. The combination of down dropped basins and adjacent fault-block mountains can produce dramaic range fronts.
Convergent boundary
Convergent boundary
convergent boundary is also termed as destructive boundary, consuming plate margin
When two plates move towards each other or converge along a line and collide wherein leading edge of one plate which is relatively formed in lighter materials overrides the heavier plate and the lighter plate is lost in mantle.
Another name of convergent boundary is destructive boundary because when lighter plate overrides the heavier plates, the less dense plates lost in hot molten magma.
The zone where lighter plate overrides the heavier plate is called as “ COLLISION ZONE OR BENIOFF ZONE OR SUBDUCTION ZONE”
lighter plate overrides the heavier plates Results in volcanic eruption , formation of fold , earthquake etc.
TYPE OF CONVERGENT PLATE BOUDARY
CONVERGENT BOUNDARY AND ASSOCIATED LANDFORM
The convergent boundary are of three type viz (i) ocean – ocean collision
(ii) Continents – continents collision (iii) ocean – continents collision
ocean – ocean collision:
it involves collision two convergent plates having oceanic crust where one oceanic crust having relatively denser materials is subducted into upper mantle.
Such collision and techtonical expression of plate collision and subduction includes deformation in crustal area, vulcanism , metamorphism, formation of oceanic trenches and island arc are formed.
Examples of ocean-ocean convergent zones are subduction of the Pacific Plate south of Alaska (creating the Aleutian Islands) and under the Philippine Plate, where it creates the Marianas Trench, the deepest part of the ocean.
Ocean – Continent collision
It involves collision of one oceanic plate having oceanic crust and other continental plate having continental crust along Benioff zone.
The resultant tectonic expressions are deformation of crustal rocks , metamorphism , volcanic eruption , folded mountain etc.
Example : collision of American plate and pacific plates is typical example of ocean – continental convergent plate boundary. Another Examples of ocean-continent convergent boundaries are subduction of the Nazca Plate under South America (which has created the Andes Mountains and the Peru Trench) and subduction of the Juan de Fuca Plate under North America (creating the Cascade Range).
CONTINENTAL – CONTINENTAL COLLISION
It involves collision of two continental plates along Benioff zone and is responsible for creation of folded mountain and occurrence of earthquake in varying magnitudes
Example : The Collision of Asiatic – Indian plate and European – African plate .
The collision between the Eurasian Plate and the Indian Plate that is forming the Himalayas.
creating the Himalaya Mountains, and the collision of the African Plate with the Eurasian Plate, creating the series of ranges extending from the Alps in Europe to the Zagros Mountains in Iran. The Rocky Mountains in North America are also a result of continent-continent collisions.
CONSERVATIVE PLATE MARGIN
CONSERVATIVE PLATE MARGIN
Two plate pass or slide each other along transform faults. These are called conservative boundary
Another name of these are shear plate margin, parallel or transform fault boundary
the significant techtonic expression of such situation is transform faults which moves parallel to the direction of plate motion.
in mid oceanic ridge oceanic transform faults is found
the best example of continental transform faults is san andres fault , carlifornia, USA
No vulcanic events, seismics events and creation of ridge and valley , fracture zone occurs there.
Triple junctions
Triple junctions
All of the plate boundaries discussed so far have involved junctions between two plates. However, there are some localities where three plates are in contact, and these are termed triple junctions. Triple junctions between three ocean ridges, such as that in the South Atlantic between the African, South American and Antarctic Plates, are known as ridge-ridge-ridge, or RRR triple junctions.
A similar notation can be used to identify triple junctions involving ocean trenches (T) or transform faults (F). For instance, a ridge-ridge-trench junction would be termed an RRT triple junction.
Some of these, such as RRR junctions, are termed stable triple junctions, which means they maintain their form over time.
However, some can only exist briefly before they evolve into another plate configuration and these are termed unstable triple junctions.
Seven types of triple junction exist in the present plate tectonic configuration. These are:
RRR (e.g. in the South Atlantic, the Indian Ocean and west of the Galapagos Islands in the Pacific)
TTT (e.g. central Japan)
TTF (e.g. off the coast of Chile)
TTR (e.g. off Moresby Island, western North America)
FFR, FFT (e.g. junction of the San Andreas Fault and the Mendocino Transform Fault off western USA)
RTF (e.g. southern end of the Gulf of California).
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