Plate Tectonics • In 1967, McKenzie and Parker suggested the theory of plate tectonics. • The theory was later outlined by Morgan in 1968. • By then, the ‘continental drift theory’ was completely discarded with the emergence of ‘convectional current theory’ and ‘see floor spreading theory’. • Both ‘convectional current theory’ and ‘see floor spreading’ paved the way for the Theory of Plate Tectonics Theory • According to the theory of plate tectonics, • the earth’s lithosphere is broken into distinct plates which are floating on a ductile layer called asthenosphere (upper mantle). • Plates move horizontally over the asthenosphere as rigid units. • The oceanic plates contain mainly the Simatic crust and are relatively thinner, while the continental plates contain Sialic material and are relatively thicker. • Lithospheric plates (sometimes called crustal plates, tectonic plates) vary from minor plates to major plates, • continental plates (Arabian plate) to oceanic plates (Pacific plate), • sometime a combination of both continental and oceanic plates (Indo-Australian plate). • The movement of these crustal plates causes the formation of various landforms and is the principal cause of all earth movements. Rates of Plate Movement • The Arctic Ridge has the slowest rate (less than 2.5 cm/yr), and • the East Pacific Rise in the South Pacific [about 3,400 km west of Chile], has the fastest rate (more than 15 cm/yr). • Indian plate’s movement during its journey from south to equator was one of the fastest plate movements Major tectonic plates • Antarctica and the surrounding oceanic plate • North American plate • South American plate • Pacific plate • India-Australia-New Zealand plate • Africa with the eastern Atlantic floor plate • Eurasia and the adjacent oceanic plate Force for the Plate Movement • The slow movement of hot, softened mantle that lies below the rigid plates is the driving force behind the plate movement. • The heated material rises to the surface, spreads and begins to cool, and then sinks back into deeper depths Interaction of Plates • These plates will interacted in three ways:- • divergent • convergent • transcurrent Divergent Edge or the Constructive Edge • As the name itself suggests, in this kind of interaction, the plates diverge [move away from each other]. • Mid-oceanic ridges are formed due to this kind of interaction. Here, the basaltic magma erupts and moves apart. • On continents, East African Rift Valley is the most important geomorphological feature formed due to divergence of African and Somali plates. • Such edges are sites of earth crust formation (hence constructive) and volcanic earth forms are common along such edges. • Earthquakes (shallow focus) are common along divergent edges. • The sites where the plates move away from each other are called spreading sites. • The best-known example of divergent boundaries is the Mid-Atlantic Ridge. • At the mid-oceanic ridge in Atlantic ocean, the American Plate(s) is/are separated from the Eurasian and African Plates. Convergent Edge or Destructive Edge • In this kind of interaction, two lithospheric plates collide against each other • The zone of collision may undergo crumpling and folding and folded mountains may emerge. • This is an orogenic collision. Himalayan Boundary Fault is one such example. • When one of the plates is an oceanic plate, it gets embedded in the softer asthenosphere of the continental plate and as a result, trenches are formed at the zone of subduction • The subducted material gets heated, up and is thrown out forming volcanic islands and dynamic equilibrium is achieved • There are mainly three ways in which convergence can occur. • between an oceanic and continental plate; • between two oceanic plates; and • between two continental plates. Transcurrent Edge • Formed when two plates move past each other. • In this kind of interaction, two plates grind against each other and there is no creation or destruction of landform but only deformation of the existing landform. • Crust is neither produced nor destroyed as the plates slide horizontally past each other. • San Andreas Fault along the western coast of USA is the best example for a transcurrent edge on continents. Evidence in Support of Plate Tectonics • Evidences for both See Floor Spreading and Plate tectonics are complimentary (almost same evidences). • Older rocks form the continents while younger rocks are present on the ocean floor. • As we move, towards ridges, still younger rocks appear. • The normal temperature gradient on the sea floor is 9.4°C/300 m but near the ridges it becomes higher, indicating an upwelling of magmatic material from the mantle. • In trenches, where subduction has taken place (convergent edge), the value of gravitational constant ‘g’ is less. This indicates a loss of material. • The fact that all plate boundary regions are areas of earthquake and volcanic disturbances goes to prove the theory of plate tectonics. Significance of Plate Tectonics • On the basis of present knowledge of crustal plate movement, the shape of landmasses in future can be guessed. • For instance, if the present trends continue, North and South America will separate. • A piece of land will separate from the east coast of Africa. • Australia will move closer to Asia. Movement Of The Indian Plate • India was a large island situated off the Australian coast, in a vast ocean. • The Tethys Sea separated it from the Asian continent till about 225 million years ago. • India is supposed to have started her northward journey about 200 million years ago at the time when Pangaea broke. • India collided with Asia about 40-50 million years ago causing rapid uplift of the Himalayas. • During the movement of the Indian plate towards the Asiatic plate • A major event that occurred was the outpouring of lava and formation of the Deccan Traps. • This started somewhere around 60 million years ago and continued for a long period of time. • Scientists believe that the process is still continuing and the height of the Himalayas is rising even to this date. Ocean – Ocean Convergence • In Ocean – Ocean Convergence, a denser oceanic plate subducts below a less denser oceanic plate forming a trench along the boundary. • As the ocean floor crust (oceanic plate) loaded with sediments subducts into the softer asthenosphere • After reaching a depth of about 100 km, plates melt. • Magma (metamorphosed sediments and the melted part of the subducting plate) has lower density and is at high pressure. • It rises upwards due to the buoyant force offered by surrounding denser medium. • The magma flows out, sometimes violently to the surface. • A continuous upward movement of magma creates constant volcanic eruptions at the ocean floor. • Constant volcanism above the subduction zone creates layers of rocks. • As this process continues for millions of years, a volcanic landform is created which in some cases rises above the ocean waters. • Such volcanic landforms all along the boundary form a chain of volcanic islands which are collectively called as Island Arcs • (Indonesian Island Arc or Indonesian Archipelago, Philippine Island Arc, Japanese Island Arc etc.). • Orogenesis sets in motion the process of building continental crust by replacing oceanic crust • this happens at a much later stage. • For example, new islands are born around Japan in every few years. • After some million years Japan will be a single landmass because continental crust formation is constantly replacing the oceanic crust • Philippine Island Arc system is formed due to subduction of Philippine Sea plate under Sunda Plate (part of Eurasian Plate). • The trench formed here is called Philippine Trench • In case of Indonesian Archipelago, Indo-Australian plate subducts below Sunda Plate (part of Eurasian Plate). • The trench formed here is called Sunda trench (Java Trench is a major section of Sunda trench). • For Caribbean islands North American Plate subducts under the Caribbean plate and forms the Puerto Rico Trench. Formation of Japanese Island Arc • Northern arc is formed due to the subduction of the Pacific Plate under the Eurasian Plate. The trench formed is Japan Trench. • Central arc is formed due to the subduction of the Pacific Plate under the Philippine Plate The trench formed is Izu Trench. • Southern Arc is formed due to the subduction of the Philippine Plate under the Eurasian Plate. The trench formed is Ryukyu Trench. Continent – Ocean Convergence • Continent – Ocean Convergence is similar to ocean – ocean convergence. • One important difference is that in continent – ocean convergence mountains are formed instead of islands • When oceanic and continental plates collide or converge, the oceanic plate (denser plate) subducts or plunges below the continental plate (less denser plate) forming a trench along the boundary. • The trenches formed here are not as deep as those formed in ocean – ocean convergence. • After reaching a certain depth, plates melt. Magma (metamorphosed sediments and the melted part of the subducting plate) has lower density and is at high pressure. • It rises upwards due to the buoyant force offered by surrounding denser medium. • The magma flows out, sometimes violently to the surface. • A continuous upward movement of magma creates constant volcanic eruptions at the surface of the continental plate along the margin. • Such volcanic eruptions all along the boundary form a chain of volcanic mountains which are collectively called as continental arc. • The Andes are formed due to convergence between Nazca plate (oceanic plate) and the South American plate (continental plate). • Peru – Chile trench is formed due to subduction of Nazca plate. • The folding process in Andes is still continuing and the mountains are constantly rising. • Volcanism is still active. • The North American plate (continental plate) moved west wards while the Juan de Fuca • plate (minor oceanic plate) and the Pacific plate (major oceanic plate) moved eastwards. The convergence gave rise to a series of parallel mountain ranges. • Unlike the Andes, the Rockies are formed at a distance from the continental margin due to the less steep subduction by the oceanic plates.