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DISTRIBUTION OF OCEANS AND CONTINENTSSARAN VARMAXI B

iNTRODUCTIONThe continents cover 29% of the earths surface and the remainder of it by the oceans.The positions of continents and the ocean bodies have not been the same in the past.

THE CONTINENTAL DRIFT THEORY Many scientists thought of the similarity between the symmetry of the coastlines of the continents & considered the possibility of joint continents.Alfred Wegener a German meteorologist & polar explorer who put forward the theory of Continental DriftStudy of maps led him to suppose that the continents had been once joined. This super continent was named PANGAEA.Subsequently, these broke into various smaller continents that exist today.

PANGAEA

EVIDENCE IN SUPPORT OF THE CONTITNETAL DRIFTApparent fit of the continentsThe coastlines of the continents appear to fit together like pieces of puzzle.Fossil CorrelationIdentical fossils have been found in the either sides of the ocean.

EVIDENCE IN SUPPORT OF THE CONTITNETAL DRIFT (CONT.)

Rocks & Mountain correlationIdentical rocks & mountain structures have been found on either side of the ocean.Paleoclimatic dataCoal has been found in cold regions and glacial evidence has been found in warm regions.

Force for drifting (WEGENER)Wegener suggested these forces responsible for the drifting of continents.Polar fleeing force:Caused by the rotation of the earth.Tidal forceDue to the attraction of the moon and the sun that develops tidesSunHowever, these forces were considered inadequate by most scholars.

POST DRIFT STUDIESA number of discoveries during the post war period added new information to this.Convectional Current TheoryThe core is radioactive and releases heat to the mantle surrounding it.The mantle floats on the core, so the mantle is directly affected by the core. The cores radioactive energy causes the lower mantle to get hot, less dense and rise. The plastic basalt rock rises, cools and it falls back. This is a convection current.

Convection currents

POST DRIFT STUDIES (cont.)Mapping of the ocean floorDetailed research revealed that the ocean floor is full of relief.Expeditions to map the oceanic floor provided a detailed picture such as the existence of submerged mountain ranges & deep sea trenches.

OCEAN FLOOR CONFIGURATION

The ocean floor may be segmented into three major divisions based on the depth as well as the forms of relief.Continental MarginsThe continental margin is the zone of the ocean floor that separates the thin oceanic crust from thick continental crust. Together, the continental shelf, continental slope, and continental rise are called the continental margin.

OCEAN FLOOR CONFIGURATION (cont.)

Abyssal PlainsExtensive plains that lie between continental margins & mid oceanic ridges.Mid Oceanic ridgesInterconnected chain of mountain system within the ocean.Has a rift at the centre & is a zone for intense volcanic activity.

OCEAN FLOOR CONFIGURATION (cont.)

Sea floor spreadingThe concept was first proposed by Harry Hess in the 1960s.Sea floor spreading occurs when:Magma rises to the surface & pushing the existing sea floor away from the ridges.The colder denser materials sink pulling the sea floor away from the ocean ridge.This way, new oceanic crust is created. The plate then, when subducts, this crust is destroyed where it melts & comes out of volcanoes or is recycled back to a new oceanic crust.

Evidence for sea floor spreadingThe rocks equidistant on either side of the mid oceanic ridges show remarkable similarities.The age of the rock increases as one moves away from the crest.The ocean crust rocks are much younger than the continental rocks.The sediments on the ocean floor are unexpectedly very thin.The deep trenches have deep seated earthquake occurrences while in the mid oceanic ridge areas, the quake foci have shallow depths.

Sea floor spreading

Plate tectonic theoryPlate Tectonic theory states that the Earths outer mechanical layer, the lithosphere is divided into large continent sized plates that are constantly moving.

PLATE TECTONICSIn 1967, McKenzie & Parker came up with this idea.Tectonic plate a massive, irregularly shaped rock.Generally composed of both continental & oceanic lithosphere.Floats on the asthenosphere.Thickness range varies from:Oceanic 5 100 kmContinental 200 kmMaybe referred to as a continental or oceanic plate depending on which occupies the larger portion.

Plate tectonics (cont.)These plates constantly move. Continents are a part of the plate.1 2 inches per year!There are three types of plate boundaries:Divergent boundariesConvergent boundariesTransform boundaries

SOME MAJOR PLATESAntarctica & the surrounding oceanic plateNorth American plateSouth AmericaPacific plateIndia Australia New Zealand plateAfrican plateEurasian plate

Some minor platesCocos plateNazca plateArabian platePhilippine plateCaroline plateFuji plate

Plate tectonics

1. Divergent boundariesNew crust is generated as plates pull away from each other.The site where the plates move away from each other is called spreading sites.Continent Rift valleyOcean Mid oceanic ridge.Eg: Mid Atlantic ridge, African Rift valley

2. Convergent boundariesWhere the crust is destroyed as one plate dives under another.The location where it occurs Subduction zoneThree ways:Between an oceanic & continental plateBetween two oceanic platesBetween two continental plates

3. Transform boundariesA transform boundary occurs where two plates grind past each other without destroying or reproducing lithosphere.Earthquakes sometimes occur near transform faults.

Movement of Indian plateThe Indian plate includes Peninsular India & the Australian continental portions.India was a large island situated off the Australian coast.India is supposed to have started her journey some 200 mya.India collided with Asia about 40 50 mya causing rapid upliftment of land (Himalayas)

Movement of Indian plate

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