Distribution of oceans and continents

CLASS XI - CBSE - GEOGRAPHY - CHAPTER 4
FUNDAMENTALS OF PHYSICAL GEOGRAPHY

1. DISTRIBUTION OF OCEANS AND CONTINENTS SARAN VARMA XI B

2. iNTRODUCTION The 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.

3. 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 Drift” Study 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.

4. PANGAEA

5. EVIDENCE IN SUPPORT OF THE CONTITNETAL DRIFT Apparent fit of the continents The coastlines of the continents appear to fit together like pieces of puzzle. Fossil Correlation Identical fossils have been found in the either sides of the ocean.

6. EVIDENCE IN SUPPORT OF THE CONTITNETAL DRIFT (CONT.) Rocks & Mountain correlation Identical rocks & mountain structures have been found on either side of the ocean. Paleoclimatic data Coal has been found in cold regions and glacial evidence has been found in warm regions.

7. 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 force Due to the attraction of the moon and the sun that develops tides Sun However, these forces were considered inadequate by most scholars.

8. POST – DRIFT STUDIES A number of discoveries during the post – war period added new information to this. Convectional Current Theory The 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 core’s 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.

9. Convection currents

10. POST – DRIFT STUDIES (cont.) Mapping of the ocean floor Detailed 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.

11. 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 Margins The 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.

12. OCEAN FLOOR CONFIGURATION (cont.) Abyssal Plains Extensive plains that lie between continental margins & mid – oceanic ridges. Mid – Oceanic ridges Interconnected chain of mountain system within the ocean. Has a rift at the centre & is a zone for intense volcanic activity.

13. OCEAN FLOOR CONFIGURATION (cont.)

14. Sea – floor spreading The 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.

15. Evidence for sea – floor spreading The 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.

16. Sea – floor spreading

17. Plate tectonic theory Plate Tectonic theory states that the Earth’s outer mechanical layer, the lithosphere is divided into large continent – sized plates that are constantly moving.

18. PLATE TECTONICS In 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 km Continental → 200 km Maybe referred to as a continental or oceanic plate depending on which occupies the larger portion.

19. 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 boundaries Convergent boundaries Transform boundaries

20. SOME MAJOR PLATES Antarctica & the surrounding oceanic plate North American plate South America Pacific plate India – Australia – New Zealand plate African plate Eurasian plate

21. Some minor plates Cocos plate Nazca plate Arabian plate Philippine plate Caroline plate Fuji plate

22. Plate tectonics

23. 1. Divergent boundaries New 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 valley Ocean → Mid – oceanic ridge. Eg : Mid – Atlantic ridge, African Rift valley

24. 2. Convergent boundaries Where the crust is destroyed as one plate dives under another. The location where it occurs → Subduction zone Three ways: Between an oceanic & continental plate Between two oceanic plates Between two continental plates

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

26. Movement of Indian plate The I ndian plate includes Peninsular India & the Australian continental portions. India was a large island situated off the A ustralian 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) Type Major Approx. Area 11,900,000 k Movement North - East Speed 26 – 36 mm/year Features Himalayas, Indian Ocean Type Major Approx. Area Movement North - East Speed 26 – 36 mm/year Features Himalayas, Indian Ocean

27. Movement of Indian plate

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