Chapter 4 Distribution of Oceans and Continents | class 11th | ncert quick revision notes geography

Notes of Ch 4 Distribution of Oceans and Continents| Class 11th Geography

Topics in the Chapter

• Continental Drift
• Evidences to support continental drift
• Forces of drifting
• Post drift studies
• Ocean floor configuration
• Distribution of volcanoes and earthquakes
• Concept of sea floor spreading
• Plate tectonics
• Major and minor plates
• Types of plate boundaries rates of plate movement
• Forces of plate movement & movement of the Indian plate

Continental Drift

• Continental drift was a theory that explained how continents shift position on Earth’s surface. Abraham Ortelius, a Dutch map maker first proposed such a possibility as early as 1596.

• Antonio Pellegrini drew a map showing – Americas, Europe and Africa – the three continents together.

• Alfred Wegener a German meteorologist put forth The Continental Drift Theory. According to him, all continents formed a single continental mass called PANGAEA.

• All oceans formed a single universal ocean called PANTHALASSA. Around 200 mya THE PANGAEA began to split into two large masses called LAURASIA and GONDWANA LAND.

→ By further splitting Laurasia formed northern continents and Gondwana land formed southern continents.

Evidences to support the Continental Drift

The matching of continents (jig-saw fit)

• The shorelines of S. America and Africa have remarkable match.It was a map that produced by Bullard in 1964 to show the jigsaw fit of these two continents.

• It was fit around 1000 fathom line of the shoreline.

• The Atlantic coasts of Africa and South America appear to fit together neatly, like the pieces of a jigsaw puzzle.

Rocks of Same Age Across the Oceans

• The radiometric dating methods developed in the recent period have facilitated correlating the rock formation from different continents across the vast ocean.

• The belt of ancient rocks of 2,000 million years from Brazil coast matches with those from western Africa.

• The earliest marine deposits along the coastline of South America and Africa are of the Jurassic age. This suggests that the ocean did not exist prior to that time.

Tillite

• It is the sedimentary rock formed out of deposits of glaciers.

• The Gondawana system of sediments from India is known to have its counter parts in six different landmasses of the Southern Hemisphere.

• At the base the system has thick tillite indicating extensive and prolonged glaciation.

• Counter parts of this succession are found in Africa, Falkland Island, Madagascar, Antarctica and Australia besides India.

• Overall resemblance of the Gondawana type sediments clearly demonstrates that these landmasses had remarkably similar histories.

• The glacial tillite provides unambiguous evidence of palaeoclimates and also of drifting of continents.

Placer Deposits

• The occurrence of rich placer deposits of gold in the Ghana coast and the absolute absence of source rock in the region is an amazing fact.

• The gold bearing veins are in Brazil and it is obvious that the gold deposits of the Ghana are derived from the Brazil plateau when the two continents lay side by side.

Distribution of Fossils

• When identical species of plants and animals adapted to living on land or in fresh water are found on either side of the marine barriers, a problem arises regarding accounting for such distribution.

• The observations that Lemurs occur in India, Madagascar and Africa led some to consider a contiguous landmass “Lemuria” linking these three landmasses.

• Mesosaurus was a small reptile adapted to shallow brackish water.

• The skeletons of these are found only in two localities : the Southern Cape province of South Africa and Iraver formations of Brazil.

• The two localities presently are 4,800 km apart with an ocean in between them.

Force for Drifting

• Wegener suggested that the movement responsible for the drifting of the continents was caused by pole-fleeing force and tidal force.

• The polar-fleeing force relates to the rotation of the earth. The earth is not a perfect sphere; it has a bulge at the equator. This bulge is due to the rotation of the earth.

• The second force that was suggested by Wegener—the tidal force—is due to the attraction of the moon and the sun that develops tides in oceanic waters.

• Wegener believed that these forces would become effective when applied over many million years. However, most of scholars considered these forces to be totally inadequate.

Post-Drift Studies
• It is interesting to note that for continental drift, most of the evidence was collected from the continental areas in the form of distribution of flora and fauna or deposits like tillite.

• A number of discoveries during the post-war period added new information to geological literature.

Particularly, the information collected from the ocean floor mapping provided new dimensions for the study of distribution of oceans and continents.

Convectional Current Theory

• Arthur Holmes in 1930s discussed the possibility of convection currents operating in the mantle portion.

• These currents are generated due to radioactive elements causing thermal differences in the mantle portion.

• Holmes argued that there exists a system of such currents in the entire mantle portion.

• This was an attempt to provide an explanation to the issue of force, on the basis of which contemporary scientists discarded the continental drift theory.

Mapping of the Ocean Floor

• Detailed research of the ocean configuration that the ocean floor is not just a vast plain but it is full of relief.

• Expeditions to map the oceanic floor in the post-war period provided a detailed picture of the ocean relief and indicated the existence of submerged mountain ranges as well as deep trenches, mostly located closer to the continent margins.

• The mid-oceanic ridges were found to be most active in terms of volcanic eruptions.

• The dating of the rocks from the oceanic crust revealed the fact that they are much younger than the continental areas.

• Rocks on either side of the crest of oceanic ridges and having equi-distant locations from the crest were found to have remarkable similarities both in terms of their constituents and their age.

Ocean Floor Configuration

• The ocean floor may be segmented into three major divisions based on the depth as well as the forms of relief.

• These divisions are continental margins, deep-sea basins and mid-ocean ridges.

Continental Margins

• These form the transition between continental shores and deep-sea basins.

• They include continental shelf, continental slope, continental rise and deep-oceanic trenches.

• The deep-oceanic trenches are the areas which are of considerable interest in so far as the distribution of oceans and continents is concerned.

Abyssal Plains

• These are extensive plains that lie between the continental margins and mid-oceanic ridges.

• The abyssal plains are the areas where the continental sediments that move beyond the margins get deposited.

Mid-Oceanic Ridges

• This forms an interconnected chain of mountain system within the ocean.

• It is the longest mountain-chain on the surface of the earth though submerged under the oceanic waters.

• It is characterised by a central rift system at the crest, a fractionated plateau and flank zone all along its length.

• The rift system at the crest is the zone of intense volcanic activity.

Distribution of Earthquakes and Volcanoes

• Plate tectonics cause earthquakes and volcanoes.

• The point where two plates meet is called a plate boundary. Earthquakes and volcanoes are most likely to occur either on or near plate boundaries.

• The focal points of the earthquake in the areas of mid-oceanic ridges are at shallow depths whereas along the Alpine-Himalayan belt as well as the rim of the Pacific, the earthquakes are deep-seated ones.

• The rim of the Pacific is also called rim of fire due to the existence of active volcanoes in this area.

Concept of Sea Floor Spreading

• Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.

• Seafloor spreading helps explain continental drift in the theory of plate tectonics.

• This theory was proposed by Hess in 1961. He argued that constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust and the new lava wedges into it, pushing the oceanic crust on either side. Thus,the ocean floor spreads.

• The younger age of the oceanic crust as well as the fact that the spreading of one ocean does not cause the shrinking of the other, made Hess think about the consumption of the oceanic crust.

• He further maintained that the ocean floor that gets pushed due to volcanic eruptions at the crest, sinks down at the oceanic trenches and gets consumed.

Plate Tectonics

• Plate tectonics is the theory that Earth’s outer shell is divided into several plates that glide over the mantle, the rocky inner layer above the core.

• The theory of plate tectonics was introduced by McKenzie, parker and Morgan in 1967.

• The plates act like a hard and rigid shell compared to Earth’s mantle. This strong outer layer is called the lithosphere. It is a massive irregularly shaped slab of solid rock.

• Plates move horizontally over the Asthenosphere. Average thickness is 100 km of oceanic part and 200 km continental part.

• Pacific plate is largest oceanic plate where as Eurasian plate is the largest continental plate.

Major Plates

1. Antarctica And Surrounding Oceanic Plate

2. North American Plate

3. South American Plate

4. Pacific Plate

5. India-Australia-New Zealand Plate

6. African Plate Eurasian Plate
7. Eurasia and the adjacent oceanic plate.

Minor Plates

(i) Cocos plate : Between Central America and Pacific plate

(ii) Nazca plate : Between South America and Pacific plate

(iii) Arabian plate : Mostly the Saudi Arabian landmass

(iv) Philippine plate : Between the Asiatic and Pacific plate

(v) Caroline plate : Between the Philippine and Indian plate (North of New Guinea)
(vi) Fuji plate : North-east of Australia.

• These plates are moving constantly throughout geological time not the continent believed by Wegener Pangaea was the convergent of all the plates.

• Position of Indian subcontinent is traced with the help of rocks analyzed from Nagpur area.

Types of Plate Boundaries

There are three types of plate boundaries:

I. Divergent Boundaries

• Where new crust is generated as the plates pull away from each other.

• 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 this, the American Plate(s) is/are separated from the Eurasian and African Plates.

II. Convergent Boundaries

• Where the crust is destroyed as one plate dived under another. (Nepal quack)

• The location where sinking of a plate occurs is called a subduction zone.

• There are three ways in which convergence can occur. These are:
(i) between an oceanic and continental plate;
(ii) between two oceanic plates; and
(iii) between two continental plates.

III. Transform Boundaries

• Where the crust is neither produced nor destroyed as the plates slide horizontally past each other.

• Transform faults are the planes of separation generally perpendicular to the mid-oceanic ridges.

• As the eruptions do not take all along the entire crest at the same time, there is a differential movement of a portion of the plate away from the axis of the earth.

• Also, the rotation of the earth has its effect on the separated blocks of the plate portions.

Rates of Plate Movement

• The strips of normal and reverse magnetic field    that parallel the mid-oceanic ridges help scientists determine the rates of plate movement.

• The Arctic Ridge has the slowest rate (less than 2.5 cm/yr), and the East Pacific Rise near Easter Island, in the South Pacific about 3,400 km west of Chile, has the fastest rate (more than 15 cm/yr).

Force for the Plate Movement

• At the time that Wegener proposed his theory of continental drift, most scientists believed that the earth was a solid, motionless body.

• However, concepts of sea floor spreading and the unified theory of plate tectonics have emphasized that both the surface of the earth and the interior are not static and motionless but are dynamic.

• The mobile rock beneath the rigid plates is believed to be moving in a circular manner.

• The heated material rises to the surface, spreads and begins to cool, and then sinks back into deeper depths.

• This cycle is repeated over and over to generate what scientists call a convection cell or convective flow.

• Heat within the earth comes from two main sources: radioactive decay and residual heat.
Arthur Holmes first considered this idea in the 1930s, which later influenced Harry Hess’ thinking about seafloor spreading. The slow movement of hot, softened mantle that lies below the rigid plates is the driving force behind the plate movement.

Movement of Indian Plate

• The Indian Plate or India Plate is a major tectonic plate straddling the equator in the eastern hemisphere. Originally a part of the ancient continent of Gondwana, India broke away from the other fragments of Gondwana 100 million years ago and began moving north.

• The Indian tectonic plate is located in the north east hemisphere.

• It is bounded by 4 major tectonic plates. North of the Indian plate is the Eurasian plate, to the south east, the Australian plate, to the south west, the African plate and to the west the Arabian plate.

• Indian plate includes India and Australia. Northern boundary is along the Himalayas. It is the place of continental convergence.

• In the east it extends up to Rakinyoma mountains of Myanmar. Eastern margin is spreading site. Western margin extends along Kirthar mountains, Makran coast red sea rift.

• The boundary between India and the Antarctic plate is also marked by oceanic ridge (divergent boundary) running in roughly W-E direction and merging into the spreading site, a little south of New Zealand.

• 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.

• About 140 million years before the present, the subcontinent was located as south as 50°S. latitude.

• The two major plates were separated by the Tethys Sea and the Tibetan block was closer to the Asiatic landmass.

• 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.

• During this time, the subcontinent was still close to the equator.

• From 40 million years ago and thereafter, the event of formation of the Himalayas took place.

• Scientists believe that the process is still continuing and the height of the Himalayas is rising even to this date.