Class -11 NCERT Fundamental of physical geography

 Distribution of oceanic and continents

Continental drift 

• Observe the shape of the coastline of the Atlantic Ocean.
•  The symmetry of the coastlines on either side of the ocean .
• No  wonder, many scientists thought of this similarity and considered the possibility of the two Americas, Europe and Africa, to be once joined together.
•  It was Abraham Ortelius , a Dutch  map maker , who first proposed such a possibility as early as 1596.
•  Antonio pellegrini drew a map showing the three continents together.
•  However, it was Alfred Werner  a German methodologist who put forth a comprehensive argument in the form of the continental drift theory in 1912. this was recording the distribution of ocean and the continent .
• According to wegener,  all the continent formed as single Continental mass and make a ocean surrounded the same .
• The he supercontinent was named PANGAEA, which mean all earth .
• The mega Ocean was called PANTHALASSA , meaning all water .
• He argued that, around 200 million a year ago, the super continent ,pangaea,  began to split .
• Pangaea first broke into two large Continental masses as laurasia and Gondwanaland forming the northern and Southern component respectively .
• Laurasia and Gondwanaland continued to break into various smaller continents that exist today .
• A variety of evidence was of birth in support of the continental drift.

Evidence in support of the continental drift

The Matching of Continent 

• The shorelines of Africa and South Africa America facing Each Other Have A Remarkable and unmistakable match.
•  A map produced using a computer program to find the best fit of the Atlantic Margin was presented by Ballard in 1964.

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 an sea and rocks of 2000 million a year 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.

Tillite

• It is the sedimentary rock formed out of deposit of glaciers.
•  The Gondwana system of sediments from India is known to have its counterpart in 6 different land masses of the southern hemisphere.
•  Counterparts of the succession are formed in Africa, Falkland Island, Madagascar Antarctica and Australia.
•  The Gondwana type sediment clearly demonstrates that these landmasses had remakably similar histories .
• The glacial tillite provides an 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 reason is an amazing fact.
•  The gold bearing veins are in Brazil and it is obvious that the gold deposits of the ghana are drived from the Brazil plateau when the two continents lay side by side.

Distribution of Fossils

• When identical species of plants and animal adapted to living on land or in freshwater are found on either side of the Marine barriers, a problem arises regarding accounting for such distribution
•  The observation that Lemuria occur in India, Madagascar  and Africa led some to consider a contiguous Landmark Lemuria  linking these three land masses.
• Mesosaurus was a small reptile adapted to Shallow Brackish water .
• The Skeleton of these are found only in two localities : the southern cape provinces of South Africa and Iraver  formation of Brazil.

Force for Drifting 

• Wegener suggested that the moment responsible for the drafting of the continents was caused by Pole fleeing force and tidal force .
• The polar fleeing Force relates to the rotation of earth.
•  It has a bluge at the equator.
•  This bluge is due to the rotation of earth.
•  The second force that was suggested by wegener the tidal force is due to the attraction of moon and the sun develop Tides in oceanicwater.

Post Drift studies 

•  The evidence was collected from the continental areas in the form of distribution of flora and fauna deposit, like tillite.
•  A number of discoveries during the post-world War II  period added new information to your geological literature.

Convectional current theory

• Arthur Holmes in 1930 as discussed the possibility of convection current operating in the mantle portion .
• These current are generated due to radioactive element causing thermal differences in the mantle portion .
• Holmes argued that there exists a system of such current in the entire mantle portion.

Mapping of the ocean floor 

• Detailed research of the ocean configuration revealed that the ocean floor is not just the vast plain but it is full of relief.
•  Expandation to map the oceanic floor in the post world war second period write a provided a detailed picture of the ocean relief and indicated the existence of submerged mountain range as well as deep trenches mostly located closer to the continent margins.
• The mid oceanic ridges were found to be most active in term of volcanic eruptions.
• The dating of the rocks from the oceanic crust the revealed and the fact that they are much younger than the continental areas.

Ocean floor configuration 

• A few things related to the ocean floor configuration that help us in the understanding of the distribution of continents and ocean.
•  The ocean floor may be segmented into three major divisions based on the depth as well as form of relief.
•  These division are continental margins, deep sea basin 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 plain that lie between the continental margins and mid oceanic ridges.
•  The abyssa  plains are the areas where the continentals the sediments that move beyond and the margins get deposit.

Mid oceanic ridges

• This form and inter-connected chain of mountains system within the ocean.
•  It  is the longest mountain chain on the surface of the earth through submerged under the oceanic waters.
•  It is characterized 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 Earthquake and Volcanoes 

• The Central part of the Atlantic ocean almost parallel to the coastlines. It further extends into the Indian Ocean.
•  It bifurcated  a little south of the Indian subcontinent with a one branch moving into East Africa and the other meeting a similar like from Myanmar to new Guiana. 
• The shaded belt showing another area of concentration can sides with the Alpine Himalayan system and the rim of the Pacific Ocean .
•  In general the foci of the earthquake in the area of mid oceanic ridges are at Shallow depth whereas along the Alpine -Himalayan belt as well as the rim of the Pacific,the earthquake are deep seated once .
• The map of volcanoes also show a similar pattern .
• The rim of Pacific is also called Rim of fire due to the existence of activity volcanoes in this area.

Concept of sea floor spreading

• The post drift studies provided considerable information that was not available at the time Wegener put forth his concept of continental drift.
•  The mapping of ocean floor and palaeomagnetic studies of rocks from oceanic region revealed the following facts:

1.  It  was realised that all along the mid oceanic ridges, volcanic eruption are common and they bring huge amounts of lava to the surface in this area .
2. The Rocks equidistant on either sides of the crest of mid- oceanic ridges show remarkable similarities  in terms of period of  formation chemical composition and magnetic properties. rocks closer to the mid oceanic ridges  have a normal polarity and are the youngest.The age of the rocks increases as one more away from the crest .
3. The ocean crust rocks are much younger than the continental rocks. the age of rocks in the oceanic crust is now where more than two hundred million a year old. 
4. The sediments on the ocean floor are unexpectedly very thin .
5. The deep trenches have deep seated earthquake occurrence while  in the mid oceanic ridges  areas, the quake foci have shallow depths. 

• These facts and a detailed analysis of magnetic properties of the rocks on either side of the mid oceanic ridge lead less hess (1961) to propose his hypothesis, known as the sea floor spreading.
•  Hess argued that constant eruption at the crest of our oceanic ridges cause the rupture of the ocean curst and the new lava into it ,pushin the oceanic crust  on either side .
• The ocean floor, thus spreads. 

Plate Tectonics

•  Since the advent of the concept of sea floor spreading, the interested in the problem of distribution of oceans and continents was revived .
• It was in 1967, McKenzie  and Parker and also Morgan, independently collected the available Ideas and came out with another concept plate tectonics.
•  A tectonic plate is a massive, irregularly shaped slab of solid Rock ,generally composed of both Continental and oceanic lithosphere.
•  Plates move horizontally over the athenosphere as ridge units.
• The lithosphere includes the crust and top Mantle with its thickness range varying  between 5 and 100 KM in Ocean parts and about 200 km in continental areas.
•  Pacific plate is largely and oceanique plate where is the Eurasian plate may be called Continental plate.
•  Young fold mountain ridges,  trenches,  and or faults surrounded these major plates.

 
The major plates are as follows :

1. Antarctica and the surrounding Oceanic plate .
2. North American plate 
3. South American plate
4.  Pacific plate
5.  India Australia New Zealand plate 
6. Africa with Eastern Atlantic floor plate
7. Eurasia and the adjacent oceanic plate. 

Some important minor plates are listed below:

1.  Cocas plate : Between Central America and Pacific plate
2. Nazca plate : Between South America and Pacific plate 
3. Arabian plate : Mostly the Saudi Arabian  landmass 
4.  Philippines plate : Between the Asiatic  and Pacific plate
5.  Caroline plate : Between the philippian and Indian plate
6. Fuji plate : North-east of Australia

•  These plates have been constantly constantly moving over the globe throughout the history of earth.
•  It is not the continent that move as believed by wegener .
• Continent are part of a plate and what move is the plate.

There are three types of plates boundaries :

Divergent Boundaries 

• The sites where the plate moves away from each other are called spreading sites.
• The best known example of divergent boundaries is the mid Atlantic Ridge.
•  The American plate is /are separated from The Eurasian and African plates. 

Convergent Boundaries 

• The location where sinking of a plate occur is called a subduction  zone .
• There are three way in which conversation can occur .

These are;

1.  Between an ocean and Continental plate 
2. Between two oceanique plate
3.  Between two Continental plates

Transform boundaries

• Transform faults are the Planes of separation generally perpendicular to the mid oceanic ridges.
•  The rotation of the earth has its effect on the separated block of the plate plate portion .

Rates of plate movement 

• The strips of normal and reverse magnetic field that parallel the mid oceanic ridges help scientist determine the rates of plate movement .
• These rate are very considerably. 
• The Arctic Ridge has the slowest rate and the east Pacific rise near Easter Island, in the south Pacific about 3400 km west of Chile, has the fastest rate.

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 .
• The fact that the plates move is now a well accepted fact .
• The mobile Rock beneath the rigid Plates is believed to be moving in a circular manner.
•  The hated material Rises to the surface, spread and being too cool,  and then sinks back into deeper depth. 
• This cycle is repeated over and over to generate what scientist call a convection cell or convective flow .
• Heat within the earth comes for from two main sources: radioactive decay and residy'all heat .
• Arthur Holmes first considered this idea in 1930 ,which letter influenced Harry hess thinking about sea floor spreading.
•  The slow movement of softened mental that lies. below the rigid plates is the driving force behind the plate movement

Movement of the Indian plate 

• The Indian Plate included peninsular India and the Australian Continental portions. 
•  The subduction zone along the Himalayas forms the northern plate boundary in the form of continent - continent convergence.
•  The Eastern margin is a spreading site lying to the east of Australia in the form of an Ocean ridge in SW Pacific .
• The Western Marine follows Kirthar mountain of Pakistan .
• The boundary between India and the Antarctica plate is also marked by oceanic ridge running in roughly W-E direction and merging into the spreading site, a little south of New Zealand.
•  India was a large Island situated of the Australian coast in a vast ocean.
• The Tethys sea separated it from the Asian continent till about 225 million a year ago.
•  India is supposed to have started her Northward journey about 200 million a year ago at the time when broke.
•  India collided with Asia about 40 50 million a year ago causing Rapid uplift of the Himalayas.
•  The position of India since about 71 million a years ago till the present are shown in the figure .
• About 140 million years before the present, the subcontinent was located in South as 5 degree as latitude.
•  The two major plates were separated by the Tethys sea and the Tibetan block was closer to Asiatic land mass.
•  During the movement of Indian Plate towards the Eurasian plate, a major event that occurred was the outpouring of lava  and formation of Deccan traps. 
• Scientists believe that the process is still containing and the height of Himalayas is raising event to date.