Distribution of Oceans and Continents Notes in English Class 11 Geography Chapter-4 Book-Fundamental of Physical Geography

 

Main Principles  

1. CONTINENTAL DRIFT THEORY 

2. CONVECTIONAL CURRENT THEORY 

3. SEA FLOOR SPREADING THEORY 

4. PLATE TECTONIC THEORY 

Introduction 

• Continents cover 29 percent of the Earth and oceans cover the rest. 

• The position of continents and oceans has changed and is still changing

CONTINENTAL DRIFT CONTINENTAL DRIFT

• The coastline on both sides of the ocean has amazing symmetry.

• Expressed the possibility of South and North America and Europe and Africa being connected together.

Continental drift was first observed in...  

• In the year 1596

• Abraham Ortelius was the first to propose this possibility

• Dutch cartographer

Antonio Pellegrini has... 

• Made a map in which all three continents were shown together.

Continental Drift Theory

• German meteorologist Alfred Wegener

• In the year 1912 

• This theory was related to the distribution of continents and oceans.

According to Alfred Wegener... 

• All the continents were connected into a single landmass.

• All the continents of today were part of this landmass and it was surrounded by a large ocean.

• This large continent was named Pangaea.

•  Pangaea means the whole earth.

• Alfred Wegener  named this vast ocean Panthalassa which means only water.

• About 200 million years ago, the division of this large continent, Paigea, began.

• Pagaea first split into two large continental bodies, Laurasia and Gondwanaland.

• Laurasia and Gondwanaland gradually broke up into several smaller parts, which are the present-day continents.

Evidence in favour of continental drift

1. Equilibrium among continents

2. Similarity in the age of rocks across oceans

3. Tealight

4. Placer deposits

5. Distribution of fossils

1. Equilibrium among continents

• The facing coastlines of South America and Africa show a wonderful and flawless similarity. 

• In 1964, Bullard, with the help of a computer program, prepared a map connecting the Atlantic coasts.

2. Similarity in the age of rocks across oceans

Radiometric dating methods developed in modern times make it easy to date the time of formation of rocks on continents across the oceans. 

A belt of 200 million years old rock formations meets each other along the coast of Brazil and the coast of West Africa.

3. Tealight

• Tillites are sedimentary rocks formed by glacial deposition. 

• The Gondwana sedimentary patterns found in India are found in six different landmasses in the Southern Hemisphere

4. Placer deposits

• The presence of large gold deposits off the coast of Ghana and the absence of origin rocks is a surprising fact. 

• Gold bearing veins are found in Brazil. 

• The gold deposits found in Ghana may have originated from the Brazilian Plateau when these two continents were connected to each other

5. Distribution of fossils

• similar species of plants and animals

• Lemurs are found in India, Madagascar and Africa; some scientists have accepted the existence of a landmass called 'Lemuria' connecting these three landmasses.

• Mesosaurus were small reptiles that could only live in shallow salt water—their bones are found only in the Southern Cape province of South Africa and the Iravar Formation of the Brazil.

Flow related forces

1. Polar flying force

2. Tidal force

1. Polar flying force

• The polar fleeing force is related to the Earth's rotation. 

• the shape of the earth is not a perfect sphere 

• It is raised at the equator. 

• This bulge is due to the rotation of the Earth.

2. Tidal force

• It is related to the attraction of the Sun and the Moon, which causes tides in the oceans. 

• Over millions of years these forces became powerful and capable of causing displacement

convection current theory

👉by Arthur Holmes

👉 in the 1930s

👉 Expressed the possibility of the effect of convection currents in the mantle part.

👉 These currents are generated in the mantle part due to temperature differences generated by radioactive elements. 

👉 Holmes said that a system of such currents exists throughout the mantle.

Mapping the ocean floor

• The oceans do not have a vast plain but they have topography.

• Expeditions after World War II provided detailed information on ocean topography 

•  In its subsurface there are submerged mountain ridges and deep trenches, which are located on the edges of the continents. 

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

• The rocks at the bottom of the oceans are newer than the rocks found in continental regions. 

texture of the ocean floor 

The ocean floor can be divided into three major parts. 

1. Continental margins

2. Deep sea basins Abyssal Plains 

3. Mid-oceanic ridges

1. Continental margin

• This is the area between the continental margin and the deep ocean basin. 

• It includes continental shelf, continental slope, continental rise and deep ocean trenches etc.

2. Deep sea basin

• These vast plains are found between the continental coasts and mid-ocean ridges. 

• Abyssal plains are areas where sediments carried from the continents are deposited away from their coasts.

3. Mid-ocean ridge

• The mid-ocean ridge forms a chain of interconnected mountains. 

• Submerged in ocean water, it is possibly the longest mountain range found on the Earth's surface.

extension of the sea floor 

• Volcanic eruptions are a common occurrence along mid-ocean ridges, and these eruptions spew large quantities of lava into the region.

• There is similarity in the formation time, structure, composition and properties of the rocks found at equal distance on both the sides of the middle part of the oceanic ridge.

• The rocks of the oceanic crust are newer than the rocks of the continental crust.

• The rocks of the oceanic crust are not more than 200 million years old.

• In deep trenches the earthquakes have their origins at greater depths. 

• Whereas in the region of mid-ocean ridges the earthquake origin centres exist at shallower depths.

Sea floor spreading

• by Hess   in 1961 

• Continuous volcanic eruptions at the top of oceanic ridges have caused differentiation of the oceanic crust, and new lava is filling the cracks, pushing the oceanic crust to both sides.

• When the expansion of one ocean did not cause the other ocean to shrink, Hess spoke of the thrusting of the oceanic crust. 

• According to Hess, if volcanic crust leads to the creation of new crust, on the other hand, the oceanic crust also gets destroyed.

Plate tectonics

• In 1967, McKenzie, Parker, and Morgan synthesized freely available ideas. 

• A tectonic plate (lithospheric plate) is a large block of solid rock composed of continental and oceanic lithospheres.

• The lithosphere includes the crust and the upper mantle, whose thickness is 5 to 100 km in the oceans and about 200 km in the continental regions.

• A plate may also be called a continental or oceanic plate, depending on whether the majority of the plate is associated with an ocean or a continent.

• These plates move horizontally as a rigid unit on the asthenosphere.

important large plate 

1. Antarctic plate

2. North American Plate

3. South American plate

4. Pacific Ocean Plate

5. Indo-Australian New Zealand Plate.

6. African plate 

7. Eurasian plate

Important small plates 

1. Cocos 

2. Nazca Plate

3. Arabian plate

4.Philippine Plate

5. Caroline Plate

Plate boundary 

1. Divergent boundary  

• When two plates move away from each other in opposite directions and new crust is formed, they are called divergent plates. 

• The place from where the plates move away from each other is also called spreading site.

2. Convergent boundary 

• When one plate subducts beneath another and where the crust is destroyed, that is a convergent boundary. 

• The place where the plates sink is also called the subduction zone. 

• Convergence can happen in three ways- 

• between oceanic and continental plates 

• between two oceanic plates 

• Between two continental plates.

3. Transform boundary 

• Where neither new crust is formed nor crust is destroyed, they are called transform boundaries. 

• This is because at this boundary the plates slide alongside each other in a horizontal direction.

Plate flow rates

• These rates of flow vary greatly. 

• The Arctic Ridge has the lowest flow rate (less than 2.5 centimeters per year). 

• The East Pacific Rise near Easter Island, 3,400 km west of Chile in the South Pacific Ocean, has the highest flow rate (more than 5 cm per year).

Movement of the Indian Plate 

• The Indian Plate is a major tectonic plate that includes much of South Asia and a portion of the Indian Ocean.

• It is part of the larger Indo-Australian Plate

• The Indian plate was part of the ancient continent Gondwana. 

• It started separating from Gondwana.

• The Indian plate moved northwards at a speed of about 15–20 cm per year.

The Indian plate collided with the Eurasian plate. 

• This collision is ongoing and has led to the uplift of the Himalayas, the Tibetan Plateau

• The most important effect of the movement of the Indian plate is the formation of the Himalayan mountain range, which continues to grow as the collision continues.