The continents and oceans have not always been in their present positions.
At present, continents cover about 29% of the Earth’s surface, while oceans cover about 71%.
Geological evidence shows that continents and ocean basins have changed their positions over millions of years. Different theories were given to explain this distribution.
The major theories related to the distribution of oceans and continents are:
- Continental Drift Theory
- Convection Current Theory
- Sea-Floor Spreading Theory
- Plate Tectonic Theory
Table of Contents
Early Ideas About Continental Movement
Abraham Ortelius, 1596
Abraham Ortelius was a Dutch cartographer.
He observed that the coastlines of the Americas and Europe-Africa looked as if they could fit together like puzzle pieces.
He suggested that continents were once joined and later separated due to natural forces such as earthquakes and floods.
Antonio Snider-Pellegrini, 1858
Antonio Snider-Pellegrini prepared maps showing Africa and South America joined together.
He proposed that they were once part of a single landmass.
He also supported his view with identical plant fossils found on both sides of the Atlantic Ocean.
These early ideas later helped Alfred Wegener develop the Continental Drift Theory.
Continental Drift Theory
The Continental Drift Theory was given by Alfred Wegener, a German meteorologist, in 1912.
Wegener gave this theory to explain the distribution of continents and oceans.
According to Wegener, all present continents were once joined together in a single large landmass called Pangaea.
Pangaea means “all lands.”
This supercontinent was surrounded by a huge ocean called Panthalassa.
Panthalassa means “all oceans.”
Wegener argued that around 200 million years ago, Pangaea began to break apart.
First, Pangaea split into two large landmasses:
- Laurasia in the north
- Gondwanaland in the south
These two landmasses were separated by the Tethys Sea.
Later, Laurasia and Gondwanaland broke into smaller continental blocks and slowly drifted to their present positions.
Evidence In Support Of Continental Drift Theory
Wegener supported his theory with several pieces of evidence.
1. Matching Of Continents
The shorelines of Africa and South America show a remarkable fit.
The fit becomes clearer when continental shelf margins are compared instead of present coastlines.
Important examples include:
- The bulge of Brazil fits into the Gulf of Guinea.
- Greenland aligns with Baffin Island.
- The west coast of India appears to have once been connected with eastern Africa.
- North and South America fit with Europe and Africa across the Atlantic Ocean.
This is called the Jig-Saw Fit of continents.
2. Rocks Of The Same Age Across Oceans
Ancient rocks of about 2,000 million years age along the Brazilian coast match similar rock formations in western Africa.
The earliest marine deposits along the coasts of South America and Africa belong to the Jurassic period.
This suggests that the Atlantic Ocean formed after that period.
3. Tillite Or Glacial Deposits
Tillite is a sedimentary rock formed from glacial deposits.
The Gondwana system of glacial sediments found in India has matching formations in:
- Africa
- Madagascar
- Antarctica
- Australia
- South America
This shows that these landmasses were once connected.
4. Placer Deposits
Rich placer deposits of gold are found along the Ghana coast.
However, source rocks are absent there.
Similar gold-bearing veins are found in Brazil.
This suggests that Ghana and Brazil were once located close to each other before drifting apart.
5. Distribution Of Fossils
Identical fossils of land and freshwater organisms are found on continents that are now separated by oceans.
Examples:
- Fossils of Mesosaurus, a freshwater reptile, are found in southern Africa and Brazil.
- These regions are now about 4,800 Km apart.
- Plant fossils such as Glossopteris are found across India, Africa, Antarctica and Australia.
These fossil records support the existence of Gondwanaland.
Forces Responsible For Continental Drift
Wegener suggested two main forces for continental drift:
- Pole-fleeing force
- Tidal force
Pole-Fleeing Force
Pole-fleeing force is related to the rotation of the Earth.
According to Wegener, centrifugal force generated by Earth’s rotation pushed continents away from the poles towards the equator.
Tidal Force
Tidal force is produced by the gravitational attraction of the Moon and the Sun.
Wegener believed that this force could contribute to continental movement.
Limitation Of Wegener’s Theory
Many scientists rejected the Continental Drift Theory because the forces suggested by Wegener were considered too weak to move massive continents across the Earth’s surface.
Convection Current Theory
The Convection Current Theory was proposed by Arthur Holmes in the 1930s.
Holmes suggested that convection currents operate within the mantle and help in the movement of continents.
These currents are generated due to internal heat inside the Earth.
The heat is mainly produced by:
- Radioactive decay
- Residual primordial heat
Due to temperature differences within the mantle:
- Hot mantle material rises.
- Cooler material sinks.
- This forms convection currents.
According to Holmes:
- Rising limbs of convection currents form mid-oceanic ridges.
- Descending limbs form oceanic trenches.
This theory provided a better explanation for the movement of continents than Wegener’s proposed forces.
Sea-Floor Spreading Theory
The Sea-Floor Spreading Theory was proposed by Harry Hess in 1961.
According to this theory, magma rises at the crest of mid-oceanic ridges due to the upwelling of mantle material.
This magma cools and solidifies to form new oceanic crust.
The newly formed oceanic crust slowly moves away from the ridge on both sides.
As a result, the ocean floor spreads outward.
When the old oceanic crust reaches oceanic trenches, it sinks back into the mantle through subduction.
In this way, oceanic crust is continuously created and recycled.
Plate Tectonic Theory
The Plate Tectonic Theory was proposed by McKenzie, Morgan and Parker in 1967.
According to this theory, the Earth’s lithosphere is divided into large and small plates.
A tectonic plate, or lithospheric plate, is a large, irregularly shaped slab of solid rock.
It may be composed of:
- Continental lithosphere
- Oceanic lithosphere
- Both continental and oceanic lithosphere
These plates move as rigid units over the semi-molten asthenosphere.
The lithosphere includes the crust and uppermost part of the mantle.
Its thickness ranges from:
- About 5-100 Km in oceanic regions
- Nearly 200 Km in continental areas
A plate may be called oceanic or continental depending on which type of crust occupies most of it.
Examples:
- The Pacific Plate is mainly oceanic.
- The Eurasian Plate is mainly continental.
Major Tectonic Plates
The Earth’s lithosphere is divided into seven major plates.
The major plates are:
- Antarctic Plate
- North American Plate
- South American Plate
- Pacific Plate
- Eurasian Plate
- African Plate
- Indo-Australian Plate
Minor Tectonic Plates
Important minor plates include:
- Cocos Plate – located between Central America and the Pacific Plate
- Nazca Plate – located between South America and the Pacific Plate
- Arabian Plate – covers most of the Arabian Peninsula
- Philippine Plate – located between the Eurasian and Pacific Plates
- Caroline Plate – located north of New Guinea
- Juan de Fuca Plate – located off the northwest coast of North America
Plate Interaction
Plate interaction occurs at plate boundaries.
There are three main types of plate boundaries:
- Divergent boundaries
- Convergent boundaries
- Transform boundaries
Divergent Boundaries
Divergent boundaries are also called constructive boundaries.
They occur where plates move apart.
At these boundaries, magma rises from the mantle and new crust is formed.
Mid-oceanic ridges are formed due to divergent boundaries.
Example:
The Mid-Atlantic Ridge separates:
- North American Plate from Eurasian Plate
- South American Plate from African Plate
Convergent Boundaries
Convergent boundaries are also called destructive boundaries.
They occur where two lithospheric plates collide.
At convergent boundaries, one plate may move beneath another plate through subduction.
This leads to destruction of crust.
Convergent boundaries form:
- Fold mountains
- Volcanic arcs
- Oceanic trenches
Example:
The Himalayas were formed due to the collision of the Indian Plate and the Eurasian Plate.
The Himalayas are still rising because the Indian Plate is still colliding with the Eurasian Plate.
Types Of Convergence
Convergence may occur in three ways:
- Between oceanic and continental plates
- Between two oceanic plates
- Between two continental plates
Transform Boundaries
Transform boundaries are also called conservative boundaries.
They occur where plates slide horizontally past each other.
At transform boundaries:
- Lithosphere is neither created nor destroyed.
- Transform faults are formed.
Example:
The San Andreas Fault along the western coast of the USA.
Importance Of Plate Tectonic Theory
Plate Tectonic Theory explains the origin and location of several major landforms and geological events.
It explains:
- Earthquakes
- Mountains
- Sea-floor features
- Oceanic trenches
- Mid-oceanic ridges
- Volcanic arcs
However, it does not explain ocean currents.
Fold Mountains And Plate Movement
Fold mountains are formed due to folding of crustal rocks by compressive forces generated from within the Earth.
The highest peaks of the world are mostly found in young fold mountains.
The Himalayas are an example of young fold mountains.
The Andes Mountain Range is located in South America.
FAQs On Distribution Of Oceans And Continents
What is the Distribution Of Oceans And Continents chapter about?
This chapter explains how continents and ocean basins changed their positions over geological time through continental drift, sea-floor spreading and plate tectonics.
Who gave the Continental Drift Theory?
Alfred Wegener gave the Continental Drift Theory in 1912.
What is Pangaea?
Pangaea was the single supercontinent made up of all present continents.
What is Panthalassa?
Panthalassa was the superocean surrounding Pangaea.
What were Laurasia and Gondwanaland?
Laurasia was the northern landmass and Gondwanaland was the southern landmass formed after Pangaea broke apart.
What separated Laurasia and Gondwanaland?
The Tethys Sea separated Laurasia and Gondwanaland.
What is Jig-Saw Fit?
Jig-Saw Fit refers to the matching shape of continental margins, especially Africa and South America.
What fossils support Continental Drift Theory?
Mesosaurus and Glossopteris fossils support Continental Drift Theory.
What were the forces proposed by Wegener?
Wegener proposed pole-fleeing force and tidal force.
Who gave the Convection Current Theory?
Arthur Holmes gave the Convection Current Theory in the 1930s.
Who gave the Sea-Floor Spreading Theory?
Harry Hess gave the Sea-Floor Spreading Theory in 1961.
What is sea-floor spreading?
Sea-floor spreading is the process in which new oceanic crust forms at mid-oceanic ridges and moves away on both sides.
Who proposed Plate Tectonic Theory?
McKenzie, Morgan and Parker proposed Plate Tectonic Theory in 1967.
What are tectonic plates?
Tectonic plates are large rigid slabs of lithosphere that move over the asthenosphere.
What is a divergent boundary?
A divergent boundary is where two plates move apart and new crust is formed.
What is a convergent boundary?
A convergent boundary is where two plates collide and crust may be destroyed through subduction.
What is a transform boundary?
A transform boundary is where plates slide horizontally past each other.
Which boundary forms new oceanic crust?
Divergent boundary forms new oceanic crust.
How were the Himalayas formed?
The Himalayas were formed due to the collision of the Indian Plate and the Eurasian Plate.
What is the San Andreas Fault?
The San Andreas Fault is an example of a transform boundary along the western coast of the USA.
Last Moment Exam Cheat Sheet – Distribution Of Oceans And Continents
- Continents cover about 29% of Earth’s surface; oceans cover about 71%.
- Alfred Wegener gave the Continental Drift Theory in 1912.
- Pangaea means “all lands”; Panthalassa means “all oceans.”
- Pangaea split into Laurasia and Gondwanaland around 200 million years ago.
- Laurasia and Gondwanaland were separated by the Tethys Sea.
- Major evidence of continental drift includes Jig-Saw Fit, same-age rocks, tillite, placer deposits and fossils.
- Mesosaurus fossils are found in southern Africa and Brazil.
- Glossopteris fossils support the existence of Gondwanaland.
- Wegener suggested pole-fleeing force and tidal force, but these were considered too weak.
- Arthur Holmes gave the Convection Current Theory in the 1930s.
- Harry Hess gave the Sea-Floor Spreading Theory in 1961.
- Sea-floor spreading creates new oceanic crust at mid-oceanic ridges.
- McKenzie, Morgan and Parker proposed Plate Tectonic Theory in 1967.
- Lithospheric plates move over the asthenosphere.
- Seven major plates include Pacific, Eurasian, African, Antarctic, North American, South American and Indo-Australian plates.
- Divergent boundaries create new crust.
- Convergent boundaries destroy crust through subduction.
- Transform boundaries neither create nor destroy crust.
- Mid-Atlantic Ridge is an example of a divergent boundary.
- Himalayas formed due to collision of Indian Plate and Eurasian Plate.
- San Andreas Fault is an example of transform boundary.
- Plate tectonics explains earthquakes, mountains and sea-floor features, but not ocean currents.