Distribution of Oceans and Continents| Class 11 Geography Notes

The theory of continental drift proposes that the continents on Earth’s surface have slowly moved and drifted across the planet over millions of years. This is caused by powerful forces originating from within the Earth’s interior, especially the convection currents in the mantle layer below the crust. The drifting and rearranging of continents over time has shaped the distribution of landmasses, oceans, climates, and life forms we see today.

Let us learn more about the distribution of oceans and continents!

Continents Drift

The theory of continental drift proposes that the continents on Earth’s surface have not always been in their present positions. Instead, they have slowly moved and drifted across the planet over extremely long periods.

The key points are:

– Long ago, around 200-300 million years ago, all the continents were joined together into one supercontinent called Pangaea, surrounded by a single vast ocean called Panthalassa.

– Over millions of years, forces inside the Earth caused Pangaea to break apart slowly into smaller pieces – the continents we recognize today.

– These continents then drifted apart, moving across the Earth’s surface at a rate of only a few centimetres per year, driven by motions in the mantle below.

– Evidence for this continental drift includes:

1) The matching shapes of the coastlines on either side of the Atlantic Ocean

2) Similar rock types, structure,s and fossils found on both sides

3) Ancient glacial deposits across multiple continents

– As the continents drifted, oceans formed in between them, and life forms on each continent evolved independently over time.

– This drifting and rearranging of continents over millions of years shaped the distribution of continents, oceans, climates, and life forms that we see today.

Evidence in Support of the Continental Drift

1) Jigsaw Fit of Continents:

The coastlines of the continents on either side of the Atlantic Ocean appear to fit together like a jigsaw puzzle if the land masses are moved closer. For example, the bulge of Brazil seems to fit neatly into the Gulf of Guinea in Africa.

2) Matching Rocks and Fossils:

Fossils of the same type of plants and animals have been found on continents that are now widely separated by oceans. There are also matching belts of rocks and minerals across different continents that were once joined together.

3) Glacial Deposits:

Evidence of ancient glacial deposits like scratches on rocks and glacial till (debris) have been found in warm tropical regions in countries like India, Africa, and Australia. This implies these warm areas were once part of a single continental landmass near the South Pole.

4) Ancient Magnetic Data:

Studying the patterns of magnetic fields preserved in ancient rocks shows that the magnetic north and south poles have switched positions many times. This supports the idea that the continents have shifted positions over time.

5) Similarities in Geology:

Geological features like mountain ranges and deposits of coal, oil, and minerals continue from one continent into another, providing evidence they were once joined.

6) Age of Oceans:

The oldest oceans on Earth like the Atlantic and Arctic are relatively young, indicating the continents separated more recently in geological time.

Forces of Drifting

The continents do not move on their own. Their slow drifting is caused by powerful forces coming from inside the Earth’s interior, especially from the mantle layer below the crust.

1) Convection Currents:

The mantle is extremely hot, almost molten rocky material. It is constantly undergoing circular motions called convection currents, similar to how water circulates when heated in a pot.

As the hot mantle material rises and colder material sinks, it generates huge forces that cause the continents above to move and drift across the Earth’s surface over millions of years.

2) Seafloor Spreading:

At the mid-ocean ridges, the hot molten rock rises from deep inside the mantle, forming a new oceanic crust that pushes the existing crustal plates apart.

As new crust is generated, it forces the attached continents on either side to drift further away from each other in opposite directions.

3) Slab Pull:

At some plate boundaries, one tectonic plate gets pulled down or subducted under another plate. This “slab pull” force can also drag the attached continents along, causing them to shift slowly over long periods.

4) Mantle Plumes:

Narrow streams of hot rising mantle material called “mantle plumes” can push up the crust and slowly move continents from below over vast time scales.

Post Drift Studies

After the continents shifted and drifted apart from their original positions, various changes and developments occurred on Earth. Scientists have studied the effects and implications of this continental drift through different fields of research:

Continental Drift Evidence

Much of the evidence supporting continental drift was initially gathered from continental regions, including the distribution of flora and fauna and deposits like tillite. Post-World War II discoveries provided further geological insights, especially through ocean floor mapping, which expanded the understanding of ocean and continent distribution.

Convectional Current Theory

In the 1930s, Arthur Holmes proposed the concept of convection currents in the mantle. These currents, driven by radioactive elements, create thermal variations within the mantle. Holmes suggested a widespread system of such currents throughout the mantle, aiming to address the issue of force that led many scientists to dismiss the continental drift theory.

Mapping of the Ocean Floor

Exploration efforts post-World War II offered detailed insights into the ocean floor’s topography, revealing it to be more than just a vast plain. Expeditions to map the ocean floor unveiled a diverse relief, including submerged mountain ranges and deep trenches, often near continental margins. Mid-oceanic ridges were identified as particularly active zones for volcanic activity. Furthermore, dating of rocks from the oceanic crust indicated their relative youth compared to continental areas. Notably, rocks located equidistant from the crest of oceanic ridges exhibited similar compositions and ages on either side of the crest.

Ocean Floor Configuration

In this section, we’ll explore key aspects of the ocean floor configuration that contribute to our understanding of continent and ocean distribution. While a detailed study of ocean floor relief is covered in Chapter 13, we can broadly divide the ocean floor into three main divisions based on depth and relief forms: continental margins, deep-sea basins, and mid-ocean ridges.

Continental Margins

Continental margins serve as the transition zone between continental shores and deep-sea basins. They encompass the continental shelf, continental slope, continental rise, and deep-oceanic trenches. Of these, deep-oceanic trenches are particularly significant in understanding ocean and continent distribution.

Abyssal Plains

Abyssal plains are vast flat areas located between continental margins and mid-oceanic ridges. These plains accumulate continental sediments that extend beyond the margins.

Mid-Oceanic Ridges

Mid-oceanic ridges form a continuous chain of underwater mountain systems. Despite being submerged beneath ocean waters, they represent the longest mountain chain on Earth’s surface. Characterized by a central rift system, a plateau, and a flank zone along its length, mid-oceanic ridges exhibit intense volcanic activity at the rift system crest. These volcanic features were introduced in the previous chapter as mid-oceanic volcanoes.

Distribution of Earthquakes and Volcanoes

Volcanoes

– Most volcanoes are found along the boundaries of tectonic plates, especially at convergent boundaries (where plates collide) and divergent boundaries (where plates move apart).

– Major concentrations of volcanoes occur along the Pacific Ring of Fire, a region encircling the Pacific Ocean where many tectonic plates meet.

– Volcanic activity is also seen at hot spots, which are stationary areas of high heat within the Earth’s mantle that can create chains of volcanoes as plates move over them.

Earthquakes

– Earthquakes predominantly occur along tectonic plate boundaries, where plates interact and create stress and friction.

– The majority of earthquakes happen at convergent boundaries, where plates collide and one plate is pushed under the other (subduction zones).

– Earthquakes also frequently occur along transform boundaries, where plates grind past each other horizontally.

– Some earthquakes can happen within plates, often due to the movement of molten rock or stress built up over time.

– Areas with high earthquake activity include the Pacific Ring of Fire, the San Andreas Fault in California, and the Himalayan region.

Concept of Sea Floor Spreading

1) Mid-ocean ridges are underwater mountain ranges that run along the ocean floors. They mark the boundaries between tectonic plates.

2) At the ridges, molten rock (magma) rises from the Earth’s mantle and erupts along the ridge, creating a new oceanic crust.

3) As new crust is formed at the ridge, it pushes the existing crust on either side away from the ridge, causing the seafloor to spread apart.

4) This spreading of the seafloor is driven by convection currents in the mantle, which circulate hot rock towards the ridges and cooler rock away from them.

5) The newly formed crust moves horizontally away from the ridge, carrying the continents along with it, at a rate of a few centimeters per year.

6) As the crust moves away and cools down, it becomes denser and eventually sinks back into the mantle at deep ocean trenches, where tectonic plates converge.

Plate Tectonics

1) Tectonic plates are made up of the Earth’s crust and the rigid upper portion of the mantle. The plates are around 100 km (60 miles) thick.

2) There are seven major plates and several minor plates that make up the lithosphere.

3) These plates move in different directions at rates of a few centimeters per year, driven by convection currents in the mantle.

4) The boundaries between plates are areas of intense geological activity, including:

– Divergent boundaries (plates moving apart) – Where new crust is formed, e.g., mid-ocean ridges

– Convergent boundaries (plates coming together) – Where subduction occurs and mountains/volcanoes form

– Transform boundaries (plates sliding past each other) – Where earthquakes occur along faults

5) Seafloor spreading at mid-ocean ridges and subduction at deep ocean trenches are part of the plate tectonic cycle.

6) The movement of plates over millions of years has shaped the Earth’s surface, forming continents, oceans, mountains, and other geological features.

Major and minor Plates

Major Plates:

1) Pacific Plate

2) North American Plate

3) Eurasian Plate

4) African Plate

5) Antarctic Plate

6) Indo-Australian Plate

7) South American Plate

These seven plates are the largest and most significant tectonic plates on Earth.

Minor Plates:

1) Arabian Plate

2) Caribbean Plate

3) Cocos Plate

4) Nazca Plate

5) Philippine Plate

6) Scotia Plate

7) Juan de Fuca Plate

Major plates typically bound these smaller plates and are often associated with significant geological activity, such as earthquakes and volcanoes.

– The major plates cover most of the Earth’s surface and include the continents and large parts of the ocean basins.

– Minor plates are generally smaller and may be partially or entirely oceanic.

– Plate boundaries occur between both major and minor plates, as well as within some of the larger plates themselves.

– The movement and interaction of both major and minor plates drive the tectonic processes that shape the Earth’s surface over time.

– While the major plates are more well-known, the minor plates also play an important role in the overall plate tectonic system.

Types of Plate Boundaries

Types of Plate Boundaries:

1. Divergent Boundaries:

– Plates move apart from each other.

– New crust is created as molten rock rises and solidifies.

– Examples: Mid-ocean ridges (like the Mid-Atlantic Ridge), continental rifts.

2. Convergent Boundaries:

– Plates move towards each other.

– Can result in subduction (one plate going under the other) or continental collision.

– Examples: Deep ocean trenches (like the Mariana Trench), a mountain ranges (like the Himalayas).

3. Transform Boundaries:

– Plates slide horizontally past each other.

– No crust is created or destroyed.

– Examples: San Andreas Fault (California and Alpine Fault (New Zealand).

Rate of Plate Movements:

Tectonic plates move at varying rates, typically ranging from 1 to 10 centimeters per year (less than the growth rate of human fingernails).

– Fastest: About 10 cm/year (e.g., the Cocos Plate near Central America).

– Slowest: Less than 1 cm/year (e.g., the North American Plate).

The rate of plate movement depends on factors like the density and thickness of the plates, the degree of resistance from surrounding plates, and the convection currents in the underlying mantle.

Generally, plates move faster along divergent boundaries (where new crust is formed) and slower along convergent boundaries (where subduction or collision occurs).

Despite their slow motion, the movement of tectonic plates over millions of years has profoundly shaped the Earth’s surface, forming mountains, oceans, and continents as we know them today.

Forces of plate movements & movements in Indian plates

Forces of Plate Movements:

The movement of tectonic plates is driven by several forces, primarily:

1. Slab Pull: As tectonic plates are subducted (pushed under) at convergent boundaries, the dense, cooler, and heavier leading edge of the plate pulls the rest of the plate along.

2. Ridge Push: At mid-ocean ridges, the upwelling of hot, less dense mantle material creates new oceanic crust, which pushes the plates horizontally away from the ridge.

3. Mantle Convection: The slow circulation of semi-molten rocks in the Earth’s mantle creates convection currents that drag the overlying tectonic plates along.

4. Gravitational Sliding: The higher elevation of continents compared to oceanic crust causes the continental plates to slowly slide downhill, driven by gravity.

These forces, combined with the rotation of the Earth and the uneven distribution of heat and density within the mantle, contribute to the continuous movement of tectonic plates.

Movement of the Indian Plate

The Indian Plate is a major tectonic plate that has undergone significant movement over geological time. Here are some key points about its movement:

– The Indian Plate was once part of the ancient supercontinent Gondwana, which began breaking apart about 180 million years ago.

– Around 90 million years ago, the Indian Plate started moving northwards, separating from the Antarctic Plate and drifting rapidly towards the Eurasian Plate.

– About 50 million years ago, the Indian Plate collided with the Eurasian Plate, causing the uplift of the Himalayan mountain range and the Tibetan Plateau.

– The Indian Plate is still moving northwards at a rate of about 5 cm per year, continuing to push into the Eurasian Plate and contributing to the ongoing formation of the Himalayas.

– The movement of the Indian Plate has been a major driving force behind the geological evolution of the Indian subcontinent and the surrounding regions.

The forces that drive plate movements, including those acting on the Indian Plate, are responsible for shaping the Earth’s surface features and causing geological events like earthquakes and volcanic activity.

Distribution of Oceans and Continents-FAQs

What are the major oceans of the world?

The major oceans of the world are the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, and the Arctic Ocean.

What are the main characteristics of the Pacific Ocean?

The Pacific Ocean is the largest and deepest of the world’s oceans. It covers an area of about 63 million square miles and has an average depth of around 4,280 meters. The Pacific Ocean is located between the Americas and Asia/Australia.

What are the main characteristics of the Atlantic Ocean?

The Atlantic Ocean is the second-largest ocean, covering an area of about 31 million square miles. It is located between the Americas and Europe/Africa. The Atlantic Ocean has an average depth of around 3,900 meters and is known for its strong ocean currents, such as the Gulf Stream.

What are the main characteristics of the Indian Ocean?

The Indian Ocean is the third-largest ocean, covering an area of about 27 million square miles. It is located between Africa, Asia, and Australia. The Indian Ocean has an average depth of around 3,900 meters and is known for its monsoon wind patterns.

What are the main characteristics of the Arctic Ocean?

The Arctic Ocean is the smallest and shallowest of the world’s major oceans, covering an area of about 5.4 million square miles. It is located entirely within the Arctic Circle and is largely covered by sea ice throughout the year.

What are the major continents of the world?

The major continents of the world are Asia, Africa, North America, South America, Europe, Australia, and Antarctica.

What is the continental drift theory, and how does it explain the distribution of continents and oceans?

The continental drift theory, proposed by Alfred Wegener, suggests that the continents have drifted apart over geological time, resulting in the current distribution of continents and oceans. This theory explains how the landmasses were once connected in a single supercontinent, called Pangaea, which gradually broke apart and drifted to their present-day locations.