DISTRIBUTION OF OCEANS AND CONTINENTS

DISTRIBUTION OF OCEANS

AND CONTINENTS 

Continents: comprises 29% of the earth's surface.

Oceans: comprises 69% of the earth's surface.

The position of continent and ocean are not the same, it changes over time.

The following theory explains the position of continents:

• Continental drift theory
• Convention cell theory
• Seafloor spreading theory
• Plate tectonic theory

Continental drift theory:

German meteorologist, Alfred Wegener gave continental drift theory in 1912 to explain the distribution of ocean and continents.

As per Wegener:

• All the continents formed from a single landmass or supercontinent was named as PANGAEA. The supercontinent is surrounded by the Mega Ocean called PANTHALASSA.
• 200 million years ago, the supercontinent began to splits.
• First, it breaks into two landmasses :
• Laurasia in the north side
• Gondwanaland in the southern side
• Laurasia and Gondwana land continued to split many landmasses.
• From Laurasia the following landmasses were created:
• North America
• Europe
• Asia except for the Indian subcontinent.
• From Gondwanaland:
• South America
• Africa
• Indian continent
• Australia

The following evidence that supports continental drift:

• The matching of the continent ( jig-saw fit)
• Rocks of the same age across the ocean
• Tillite deposits
• Placer deposits
• Distribution of fossils

The Matching of continents( Jig-saw-fit):

• The shoreline of Africa and South America facing each other match and fit.
• The shoreline of North America and Europe are matching and fit.

Rocks of the same age across the oceans;

• By the radiometric dating method, rocks age of Brazil coast matches with western Africa. This means the ocean does not exist between these continents during Jurassic age.

Tillite;

• Tillite is a sedimentary rock formed by the deposition of glaciers. The sediments from India are matched with six different landmasses of the southern hemisphere such as Africa, Falklands Island, Madagascar, Antarctica, and Australia. These landmasses also have a similar history.

Placer deposits:

• In Ghana coast, deposits of golds are found but sources rock of gold is not present on Ghana coast. But, sources of gold rocks do present on Brazil coast. That means, gold deposits of Ghana is derived from Brazil plateau. That means, in past Africa and South America were the same landmasses.

Distribution of Fossils:

• Lemur occurs in India, Madagascar, and Africa. That means these landmasses were attached in the past.

Forces of drifting as per Wegener;

Wegener suggested two forces for drifting of the continent:

• Pole fleeing force
• Tidal force

Pole fleeing force:

It is related to the rotation of the earth. Earth is not a perfect sphere, the bulge at equator and earth rotation leads to movement of continents in pole war direction; this force is called poles fleeing force.

Tidal forces;

Due to the attraction of the moon and sun that develop tides in oceans Water. Tidal force leads to the movement of the continent.

Arthur Holmes gave convention current theory in 1930 and try to explain the forces that lead to drifting the large landmasses.
Convention current is generated in the mantle portion. Disintegration Radioactive elements in mantle create a thermal gradient which causes current to flow in a circular manner in earth mantle.
Earth has many convention currents.
Convention currents are mainly driving force of continents.

Mapping of the ocean floor:
By research and observation, it was found that the ocean floor is not vast plain but full of reliefs.

The following types of landforms are found in oceans:

• Submerged mountain
• Deep trenches located to the continent margin.
• Mid-ocean ridge was found in volcanoes eruption region on the ocean.
• Rocks Closer to the mid-ocean ridge is much younger as compared to rocks farther to the mid-ocean ridge.
• Rocks on either side of the ridge having equal distance have equal similarities such as age and constituent.

Ocean floor configuration:
Ocean floor may be segmented into three major division:

• Continental margin
• Deep-sea basins.
• Mid-ocean ridges

Continental margins:
These are the transition between continental shores and deep-sea basins.
It includes:

• Continental shelf
• Continental slope
• Continental rise
• Deep ocean trenches

Abyssal plains or deep-sea basins:
Abyssal plains lie between continental margin and mid-ocean ridge. Sedimentary of continental beyond continental margin settled here.

Mid oceanic ridges:
This forms an interconnected chain of the mountain system in oceans. It is the largest mountain chain on the earth's surface. Intense volcanic and earthquake activities are found in these regions.

Distribution of foci earthquake and volcanoes:
The following areas:

• Middle of Atlantic ocean almost parallel to the coastline. It further extended to Indian oceans.
• Alpine Himalayan region
• The rim of the pacific ocean or rim of fire.

Seafloor spreading:

• The mapping of the ocean floor and palaeomagnetic studies of rock from the ocean floor revealed the following facts:
• The volcanic eruption is common in the mid-ocean ridge and they bring a huge amount of lava to the surface.
• Rocks equidistant from either side of mid-oceanic ridge show remarkable similarities such as the period of rock formation, the chemical composition of the rock, magnetic properties.
• Rocks closer to the ridge are youngest and when we go away from the ridge, the age of rocks increases.
• Oceanic crust rocks are much younger than continental rocks. The age of oceanic crust is about 200 million years and the age of continental crust is about 3,200 million years old.
• Sediments on the ocean floor are older than 200 million years.
• Foci of the earthquake in the mid-ocean ridge area are shallow depth while deep in trenches areas.

Plate tectonic theory:
In 1967, McKenzie, Parker, and Margin gave the concept of plate tectonic. Seafloor theory and convection current theory are the base theory that helps to conceptualize the plate tectonically.

What are tectonic plates:
Tectonic or lithosphere plate is a massive, irregular shaped solid plate, can be made of the only continent, only ocean or by both.

The depth of the ocean plate is approximately 5 to 100 km and the depth of the continental plate is approximately 10 to 200 km.

The tectonic or lithosphere plate moves horizontally over the asthenosphere.
There are 7 major plates;
Antarctica and surrounding ocean plates.

• North American plate
• South American plate
• Pacific plate.
• India Australia New Zealand plate
• Africa and easter Atlantic plate
• Eurasia and the adjacent ocean except for the Indian subcontinent.

The continent is part of the plate, it is not the only continent that moves as Wegener suggested but ocean and continents both move.

There can be three types of plate boundary:

• Divergent
• Convergence
• Transverse

Divergence boundary:
A new crust is generated as a plate pulls away from each other.
There can three types of divergence boundary:

• Ocean to ocean
• Ocean to continent
• Continent to continent

Example,
Mid Atlantic Ridge is an ocean to ocean types of divergence boundary
The boundary between Eurasia and the African plate is continent to continent divergence boundary.

Convergent boundary:
In the convergent boundary, one plate which is lighter destroys by sinking under other plates.

The sinking of the plate is called a subduction zone.
Three types of convergence can occur:

• Between ocean and continent plate
• Between ocean and ocean plate
• Between continents and continent, example, Indian plate, and Eurasian plate

Transverse boundary:

In the transverse boundary, plate neither created nor destroyed as plate slide horizontally in the opposite direction.

Rate of plate movement:

The strips of the normal and reverse magnetic fields that parallel to the ridge determined the rate of plate movement.

• Arctic ridge is the slowest rate:2.5 cm per year.
• South Pacific, moving fastest: 15 cm per year.

Force for plate movement:

• Convection current in mantle helps to the movement of the plate.
• Divergence boundary is formed when two adjacent convection cell moves in the opposite direction.
• Convergence boundary is formed when two adjacent convection cells in the same direction.
• Movement of Indian plate:

Indian Plates:

• The Indian plate is comprised of the Indian subcontinent and Australia.
• Around 200 million years ago, the Indian subcontinent was an island surrounded by Tethys sea.
• 50 million years ago, it collides with Eurasia and subduction zone is created along with Himalayan
• It is continent to continent convergence types of boundary.
• During a movement toward the Eurasian plate, major even happened such as the outpouring of lava and formation of the Deccan trap around 60 million years ago.
• Movement is still happening, Himalayan is rising even now.