DISTRIBUTION OF OCEANS AND CONTINENTS
You know that continents cover 29 per cent of the surface of the earth and the remainder is under oceanic waters.
- Alfred Wegener—a German meteorologist who put forth a comprehensive argument in the form of “the continental drift theory” in 1912.
- This was regarding the distribution of the oceans and the continents.
- According to Wegener, all the continents formed a single continental mass and Mega Ocean surrounded the same.
- The super continent was named PANGAEA, which meant all earth.
- The mega-ocean was called PANTHALASSA, meaning all water.
- He argued that, around 200 million years ago, the super continent, Pangaea, began to split into Laurasia and Gondwanaland forming the northern and southern components respectively.
- Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that exist today
Evidence in Support of the Continental Drift
- The Matching of Continents (Jig-Saw-Fit):
- The shorelines of Africa and South America facing each other have remarkable match.
- The best fit of the Atlantic margin was presented by Bullard in 1964.
- Rocks of Same Age Across the Oceans:
- The belt of ancient rocks of 2,000 million years 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.
- It is the sedimentary rock formed out of deposits of glaciers.
- The Gondawana system of sediments from India is known to have its counter parts in six different landmasses of the Southern Hemisphere.
- Counter parts of this succession are found in Africa, Falkland Island, Madagascar, Antarctica and Australia besides India.
- Placer Deposits:
- The occurrence of rich placer deposits of gold in the Ghana coast and the absolute absence of source rock in the region is an amazing fact.
- The gold bearing veins are in Brazil and it is obvious that the gold deposits of the Ghana are derived from the Brazil plateau when the two continents lay side by side.
- Distribution of Fossils:
- When identical species of plants and animals adapted to living on land or in fresh water are found on either side of the marine barriers, a problem arises regarding accounting for such distribution.
- The observations that Lemurs occur in India, Madagascar and Africa led some to consider a contiguous landmass “Lemuria” linking these three landmasses.
- Mesosaurus was a small reptile adapted to shallow brackish water. The skeletons of these are found only in two localities : the Southern Cape province of South Africa and Iraver formations of Brazil. The two localities presently are 4,800 km apart with an ocean in between them.
Force for Drifting
Wegener suggested that the movement responsible for was caused by
- Pole-fleeing force
- Tidal force
Convectional Current Theory:
- Arthur Holmes in 1930s discussed the possibility of convection currents operating in the mantle portion.
- These currents are generated due to radioactive elements causing thermal differences in the mantle portion.
Mapping of the Ocean Floor:
- Detailed research of the ocean configuration revealed that the ocean floor is not just a vast plain but it is full of ocean relief and indicated the existence of submerged mountain ranges as well as deep trenches, mostly located closer to the continent margins.
- The mid-oceanic ridges were found to be most active in terms of volcanic eruptions.
- The dating of the rocks from the oceanic crust revealed the fact that they are much younger than the continental areas.
- Rocks on either side of the crest of oceanic ridges and having equi-distant locations from the crest were found to have remarkable similarities both in terms of their constituents and their age.
CONCEPT OF SEA FLOOR SPREADING
Mapping of the ocean floor and palaeomagnetic studies of rocks from oceanic regions revealed the following facts :
- It was realised that all along the midoceanic ridges, volcanic eruptions bring huge amounts of lava.
- The rocks equidistant on either sides show remarkable similarities in terms of period of formation, chemical compositions and magnetic properties. Rocks closer to the mid-oceanic ridges have normal polarity and are the youngest. The age of the rocks increases as one moves away from the crest.
- The ocean crust rocks are much younger than the continental rocks. The age of rocks in the oceanic crust is nowhere more than 200 million years old. Some of the continental rock formations are as old as 3,200 million years.
- The sediments on the ocean floor are unexpectedly very thin.
- The deep trenches have deep-seated earthquake occurrences while in the midoceanic ridge areas, the quake foci have shallow depths.
These facts led Hess (1961) to propose his hypothesis, known as the “sea floor spreading”.
- In 1967, McKenzie, Parker and Morgan, independently collected the available ideas and came out with another concept termed Plate Tectonics.
- A tectonic plate (also called lithospheric plate) is a massive, irregularly-shaped slab of solid rock, generally composed of both continental and oceanic lithosphere.
- Plates move horizontally over the asthenosphere as rigid units.
- A plate may be referred to as the continental plate or oceanic plate depending on which of the two occupy a larger portion of the plate.
- The theory of plate tectonics proposes that the earth’s lithosphere is divided into seven major and some minor plates.
The major plates are as follows:
- Antarctica and the surrounding oceanic plate
- North American (with western Atlantic floor separated from the South American plate along the Caribbean islands) plate
- South American (with western Atlantic floor separated from the North American plate along the Caribbean islands) plate
- Pacific plate
- India-Australia-New Zealand plate
- Africa with the eastern Atlantic floor plate
- Eurasia and the adjacent oceanic plate.
Some important minor plates are listed below:
- Cocos plate : Between Central America and Pacific plate
- Nazca plate : Between South America and Pacific plate
- Arabian plate : Mostly the Saudi Arabian landmass
- Philippine plate : Between the Asiatic and Pacific plate
- Caroline plate : Between the Philippine and Indian plate (North of New Guinea)
- Fuji plate : North-east of Australia.
There are three types of plate boundaries:
- Where new crust is generated as the plates pull away from each other.
- The sites where the plates move away from each other are called spreading sites.
- The best-known example of divergent boundaries is the Mid-Atlantic Ridge.
- At this, the American Plate(s) is/are separated from the Eurasian and African Plates.
- Where the crust is destroyed as one plate dived under another.
- The location where sinking of a plate occurs is called a subduction zone.
- There are three ways in which convergence can occur.
- Between an oceanic and continental plate.
- Between two oceanic plates.
- Between two continental plates.
- Where the crust is neither produced nor destroyed as the plates slide horizontally past each other.
- Transform faults are the planes of separation generally perpendicular to the midoceanic ridges.
- As the eruptions do not take all along the entire crest at the same time, there is a differential movement of a portion of the plate away from the axis of the earth.
- The rotation of the earth has its effect on the separated blocks of the plate portions.
Rates of Plate Movement
- The strips of normal and reverse magnetic field that parallel the mid-oceanic ridges help scientists determine the rates of plate movement.
- The Arctic Ridge has the slowest rate (less than 2.5 cm/yr), and the East Pacific Rise near Easter
- Island, in the South Pacific about 3,400 km west of Chile, has the fastest rate (more than 15 cm/yr).
Force for the Plate Movement
- Concepts of sea floor spreading and the unified theory of plate tectonics have emphasised that both the surface of the earth and the interior are not static and motionless but are dynamic.
- The heated material rises to the surface, spreads and begins to cool, and then sinks back into deeper depths.
- This cycle is repeated over and over to generate what scientists call a convection cell or convective flow.
- Heat within the earth comes from two main sources: radioactive decay and residual heat.
- Arthur Holmes first considered this idea in the 1930s, which later influenced Harry Hess’ thinking about seafloor spreading.