Chapter 4. Distribution of Oceans and Continents

Continental Drift
• From known records of history of science, it was Abraham Ortelius, a Dutch mapmaker, who first proposed such [that at some point of time all continents were joined together] a possibility as early as 1596.
• Antonio Pellegrini drew a map showing three continents together.
• However, it was Alfred Wegener—a German meteorologist who put forth a comprehensive argument in form of ‘the continental drift theory’ in 1912.
• According to Wegener, all continents formed a single continental mass and a mega ocean surrounded same.
• supercontinent was named Pangaea, which meant all earth. mega-ocean was known as Panthalassa, meaning all water.
• Pangaea first broke into two large continental masses Laurasia and Gondwanaland forming northern and southern components respectively.

Evidence in Support of Continental Drift
There are numerous pieces of evidence found in support of Continental Drift. Let us understand them one by one.

Matching of Continents [Jig-Saw-Fit]
• shorelines of Africa and South America facing each other have a remarkable and unmistakable match. It may be noted that a map produced using a computer program to find best fit of Atlantic margin was presented by Bullard in 1964.

• This is sedimentary rock formed out of deposits of glaciers.
• Gondwana system of sediments from India is known to have its counterparts in six different landmasses of Southern Hemisphere.
• Counterparts of this succession are found in Africa, Falkland Island, Madagascar, Antarctica and Australia.
• glacial tillite provides unambiguous evidence of palaeoclimates and of drifting of continents.

Rocks of Same Age Across Oceans
• radiometric dating methods developed in recent period have facilitated correlating rock formation from different continents across vast ocean.
• belt of ancient rocks of 2,000 million years from Brazil’s coast matches with those from western Africa.

Distribution of Fossils
• When identical species of plants and animals adapted to live on land or in freshwater are found on either side of marine barriers, a problem arises regarding accounting for such distribution.
• 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. skeletons of these are found only in two localities: Southern Cape province of South Africa and Iraver formations of Brazil.

Placer Deposits
• occurrence of rich placer deposits of gold on Ghana coast and absolute absence of source rock in region is an amazing fact.
• gold-bearing veins are in Brazil and it is obvious that gold deposits of Ghana are derived from Brazil plateau when two continents lay side by side.

Force for Drifting
• Wegener suggested that movement responsible for drifting of continents was caused by polefleeing force and tidal force.
• polar-fleeing force relates to rotation of earth. You are aware of fact that earth is not a perfect sphere; it has a bulge at equator.
• Wegener believed that these forces would have become effective when applied over many million years. However, most scholars considered these forces to be inadequate.

Post-drift Studies
• Several discoveries during post–World War 2 period added new information to geological literature. Particularly, information collected from ocean floor mapping provided new dimensions for study of distribution of oceans and continents.

Convectional Current Theory
• Arthur Holmes, in 1930s, discussed possibility of convection currents operating in mantle portion.
• These currents are generated due to radioactive elements causing thermal differences in mantle portion.
• Holmes argued that there exists a system of such currents in entire mantle portion.

Mapping of Ocean Floor
• Detailed research of ocean configuration revealed that ocean floor is not just a vast plain but it is full of relief.
• Expeditions to map oceanic floor in post– World War 2 period provided a detailed picture of ocean relief and indicated existence of submerged mountain ranges as well as deep trenches, mostly located closer to continental margins.
• mid-oceanic ridges discovered to be most active in terms of volcanic eruptions.
• Rocks on either side of crest of oceanic ridges and having equidistant locations from crest discovered to have remarkable similarities both in terms of their constituents and their age.

Ocean Floor Configuration
• ocean floor may be segmented into three major divisions based on depth as well as forms of relief.
• These divisions are continental margins, deep-sea basins and mid-ocean ridges.

Continental Margins
• These form transition between continental shores and deep-sea basins.
• Of these, deep-oceanic trenches are areas that are of considerable interest in so far as distribution of oceans and continents is concerned.

Mid-Oceanic Ridges
• This forms an interconnected chain of mountain systems within ocean. This is longest mountain chain on surface of earth submerged under oceanic waters.
• rift system at crest is zone of intense volcanic activity.

Abyssal Plains
• These are extensive plains that lie between continental margins and mid-oceanic ridges.
• abyssal plains are areas where continental sediments that move beyond margins get deposited.

Distribution of Earthquakes and Volcanoes
• It bifurcates a little south of Indian subcontinent with one branch moving into East Africa and other meeting a similar line from Myanmar to New Guiana.
• In general, foci of earthquake in areas of mid-oceanic ridges are at shallow depths whereas, along Alpine-Himalayan belt as well as rim of Pacific, earthquakes are deep-seated ones.

Concept of Seafloor Spreading
• It was realised that all along mid-oceanic ridges, volcanic eruptions are common and they bring huge amounts of lava to surface in this area.
• rocks equidistant on either side of crest of midoceanic ridges show remarkable similarities in terms of period of formation, chemical compositions and magnetic properties.
• sediments on ocean floor are unexpectedly very thin. Scientists were expecting, if ocean floors were as old as continent, to have a complete sequence of sediments for a period of much longer duration.
• deep trenches have deep-seated earthquake occurrences while in mid-oceanic ridge areas, quake foci have shallow depths.

Plate Tectonics
• Since advent of concept of seafloor spreading, interest in problem of distribution of oceans and continents was revived. It was in 1967, that McKenzie and Parker and Morgan, independently collected available ideas and came out with another concept termed Plate Tectonics.
• A tectonic plate [also known as a lithospheric plate] is a massive, irregularly-shaped slab of solid rock, usually composed of both continental and oceanic lithosphere.
• Plates move horizontally over asthenosphere as rigid units.
• A plate may be referred to as continental plate or oceanic plate depending on which of two occupy a larger portion of plate.
• Pacific plate is largely oceanic whereas Eurasian plate may be known as a continental plate.
• There are three types of plate boundaries: Divergent Boundaries Where new crust is generated as plates pull away from each other. Convergent Boundaries where crust is destroyed as one plate dives under another. Transform Boundaries where crust is neither produced nor destroyed as plates slide horizontally past each other.
• major plates are as follows:
(1) Antarctica and surrounding oceanic plate
(2) North American [with western Atlantic floor separated from South American plate along Caribbean islands] plate
(3) South American [with western Atlantic floor separated from North American plate along Caribbean islands] plate
(4) Pacific plate
(5) India-Australia-New Zealand plate
(6) Africa with eastern Atlantic floor plate
(7) Eurasia and 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
Philippine plate: Between Asiatic and Pacific plate
Arabian plate: Mostly Saudi Arabian landmass

Rates of Plate Movement
• strips of normal and reverse magnetic field that parallel mid-oceanic ridges help scientists to determine rates of plate movement.
• These rates vary considerably. Arctic Ridge has slowest rate [less than 2.5 cm/yr], and East Pacific Rise near Easter. Island, in South Pacific about 3,400 km west of Chile, has fastest rate [more than 15 cm/yr].

Force For Plate Movement
• At time that Wegener proposed his theory of continental drift, most scientists believed that earth was a solid, motionless body.
• mobile rock beneath rigid plates is believed to be moving circularly. heated material rises to surface, spreads & begins to cool, and then sinks back into deeper depths.
• Heat within earth comes from two main sources: radioactive decay and residual heat.

Movement of Indian Plate
• Indian plate has a piece of Peninsular India and a piece of Australian continent. northern plate boundary is formed by subduction zone along Himalayas, which is where two continents meet.
As an oceanic ridge in SW Pacific, eastern margin is a spreading site east of Australia. boundary between India plate and Antarctic plate is an oceanic ridge that runs roughly west to east and meets a spreading site a little south of New Zealand.
• India was a big island in a big ocean, not far from coast of Australia.
• Until about 225 million years ago, Tethys Sea was between it and Asia. India is thought to have started to move north when Pangaea broke up about 200 million years ago.
• India and Asia crashed into each other about 40 to 50 million years ago. This caused Himalayas to rise quickly.
• When Indian plate moved toward Eurasian plate, lava came out of ground and Deccan Traps were made.
• Himalayas were formed between 40 million years ago and after that.

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