🌎DISTRIBUTION OF OCEANS AND CONTINENTS

Continental Drift :  

Continental drift was a theory that explained how continents shift position on Earth's surface. Abraham Ortelius, a Dutch map maker first proposed such a possibility as early as 1596. Antonio Pellegrini drew a map showing - Americas, Europe and Africa - the three continents together.  

Alfred Wegener, a German meteorologist put forth The Continental Drift Theory. According to him all continents formed a single continental mass called Pangaea. All oceans formed a single universal ocean called Panthalassa. Around 200 million years ago the Pangaea began to split into two large masses called Laurasia and Gondwanan land. By further splitting Laurasia formed northern continents and Gondwanaland formed southern continents.

Evidence in Support of the Continental Drift 

The matching of continents (jig-saw fit): The shorelines of S. America and Africa have remarkable match. It was a map that was produced by Bullard in 1964 to show the jigsaw fit of these two continents. It was fit around 1000 fathom line of the shoreline. The Atlantic coasts of Africa and South America appear to fit together neatly, like the pieces of a jigsaw puzzle.  

Rocks Of Same Age Across The Oceans: The belt of ancient rocks of 2000 million years from Brazil coast matches with those of Western Africa. Marine deposits of South America and Africa belong to the Jurassic age. This suggests that the ocean did not exist prior to that time.  

Tillite: It is the sedimentary rock formed out of deposits of glaciers. The Gondwanan system of sediments from India is known to have its counterparts in six different landmasses of the Southern Hemisphere. At the base the system has thick tillite indicating extensive and prolonged glaciation. Counter parts of this succession are found in Africa, Falkland Island, Madagascar, Antarctica and Australia besides India. Overall resemblance of the Gondwanan type sediments clearly demonstrate that these landmasses had remarkably similar histories. The glacial tillite provides unambiguous evidence of paleoclimates and also of drifting of continents.  

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:  Identical species of animals and plants are found along the coastal regions of the different continents. The observation of lemurs occurs in India, Madagascar and Africa led some to consider a contiguous landmass called LEMURIA. The fossils of mosasaurs were found in only South Africa and Brazil. 

Force Of Drifting  : Wegener suggested that the movement responsible for the drifting of the continents was caused by pole-fleeing force and tidal force. The polar-fleeing force relates to the rotation of the earth. The earth is not a perfect sphere; it has a bulge at the equator. This bulge is due to the rotation of the earth. The second force that was suggested by Wegener is the tidal force is due to the attraction of the moon and the sun that develops tides in oceanic waters. Wegener believed that these forces would become effective when applied over many million years. However, most scholars considered these forces to be totally inadequate.

Post Drift Studies

Information collected from the ocean mapping is more useful to study the continental drift.  

Convectional Current Theory  : It was proposed by Arthur Holmes in 1930. These currents are generated due to radioactive elements causing thermal differences in the mantle portion. Holmes argued that there exists a system of such currents in the entire mantle portion.  

Mapping Of The Ocean Current  : Detailed research revealed that the ocean floor is not just a vast plain but it is of relief. Expeditions to map the oceanic floor in the post-war period provided a detailed picture of the ocean relief and indicated the existence of submerged mountain ranges as well as deep trenched 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 equip-distant locations from the crest were found to have remarkable similarities both in terms of their constituents and their age.  

Ocean Floor Configuration

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

Continental Margins :  These form the transition between continental shores and deep-sea basins. They include continental shelf, continental slope, continental rise and deep-oceanic trenches. The deep-oceanic trenches are the areas which are of considerable interest in so far as the distribution of oceans and continents is concerned.

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

Mid-Oceanic Ridges  :This forms an interconnected chain of mountain system within the ocean. It is the longest mountain-chain on the surface of the earth though submerged under the ocean waters.  

It is characterized by a central rift system at the crest( which is a zone of intense volcanic activity), a fractionated plateau and flank zone all along its length. 

Distribution of Earthquakes And Volcanoes 

Plate tectonics cause earthquakes and volcanoes. The point where two plates meet is called a plate boundary. Earthquakes and volcanoes are more likely to occur either on or near plate boundaries. The focal points of the earthquake in the areas of mid-oceanic ridges are at shallow depths whereas along the Alpine-Himalayan belt as well as the rim of the Pacific, the earthquakes are deep-seated ones. The rim of the Pacific is also called Rim of fire due to the existence of active volcanoes in this area. 

Concept Of Seafloor Spreading 

Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Concept of seafloor spreading was proposed by Hess in 1981 who believed that new lava pushes out the plates from the mid-oceanic ridge. Particularly, the mapping of the ocean floor and paleomagnetic studies of rocks from oceanic regions revealed the following facts :  

Constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust and the new lava wedges into it, pushing the oceanic crust on either side. The ocean floor, thus spreads. The younger age of the oceanic crust as well as the fact that the spreading of one ocean does not cause the shrinking of the other, made Hess think about the consumption of the oceanic crust. He further maintained that the ocean floor that gets pushed due to volcanic eruptions at the crest, sinks down at the oceanic trenches and gets consumed.

Plate Tectonics   

Plate tectonics is the theory that Earth's outer shell is divided into several plates that glide over the mantle, the rocky inner layer above the core. The theory of plate tectonics was introduced by McKenzie, Parker and Morgan in 1967. The plates act like a hard and rigid shell compared to Earth's mantle. This strong outer layer is called the lithosphere. It is a massive irregularly shaped slab of solid rock. Plates move horizontally over the Asthenosphere. Average thickness is 100 km of oceanic part and 200 km continental part. Pacific plate is the largest oceanic plate whereas Eurasian plate is the largest continental 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 Surrounding Oceanic Plate  

- North American Plate  

- South American Plate  

- Pacific Plate  

- India-Australia-New Zealand Plate  

- African Plate Eurasian Plate  

- Eurasia and the adjacent oceanic plate.  

Some of the minor plates are:  

- 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 

Wegener had thought of all the continents to have initially existed as a supercontinent in the form of Pangaea, and Pangaea was a result of converging of different continental masses that were parts of one or the other plates.  

These plates are moving constantly throughout geological time not the continent believed by Wegener Pangaea was the convergent of all the plates. Position of Indian subcontinent is traced with the help of rocks analyzed from the Nagpur area

Types of Plate Boundaries  

There are three types of plate boundaries:  

1. Divergent 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. 

2. Convergent Boundaries : 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. These are: Between an oceanic and continental plate; Between two oceanic plates; and Between two continental plates.  

3. Transform Boundary  : 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 mid oceanic 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. Also, the rotation of the earth has its effect on the separated blocks of the plate portions.  

4. 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). 

5. Force For The Plate Movement  : At the time that Wegener proposed his theory of continental drift, most scientists believed that the earth was a solid, motionless body. However, concepts of sea floor spreading and the unified theory of plate tectonics have emphasized that both the surface of the earth and the interior are not static and motionless but dynamic. The mobile rock beneath the rigid plates is believed to be moving in a circular manner. 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 i.e., Radioactive decay and Residual heat. Arthur Holmes first considered this idea in the 1930s, which later influenced Harry Hess’ thinking about seafloor spreading. The slow movement of hot, seemed mantle that lies below the rigid plates is the driving force behind the plate movement.  

Movement Of The Indian Tectonic Plate

1. The Indian plate includes Peninsular India and the Australian continental portions. The subduction zone along the Himalayas forms the northern plate boundary in the form of continent-continent convergence.  

2. In the east, it extends through Rakhine Yoma Mountains (Arakan Yoma) of Myanmar towards the island arc along the Java Trench.  

3. The eastern margin is a spreading site lying to the east of Australia in the form of an oceanic ridge in SW Pacific.  

4. The Western margin follows Kirthar Mountain of Pakistan. It further extends along the Makrana coast (Pakistan and Iranian coasts) and joins the spreading site from the Red Sea rift (Red Sea rift is formed due to the divergence of Somali plate and Arabian plate) south-eastward along the Chagos Archipelago (Formed due to hotspot volcanism).  

5. The boundary between India and the Antarctic plate is also marked by an oceanic ridge (divergent boundary) running in roughly W-E direction and merging into the spreading site, a little south of New Zealand. 

Movement 

India was a large island situated off the Australian coast. The Tethys Sea separated it from the Asian continent till about 225 million years ago. India is supposed to have started her northward journey about 200 million years (Pangaea broke). About 140 million years ago, the subcontinent was located as south as 50' S latitude.  

The Tethys Sea separated the Indian plate and the Eurasian plate and The Tibetan block was a part of the Asiatic landmass. India collided with Asia about 40-50 million years ago causing rapid uplift of the Himalayas (the Indian plate and the Eurasian plate were close to the equator back then).  

It’s thought that India’s coastline was denser and more firmly attached to the seabed, which is why Asia’s softer soil was pushed up rather than the other way around. The process is continuing, and the height of the Himalayas is rising even to this date.  

The northward movement of the Indian tectonic plate pushing slowly against the Asiatic plate is evident by the frequent earthquakes in the region.  

During the movement of the Indian plate towards the Asiatic plate, a major event that occurred was the outpouring of lava and formation of the Deccan Traps (shield volcano). The shield volcanism started somewhere around 60 million years ago and continued for a long period.  

Share

& Comment

Tweet