Distribution of Oceans and Continents

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (Fundamentals of Physical Geography), Chapter 4.

Do the continents fit together like a jigsaw?

Look at a world map and slide South America towards Africa. The bulge of Brazil sits in the bend of West Africa as if it had once been part of it. Continents cover 29 per cent of the earth's surface and the rest is ocean, and these positions are not permanent. This chapter asks what the positions were in the past, how scientists know, and why and how they change. The answer took more than fifty years of argument, and it changed geography for good.

Wegener's continental drift theory

In 1912 Alfred Wegener, a German meteorologist, put forward a full argument called the continental drift theory. He said all continents once formed a single mass, surrounded by a mega ocean.

About 200 million years ago Pangaea began to split. It first broke into Laurasia and Gondwanaland, which then broke into the smaller continents we know today.

Wegener was not the first to notice the fit. From the known records, the Dutch map maker Abraham Ortelius proposed the possibility as early as 1596, and Antonio Pellegrini drew a map showing three continents together. But Wegener gave the first comprehensive argument.

The evidence for drift

Why the fossil clue is strong. Mesosaurus was adapted to shallow brackish water, not to open ocean. The simplest explanation for the same reptile on two continents is that the two lay together when it lived.

Pause and predict: gold is found on the Ghana coast, but no gold bearing rock exists anywhere nearby. Where is the source, and what does that suggest?

Check your answer: the source is in the Brazil plateau. The deposits only make sense if Africa and South America were joined, and the gold was carried from Brazil into what is now Ghana.

Why most scientists rejected Wegener

The weak point was the force. Wegener suggested two.

He believed these forces would work if they acted over many millions of years, but most scholars considered them totally inadequate. The theory lost support, not because the evidence was wrong, but because nobody could explain what moved the continents.

After drift: new clues from the ocean floor

Wegener's evidence came from the continents (distribution of plants, animals and deposits like tillite). After World War II, the mapping of the ocean floor changed everything.

Convection currents. In the 1930s Arthur Holmes suggested that convection currents operate in the mantle. They are generated by thermal differences due to radioactive elements. His idea was an attempt to supply the missing force, the very thing for which contemporaries had thrown out drift.

The ocean floor is not a flat plain. Mapping showed a floor full of relief, with submerged mountain ranges and deep trenches mostly near the continental margins. The mid oceanic ridges are the most active in volcanic eruptions. Rocks of the oceanic crust are much younger than the continents. Rocks on either side of a ridge crest, at equal distances from it, are remarkably similar in both composition and age.

The ocean floor has three major divisions.

Where earthquakes and volcanoes occur. A line of earthquake and volcano points runs through the central Atlantic Ocean almost parallel to the coastlines, and it coincides with the mid oceanic ridge. It extends into the Indian Ocean and divides a little south of the Indian subcontinent, with one branch going to East Africa and the other meeting a similar line from Myanmar to New Guinea. Another belt coincides with the Alpine Himalayan system and the rim of the Pacific Ocean, which is called the rim of fire because of its active volcanoes.

Sea floor spreading: the missing mechanism

Ocean floor mapping and studies of the magnetism of ocean rocks revealed five facts.

From these, Hess (1961) proposed the hypothesis of sea floor spreading. Constant eruptions at the crest of an oceanic ridge rupture the oceanic crust, and new lava wedges in, pushing the crust on either side apart. The ocean floor spreads. Because one ocean spreading does not make another shrink, and because the crust is young, Hess concluded that old ocean floor must be consumed. The crust pushed away from the crest sinks down at the oceanic trenches and is consumed.

The cycle in one line: lava erupts at the ridge → new crust forms and spreads → crust travels across the ocean → sinks at a trench and is consumed.

Why it works this way. If ocean floors were as old as the continents, they would carry a complete sediment record of a much longer time. The ocean crust is recycled. It is made at the ridge and destroyed at the trench, which keeps its age below about 200 million years. The continents are not recycled in the same way, so their rocks can be far older.

Pause and predict: you sample rocks at the crest of a mid oceanic ridge and again 1,000 km away. Which sample is older, and what does this tell you about the ocean floor?

Check your answer: the sample 1,000 km away is older. Age increases with distance from the crest, which shows that new crust forms at the ridge and spreads outwards.

Plate tectonics: the unified theory

In 1967, McKenzie and Parker, and Morgan, independently put the ideas together as plate tectonics. A tectonic plate (lithospheric plate) is a massive, irregular slab of solid rock, usually made of both continental and oceanic lithosphere. Plates move horizontally over the asthenosphere as rigid units. The lithosphere includes the crust and uppermost mantle. Its thickness varies from 5 to 100 km in oceanic parts and is about 200 km in continental areas.

A plate is called continental or oceanic according to which type fills the larger part. The Pacific plate is largely oceanic, while the Eurasian plate is called a continental plate. The lithosphere is divided into seven major plates and several minor ones, which are surrounded by young fold mountains, trenches or faults.

Seven major plates. Antarctica and the surrounding oceanic plate; North American; South American; Pacific; India-Australia-New Zealand; Africa with the eastern Atlantic floor; Eurasia and the adjacent oceanic plate. The North American and South American plates, with the western Atlantic floor, are separated along the Caribbean islands.

A key correction to Wegener. It is not the continent that moves. Continents are part of a plate, and the plate moves. Every plate has moved in the geological past and will keep moving. Pangaea itself was the result of different continental masses, parts of different plates, converging; the continents had been wandering throughout geological time.

Two views. For the exam: Wegener's drift theory said the continents moved, and plate tectonics says plates move, carrying continents with them. Another reading: scientists often describe the two as stages of one story, with Wegener giving the evidence of movement, sea floor spreading giving the mechanism of new crust and plate tectonics bringing it all together as one theory. Wegener's central idea that the continents were once joined survived, but his forces did not.

Three kinds of plate boundary

Transform faults exist because eruptions do not occur all along the ridge crest at the same time, so a portion of the plate moves differently, and the earth's rotation also affects the separated blocks.

Rates of plate movement. The strips of normal and reverse magnetic field that parallel the mid oceanic ridges let scientists work out how fast plates move. The rates vary widely. The Arctic Ridge is the slowest at less than 2.5 cm a year, and the East Pacific Rise near Easter Island, about 3,400 km west of Chile, is the fastest at more than 15 cm a year.

The force behind plate movement. At the time of Wegener most scientists thought the earth was solid and motionless. Sea floor spreading and plate tectonics show that both its surface and its interior are dynamic. The soft mobile rock beneath the rigid plates moves in a circular way: heated material rises, spreads and cools, and then sinks again. This repeated cycle is a convection cell. The heat comes from two main sources, radioactive decay and residual heat. Holmes first considered the idea in the 1930s, and it later influenced Hess. The slow movement of hot softened mantle below the rigid plates is the driving force of plate movement.

The Indian plate's long journey

The Indian plate includes Peninsular India and the Australian continental portion.

Beyond the textbook. For the exam: India collided with Asia 40 to 50 million years ago, the Himalayas are formed by continent continent convergence, and the Indian plate is still moving north. In reality: because both plates carry light continental crust, neither sinks easily, so the crust piles up into the thick crust and high mountains of the Himalayan region, and the same squeeze builds the stress that causes earthquakes there. Why it matters: it links this chapter to the crust thickness under the Himalayas in the previous chapter and to the earthquake zones in Natural Hazards and Disasters.

Beyond the textbook. For the exam: Wegener's date for the breakup of Pangaea is about 200 million years ago. In reality: Pangaea itself was assembled from earlier continental pieces, and the continents have repeatedly come together and split apart. Pangaea is the latest supercontinent that we can reconstruct well, not the beginning of the story. Why it matters: it explains why the textbook says that Pangaea was a result of converging continental masses and not a permanent first state.

Exam traps

60-second revision

Where this chapter connects

• Previous: Interior of the Earth described the lithosphere, asthenosphere and mid ocean volcanoes that this chapter sets in motion.
• Next: Geomorphic Processes shows how the movements inside the earth, including plate motion, build landforms.
• Ocean link: Water (Oceans) studies the ocean floor relief that this chapter introduces.
• India link: Structure and Physiography places the Himalayas, Peninsular block and Deccan Traps in India's geological structure.
• Hazard link: Natural Hazards and Disasters uses the northward push of the Indian plate to explain Himalayan earthquakes.