Look at a world map: the continents and oceans do not appear to be randomly scattered. South America and Africa fit together like pieces of a jigsaw puzzle. The distribution of oceans and continents on the Earth's surface is the result of a long geological history involving the movement of large landmasses. The study of how continents and oceans are distributed, and how they have moved over time, is the subject of this chapter. Today we know that the present arrangement of continents and oceans was very different in the geological past, and it will continue to change in the future.
The distribution of land and water is highly unequal. The Northern Hemisphere has about 60% of the Earth's land and is therefore called the land hemisphere, while the Southern Hemisphere, with about 81% water, is called the water hemisphere. This unequal distribution raises a fundamental question: why are the continents where they are? Two main theories attempt to answer this question: the theory of continental drift and the theory of plate tectonics. These theories explain the present distribution of oceans and continents and the processes that continue to reshape them.
The Continental Drift Theory was proposed by the German meteorologist Alfred Wegener in 1912. According to Wegener, all the continents were once joined together as a single supercontinent called Pangaea, surrounded by a single global ocean called Panthalassa. About 200 million years ago, Pangaea began to split. It first broke into two large landmasses: Laurasia (in the north) and Gondwanaland (in the south), separated by a T-shaped sea called the Tethys Sea. Later, Laurasia and Gondwanaland broke further into the present continents, which drifted to their current positions.
Wegener gathered evidence from many fields: - Geological evidence: The mountain belts of eastern North America and northwestern Europe match in age and structure; the coal fields and glacial deposits of present-day India, South America, Africa and Australia match. - Fossil evidence: Fossils of the same species of plants (like Glossopteris) and animals (like Mesosaurus and Lystrosaurus) are found in continents now separated by oceans, proving they were once connected. - Climatic evidence: Glacial deposits of the same age are found in regions that are now tropical (such as India and South America), while coal (a tropical deposit) is found in cold regions. - Paleomagnetic evidence: The magnetic properties recorded in rocks of the same age on different continents point to different ancient positions of the magnetic poles, supporting drift.
Wegener's theory faced serious objections. He could not provide a satisfactory mechanism for the movement of continents. His idea that the continents "ploughed" through the oceanic crust was rejected because the crust is too strong and the tides, which he suggested as a driving force, are far too weak. This weakness in explaining the "how" of continental movement caused the theory to fall out of favour until mid-century.
To provide a mechanism for continental drift, the British geologist Arthur Holmes proposed the Convectional Current Theory in the 1930s. Holmes suggested that heat generated by radioactive decay inside the Earth creates convection currents in the mantle. Hot material rises from the deep interior, cools near the surface, and then sinks back down. These currents, according to Holmes, could drag the continents along, splitting them and driving them apart. Holmes's ideas anticipated the modern plate tectonic theory and are now widely accepted as an important mechanism for plate movement.
In 1961, the American geologist Harry Hess proposed the theory of Sea Floor Spreading. He observed that the ocean floors are not old, flat plains but young, dynamic regions. Along the mid-oceanic ridges, magma rises from the mantle and creates new oceanic crust. As new crust is created, it spreads out on either side of the ridge, pushing the ocean floor apart. This process is called sea floor spreading.
The strongest evidence came from the study of the magnetic stripes on the ocean floor. As new lava cooled at the mid-oceanic ridges, it recorded the direction of the Earth's magnetic field at that time. Because the Earth's magnetic field has reversed polarity many times in the past, the ocean floor shows symmetric bands of normal and reversed magnetism on either side of the ridge. The age of the ocean floor also increases with distance from the ridge, and nowhere is the oceanic crust older than about 200 million years, unlike continental rocks that can be billions of years old. This proves that new crust is continuously being created at the ridges.
The Theory of Plate Tectonics was developed in the 1960s, synthesising the ideas of continental drift and sea floor spreading. According to this theory, the Earth's lithosphere is broken into a number of large and small rigid slabs called tectonic plates. There are about 7 major plates and many smaller ones. The major plates include the Pacific Plate, North American Plate, South American Plate, African Plate, Eurasian Plate, Indo-Australian Plate and Antarctic Plate. These plates float on the asthenosphere and move slowly relative to one another.
Plates interact at their boundaries in three ways: 1. Divergent boundaries: Plates move away from each other. New crust is formed as magma rises, creating mid-oceanic ridges and rift valleys. Example: the Mid-Atlantic Ridge. 2. Convergent boundaries: Plates move towards each other. Where an oceanic plate meets a continental plate, the heavier oceanic plate subducts, forming trenches and mountain ranges and causing volcanoes and earthquakes. Example: the Himalayas (India colliding with Eurasia). 3. Transform boundaries: Plates slide past each other horizontally, causing earthquakes. Example: the San Andreas Fault in California.
Plates are driven by convection currents in the mantle, ridge push (the weight of the elevated ridge pushes the plate away) and slab pull (the sinking of the dense subducting slab pulls the rest of the plate). The combination of these forces keeps the plates in slow but continuous motion, about 2 to 10 centimetres per year.
The present continents are remnants of the ancient supercontinents. The Earth has 71% of its surface covered by oceans and 29% by land. There are four major oceans: the Pacific (the largest), the Atlantic, the Indian and the Arctic. The seven continents are Asia (the largest), Africa, North America, South America, Antarctica, Europe and Australia (the smallest). The arrangement of these continents and oceans has changed through time and continues to change at a rate of a few centimetres per year, driven by plate tectonics.
| Theory | Proponent | Year | Core Idea |
|---|---|---|---|
| Continental Drift | Alfred Wegener | 1912 | Continents were once one landmass (Pangaea) and drifted apart |
| Convectional Current | Arthur Holmes | 1930s | Mantle convection currents drag continents |
| Sea Floor Spreading | Harry Hess | 1961 | New crust forms at mid-oceanic ridges |
| Plate Tectonics | Modern synthesis | 1960s | Lithosphere is broken into moving plates |
| Boundary Type | Plate Motion | Features | Example |
|---|---|---|---|
| Divergent | Move apart | Mid-oceanic ridges, new crust | Mid-Atlantic Ridge |
| Convergent | Move together | Mountains, trenches, subduction | Himalayas, Andes |
| Transform | Slide past | Earthquakes | San Andreas Fault |
| Land / Water Body | Area Rank | Key Fact |
|---|---|---|
| Asia | 1st continent | Largest continent |
| Pacific Ocean | 1st ocean | Largest and deepest ocean |
| Australia | Smallest continent | --- |
| Arctic Ocean | Smallest ocean | Mostly covered by ice |
The present distribution of oceans and continents is the outcome of a long and dynamic geological history. Wegener's continental drift theory, supported by fossil, geological, climatic and paleomagnetic evidence, proposed that the continents were once joined as Pangaea. Arthur Holmes supplied a mechanism through mantle convection, and Harry Hess's concept of sea floor spreading explained how new oceanic crust forms at mid-oceanic ridges. These ideas were integrated into the modern theory of plate tectonics, which describes the lithosphere as broken into moving plates interacting at divergent, convergent and transform boundaries. The continuous motion of these plates explains earthquakes, volcanoes, mountain building and the unequal distribution of land and water, and it guarantees that the map of the world will keep changing in the future.
Keywords: Pangaea, Panthalassa, Wegener, Laurasia, Gondwanaland, Tethys, Hess, sea floor spreading, plate tectonics, lithospheric plates.