Plate Tectonics & Distribution of Oceans and Continents

Look at a world map. Notice how the east coast of South America and the west coast of Africa seem to fit together like puzzle pieces. This is not a coincidence. These two continents were once joined together and have since drifted thousands of kilometres apart over hundreds of millions of years.

The theory that explains this is called Plate Tectonics, one of the most powerful ideas in all of science. It explains why earthquakes happen where they do, why volcanoes form in certain belts, why some mountain ranges are found near ocean coasts, and why the shapes of continents match across oceans. NDA tests this chapter through questions on what happens at different types of plate boundaries, which plates are major and which are minor, and what the Ring of Fire is.

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The Basic Idea: The Earth’s Crust is in Pieces

The Earth’s lithosphere (crust + upper mantle) is not one continuous shell. It is broken into large pieces called tectonic plates. There are about 7 major plates and several smaller minor plates.

These plates are not stationary. They move very slowly, roughly the speed your fingernails grow (about 2–10 cm per year). Over millions of years this slow movement adds up to enormous distances. This movement is driven by convection currents in the mantle: hot material rises, spreads sideways, cools, and sinks, dragging the plates with it.

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Continental Drift: History of the Idea

The idea that continents move was first proposed by Alfred Wegener, a German scientist, in 1912. He called it the theory of Continental Drift. He proposed that all the continents were once joined in one supercontinent called Pangaea: meaning “all land.”

Wegener’s four pieces of evidence:

The coastlines of Africa and South America fit together remarkably well, like matching pieces of a broken puzzle.

Similar fossils are found on continents now separated by oceans, suggesting these landmasses were once connected.

Similar rock types and mountain ranges match across continents separated by oceans.

Evidence of ancient ice ages has been found in tropical regions, suggesting those regions were once near the poles.

However, Wegener could not explain what force was driving the continents. His theory was rejected by many scientists during his lifetime. After his death, the discovery of sea-floor spreading in the 1950s and 1960s provided the mechanism, and the modern theory of plate tectonics was born.

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Major and Minor Tectonic Plates

The 7 Major Plates

Major Plate	Key Feature
Pacific Plate	Largest plate of all; mostly oceanic [NDA 2019-II]
North American Plate	—
South American Plate	—
Eurasian Plate	—
African Plate	—
Indo-Australian Plate	Collision with Eurasian plate formed Himalayas
Antarctic Plate	Major plate: NOT minor [NDA 2021-I]

Minor Plates

Minor plates include: Cocos, Nazca, Arabian, Philippine, Caribbean, and Caroline plates.

The Cocos, Nazca, Arabian, and Philippine plates are all minor plates. [NDA 2019-II | NDA 2021-I]

The Saudi Arabian plate is a minor plate: not a major one.

★ IMPORTANT The distinction between major and minor plates is directly tested in NDA. Pacific Plate = major (largest). Antarctic Plate = major (not minor). Cocos, Nazca, Arabian, Philippine = minor. [NDA 2019-II | NDA 2021-I]

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Types of Plate Boundaries

Where two plates meet is called a plate boundary. Almost all earthquakes, volcanoes, and mountain-building activity happen at or near plate boundaries. There are three types.

Convergent Boundaries (Collision Zones)

Where two plates move toward each other and collide. What happens depends on which types of crust are involved.

Oceanic plate meets Continental plate (Subduction): The oceanic plate is denser (SIMA) than the continental plate (SIAL). So the oceanic plate is forced downward beneath the continental plate, a process called subduction. The oceanic plate sinks into the mantle and melts. [NDA 2013-II]

The oceanic plate is forced below the continental plate: NOT the other way around. [NDA 2013-II]

Results: A deep ocean trench forms at the subduction zone. Volcanoes form on the continental plate above. Earthquakes are frequent.

Oceanic meets Oceanic: One plate sinks beneath the other. A deep trench forms and volcanic island arcs appear behind the trench.

Continental meets Continental: Neither plate sinks, as both are relatively light (SIAL). Instead, the crust crumples and folds upward, forming fold mountain ranges. The Himalayas formed when the Indo-Australian Plate crashed into the Eurasian Plate. The sediments of the ancient Tethys Sea were folded and pushed upward to form the world’s highest mountains.

The intense earthquakes along the eastern margins of Asia are caused by the subduction of the Pacific Plate under the Asiatic Plate.

Divergent Boundaries (Spreading Zones)

Where two plates move apart from each other. Hot magma from the mantle wells up through the gap, cools, and creates new oceanic crust, a process called sea-floor spreading.

Results: Mid-ocean ridges form: long underwater mountain ranges where new crust is being created. The Atlantic Ocean is getting wider as North America and Europe slowly move apart. On land, divergent boundaries create rift valleys: the East African Rift Valley is a current example.

Subduction happens at convergent boundaries: NOT at divergent boundaries. Along divergent boundaries, plates move apart and new crust forms.

Transform Boundaries

Where two plates slide past each other horizontally, neither coming together nor moving apart.

Results: No volcanoes (no subduction, no magma). Frequent, powerful earthquakes as plates grind past each other. The San Andreas Fault in California is a famous transform boundary.

Boundary Type	Movement	Results	Key Example
Convergent	Plates move toward each other	Subduction, Fold mountains, Trenches, Volcanoes	Himalayas; Pacific Ring of Fire
Divergent	Plates move apart	Mid-ocean ridges, Sea-floor spreading, Rift valleys	Mid-Atlantic Ridge; East African Rift
Transform	Plates slide past each other	Earthquakes only; no volcanoes	San Andreas Fault, California

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The Ring of Fire

The Ring of Fire is a zone of intense volcanic and earthquake activity that forms a roughly horseshoe shape around the edges of the Pacific Ocean. It follows the boundaries of the Pacific Plate.

All three of the following statements about the Ring of Fire are correct: [NDA 2018-I]

It is a zone of convergent plate boundaries: TRUE. (The Pacific Plate is being subducted at many points around its edges.) It is an active seismic and volcanic zone: TRUE. It is associated with deep trenches: TRUE (subduction creates deep ocean trenches).

The Ring of Fire includes the western coasts of North and South America, Alaska, Japan, the Philippines, Indonesia, and New Zealand. About 90% of the world’s earthquakes and 75% of the world’s volcanoes occur along or near the Ring of Fire.

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Africa and South America: Mirror Image

Africa and South America form a mirror image of each other. Their coastlines fit together remarkably well. This is one of the key pieces of evidence for Wegener’s Continental Drift theory.

These two continents were once part of the supercontinent Gondwana. As the South Atlantic Ocean opened up through sea-floor spreading at the Mid-Atlantic Ridge, they drifted apart to their current positions.

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