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Continental drift, sea-floor spreading and plate tectonics

Geomorphology: plates, earthquakes, volcanoes and landforms · Part 1 / 4 · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

Endogenic and exogenic forces, Wegener's continental drift and its evidence, Holmes and convection currents, the mapping of the ocean floor, Hess and sea-floor spreading, palaeomagnetism, the major and minor plates, the three kinds of plate boundary, and India's journey to the Himalaya.

Geomorphology is the study of landforms: how mountains, plateaus, valleys and coasts come into being and how they change. Two sets of forces shape the surface, one building relief up from inside the Earth and the other wearing it down from outside. This chapter takes them in that order. Part 1 explains the big idea that ties the internal forces together: the continents and ocean floors ride on moving plates. Part 2 turns to earthquakes and volcanoes, which happen mostly where plates meet. Parts 3 and 4 follow the external forces, from weathering to the landforms made by rivers, glaciers, wind, groundwater and waves.

endogenic forces come from the Earth's internal heat, much of it released by radioactive decay, and act from within. Slow movements of the crust are called diastrophism: orogeny is mountain building, in which rock is squeezed into folds and broken along faults, while epeirogeny is the gentle up-warping or down-warping of large parts of a continent. Sudden movements include earthquakes and volcanic eruptions. exogenic forces draw their energy from the Sun, working through the atmosphere, and from gravity. Running water, glaciers, wind, waves and groundwater break rock down by weathering, carry it away by erosion and lay it down elsewhere by deposition; this levelling work is called gradation. Put simply, the inside of the Earth keeps pushing parts of the surface up while the outside keeps grinding them down, and every landscape records how far each side has got.

The idea that continents move was set out most fully by Alfred Wegener, a German meteorologist, in 1912. He was struck by how the coastlines on either side of the Atlantic, especially those of South America and Africa, match like pieces of a jigsaw. In Wegener's view, today's continents were once fused into one landmass. He called it Pangaea ('all land') and gave the name Panthalassa ('all sea') to the single ocean around it. Roughly 200 million years ago Pangaea started to come apart; it divided first into Laurasia in the north and Gondwanaland in the south, and these later broke into the continents of today. India belonged to Gondwanaland, along with South America, Africa, Australia and Antarctica. This theory of continental drift said the continents had moved, and were still moving, across the globe.

Wegener gathered several kinds of evidence. Besides the jigsaw fit, rocks of the same age and type appear on coasts now separated by oceans, such as the facing shores of Brazil and West Africa. Tillite, rock formed from the debris of ancient glaciers, is found in India, Africa, South America, Australia and Antarctica; in India it belongs to the Gondwana deposits, suggesting these lands once lay together under the same ice sheet. Ghana's coast holds rich placer gold with no source rock anywhere near it, yet matching gold-bearing rock turns up in Brazil, which suggests the two coasts once lay side by side. Fossils agree: remains of Mesosaurus, a small reptile of fresh water, turn up only in Brazil and southern Africa, and Glossopteris plants are found across the southern continents and India. Neither could have crossed a wide ocean.

Despite this evidence, most scientists of Wegener's day rejected continental drift because of its mechanism. Wegener suggested that continents were moved by a 'pole-fleeing' force linked to the Earth's rotation and by tidal forces, but both were far too weak to shift whole continents, and it seemed impossible for continents to plough through solid ocean floor. A more promising engine was proposed in the 1930s by the British geologist Arthur Holmes. Heat from radioactive elements in the mantle, he argued, sets up slow convection currents: hot material rises, spreads sideways and sinks again as it cools, like water circulating in a pot on a stove. Such currents could drag the crust above them.

After the Second World War, echo-sounding and magnetic surveys mapped the ocean floor in detail, and it turned out to be neither flat nor ancient. A chain of underwater mountains, the mid-ocean ridge, winds through all the oceans, with a rift valley along its crest and frequent volcanic activity; the Mid-Atlantic Ridge runs down the middle of the Atlantic and rises above the sea in Iceland. Deep, narrow trenches lie along some continental edges and island chains, the deepest being the Mariana Trench in the western Pacific, close to 11 km deep. Three findings stood out. Rocks are youngest at the ridges and grow older away from them. Sediment on the sea floor is thinnest near the ridges. And no ocean floor is older than about 200 million years, whereas some continental rocks are around 4,000 million years old.

In the early 1960s the American geologist Harry Hess drew these findings together as sea-floor spreading. Magma rises along the mid-ocean ridges, cools and forms new oceanic crust, which is pushed aside in both directions as still newer crust forms behind it. The ocean floor thus moves away from the ridges like two slow conveyor belts, which is why the youngest rocks lie at the crest. Since the Earth is not growing, old crust must be destroyed somewhere at the same rate, and Hess proposed that it sinks back into the mantle at the trenches. Strong support came from palaeomagnetism. The Earth's magnetic field has reversed many times, and lava records the field's direction as it cools. Bands of normal and reversed magnetism run parallel to the ridges and match on either side, exactly as a conveyor belt would record them.

By the late 1960s continental drift and sea-floor spreading had been combined into the theory of plate tectonics. It holds that the lithosphere is broken into rigid slabs, called plates, which move over the weak asthenosphere. A plate may carry continental crust, oceanic crust or both, so the continents do not drift on their own but ride on plates. Most lists name seven large plates: Pacific, Eurasian, African, Antarctic, Indo-Australian, North American and South American. Largest of all is the Pacific plate, which is made almost entirely of ocean floor. Many minor plates lie between them, among them the Nazca, Cocos, Caribbean, Arabian, Philippine and Juan de Fuca plates, and many geologists now treat the Indian and Australian parts as separate plates. Plates move only a few centimetres a year, but over millions of years that adds up to thousands of kilometres.

Most geological action happens at plate boundaries, of which there are three kinds. At a divergent, or constructive, boundary, plates move apart and new crust forms from rising magma; the mid-ocean ridges are the main examples, and the East African Rift Valley shows the same process beginning to split a continent. At a transform, or conservative, boundary, plates slide past each other sideways, so crust is neither made nor destroyed, but the grinding causes frequent earthquakes; the San Andreas Fault in California is the best-known example. At a convergent, or destructive, boundary, plates move towards each other and crust is consumed or crumpled. What happens there depends on what the two plates carry, and convergent boundaries produce the deepest trenches, the most explosive volcanoes and the highest mountains on Earth.

When ocean crust runs into a continent, the heavier oceanic plate is forced under and descends into the mantle; this is subduction. A trench forms offshore, and melting at depth feeds a line of volcanoes on the land; the Nazca plate sinking beneath South America has raised the volcanic Andes in this way. Where two oceanic plates meet, one is subducted beneath the other, producing a deep trench and a curved chain of volcanic islands, an island arc, as in Japan and the Mariana Islands. Where two continental plates meet, neither sinks easily, because continental crust is too light. Instead the crust is squeezed, folded and thickened into great fold mountains. The best-known example is the Himalaya, pushed up as the Indian plate drove into the Eurasian plate; with no plate sinking there, the Himalaya has no active volcanoes, while the Andes do.

India's own journey shows plate tectonics at work. As Gondwanaland broke up, the Indian landmass separated from Africa, Madagascar and Antarctica and moved north across the ocean, unusually fast for a plate. On the way it passed over a hotspot, a stationary plume of hot material rising from deep in the mantle; the vast lava outpourings that built the Deccan Traps about 66 million years ago are linked to the hotspot that now lies beneath Réunion Island. A similar hotspot under the Pacific has built the Hawaiian island chain as the plate moves over it. Roughly 50 million years ago India collided with Asia. The Tethys Sea between them closed, and its sediments, with the edge of the Indian plate, were crumpled and lifted to form the Himalaya. The Indian plate is still pushing north by a few centimetres a year, so the Himalaya is still rising and the region remains earthquake-prone.

Seen together, the ideas of this part form one story. Wegener showed that the continents had moved but could not explain how. Holmes offered convection in the mantle as a driving force, and mapping of the ocean floor revealed spreading at the ridges and destruction of crust at the trenches. Plate tectonics combined them: rigid lithospheric plates are created at divergent boundaries, consumed or crumpled at convergent ones, and slide past each other at transform faults. The theory explains why fold mountains, trenches and island arcs lie where they do, why the ocean floor is young, and why India sits at the foot of the Himalaya. It also explains why earthquakes and volcanoes cluster along plate boundaries, the subject of the next part.

Exam traps

• Names and ideas are often mixed up: Wegener proposed continental drift (1912), Holmes mantle convection currents (1930s) and Hess sea-floor spreading (early 1960s).
• Pangaea was surrounded by Panthalassa and split into Laurasia in the north and Gondwanaland in the south; India was part of Gondwanaland, not Laurasia.
• Ocean-floor rocks are youngest at the mid-ocean ridges and older towards the trenches; no part of the ocean floor is much older than about 200 million years, yet some rocks on the continents are close to 4,000 million years old.
• Divergent = constructive, convergent = destructive, transform = conservative. At transform boundaries crust is neither created nor destroyed.
• A continent-continent collision builds fold mountains without active volcanoes (Himalaya); ocean-continent subduction builds volcanic mountains (Andes).
• Plates are pieces of the lithosphere (crust plus uppermost mantle) moving over the asthenosphere; continents ride on plates rather than drifting on their own.
• Orogeny is mountain building; epeirogeny is the broad up- or down-warping of continents. Both are endogenic; weathering, erosion and deposition are exogenic.
• Wegener's own forces (pole-fleeing and tidal) were too weak, which is why his theory was rejected; the evidence for drift itself was not the problem.

Recap

• Endogenic forces (internal heat: diastrophism, volcanism, earthquakes) build relief; exogenic forces (Sun and gravity: weathering, erosion, deposition) wear it down.
• Wegener (1912): Pangaea and Panthalassa; break-up from about 200 million years ago into Laurasia and Gondwanaland; India in Gondwanaland.
• Evidence: jigsaw fit, matching rocks across oceans, tillite, Ghana's placer gold and Brazil, Mesosaurus and Glossopteris.
• Rejected for want of a mechanism; Holmes (1930s) proposed mantle convection currents.
• Ocean floor: mid-ocean ridges, trenches (Mariana, about 11 km); youngest rocks at ridges; ocean floor no older than about 200 million years.
• Hess (early 1960s): sea-floor spreading; crust made at ridges, destroyed at trenches; confirmed by symmetrical magnetic stripes.
• Plate tectonics: seven major plates and many minor ones on the asthenosphere; divergent, convergent and transform boundaries.
• India: drifted north from Gondwanaland, passed over the Réunion hotspot (Deccan Traps), hit Asia about 50 million years ago; the Himalaya still rises.

Practice questions

Sources

• United States Geological Survey: educational material on landforms and the processes that shape the Earth's surface (US government) — https://www.usgs.gov/programs/earthquake-hazards
• United States Geological Survey: 'This Dynamic Earth: The Story of Plate Tectonics' (W. J. Kious and R. I. Tilling, US government publication) — https://pubs.usgs.gov/publication/7000097
• NOAA Ocean Exploration and National Ocean Service: mid-ocean ridges, ocean trenches and the Mariana Trench (National Oceanic and Atmospheric Administration, US government) — https://oceanexplorer.noaa.gov
• Geological Survey of India, Ministry of Mines: geology of India, the Gondwana deposits, the Deccan Traps and the Himalaya — https://www.gsi.gov.in