Continental Drift and Seafloor Spreading Evidence

Continental drift is the idea, proposed by Alfred Wegener in 1912, that the continents have moved slowly across Earth’s surface over geological time, and seafloor spreading is the process by which new oceanic crust forms at mid-ocean ridges and moves away on both sides; together they led to the theory of plate tectonics. In brief: the evidence includes the jigsaw fit of the continents, matching rocks and mountain belts, identical fossils on separate continents, ancient climate and glacial evidence, the symmetrical magnetic stripes and the increasing age of the ocean floor away from the ridges, and direct measurement of plate motion by GPS. Plates move at about 1–10 cm per year, roughly the speed at which fingernails grow.

The theory of plate tectonics ties together earthquakes, volcanoes, mountains and ocean basins. This guide describes the evidence for continental drift and seafloor spreading, compares the lines of evidence in a table, works through rate and time calculations, outlines the history of the idea and corrects common mistakes.

What is continental drift?

Alfred Wegener, a German meteorologist and geophysicist, proposed in 1912 (and published in his book The Origin of Continents and Oceans in 1915) that all the continents were once joined in one supercontinent, which he called Pangaea (“all land”), surrounded by one ocean, Panthalassa. Pangaea began to break up about 200 million years ago into northern Laurasia and southern Gondwana, which then split into the present continents. Wegener’s weakness was that he could not explain the force that moved the continents (he suggested the tides and Earth’s rotation, which are far too weak), and so most geologists rejected the idea for decades.

What was the evidence for continental drift?

• Fit of the continents: the coasts of South America and Africa fit together like jigsaw pieces, and the fit is even better if the edges of the continental shelves (about 200 m depth), not the coastlines, are compared (a computer fit by Edward Bullard and colleagues in 1965).
• Matching rocks and mountain belts: rock types, ages and structures continue across the ocean; the Appalachian Mountains of North America match the Caledonian mountains of Scotland and Scandinavia.
• Fossils: the freshwater reptile Mesosaurus is found only in South America and southern Africa, and the seed fern Glossopteris in South America, Africa, India, Australia and Antarctica. These could not have crossed a wide ocean.
• Climate evidence: glacial deposits and scratches from ice about 300 million years ago are found in now tropical regions (India, southern Africa, South America), while coal beds from tropical swamps are found in Antarctica, which suggests that the continents have moved to different latitudes.
• Paleomagnetism: rocks record the direction of Earth’s magnetic field when they formed. Rocks of the same age on different continents point to different “magnetic poles”, which are brought together when the continents are put back together.

What is seafloor spreading and what is its evidence?

In the 1950s and 1960s, mapping of the ocean floor showed the world-encircling system of mid-ocean ridges and deep trenches. Harry Hess proposed in 1960–1962 that new crust is created by magma rising at the ridge, and the older crust moves away on both sides like a conveyor belt, until it sinks back into the mantle at a trench (subduction). Evidence: magnetic stripes — the basalt records the magnetic field direction at the time it cooled, and Earth’s field has reversed many times, producing a pattern of normal and reversed stripes parallel to the ridge and symmetrical on both sides (Frederick Vine, Drummond Matthews and Lawrence Morley, 1963); age of the sea floor — the rocks are youngest at the ridge and become progressively older with distance, with none older than about 180 million years; sediment thickness — thin near the ridge and thicker farther away; heat flow and volcanism — high at the ridge; and drilling by the ship Glomar Challenger from 1968, which confirmed the ages. Today, GPS and satellite measurements show the plates moving by centimeters per year.

Evidence compared

Worked example: speed and age

1. Spreading rate to speed: a spreading rate of 2.5 cm per year means that the two sides of the ridge move apart by 2.5 cm each year. In an 80-year lifetime that is 2.5 × 80 = 200 cm = 2.0 m.
2. Age of the Atlantic: the narrowest part of the South Atlantic (between Brazil and West Africa) is about 2,850 km wide. At 2.5 cm a year the separation would take 2,850 km ÷ 2.5 cm/yr = 285,000,000 cm ÷ 2.5 = 114 million years, of the same order as the generally accepted age of about 100–130 million years for the opening of the South Atlantic (the rate is not constant, so this is only a rough check).
3. Distance in time: in 10 million years at 2.5 cm a year, the plates move 2.5 cm × 10,000,000 = 25,000,000 cm = 250 km.
4. Distance in 200 million years: 2.5 × 200,000,000 cm = 500,000,000 cm = 5,000 km, about the width of the present Atlantic in its wider parts.
5. Half-rate: the figure of 2.5 cm a year used above is the full spreading rate, the speed at which the two plates separate. Each plate moves away from the ridge at the half-rate, 2.5 ÷ 2 = 1.25 cm a year.
6. Age of ocean crust from distance: crust 1,000 km from the ridge (100,000,000 cm) moving away at 1.25 cm a year has an age of 100,000,000 ÷ 1.25 = 80,000,000 years, or 80 million years.

How did the theory develop?

Abraham Ortelius noted the fit of the continents in 1596, and Antonio Snider-Pellegrini and others made similar suggestions in the 19th century. Wegener collected the evidence in 1912–1929, but was criticized, and he died in 1930 on an expedition in Greenland. Arthur Holmes proposed mantle convection as a mechanism in the 1930s. After the Second World War, sonar mapping by Marie Tharp and Bruce Heezen showed the Mid-Atlantic Ridge and its rift valley, Harry Hess proposed seafloor spreading, and the magnetic stripes found by Vine, Matthews and Morley in 1963 provided strong support. By the late 1960s, the unified theory of plate tectonics was accepted, with contributions from J. Tuzo Wilson, Dan McKenzie, Jason Morgan and others.

Which mistakes are common?

Mistake 1: Thinking that the continents plough through the ocean floor

They are part of the plates and move with the ocean crust, not through it.

Mistake 2: Believing that Wegener was proved right in every respect

He was right about drift, but wrong about the mechanism. Plate tectonics explains the movement by mantle convection, slab pull and ridge push.

Mistake 3: Thinking that Pangaea was the first supercontinent

Earlier supercontinents, such as Rodinia, are thought to have formed and broken up in cycles.

Mistake 4: Confusing the half-rate and the full spreading rate

The rate on each side of the ridge (half-rate) is half the rate at which the two plates separate (full rate).