Plate Tectonics Revolution (1960s): Seafloor Spreading Proven

The plate tectonics revolution was the rapid transformation of the earth sciences in the 1960s, when evidence from the ocean floor showed that Earth’s surface is made of moving plates. Between Harry Hess’s seafloor spreading paper of 1962 and the first global plate models of 1967–1968, an idea long dismissed as fringe became the foundation of modern geology. It explained, in one framework, why continents drift, why oceans open and close, and where earthquakes and volcanoes occur.

Background: an idea without a mechanism

In 1912 the German scientist Alfred Wegener proposed that the continents had once formed a single landmass and had since drifted apart. His evidence, described in Continental Drift (1912), included the matching coastlines of South America and Africa and identical fossils on widely separated continents, such as the seed fern Glossopteris.

Most geologists, especially in Britain and North America, rejected the theory. Wegener could not explain what force moved continents through the solid ocean floor, and the forces he suggested were far too weak. A few scientists, notably Arthur Holmes in Britain and Alexander du Toit in South Africa, kept the idea alive, but for decades it remained a minority view.

New evidence from the oceans

The breakthrough came from the sea floor, which was barely known before the Second World War. Wartime and Cold War naval research, including echo sounding and submarine detection, produced detailed measurements of ocean depth, gravity and magnetism.

In the early 1950s Marie Tharp, working with Bruce Heezen at Columbia University’s Lamont Geological Observatory, identified a deep rift valley running along the crest of the Mid-Atlantic Ridge. Further surveys showed that mid-ocean ridges form a connected system tens of thousands of kilometres long, winding through every ocean basin.

At the same time, studies of magnetism preserved in old rocks, known as palaeomagnetism, showed that the magnetic poles appeared to have wandered over geological time. British researchers such as Keith Runcorn and Edward Irving found that the wandering paths for Europe and North America made sense only if the two continents had moved relative to each other.

Seafloor spreading: Hess and Dietz

Harry Hess, a Princeton geologist and former US Navy officer, combined these clues. In a paper circulated in 1960 and published in 1962 as “History of Ocean Basins”, he proposed that hot material rises beneath mid-ocean ridges, forms new ocean crust and spreads outward, while old crust sinks back into the mantle at deep ocean trenches. Hess modestly called his paper “an essay in geopoetry”.

The American oceanographer Robert Dietz, then working for the US Navy, independently published a similar idea in Nature in 1961 and gave it the name “seafloor spreading”. The concept solved Wegener’s problem: continents did not plough through the ocean floor but were carried along with it.

Magnetic stripes: the decisive test

The strongest proof came from magnetic surveys. In the late 1950s, Ronald Mason and Arthur Raff mapped a striking zebra-like pattern of strong and weak magnetic readings on the sea floor off the west coast of North America, but its cause was unclear.

In 1963 Fred Vine, a Cambridge research student, and his supervisor Drummond Matthews published “Magnetic Anomalies over Oceanic Ridges” in Nature. They argued that as new crust cools at a ridge, it records the direction of Earth’s magnetic field. Because the field reverses at irregular intervals, spreading crust should form parallel stripes of normal and reversed magnetism, mirrored on both sides of the ridge. The Canadian geophysicist Lawrence Morley reached the same idea independently, but his paper was rejected by journals in 1963; the theory is now often called the Vine–Matthews–Morley hypothesis.

The test depended on a reliable timescale of magnetic reversals, which Allan Cox, Richard Doell and Brent Dalrymple in the United States and researchers in Australia built up in 1963–1966. In 1966 Walter Pitman and James Heirtzler of Lamont published the Eltanin-19 profile across the Pacific–Antarctic Ridge. Its stripes were almost perfectly symmetrical and matched the reversal timescale, persuading many former sceptics.

From spreading to plates

Other pieces fell into place quickly. In 1965 the Canadian geologist J. Tuzo Wilson described transform faults, a new class of fault that links ridges and trenches, and suggested that Earth’s surface is divided into rigid plates. In 1967 Lynn Sykes used earthquake records to confirm that transform faults move in the direction Wilson predicted.

Seismology was crucial. The World-Wide Standardized Seismograph Network, set up in the early 1960s partly to monitor nuclear tests, gave precise earthquake locations. These showed narrow belts of quakes along ridges and dipping zones of deep quakes beneath trenches, where one plate sinks under another. Studies of the 1964 Alaska earthquake helped show that such great quakes occur on these sinking plate boundaries.

In 1967–1968 the theory was given its full form. Dan McKenzie and Robert Parker described plate motions on a sphere for the North Pacific, W. Jason Morgan of Princeton presented a model of rigid crustal blocks, and Xavier Le Pichon calculated the motions of six major plates. In 1968 Bryan Isacks, Jack Oliver and Lynn Sykes showed how global earthquake data fitted this “new global tectonics”.

Confirmation and acceptance

Drilling soon supplied direct evidence. From December 1968 the research ship Glomar Challenger of the Deep Sea Drilling Project drilled a line of holes across the South Atlantic. The oldest sediments resting on the oceanic crust grew steadily older with distance from the Mid-Atlantic Ridge, as seafloor spreading predicted. Later drilling confirmed that the present ocean floor is young by geological standards, nowhere much older than about 180–200 million years.

By the end of the 1960s most earth scientists in North America and Western Europe had accepted plate tectonics, a strikingly fast shift. Acceptance was slower in some countries, including the Soviet Union, where many geologists held to other theories into the 1970s and 1980s.

Myths and facts

A common belief is that Wegener was simply ignored because he was an outsider. In fact his ideas were widely debated; they were rejected mainly because he lacked a mechanism and some of his estimates, such as very fast drift rates, were wrong. Plate tectonics also differs from his theory: whole plates, including the ocean floor, move, not continents alone.

Another myth is that one scientist “discovered” plate tectonics. It was a collective achievement of dozens of researchers in several countries, often working in competition and building on decades of naval and oceanographic data.

Timeline

• 1912, 6 January: Alfred Wegener first presents his theory of continental drift in Frankfurt.
• 1953: Marie Tharp identifies a rift valley along the crest of the Mid-Atlantic Ridge.
• 1961: Robert Dietz names the process “seafloor spreading” in Nature.
• 1962: Harry Hess’s “History of Ocean Basins” is published.
• 1963, 7 September: Vine and Matthews publish their magnetic-stripe hypothesis in Nature.
• 1965: J. Tuzo Wilson describes transform faults.
• 1966: Pitman and Heirtzler publish the symmetrical Eltanin-19 magnetic profile.
• 1967–1968: McKenzie and Parker, Morgan, and Le Pichon publish the first global plate models.
• 1968, December: Glomar Challenger drilling in the South Atlantic confirms seafloor ages increase away from the ridge.

Why it matters

Plate tectonics did for geology what evolution did for biology: it gave a single explanation for many separate observations. It explains the break-up of supercontinents such as Pangaea, the rise of mountain ranges, and the distribution of earthquakes, volcanoes, minerals and fossils. Today it underpins earthquake hazard assessment, the search for natural resources and our understanding of Earth’s long-term climate.