Earth Science 1962

History of Ocean Basins

Harry Hammond Hess

The ocean floor is a conveyor belt — born at the ridges, spreading outward, and diving back into the Earth.

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In depth · the introduction

The continents puzzled everyone because they seemed to plough through solid rock. Hess's answer: they don't move through the ocean floor — they ride on it, because the ocean floor is itself moving.

The idea, unpacked

Hess proposed that the sea floor is being made, all the time, along the great undersea mountain chains called mid-ocean ridges. Hot rock rises from deep in the Earth, hardens into new ocean floor at the ridge, and is slowly carried away to either side — like two conveyor belts running in opposite directions from the same source.

Where does the old floor go? Down. At the deep ocean trenches, the floor bends and sinks back into the Earth to be melted and recycled. So the whole ocean floor is young: born at the ridges, ageing as it travels, and swallowed at the trenches. The continents simply ride along on top.

A geologist who read the sea floor from a warship

Harry Hess was a Princeton geologist who served as a US Navy captain in the Second World War. He left the ship's echo-sounder running across the Pacific and charted dozens of strange flat-topped undersea mountains — drowned islands he named guyots. After the war, sonar and other surveys revealed the mid-ocean ridges and the thinness of ocean sediment. In 1962 Hess put it all together. Knowing he was reasoning ahead of the proof, he disarmingly called his paper “an essay in geopoetry.” The poetry turned out to be right: within a few years the magnetic stripes of the ridges confirmed the spreading floor, and Wegener's long-rejected drifting continents were vindicated at last.

Why it mattered

Fifty years earlier, Wegener had argued the continents move but could not say what pushed them, and was dismissed. Hess supplied the missing engine — and made it the ocean floor's, not the continents'. That single shift unlocked everything: it explained why the sea floor is young, why earthquakes and volcanoes cluster where they do, and how mountains rise. By the end of the 1960s it had grown into plate tectonics, the theory that now underlies all of geology — the reason California quakes, the Atlantic widens, and the Himalayas keep growing.

Two conveyor belts

Picture a pair of luggage conveyor belts laid end to end, running away from each other from a slot in the middle. New rock keeps emerging from the slot — the ridge — and is carried outward on both belts at the same speed. The farther a piece has travelled, the older it is and the colder; at the far end it tips off the belt and drops out of sight — the trench. The luggage riding on top, never sinking, is a continent.

Before and after

Arthur Holmes had imagined a convecting mantle dragging the crust back in the 1930s, but without the sea-floor data to anchor it. Robert Dietz reached the same picture in 1961 and named it “sea-floor spreading.” Then Vine and Matthews (1963) found the proof in the ridges' magnetic stripes, and by 1968 the rigid-plate framework — plate tectonics — was complete. In the Library, Hess is the answer to the question Wegener (1912) could not: what moves the continents.

The original document

Original source text

Harry H. Hess (1906–1969) · “History of Ocean Basins,” in Petrologic Studies: A Volume in Honor of A. F. Buddington · Geological Society of America, 1962, pp. 599–620

An essay in geopoetry

Hess opens by naming the spirit of the paper. He is reasoning ahead of the data, and says so — a frank disclaimer rare in a scientific paper, and one that became its most quoted line.

Like Umbgrove, I shall consider this paper an essay in geopoetry. In order not to travel any further into the realm of fantasy than is absolutely necessary I shall hold as closely as possible to a uniformitarian approach; even so, at least one great catastrophe will be required early in the Earth's history.

The puzzle of the ocean floor

(Paraphrase.) Hess assembles the wartime and post-war findings about the deep sea: the mid-ocean ridges form a single globe-encircling mountain system; the ocean floor carries surprisingly little sediment and no rocks older than the Mesozoic; heat flow is high over the ridges; and the deep trenches mark belts of earthquakes. The oceans, he argues, are young and active, not the ancient permanent basins geology had assumed.

The mechanism: mantle convection and a spreading floor

(Paraphrase.) His proposal: slow convection cells in the mantle rise beneath the mid-ocean ridges, where hot mantle wells up and freezes into new basaltic ocean floor. That floor is then carried laterally away from the ridge — like a conveyor belt riding on the top of the convecting mantle — and descends again at the trenches, where the cell turns downward. The whole ocean floor is therefore created, transported, and recycled on a timescale of only a few hundred million years, which is why it is so young and so thinly veneered with sediment.

Continents ride passively; guyots record the ride

(Paraphrase.) Crucially, the continents do not plough through the ocean floor — Wegener's fatal difficulty — but ride passively atop the moving mantle, between a rising ridge and a descending trench. Hess uses the flat-topped drowned seamounts he had charted from the deck of USS Cape Johnson during the war (he named them guyots) as evidence: volcanoes built near a ridge, planed flat by waves, then carried down and away as the floor spread and cooled and sank.

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Princeton, New Jersey · 1962