Harry Hess, a geologist and U.S. Naval Reserve officer, is recognized for his groundbreaking work that changed the understanding of Earth’s dynamics. The central theme was the Seafloor Spreading Hypothesis, formally presented in 1962. This theory provided the first plausible mechanism to explain Alfred Wegener’s earlier idea of continental drift. Hess’s model described a continuous process of creation and destruction of the oceanic crust, resolving several long-standing puzzles in geology.
Foundational Work: Mapping the Ocean Floor
Hess’s ideas were built upon his service in the U.S. Navy during World War II. While commanding the USS Cape Johnson, he kept the echo-sounding equipment running constantly. This sonar technology allowed him to create thousands of miles of detailed bathymetric surveys mapping the deep ocean floor.
These continuous soundings revealed that the ocean floor was not the uniformly flat, ancient plain that most scientists had long presumed it to be. Instead, Hess discovered vast, submerged features, including the continuous, globe-circling Mid-Ocean Ridge system. He also identified hundreds of submerged, flat-topped mountains, which he named “guyots.”
The existence of these guyots—volcanic peaks whose tops appeared to be eroded by wave action but were now kilometers below sea level—was particularly puzzling. The presence of a massive, continuous mountain range in the middle of the oceans, often with a prominent rift valley running along its crest, further challenged the static view of the ocean basin. Hess realized that these observations about the deep-sea topography required a dynamic, not a static, explanation.
The Seafloor Spreading Hypothesis
Hess proposed the Seafloor Spreading Hypothesis in his 1962 paper, History of Ocean Basins. This hypothesis posited that new oceanic crust is continuously formed at the Mid-Ocean Ridges. Hot magma rises from the mantle into the rift valleys at the ridge crests.
As this magma cools and solidifies, it creates new seafloor, which then moves laterally away from the ridge in both directions. This continuous production of new crust slowly pushing the older crust outward. This process accounted for why the ocean floor was found to be much younger than the continents, with the oldest seafloor rocks dating back no more than about 180 million years.
The process also explained the fate of the older crust, which Hess theorized was recycled back into the mantle at deep-ocean trenches. These trenches were identified as subduction zones where the cold, dense oceanic crust sinks back into the Earth’s interior. This recycling mechanism resolved the puzzle of why the ocean floor held relatively little sediment.
The Mechanism Driving Continental Movement
The Seafloor Spreading Hypothesis provided the necessary driving mechanism that had been missing from earlier theories of continental drift. Hess proposed that the continents were passively carried along as the ocean floor beneath them spread, rather than plowing through the ocean floor.
Hess proposed that this movement was powered by thermal convection cells within the Earth’s mantle. He suggested that hot, buoyant mantle material rises beneath the Mid-Ocean Ridges. This rising material then spreads out horizontally beneath the crust, dragging the seafloor and the continents resting on it away from the ridge.
As the mantle material moves laterally, it cools and becomes denser, causing it to sink back down into the deeper mantle beneath the deep-ocean trenches. This circulation provides the engine for the creation of new crust at the ridges and the destruction of old crust at the trenches. Hess’s proposal unified the movement of the ocean floor with the movement of the continents, laying the foundation for the modern theory of Plate Tectonics.