While he was the first to bring the concept to the forefront, there were others who had prior ideas and observations:

* 1947: Hess, while surveying the ocean floor with sonar, noticed "mid-ocean ridges" and "deep-sea trenches."

* 1950s: Hess proposed the idea of "convective currents" within the Earth's mantle as the driving force behind the movement of tectonic plates.

* 1962: Hess published his groundbreaking paper "History of Ocean Basins" where he formally presented his theory of seafloor spreading.

Therefore, while not a specific date, the 1960s mark the period when Harry Hess formally developed and shared his theory of seafloor spreading.