Final answer:
Harry Hess expanded on Wegener's continental drift theory by proposing the concept of seafloor spreading in the 1960s, which contributed to the modern understanding of plate tectonics.
Step-by-step explanation:
The theory of seafloor spreading in the early 1960s was expanded upon by geologist Harry Hess, building upon the earlier work of Alfred Wegener's hypothesis of continental drift. Wegener's pioneering work in the early 20th century proposed that continents were once connected in a supercontinent called Pangea, and he provided several kinds of evidence to support his theory, such as the jigsaw fit of continents and coinciding fossils on separate continents. Hess furthered this understanding by proposing that new ocean floor was being created at mid-ocean ridges, explaining the mechanism behind the movement of the continents. Despite initial skepticism, this idea was crucial in developing the modern theory of plate tectonics which explains the dynamics of Earth's surface.