Harry Hess (1906–1969), an American geologist and professor of geology at Princeton University, discovered seafloor spreading. Based on material brought up from the ocean floor during a drilling project, he determined that rocks on the ocean floor were younger than those on the continental landmasses. He also discovered rocks on the ocean floor varied in age: there were older rocks farther from the mid-ocean ridges and younger rocks around mid-ocean ridges. Hess proposed that the seafloor was spreading as magma erupted from Earth’s interior along the ocean’s mid-ocean ridges. The newly created seafloor slowly spreads away from the ridges, and later sinks back into Earth’s interior around deep-sea trenches.