1. Seafloor Spreading: Hess proposed the theory of seafloor spreading in the 1960s, which explained the movement of tectonic plates. He observed mid-ocean ridges where new oceanic crust is formed, and trenches where older crust is subducted back into the mantle. He also noticed a pattern of magnetic striping on the ocean floor, which supported his theory.
2. Hot Spots: Hess also identified areas of volcanic activity known as hotspots. These are areas where plumes of hot mantle material rise up through the Earth's crust, creating volcanoes even in the middle of tectonic plates. Examples of hotspots include Hawaii and Yellowstone.
Both of these discoveries played a crucial role in the development of the theory of plate tectonics, which is now the unifying theory in geology.