| 1. Earth's Core (1906) Seismologist Richard Oldham determines that earthquake waves move through the central part of the Earth much slower than through the mantle around it. He surmises that the Earth has a core composed of liquid. 2. Earth's Inner Core (1930s) Inge Lehmann documents that some seismic waves from deep inside the Earth's core do not pass through, but are reflected back. It becomes clear that the Earth has an inner core consisting of a small, solid iron sphere that is surrounded by a thick outer core composed of liquid iron. 3. Continental Drift (1911) Alfred Wegener proposes that all the continents in the world once formed a single, giant landmass that was eventually split apart in a process called "continental drift." Wegener's evidence consists of the "fit" of South America with Africa, fossil distribution and geological similarities. 4. Seafloor Spreading (1950s -- 1960s) Adding his own data on changes in seafloor depth and geology to discoveries of his peers, Harry Hess proposes that Wegener's theory of continental drift is a result of seafloor spreading. He hypothesizes that molten magma from beneath the Earth's crust is oozing up between the plates in the Great Global Rift (now referred to as the Mid-Ocean Ridge). As the hot magma cools, it expands and pushes the plates out from the rift, causing the Atlantic Ocean to get wider over time. 5. Plate Tectonics (1960s) The work of many scientists reveals that the Earth's surface is broken into several ... |
