2Active Earth: From
Continental Drift to Plate Tectonics

What evidence do we have to support the plate tectonic theory?



In 1915, German meteorologist Alfred Wegener proposed his hypothesis of Continental Drift. Wegener proposed that the continents were not of fixed position, rather they moved very slowly over time. Wegener suggested that all the continents were once joined together as a supercontinent, Pangea, that existed during the late Paleozoic and Early Mesozoic and had since broke into smaller plates that had drifted apart.  Wegener’s hypothesis of Continental Drift was widely rejected by the scientific community, even with the evidence of the supercontinent Pangea, because it lacked a viable driving mechanism to explain the proposed motion of the plates.


It was another 50 years before Wegener’s theory was resurrected by research being conducted during World War II. Exploration of the ocean floor provided evidence in support of Wegener’s hypothesis including an extremely long mid ocean-ridge system, and that oceanic crust was relatively young relative to the continents. Considerable research was conducted to find a means of detecting large steel objects beneath the ocean surface (submarines). As a result, sensitive equipment was constructed that could detect very small changes in the Earth’s magnetic field – and could be carried in aircraft and used to survey large areas from the air. Surveys of the oceans, and in particular, areas on either side of the mid-ocean ridges, showed patterns of magnetic anomalies that were repeated (mirrored) on either side of the spreading ridge.


The Modern Plate Tectonic Theory states that the lithosphere is divided into rigid plates that diverge (pull apart), converge (come together), or slide past each other as they move over the asthenosphere. The motion of the lithospheric plates is a result of convection with in the Earth’s mantle, the outward push of the diverging plates (ridge push) along the mid-ocean ridge system, and the pull of the subducting oceanic plates (slab pull) back into the mantle along convergent plate boundaries. The heat that drives the convection within the mantle is in part left over primordial heat from the formation of early Earth and the radioactive decay of elements such as uranium and thorium and their decay products from deep in the interior of the Earth. 


The movements of the plates and the interactions of plates along their boundaries cause volcanic eruptions, earthquakes, the formation of mountain ranges and new ocean basins, and is responsible for the recycling of rock material and production of many of Earth’s naturally occurring economic resources. Oil, natural gas, and coal are the byproducts of the accumulation of organic materials, deep burial, and decomposition within a sedimentary basin that flanks mountain ranges formed by plate tectonic processes. Oil, natural gas, and coal found in Ohio are by in part a result of Ohio being a sedimentary basin for the Appalachian Mountains formed millions of years ago. The theory of Plate tectonics is considered a unifying explanation for many geologic features and events and provides a framework for interpreting the composition, structure, and internal processes of Earth on a global scale.