Theories and Plate Tectonic

• Pangaea

  Supercontinent that began to split apart around 200 million years ago
• Plate tectonics theory


  Theory regarding how continents/plates move
• Abraham Ortelius


  Flemish cartographer (map maker) and geographer who first imagined that the continents were joined together before drifting to their present positions

  
• Continental drift


  The gradual movement of the continents across the earth
• Nicolaus Steno


  Danish Anatomist and Geologist who observed the changes in a sequence of rock layers in the mountains of Italy and proposed the Law of Superposition
• Law of Superposition


  In a sequence of sedimentary rock layers, each layer of rock is older than the layer above it and younger than the rock layer below it
• James Hutton


  Scottish geologist who presented the uniformitarian principle in 1785, suggesting that the landscape developed over long periods of time through a variety of slow geologic and geomorphic processes
• Uniformitarianism


  The theory that the rates of all geologic processes had been the same throughout time
• Alfred Wegener


  German geophysicist who proposed the Supercontinent Theory in 1912, suggesting that around 200 million years ago, the supercontinent Pangaea began to split apart
• Wegener's theory was based in part on the remarkable fit of the South American and African continents, first noted by Abraham Ortelius three centuries earlier
• Wegener's theory of continental drift was not accepted until the 1950's
• Geologic evidence of continental drift

  • The shapes of the continents match
  • Same fossils found on different continents
  • The same rock structures on different continents
• A fossilized tropical plant was found on Antarctica, which used to be in a warmer climate before the continents moved
• Wegener's theory of continental drift was not well received, even though it seemed to agree with the scientific information available at the time
• Arthur Holmes


  British geologist who proposed the Convection Currents Theory in 1929, suggesting that the mantle undergoes thermal convection, which may be enough to cause continents to move
• Convection currents


  As magma is heated, it tends to rise, then cools and sinks again, resulting in a current which may be enough to cause continents to move
• Even though the theory of continental drift was proposed in 1912 by Alfred Wegener, the idea of moving continents wasn't generally accepted until the early 1960s
• Geologists now understand the forces that cause tectonic plates to move, and the force that causes plates to move is called convection
• Convection currents in the mantle


  Move tectonic plates as the plastic-like asthenosphere circulates due to the heat present in the core
• Harry Hess


  American geologist who proposed the Sea Floor Spreading Theory in 1962, suggesting that the seafloor itself moves and carries continents with it, as it expands from a central point
• Sea floor spreading


  Molten magma from beneath the earth's crust oozes up between the plates in the rift in the ocean floor, causing the hot magma to cool and expand, pushing the plates beside it
• Dan McKenzie


  English geophysicist who proposed the Theory of Plate Tectonics in 1968, which is a combination of continental drift and sea-floor spreading
• Plate tectonics


  • Earth's outermost layer, the lithosphere, is broken into 7 large, rigid pieces called plates, which are all moving in different directions and at different speeds
  • The ocean floors are continually moving, spreading from the center, sinking at the edges, and being regenerated
  • Convection currents beneath the plates move the crustal plates in different directions
  • The source of heat driving the convection currents is from radioactivity deep in the Earth's mantle