1. From the time maps of the globe became available, people wondered about the arrangement of the continents and oceans. Hundreds of years later, valid explanations were constructed. Continental Drift and Plate Tectonics

2. Leonardo da Vinci and Francis Bacon wondered about the possibility of the American and African continents having broken apart, based on their shapes. Early Observations

3. 20 th century meterologist, Wegener, revived the early idea of continental drift, contending that all of the present-day continents were connected. He called the supercontinental mass Pangaea, Greek for ‘all lands’. Pangaea

4. Wegener’s summary was based on a number of careful observations: Wegener’s Evidence -- matching rock, fossil, glacier, and structural relations among different parts of different continents

5. Continental Drift: Fossil Evidence Similar plant and animal fossils found on different continents

6. Continental Drift: Glacial Evidence Large ice masses carve grooves in the rocks over which ice flows. Such masses tend to flow outward (generally downhill) from a central locality.

7. Continental Drift: Rock Ages Even before geochronology, the relative framework of rock ages showed strong correlation across the Atlantic, as did mountain ranges of similar age.

8. Mantle Convection How do continents drift? Arthur Holmes described mantle heat flow in terms of convection. Deep materials, hotter than their surroundings (and hence buoyant), would tend to flow upward. In approaching the cool surface of the Earth, the material would lose its thermal energy, cool and sink, having lost buoyancy.

9. Mantle Convection Materials that can flow tend to lose thermal energy by the convection process. A pot of water heated is similar to under the Earth’s crust

10. From the 1940’s to the 60’s, Harry Hess made many key intellectual contributions to the coming revolution in geologic thought: Harry Hess and Marine Geology

11. echo-sounding of sea floor revealed deep sea features like guyots and seamounts, and the topography of mid- ocean ridges -- ridges are areas of high heat flow and volcanic activity -- young age of ocean floor, based on thickness of sediment

12. Topography and Age of the Sea Floor thin sediment cover thick sediment cover As ocean crust ages, it cools and is less buoyant. The cool mantle root on this crust helps pull it down into the mantle, resulting in deeper sea floor progressively away from the ridges.

13. Sea Floor Spreading Hess combined his observations with the earlier ideas of Wegener and the mechanism of Holmes into the concept of sea floor spreading, which lead to plate tectonics.

14. Credits Some of the images in this presentation come from: Plummer, McGeary and Carlson, Physical Geology, 8/e; Hamblin and Christiansen, Earth’s Dynamic Systems, 8/e; Press and Siever, Understanding Earth, 3/e; Paul Tomascak (University of Maryland)