Plate Tectonics

1. Plate Tectonics Prepared by Betsy Conklin for Dr. Isiorho

2. Plate Tectonics • plate tectonics: the idea that the earth’s surface is divided into a few large, thick plates that move slowly and change in size • continental drift: the idea that continents move freely over the earth’s surface, changing their positions relative to one another • sea-floor spreading: a hypothesis that the sea floor forms at the crest of the mid-oceanic ridge, then moves horizontally away from the ridge crest toward an oceanic trench

3. The Early Case for Continental Drift • continents can be made to fit together like pieces of a picture puzzle • Alfred Wegener purposed that the continents were originally one giant supercontinent which he called Pangaea. Pangaea then split into two parts - Laurasia which is now North America and Eurasia, and Gondwanaland which is now the southern-hemisphere continents and India • polar wandering: an apparent movement of the earth’s poles • the distribution of fossils on various continents

4. Skepticism about Continental Drift • fossil plants could have been spread from one continent to another by winds or ocean current • polar wandering might have been caused by moving poles rather than by moving continents

5. Sea-Floor Spreading • subduction: the sliding of the sea floor beneath a continent or island arc • convection: a circulation pattern driven by the rising of hot material and\or the sinking of cold material

6. Plates and Plate Motion • plate: a large, mobile slab of rock that is part of the earth’s surface • lithosphere: the relatively rigid outer shell of the earth of which the plates are a part • asthenosphere: a zone of low seismic-wave velocity that behaves plastically because of increased temperature and pressure • transform fault: the portion of a fracture zone between two offset portions of a ridge crest

7. Divergent Plate Boundaries • divergent plate boundary: a boundary between plates that are moving apart

8. Transform Plate Boundaries • transform plate boundary: a boundary between plates that are moving horizontally past one another Transform boundaries between two ridges Transform boundaries between a ridge and a trench Transform boundaries between two trenches

9. Convergent Plate Boundaries • convergent plate boundary: a boundary between plates that are moving toward each other • ocean-ocean convergence: when two plates capped by sea floor converge and one plate subducts under the other • island arc: a curved line of volcanoes that form a string of islands parallel to the oceanic trench

10. Convergent Plate Boundaries (cont.) • ocean-continent convergence: when a plate captured by oceanic crust is subducted under the continental lithosphere and an accretionary wedge and forearc basin form an active continental margin between the trench and the continent • magmatic arc: a broad term used both for island arcs at sea and for belts of igneous activity on the edges of continents

11. Convergent Plate Boundaries (cont.) • continent-continent convergence: when two continents approach each other an collide

12. What causes Plate Tectonics • mid-oceanic ridge crests are hot and elevated, while trenches are cold and deep • ridge crests have tensional cracks • the leading edges of some plates are subducting sea floor, while the leading edges of other plates are continents (which cannot subduct)

13. Mantle Plumes and Hot Spots • mantle plumes: narrow columns of hot mantle rock that rise through the mantle, much like smoke rising from a chimney Continental breakup caused by a mantle plume. (A) A dome forms over a mantle plume rising beneath a continent. (B) Three radial rifts develop due to outward radial flow from the top of the mantle plume. (C) Continent separates into two pieces along two of the three rifts, with new ocean floor forming between the diverging continents. The third rift becomes an inactive “failed rift” (or aulacogen) filled with continent sediment.

14. Pictures • All pictures used in this power point presentation were taken from the following: • Carlson, Diane H., David McGeary and Charles C. Plummer. Physical Geology: Updated Eighth Edition. New York City, McGraw-Hill Higher Education, 2001.