Plate tectonics Geology 103. Why does the Earth have mountains and basins? Basic question asked by...
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Transcript of Plate tectonics Geology 103. Why does the Earth have mountains and basins? Basic question asked by...
![Page 1: Plate tectonics Geology 103. Why does the Earth have mountains and basins? Basic question asked by many: erosion is evident everywhere, so why haven’t.](https://reader035.fdocuments.us/reader035/viewer/2022062408/56649e495503460f94b3c0f1/html5/thumbnails/1.jpg)
Plate tectonics
Geology 103
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Why does the Earth have mountains and basins?
• Basic question asked by many: erosion is evident everywhere, so why haven’t all mountains eroded and filled in all basins?
• Alfred Wegener proposed continental drift in 1912.
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Evidence for continental drift
• Continental “fit” – South America and Africa separated by the Atlantic
• Similar rocks separated by the Atlantic
• Similar climate markers (glacial striations) separated by the Atlantic
• Similar fossils separated by the Atlantic
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Unifying theory of geology
• Extension of Wegener’s continental drift theory from the early 20th century
• Needed not only information about rocks but also breakthroughs in geochronology and geophysics
• Continental drift failed to provide a sufficient mechanism (tides and wind were not enough); plate tectonics does
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Mechanism(s) of tectonics
• First, and foremost, the second law of thermodynamics says that heat is transferred from hot bodies to cold bodies (e.g., Earth’s core to space)
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Mechanism(s) of tectonics
• Heat transfers in three ways: radiation, conduction and convection. It is the last of these that occurs within the mantle.
• Convection (and internal heating) seems to keep the mantle hot and plastic enough for plates to sink into it -- may not be enough to move plates
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Mechanism(s) of tectonics• Slab pull seems to be the major “drag”,
ridge push and plate sliding are more minor forces
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Tenets of plate tectonics
• Plates exist
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Plate materials• Remember: lithosphere, not crust
• Continental plates are made of granite
• Oceanic plates are made of basalt
• Granite and basalt are types of igneous rocks
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Tenets of plate tectonics
• Plates move
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Hotspots
• Focused convection plumes from mid-mantle
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Hotspots show plate movement
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Problems with motion
• Hotspots may not be fixed in position and they are certainly not permanent
• GPS does a better job of measuring annual rates, though over the short-term only
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Tenets of plate tectonics
• Plate boundaries are areas of geological phenomena such as earthquakes, volcanoes and mountain building
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Plate interactions
• Convergent
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Types of convergent boundaries
• Continental plate - continental plate convergence results in non-volcanic mountain ranges
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Types of convergent boundaries
• Oceanic plate - oceanic plate convergence results in a volcanic oceanic island arc
• Subduction occurs
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Types of convergent boundaries
• Continental plate - oceanic plate convergence results in a volcanic continental arc
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Plate interactions
• Divergent
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Types of divergent boundaries
• Between two oceanic plates
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Sea floor spreading
• Oceanic divergent boundaries provided evidence for plate tectonics. Paleomagnetism (the measurement of “stripes” of different polarity minerals in ocean floor rock of different ages) allowed Hess and others (1950s) to postulate the movement of the sea floor itself.
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Types of divergent boundaries
• Between two continental plates
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Plate interactions
• Transform
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Tenets of plate tectonics
• Plate interiors are geologically quiet, except for hotspots
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Synthesis
• All three types
Nearby!
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Grander synthesis: the Wilson (supercontinent) cycle
• J. Tuzo Wilson (1970) suggested that plate tectonics allow supercontinents to rift apart and reform over and over on a roughly half billion year cycle
• Evidence for this: evidence of pre-Pangea supercontinents, such as Rodinia
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