Geog 101 Geog 101
Minnesota State University, Mankato
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This 3 page Class Notes was uploaded by Hallie Notetaker on Friday September 16, 2016. The Class Notes belongs to Geog 101 at Minnesota State University - Mankato taught by Phillip Larson in Fall 2016. Since its upload, it has received 12 views. For similar materials see Introductory Physical Geography in Geography at Minnesota State University - Mankato.
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Date Created: 09/16/16
Plate Tectonics Early Ideas Sir Francis Bacon o Africa and South America look like they fit together o 1620 he noted the parallelism of the shorelines across theAtlantic Ocean Ben Franklin o 1782 the crust of the Earth must be a shell floating on a liquid interior Thus the surface of the globe would be capable of being broken and disordered by violent movements of the liquid The Scientific Paradigm Alfred Wegener – German meteorologist o The Pangaea Hypothesis o Fascinated by the idea that the continents drifted (continental drift) o Compiled the observations of others and his own and made a case in 1915 for moving continents Coastlines fit Matching fossil records Matching rock types Matching mountain ranges Exotic animals inAustralia isolated by continental drift o Suggested that tidal (gravitational) forces between Moon and Sun moved them and that continental lithosphere plowed through oceanic lithosphere No evidence for this and tidal forcing that strong would have stopped the earth from spinning Solution later in time – plate tectonics The vast majority of geologists were heavily prejudiced against an atmospheric scientist for holding heretical beliefs that continents moved Was not until the 1970s that people started to accept Wegener’s hypothesis Plate Tectonics All views involve too much heat being produced from radioactive decay o K, Ra, Th and intense pressure force convection currents Best guess is these currents run through the mantle What goes down eventually comes back up and completes the cycle Plate boundaries o Convergent – comes together o Divergent – spreads apart (driving force of plate tectonics) o Transform – slides past Divergent Boundary Driven by convection in the mantle o Hotter, less dense partially to full molten material from the mantle forces its way to the surface where it erupts This lava then cools and is forced apart as new material arrives from the mantle This process serves to push the plates apart over time and drives the movement of the continents How do we know divergence is happening? o Half-lives of isotopes Radiometric dating o Crustal age o Paleomagnetism o Not only in oceans Started rifting 22-25 mya (EastAfrican Rift Zone) Rifting at a rate of 6-7 mm annually In 10 million years the Somalian plate will rift off and a new ocean basin will form Big question o If new crust is being made at spreading/divergent plate boundaries, why isn’t earth expanding? Crust is “recycled” at subduction zones – convergent plate boundaries (e.g. ocean trenches) Convergent Boundary Proof – reabsorption of old rock material Benioff-Wadati Zone Volcanoes often rest on top of mountain chains at these boundaries o Not if two continents collide though The bulk of mountain building is not from volcanoes, but from compression Ocean-Continent o Examples – Andes, Cascades,Alaska, British Columbia, New Zealand o This process is observed along the entire western coast of theAmericas Except California where it used to, creating the Sierra Nevadas, but is now a transform boundary o Creates large mountain belts that parallel the margin of continents o Oceanic crust is more dense than the continental, therefore, it subducts Ocean-Ocean o Examples – island arcs in the Pacific and Caribbean Continent-Continent o Examples – Himalayas,Alps Transform Boundary No volcanoes at these boundaries No massive mountain chains (some topography) Strong, near surface earthquakes are very problematic o California, Turkey, Israel Right lateral example – San Andreas Fault Left lateral example – Levant Fault (Dead Sea Fault) Hot Spots Other patterns that do not exactly fit the theory of continental drift Areas of volcanism located in the interior of a tectonic plate o Do not know how they form Hawaii island formation o Oldest islands were formed first when they passed over the hot spot Move farther and farther away As islands are cut off from the lava source the ocean wears them down o New island (Loihi) is being built beneath the ocean Yellowstone Super volcano o North American plate moving over hot spot IsostacyAdjustments Earth’s crust seeking equilibrium o Earth’s crust is elastic Step 1 – crust is depressed by putting mas on top of it Step 2 – mass removed and crust rises (isostatic rebound)