EAR 105 EAR 105 - M001
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This 4 page Class Notes was uploaded by Lucy Kassel on Saturday February 6, 2016. The Class Notes belongs to EAR 105 - M001 at Syracuse University taught by L. Ivany in Winter 2016. Since its upload, it has received 27 views. For similar materials see Earth Science in Earth Sciences at Syracuse University.
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Date Created: 02/06/16
February 1, 2015 Plate Tectonics and the structure of the Earth The Structure of Earth’s Interior ● Crust ● Mantle ● Inner Core ● Outer Core Crust ● Oceanic Crust ○ Basalt, dense, thin (5 km) ● Continental Crust ○ Granite, lighter, thick, (3570 km) ● Moho (Mohorovicic discontinuity) ○ separates crust from mantle ○ marks an increase in density ● Both kinds of crust “float” on the mantle because they are less dense ○ → Isostasy Isostasy The Iceberg Effect ● Wood blocks (and icebergs) float because they are less dense than water ● Larger blocks sit higher, but have a deeper keel too ● Areas with thicker crust, like mountain belts, sit lower in the mantle (like bigger icebergs float lower in the ocean) ● If you add weight to the crust (e.g. a glacier) it will float lower into the mantle Lithosphere ● Crust plus top of mantle ● rigid slabs ● Continental lithosphere is 100200 km thick Asthenosphere ● Upper mantle ● “tar”, flows Core ● Iron and nickel ● Inner is solid ● Outer is liquid ● Circulation of molten iron in the outer core generates Earth’s magnetic field What drives circulation in the outer core? ● Brand new research suggests that magnesium minerals are precipitating out of the liquid outer core and rising, making remaining Ferich liquid more dense, driving circulation Plate Tectonics ● the lithosphere is broken up into rigid plates that float on the asthenosphere and slide past each other ● the continents are not fixed in place! Evolution of an idea... ● Alfred Wegener ● 1915 ● noticed that the continents fit pretty well if you push them together Continental Drift ● similar rock types were found on both sides of the suture ● similar fossils were found on both sides of the suture ● evidence for glaciers on all southern continents, but some are tropical now ● the reconstructions make sense! ● Pangaea a “supercontinent” ○ when the continents were all together forming one continent Evidence for Continental Drift ● Similar continental outlines ● Similar rock types ● Similar fossils ● Glacial deposits match up ● Paleoclimate reconstructions make more sense ● BUT...how do you make it work? How can continents plow through ocean crust??? Need a mechanism... ● So the idea languished... Then in the 1960s, we mapped the ocean floor...(Marie Tharp ) ● Rocks get older as you move away from the ridge in either direction symmetric ● Sea floor shows stripes of alternating magnetic polarity symmetric around the ridge SeaFloor Spreading ● Creation of new oceanic crust (basalt) at midatlantic ridges ● But... the Earth is not expanding ● If seafloor spreading is correct, plates must be being destroyed somewhere also ● Where? ○ Trenches ■ Marianas Trench 35,798 ft deep! (10,911 m) ● Earthquake distribution is related... as is the distribution of volcanoes... Subduction ● Destruction of old crust ● Creation of new crust Plate Tectonics ● Lithosphere plates can contain both oceanic and continental crust February 3, 2015 Plate Tectonics ● Lithospheric plates can contain both oceanic and continental crust Plate Boundaries ● Transform sliding past each other ● Convergent coming together ● Divergent spreading apart 1) Divergent Plate Boundaries ● Sea floor spreading ● New crust is always basalt, from partial melting of the ultramafic mantle due to decompression ● Why is there a ridge? ○ Young crust is hot, less dense, sits high ○ As crust cools, becomes more dense, contracts ○ Ex: MidAtlantic Ridge ■ comes to the surface of Iceland ● Ex: Earthquakes ○ generally small/not disastrous, and frequently moving ● Rifting ○ spreading starts under a continent ○ Ex: East Africa 2) Convergent Plate Boundaries ● Two different types of crust, so three possibilities: ○ Oceanic oceanic ○ Oceanic continental ○ Continental continental ● Ocean Ocean Convergence ○ Subduction of denser crust at the trench ○ melting of downgoing slab, volcanism ○ creation of Island Arc ○ earthquake foci deeper away from trench ○ Ex: Japan is a classic island arc ○ Partial melting of downgoing slab and sediment only the lowertemperature minerals melt ○ Island arc rocks are therefore not quite as mafic ○ Probably how the continents were originally formed ● Oceanic Continental Convergence ○ Subduction of oceanic slab at the trench ○ Volcanism and mountain building along edge of continent “active margin” ○ Ex: Andes Mountains ■ West Coast of South America ■ “Active margin” ○ Ex: Cascade Mountains: US Pacific Northwest ■ Mount St. Helens ● Continent Continent Collision ○ Continents cannot be subducted ○ Intense deformation and mountain building during collision ○ Ex: Himalayas (Mount Everest) ○ Ex: India Asia collision ○ Ex: Folding in the Swiss Alps ○ Ex: Appalachians ■ Formation of Appalachians ● Much older collision, about 250 mya (end of Paleozoic Era) ● Collision of North America and Europe/ North Africa to make Pangea 3) Transform Plate Boundaries ● “strikeslip” motion sliding ● offsets in spreading ridges ● Ex: San Andreas Fault
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