2 the evidence

2 the evidence - Plate Tectonics The Evidence 15:01:00 1....

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  Plate Tectonics- The Evidence 26/08/2008 16:01:00 1. Divergent Plate Boundaries Rifting o East African Rift—splitting Africa into two and starting to form a new ocean (Red Sea is an example of a divergent boundary; the sea is spreading yearly) o Crow’s foot shape (see drawing) is typical of continental splitting; typically, two of the rift arms will succeed and become ocean, but a third will fail (aulochogene = failed rift arm ) Intra-Plate Earthquakes: rare but sometimes large New Madrid, MO —Three very large eqs (M7-7.5) Winter of 1811-1812 o Sometimes referred to as “the largest that have ever struck the United States”; felt over a huge area, even ringing bells in Boston o Missouri is not on a plate boundary; the rocks out in California are really chewed up so they suck up the energy and cause the shaking to die out sooner; water-saturated sediments shake a lot longer than dry sediments so people living in Missouri on the Mississippi riverbank felt a lot of shaking and the buildings on the sand collapsed but those further inland were okay o Mississippi is located in the lowest spot so everything from the higher levels drains into the lowest (blue) level; why is it a low spot? It is an old failed rift arm. 2. Convergent Plate Boundaries Convergent boundaries are the sources of the REALLY big earthquakes Earthquakes caused by convergent boundaries look different from other earthquakes Belt of earthquakes extending downward from where one plate subducts under another—called subduction zones (see drawing) o Ex. From the Japan Trench all the way down to 600km depth o At 125km depth, the rocks within the slab undergo mineral transformations in response to increase Pressure and Temperature o Dehydration reactions (see drawing) expel H20 from xl lattice water causes the rocks in the overriding slab to melt and become magma and rise to the top California—state mineral: Serpentine; state rock: Serpentinite
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o altered hydrous oceanic Lithospheric rock) o ultramafic rock (extremely dense) Deepest eqs in the world are at about 600-700km depth Pacific Ring of Fire —wide belt of earthquakes that surrounds the pacific; association of subduction zones and volcanoes [also a Johnny Cash and Wallavoodoo song ;) haha] Exploding volcanoes (glowing avalanche) o Andesite—lava erupted at convergent-margin volcanoes Rich in silica and low in iron and magnesium MUCH more viscous (thicker) than basalt (which is kind of runny) o Lava is FULL of gases; these bubbles expand as the pressure decreases when they rise and they want to pop when the reach the surface. The magma is too thick to release the gases, so the pressure builds up; huge landslide of the lava comes out of the volcano, which releases the pressure so the pent up gases all of a sudden explode (ex. Mt. Saint Helens) Most convergent boundaries have at least one oceanic plate diving down under another plate; characterized by subduction of an oceanic Lithospheric plate 1.
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2 the evidence - Plate Tectonics The Evidence 15:01:00 1....

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