Seamounts The Oceanic Crust o Oceanic crust is covered by sediment Thickest

Seamounts the oceanic crust o oceanic crust is

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Seamounts The Oceanic Crust o Oceanic crust is covered by sediment Thickest near the continents Thinnest (or absent) at the mid-ocean ridge o Oceanic crust is mafic No granitic or metamorphic rocks o Heat flow is greater at the mid-ocean ridges Sea-Floor Spreading o Seediment thickens away from ridges o Earthquakes at mid-ocean ridges indicate cracking High heat flow from molten rock rises into the cracked crust o The “Sea-floor spreading” theory Upwelling mantle erupts at the mid-ocean ridges New crust moves away from ridges At trenches, the sea-floor subducts back into the mantle o Instantly provided a mechanism for continental drift Continents move apart during sea-floor spreading Continents move together as sea-floor is subducted Evidence for Sea-Floor Spreading o Magnetic anomalies map as stripes of positive and negative intensity o Magnetic stripes form a pattern o The pattern is symmetric on either side of the MOR
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Magnetic Reversals o The magnetic field sometimes “flips”; we don’t know why o A reversed N magnetic pole is near the S geographic pole o Can be used as time markers o Reversals occur at uneven intervals Longer intervals (500 to 700+ Ka) Shorter intervals (~200 Ka) Sea-Floor Spreading o Sea-floor spreading explains the striped Magnetic polarity reversals are imprinted in sea-floor rock We can determine rates of sea-floor spreading Mid-Atlantic Ridge ~2cm/yr East Pacific Rise ~10 cm/yr o The width of the magnetic anomaly stripes is related to the spreading rate Faster spreading = wide stripes Slower spreading = narrow stripes o Age increases with distance from MORs Plate tectonics o Earth’s outer shell is broken into rigid plates that move o Moving plates change the face of planet Earth o Tectonics is able to explain almost everything o Plate tectonics thwory provides a unifies mechanism explaining: Igneous, sedimentatry, and metamorphic rocks Distribution of earthquakes and volcanes Origin of continents and ocean basins Distribution of fossil plants and animals Genesis and destruction of mountain chains Continental drift Lithosphere o Tectonic plates are fragments of lithosphere Lithosphere is made of both crust and the upper mantle In motion over the asthenosphere Bends elastically Two types of Lithosphere o Continental: ~150 km thick Lighter (less dense) More boyant—floats higher o Oceanic: ~7-100 km thick Heavier (more dense) Less boyant o Lithosphere is fragmented into ~20 tectonic plates o Plates move continupously
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o Plates interact along their boundaries o Display a variety of sizes and shapes Changes throughout history Plate Boundaries o Locations on Earth where tectonic plates meet Identified by concentrations of Earthquakes Plate interiors are almost earthquake-free Continental Margins o Where land meets the ocean Margins near plate boundaries are “active.” Magins far from plate boundaries are “passive.” o Passive-margin continental crust thins seaward Transitions into oceanic crust Traps eroded sediment Develops into the continental shelf Plate Boundaries: 3 types o
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