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Aulacogens and ophiolites
Geology 101, Fall 2010
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When a heat must escape through a surface
Maximize the length of “gaps” through which heat can escape Hexagonal cracking of the surface does the trick Basalt columns (Giants Causeway in Ireland)
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When a rift occurs Second law of thermodynamics as the heat of the mantle gets through the lithosphere. Three rift valleys form: three divergent boundaries
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As time progresses Basic space problem: three rifts cannot continue producing new sea floor without the sea floor “bunching up”. Solution: kill one of the rifts (“dead” arm called the “aulacogen”).
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Current version East African Rift: along with the Red Sea and Gulf of Aden arms, part of the three-arm rift system. The East African rift is the aulacogen.
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Throughout time Cambrian period version in Arkasas and Missouri
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Evolution of an aulacogen
Though “dead”, an aulacogen rift valley is a zone of weakened, broken rock -- perfect for rivers The Rhine graben in northern Europe
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Grabens and rifts Grabens are valleys bounded by “normal” faults, which are characteristic of divergent boundaries A low point in an uplifting area!
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New sea floor -- ophiolites
An ophiolite (or ophiolite sequence) is a set of rocks associated with sea floor spreading -- literally, the rocks you would find in a cross-section of the sea floor
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Ophiolites Ophiolites are found on land, and provide evidence of the accretion (or at least uplift) of the sea floor onto continents
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Local ophiolite Green Park, Fidalgo Island (near San Juan Islands ferry terminal in Anacortes)
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