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Aulacogens and ophiolites

Aulacogens and ophiolites. Geology 101, Fall 2010. 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). When a rift occurs.

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Aulacogens and ophiolites

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  1. Aulacogens and ophiolites Geology 101, Fall 2010

  2. 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)

  3. 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

  4. 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”).

  5. 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.

  6. Throughout time • Cambrian period version in Arkasas and Missouri

  7. 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

  8. Grabens and rifts • Grabens are valleys bounded by “normal” faults, which are characteristic of divergent boundaries • A low point in an uplifting area!

  9. 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

  10. Ophiolites • Ophiolites are found on land, and provide evidence of the accretion (or at least uplift) of the sea floor onto continents

  11. Local ophiolite • Green Park, Fidalgo Island (near San Juan Islands ferry terminal in Anacortes)

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