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)