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

• Second law of thermodynamics as the heat of the mantle gets through the lithosphere.

• Three rift valleys form: three divergent boundaries

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”).

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.

Throughout time

• Cambrian period version in Arkasas and Missouri

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

Grabens and rifts

• Grabens are valleys bounded by “normal” faults, which are characteristic of divergent boundaries

• A low point in an uplifting area!

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

Ophiolites

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

Local ophiolite

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