Download aulacogens

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)