Last Time
UNIVERSITY OF SOUTH ALABAMA
MAS 603: Geological Oceanography
A) Seismic waves and the Earth's interior
B) Seismic Tomography
C) Mineral phase changes, inner core nuclear
reactors, and outer core magnetism (weird stuff)
Lecture 3: Divergent Plate
Boundaries and Evolution of Oceans
Deep Seismic
Seismic Waves
But wave refraction results in
the formation of “shadow
zones” where P or S-waves do
not occur.
The speed that Sand P- waves travel
through the Earth
varies an many
places. The most
dramatic changes
are associated with
the 4 major layers…
The S-wave shadow zone is wider
than the P-wave shadow zone
P-wave shadow zones
… but 5-50 km
down, you get sharp
drop in wave speed.
Deep Seismic
Polymorphs of SiO2
Quartz has 6 polymorphs related to pressure and temperature
β-quartz
Really impressive.
COCORP lines are
being run all around the
world these days
α-quartz
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Seismic Tomography
Seismic tomography is a method of using seismic waves
from earthquakes (plus some other data) to create 3D
images of the mantle. These studies pick out areas of fast or
slow mantle, which correspond to areas of high and low
temperature.
Today’s Agenda
Seismic Tomography
and descending lithospheric
plates
Today’s Agenda
A) Composition of Oceanic crust
B) Origin of magma in divergent plate boundaries
C) Evolution of oceans (the Wilson Cycle)
Oceanic Crust
Oceanic crust is
approximately 5 km thick.
Oceanic Crust
Oceanic crust is
approximately 5 km thick.
1) We can easily get samples
of the upper crust.
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Oceanic Crust
Oceanic Crust
Oceanic crust is
approximately 5 km thick.
Oceanic crust is
approximately 5 km thick.
1) We can easily get samples
of the upper crust.
1) We can easily get samples
of the upper crust.
2) We can core through 2 or
3 km with only moderate
difficulty
2) We can core through 2 or
3 km with only moderate
difficulty
Oceanic Crust
3) We can study it via seismology
(Remote sensing)
Oceanic Crust
Or we can visit outcrops of
oceanic crust (ophiolite
complexes)
Ophiolites are fragments of
oceanic crust that have been
thrust onto continental plates
at convergent plate
boundaries.
Ophiolites in Oman
Oceanic Crust
Ophiolites can be thick.
Composite sections permit
reconstruction of the entire
oceanic crust
Layer
Composition
Thickness
Density
1
2
sediment
500 m
2.3
basalt
1750 m
2.7
3
gabbro
to 4700 m
3.0
4
Peridotite
Moho Discontinuity
1000m+
Ophiolites in Oman
Basalts
Basalts come in several
different “flavors”. In the
marine regime (i.e., pillow
lavas), we recognize two
main types on the basis of
geochemical composition.
1
2
3
Alkali basalt (K+, Ti4+, U4+, Th2+, Sr2+, Cs2+)
Tholeiite basalt (Al3+, Cr3+, Cu2+, Ca2+, Mg2+))
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3.4
Chalk Board
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Basalts come in several
different “flavors”. In the
marine regime (i.e., pillow
lavas), we recognize two
main types on the basis of
geochemical composition.
wt% Alkalis (Na+, K+ etc)
Basalts
ka
Al
Origin of Oceanic Crust
al t
as
il b
Th
b
iite
ole
as a
lt
Divergent plate
boundaries are areas
where molten rock makes
it to the Earth’s surface.
wt% SiO2
Alkali basalt (K+, Na+, Ti4+, U4+, Th2+, Sr2+, Cs2+)
Tholeiite basalt (Al3+, Cr3+, Cu2+, Ca2+, Mg2+))
Chalk Board
Origin of Oceanic Crust
Divergent plate
boundaries are areas
where molten rock makes
it to the Earth’s surface.
Origin of Oceanic Crust
Two hypotheses explaining the origin of divergent plate
boundary basalts have survived.
1) one magma hypothesis (fractionation)
2) multiple magma hypothesis
Any model explaining the intrusion
mechanism must also explain the
layered structure of ophiolites
Evolution of Oceans
Six stages in the ocean cycle are recognized by geologists:
1) Embryonic (e.g., East African Rift)
2) Young (e.g., Red Sea)
3) Mature (e.g., Atlantic Ocean)
4) Declining (e.g., Pacific Ocean)
5) Terminal (e.g., Mediterranean Sea)
6) Relict (e.g., ophiolites)
Chalk Board
Evolution of Oceans
Seven? stages in the ocean cycle are recognized by geologists:
0) Pre-embryonic
1) Embryonic (e.g., East African Rift)
2) Young (e.g., Red Sea)
3) Mature (e.g., Atlantic Ocean)
4) Declining (e.g., Pacific Ocean)
5) Terminal (e.g., Mediterranean Sea)
6) Relict (e.g., ophiolites)
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0) Pre-embryonic Stage
Imagine an ideal “plate” underlain by a hot spot
0) Pre-embryonic Stage
0) Pre-embryonic Stage
A volcanic plume would rise up through the lithosphere and
eventally burn its way through the surface.
0) Pre-embryonic Stage
Ternary rift patterns or…
At the surface, in the vicinity of the hotspot, tension would
essentially put the rocks apart. The result….
0) Pre-embryonic Stage
….Triple Junction
1) Embryonic Stage
Over large areas, several ternary rift patterns might over lap
resulting in continuous spreading…
• In overlapping triple junction sets, one of the “arms” in each set will become
a failed rift or an Aulcogen. Two will continue to spread into an ocean
(embryonic oceans)
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1) Embryonic Stage
• The best modern example of a “failed arm” is the East African
Rift. This is analogous to an embryonic ocean.
2) Young Ocean Stage
1) Embryonic Stage
• The best ancient example of a failed arm is the Niger River
basin of Africa.
2) Mature Ocean Stage
Mature oceans are underlain by
basalt, and are flanked by thick
deposits of continent-derived
sediment (“shelves”).
Young Oceans are narrow, deep
and not underlain by basalt
4) Declining Ocean Stage
5) Terminal Ocean Stage
Declining oceans are surrounded by convergent plate boundaries
and are shrinking in size
Terminal oceans are near death…..
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6) Relict Ocean Stage
Ocean Evolution
And relict oceans are dead….
• So oceans go through a growth phase….
Ocean Evolution
Ocean Evolution
• and a death phase….
• This is now called the
Wilson Cycle in honor of
J. Tuzo Wilson who first
suggested it for the
Atlantic Ocean.
• And sometimes they
repeat this cycle more than
once (e.g., the Atlantic
Ocean)
The Wilson Cycle
Was named after Wilson’s
classic paper on
Newfoundland (an island at
the northern end of the
Appalachian Mountains). The
single coolest place on the
planet to live for its geology
and its lifestyle.
http://iz.carnegiemnh.org/crayfish/country_pages/newfoundland.htm
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The Wilson Cycle
Was named after Wilson’s
classic paper on
Newfoundland (an island at
the northern end of the
Appalachian Mountains). The
single coolest place on the
planet to live for its geology
and its lifestyle.
The Wilson Cycle
http://iz.carnegiemnh.org/crayfish/country_pages/newfoundland.htm
Paleozoic Paleogeography
The island is divisible into several
geological zones, each produced
within a specific tectonic
environment that tells a story of
ocean closure.
Paleozoic Paleogeography
Laurentia
Paleozoic Paleogeography
Paleozoic Paleogeography
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Paleozoic Paleogeography
Paleozoic Paleogeography
Paleozoic Paleogeography
Next Time
1. No Monday Lecture
2. Wed: Gulf of Mexico Discussion (led by Zeb)
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