Transcript The Origin of Ocean Basins

Evidence for Continental Drift: Puzzle
Figure 1-8a Tectonic Plates
According to Wegner, the continents are sections of a past super
continent called Pangea, which broke apart and drifted to their
present locations.
Evidence for Continental Drift: Fossils
Now-Extinct Life
Forms Preserved
in the Geologic
Record tell a story
Source: J. C. Carton/Carto/Bruce Coleman, Inc. New York
Evidence for Continental Drift: Rock Record
Source: William E. Ferguson
Pangaea 200 to 300 Millions of Years
Before the Present
Sea-Floor Spreading:
3-2 Movement at ridges
• Axis of the oceanic ridge is offset by transform
(strike-slip) faults which produce lateral
displacement. Ridges and rifts indicate movement.
Segmented Ocean Ridge
3-3 Global Plate Tectonics
Earth’s geomagnetic field is recorded as
new crust cools.
New crust.
Parallel bands of
crust with the
same magnetism
form along the
ridge.
Driving Mechanisms for Plate Motions
Geomagnetic Polarity Reversals
When new crust materials crystallizes,
some minerals align themselves with
Earth’s magnetic field, as it exists at that
time, imparting a permanent magnetic
field, called paleomagnetism, to the rock.
Periodically Earth’s magnetic field
polarity (direction) reverses poles.
Magnetic Anomalies
Geologic Time
Modern humans
1_14
Extinction of dinosaurs
Flowering plants and grasses
First mammals
Earliest dinosaurs
Early reptiles
Primitive
fish
3-2 Sea-Floor Spreading
• Rocks forming at the ridge crest record the
magnetism existing at the time they solidify.
• Sea floor increases in age and is more deeply
buried by sediment away from the ridge
because sediments have had a longer time to
collect.
• Rates of sea-floor spreading vary from 1 to 10
cm per year for each side of the ridge and can
be determined by dating magnetic anomaly
stripes of the sea floor and measuring their
distance from the ridge crest.
• Continents are moved by the expanding sea
floor.
3-3 Global Plate Tectonics
Because Earth’s size has not changed,
expansion of the crust in one area
requires destruction of the crust
elsewhere.
• Currently, the Pacific Ocean basin is
shrinking as other ocean basins expand.
• Seismicity is the frequency, magnitude and
distribution of earthquakes. Earthquakes
are concentrated along oceanic ridges,
transform faults, trenches and island arcs.
• Tectonism refers to the deformation of
Earth’s crust.
3-3 Global Plate Tectonics
• Destruction of sea floor occurs in
subduction zones.
• Subduction is the process at a trench
whereby one part of the sea floor plunges
below another and down into the
asthenosphere.
3-3 Global Plate Tectonics
• Benioff Zone is an area of increasingly deeper
seismic activity, inclined from the trench
downward in the direction of the island arc.
South Figi Basin and Cross Section
Showing Benioff Zone
3-3 Global Plate Tectonics
Earth’s surface is composed of a series
of lithospheric plates. Plate edges
extend through the lithosphere and are
defined by seismicity.
• Plate edges are trenches, oceanic ridges
and transform faults.
• Seismicity and volcanism are concentrated
along plate boundaries.
3-3 Global Plate Tectonics
Movement of plates is caused by thermal
convection of the “plastic” rocks of the
asthenosphere which drag along the
overlying lithospheric plates.
Driving Mechanisms for Plate Motions
Plate Rifting (cont'd): Earths Internal Heat Engine
Figure 1-10
Mid-Ocean
Ridge
3-3 Global Plate Tectonics
• Mantle plumes originate deep within the
asthenosphere as molten rock which rises
and melts through the lithospheric plate
forming a large volcanic mass at a “hot
spot”.
Mantle Plume
3-3 Global Plate Tectonics
Wilson Cycle refers to the sequence of
events leading to the formation,
expansion, contracting and eventual
elimination of ocean basins.
• Stages in basin history are:
– Embryonic - rift valley forms as continent begins to
split.
– Juvenile - sea floor basalts begin forming as
continental fragments diverge.
– Mature - broad ocean basin widens, trenches
eventually develop and subduction begins.
– Declining - subduction eliminates much of sea floor
and oceanic ridge.
– Terminal - last of the sea floor is eliminated and
continents collide forming a continental mountain
chain.
The Wilson Cycle
3-4 Transform Faults
The San Andreas fault in
southern California is a
transform fault that
connects the sea-floor
spreading ridge of the
Gulf of California with
the spreading ridge off
Oregon and Washington.
• If these plate motions
continue, Baja will
splinter off California.
3-4 Transform Faults
Because the San Andreas fault has an
irregular trace, strike-slip motion can
cause local compression or tension.
Fault Geometry
3-4 Juvenile Ocean Basin
• The Red Sea is a juvenile ocean basin that
is forming as the African plate diverges
from the Arabian plate.
• New basaltic ocean crust is just beginning
to form in the center of the Red Sea.
3-4 Juvenile Ocean Basin
Hot, salty groundwater is dissolving metals from
the rocks and depositing them as metal sulfides
in dense brine pools like the Atlantis II Deep.
Atlantis II Deep