Figure 1-8a Tectonic Plates

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Figure 1-8a Tectonic Plates
Evidence for Continental Drift Puzzle
According to Wegner, the continents are sections
of a past super continent called Pangea, which
broke apart and drifted to their present
locations.
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Now-Extinct Life Forms Preserved in the
Geologic Record tell a story
Evidence for Continental Drift Fossils
Source J. C. Carton/Carto/Bruce Coleman, Inc.
New York
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Evidence for Continental Drift Rock Record
Source William E. Ferguson
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Pangaea 200 to 300 Millions of Years Before the
Present
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Sea-Floor Spreading Movement at ridges
3-2

• Axis of the oceanic ridge is offset by transform
  (strike-slip) faults which produce lateral
  displacement. Ridges and rifts indicate movement.

Segmented Ocean Ridge
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Earths geomagnetic field is recorded as new
crust cools.
3-3
Global Plate Tectonics
New crust. Parallel bands of crust with the same
magnetism form along the ridge.
Driving Mechanisms for Plate Motions
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Geomagnetic Polarity Reversals
When new crust materials crystallizes, some
minerals align themselves with Earths magnetic
field, as it exists at that time, imparting a
permanent magnetic field, called paleomagnetism,
to the rock. Periodically Earths magnetic field
polarity (direction) reverses poles.
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Magnetic Anomalies
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1_14
Geologic Time
Modern humans
Extinction of dinosaurs
Flowering plants and grasses
First mammals
Earliest dinosaurs
Early reptiles
Primitive
fish
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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.

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Because Earths size has not changed, expansion
of the crust in one area requires destruction of
the crust elsewhere.
3-3
Global Plate Tectonics

• 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 Earths
  crust.

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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.

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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
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Earths surface is composed of a series of
lithospheric plates. Plate edges extend through
the lithosphere and are defined by seismicity.
3-3
Global Plate Tectonics

• Plate edges are trenches, oceanic ridges and
  transform faults.
• Seismicity and volcanism are concentrated along
  plate boundaries.

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Movement of plates is caused by thermal
convection of the plastic rocks of the
asthenosphere which drag along the overlying
lithospheric plates.
3-3
Global Plate Tectonics
Driving Mechanisms for Plate Motions
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Plate Rifting (cont'd) Earths Internal Heat
Engine
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Figure 1-10 Mid-Ocean Ridge
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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
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Wilson Cycle refers to the sequence of events
leading to the formation, expansion, contracting
and eventual elimination of ocean basins.
3-3
Global Plate Tectonics

• 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.

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The Wilson Cycle
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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.

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Because the San Andreas fault has an irregular
trace, strike-slip motion can cause local
compression or tension.
3-4
Transform Faults
Fault Geometry
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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.

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Hot, salty groundwater is dissolving metals from
the rocks and depositing them as metal sulfides
in dense brine pools like the Atlantis II Deep.
3-4
Juvenile Ocean Basin
Atlantis II Deep