1. 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.
2. 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: Fossils
5. • Axis of the oceanic ridge is offset by transform
(strike-slip) faults which produce lateral
displacement. Ridges and rifts indicate movement.
3-2
Sea-Floor Spreading:
Movement at ridges
Segmented Ocean Ridge
6. Earth’s geomagnetic field is recorded as
new crust cools.
3-3 Global Plate Tectonics
Driving Mechanisms for Plate Motions
New crust.
Parallel bands of
crust with the
same magnetism
form along the
ridge.
7. 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.
9. 1_14
Modern humans
Extinction of dinosaurs
Flowering plants and grasses
First mammals
Earliest dinosaurs
Early reptiles
Primitive
fish
Geologic Time
10. • 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.
3-2 Sea-Floor Spreading
11. 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
12. • 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
13. • Benioff Zone is an area of increasingly deeper
seismic activity, inclined from the trench
downward in the direction of the island arc.
3-3 Global Plate Tectonics
South Figi Basin and Cross Section
Showing Benioff Zone
14. 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
15. 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
18. • 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”.
3-3 Global Plate Tectonics
Mantle Plume
19. 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.
3-3 Global Plate Tectonics
21. 3-4 Transform Faults
• If these plate motions
continue, Baja will
splinter off California.
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.
22. Because the San Andreas fault has an
irregular trace, strike-slip motion can
cause local compression or tension.
3-4 Transform Faults
Fault Geometry
23. • 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
24. 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
