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The Origin of Ocean Basins

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Periodically Earth s magnetic field polarity (direction) reverses poles. ... The Origin of Ocean Basins Author: Lianne Ames Last modified by: CCMI Created Date: – PowerPoint PPT presentation

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Title: The Origin of Ocean Basins


1
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
Evidence for Continental Drift Fossils
Source J. C. Carton/Carto/Bruce Coleman, Inc.
New York
3
Evidence for Continental Drift Rock Record
Source William E. Ferguson
4
Pangaea 200 to 300 Millions of Years Before the
Present
5
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
6
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
7
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.
8
Magnetic Anomalies
9
1_14
Geologic Time
Modern humans
Extinction of dinosaurs
Flowering plants and grasses
First mammals
Earliest dinosaurs
Early reptiles
Primitive
fish
10
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.

11
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.

12
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.

13
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
14
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.

15
Movement of plates is caused by thermal
convection of the plastic rocks of the
asthenosphere which drag along the overlying
lithospheric plates.
Global Plate Tectonics
3-3
Driving Mechanisms for Plate Motions
16
Plate Rifting Earths Internal Heat Engine
17
Mid-Ocean Ridge
18
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
19
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.

20
The Wilson Cycle
21
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.

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
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.

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
25
Plate Subduction
26
Continental Collision
27
Continental Collision (cont'd)
28
Transform Motion
29
Convection Cells
30
Slice of Earth System Interactions (Atmosphere,
Biosphere, Lithosphere, and Hydrosphere)
31
Plate Boundaries
32
Plate Boundaries (contd)
33
Plate Boundaries (cont'd)
34
Plate Boundaries (cont'd)
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