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Chapter Twelve
Plate Tectonics:
Creating Oceans
and Continents
Key elements of Plate Tectonics
Earth’s lithosphere consists of rigid plates
 Plates move relative to one another by divergence,
convergence, or transform motion
 Oceanic lithosphere forms at divergent plate
boundaries and are consumed at subduction zones
 Most earthquakes, volcanism, faulting and
mountain building take place at plate boundaries.
 Plate centers tend to be geologically stableKey
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Basic Assumptions of Plate Tectonic Theory
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Lithosphere consists of rigid plates (100 km average; 70
km for ocean & 150 km for continents)
Plates move relative to one another by Divergence,
Convergence, or Transform motion
Formation of Oceanic lithosphere at divergent plate
boundaries and is consumed at subduction zone
Most earthquake activity, volcanism, faulting, and
mountain building take place at plate boundaries
Centers of plates are stable
Plate velocity - determination
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Satellite based studies- plate motion and geomagnetic
research
 Velocity of plates measures directly by ground-based laser
which beams off reflectors on selected satellites
 Hot Spots- can serve as fixed reference point to measure
absolute plate velocities
 Structures formed as plates move over hot spots also reveal
speed and direction of plates
 Marine Magnetic Anomalies are used to estimate rates of
divergence, and therefore plate motion
 Eco-sounder sonar- topography of sea floor and seismic
profiling- study underlying layers
Relative Plate velocities
Relative Plate velocities-contd.
Satellite Global system
Mid-Pacific Hot spot
Tracking Magnetic Field Reversals
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DISCOVERY OF MARINE MAGNETIC
ANOMALIES (WW-II MAGNETOMETER
OBSERVATIONS IN THE SHIP)
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VINE & MATHEWS’ HYPOTHESIS OF
MAGNETIC REVERSALS (EVIDENCE OF
SEA-FLOOR SPREADING AT DIVERGENT
PLATE BOUNDARIES)
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MEASURING PLATE MOVEMENT BY
MAGNETIC ANOMALIES (ANOMALY’S
DISTANCE FROM SPREADING RIDGE)
 PLATE SPEED: 1 TO 10 CM PER YEAR
Marine magnetic anomalies
Marine magnetic anomaly-contd.
Marine magnetic anomaly-contd.
Directions and Rates of Plate movement
Nature and Origin of the Ocean Floor
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Rifting
 rising mantle over hot spot- 3 radiating valleys. One fails to
open and later becomes filled with sediments- aulacogen.
When rifting stops- the rift edge becomes inactive tectonically
and therefore have passive continental margin
Divergent Plate Boundaries
 as divergence continues, full seaway forms and new oceanic
lithosphere forms at the mid-oceanic ridge as up-welling
ultramafic melt produce basaltic magma
Transform Boundaries/offset mid-oceanic ridges
 transform fault becomes divided into short offset segments by
ocean
 Information about the Ocean Floor – Echo-Sounding sonar,
Seismic Profiling, Deep-Sea Drilling Project & Submersible
vessels
Active Rifting
Rifting and Origin of Ocean Basins
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Active Arms marked by:
– High heat flow
– Normal faulting
– Frequent shallow earthquakes
– Widespread basaltic volcanism
Rift valley widening leads to the development of
new seaways and evaporite belts
In Ocean, it leads to the formation of sediment
deposition and development of Passive
continental margins
The East Africa Rift Zone
The growth of oceanic basin
Growth of ocean basin – contd.
Growth of ocean basin – contd.
Nature and Origin of the Ocean floor – contd.
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Oceanic trench
 forms where dense oceanic plate plunges (subducts) under
less dense plates forming a depression in the earth’s surface
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Melange
 mixture of oceanic sediments and ophiolite rocks- form
massive accretionary wedge that may be attached to the edge
of the overriding plate
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Convergent
 collision of two plates resulting in suture zones
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Volcanic arc
 chain of volcanoes formed from subduction
Structure of Oceanic Lithosphere
Upper surface – 200 M: sediment of
siliceous or carbonate ooze and/or reddish
clay
 200 m- 2 km: Oceanic basalt with top layer
of pillow structures
 3-6 km: Gabbro
 Below Gabbro is Peridodite
 Serpentinite formed by alteration of rocks
by water – Entire sequence of ocean-floor
rock may be altered its faults & fissures
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Structure of Oceanic Lithosphere-contd.
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H2O + Pyroxene (in basalt, gabbro) – chlorite
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H2O +Mg olivine (Umafic peridotite)Serpentine (Mg-Si mineral)
Layer of Ophiolite suite
Transform Boundaries & Offset Mid-Ocean
Ridges
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Occur where plates slide past one another in
opposite direction
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15% total length of plate margins
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Faulted blocks move in opposite directions --produce stress --- leads to earthquakes
Direction of motion of a plate
Subduction-zone feature
Breadth of arc-trench gap
Breadth of arc-trench gap-contd
Anatomy of a continent
Continental shield
Origin of a Supercontinent
Origin of the supercontinent Pangaea
Earth’s plates before Pangaea
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Gondwana: ~500 million yrs BP: Near South
Pole; 4 Northern landmasses
 Laurasia: 3 Landmasses in the Northern
Hemisphere; North America, Northern Europe,
Southern Europe + Parts of Africa & Siberia
 Pangaea: Laurasia and Gondwana colloided to
form Pangaea
Future Events
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50-100 Million yrs from now: Australia will collide with
Southeast Asia
Western part of California will separate from North
America along the San Andreas fault and become a
separate microcontinent
Mediterranean will close as the African and Eurasian
Plates collide
Subduction may occur along the East coast of North
America
All of the Earth’s landmasses may reunite into another
Pangaea-like supercontinent
Driving Forces of Plate Motion
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Uncertainty on what drives Plate Motion
 Slow Asthenosphere convection currents
 Evidence against plate pushing by rising magma
wedges at divergent boundaries
 Evidence in favor of plate pulling by descending
slabs at subduction zones
 Gravity forces plates away from uplifted midocean ridge
Convection Cells Hypothesis
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Deep convection cell hypothesis: Lower mantle to
the surface of the earth
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Shallow convection cell hypothesis: Cells are
driven within the Asthenosphere
Three factors that may drive plate tectonics
Convection cell hypothesis
Convection cell hypothesis-contd.
Configuration cell hypothesis
Seismic Topography
Seismic topography-contd.
Thermal plumes
Two patterns of Marine anomalies