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
Basic Assumptions of Plate Tectonic Theory
 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
 Satellite based studies- plate motion and geomagnetic research
 Velocity of plates measures directly by ground-based laser which beams off reflectors on selected
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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
 DISCOVERY OF MARINE MAGNETIC ANOMALIES (WW-II MAGNETOMETER
OBSERVATIONS IN THE SHIP)
 VINE & MATHEWS’ HYPOTHESIS OF MAGNETIC REVERSALS (EVIDENCE OF SEAFLOOR SPREADING AT DIVERGENT PLATE BOUNDARIES)
 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
 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
 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.
 Oceanic trench
 forms where dense oceanic plate plunges (subducts) under less dense plates forming a depression in
the earth’s surface
 Melange
 mixture of oceanic sediments and ophiolite rocks- form massive accretionary wedge that may be
attached to the edge of the overriding plate
 Convergent
 collision of two plates resulting in suture zones
 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
Structure of Oceanic Lithosphere-contd.
 H2O + Pyroxene (in basalt, gabbro) – chlorite
 H2O +Mg olivine (Umafic peridotite)- Serpentine (Mg-Si mineral)
Layer of Ophiolite suite
Transform Boundaries & Offset Mid-Ocean Ridges
 Occur where plates slide past one another in opposite direction
 15% total length of plate margins
 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
 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
 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
 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 mid-ocean ridge
Convection Cells Hypothesis
 Deep convection cell hypothesis: Lower mantle to the surface of the earth
 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
Chapter Summary
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Marine magnetic anomaly
Plate velocity
Info on layers beneath the ocean floor
Plate edges are considered to be passive continental margins – when?
Mid-ocean ridges
Backarc spreading – rifting of the overriding plate
First landmasses – volcanic islands
Pangaea, Gondwana, Laurasia
Evidence of forces that drive Plates
Spreading Ridge of the East Pacifc rise is not in the center of the Pacific Ocean because --- Eastern part
of the Pacific has been subducted beneath the American Plates.