Transcript Earth Science Honors

Theory of Plate
Tectonics
Lecture 4
Review
Wegener - continental drift hypothesis
Continental puzzle
Fossil records
Matching mountain ranges
Ancient climate
Main objection to Wegener's proposal
Inability to provide a mechanism
Theory of Plate tectonics
4 scientific developments
• Ruggedness and youth of ocean floor
• Earthquakes and volcanoes active along ocean mountain
ranges and trenches
• Repeated changes of earth’s magnetic field
• Spreading of the ocean floor hypothesis
Review
Plate boundaries
Divergent
• Oceanic and continental
Convergent
• Oceanic-continental
 Continental volcanic arcs
• Oceanic-oceanic
 Volcanic island arcs
• Continental-continental
 Mountain ranges - Himalayas
Transform
• Aid in crustal movement
Theory of Plate TectonicsPaleomagnetism
Paleomagnetism is probably the most
persuasive evidence supporting the theory
of plate tectonics
Theory of Plate tectonics Evidence from ocean drilling
Seafloor spreading
Some of the most convincing evidence
confirming seafloor spreading
Evidence from drilling directly into ocean-floor sediment
 Age of deepest sediments - furthest away from boundary
Theory of Plate Tectonics Hot spots and mantle plumes
 Hot spots - caused by
rising plumes of mantle
material
 Volcanoes - Hawaiian
islands over hot spot
 The further from Hawaii
the older the island
 Pacific plate moves over
hot spot - volcanic
mountains form
 Evidence plates moved
What drives plate motion
Driving mechanism of plate tectonics
• Several mechanisms contribute to plate
motion
• Scientists generally agree that convection
occurring in the mantle is the basic driving
force
• Warm less dense material rises
• Cooler, denser material sinks
• Movement of the plates and mantle is driven by the
unequal distribution of Earth’s heat
• Heat is generated by radioactive decay of elements such
as Uranium found in the mantle and crust
Several models to explain plate
movement have been proposed
• Model 1 - Slab-pull and slab-push model
• Descending oceanic crust pulls the plate downward arm of convective flow
• Elevated ridge system pushes the plate
Figure 15.26
Several models to explain plate
movement have been proposed
• Model 2 - Plate-mantle convection
• Mantle plumes extend from mantle-core
boundary and cause convection within
the mantle
• 2 different models for plate-mantle
convection
• see the following figures
Whole-mantle convection
Cold oceanic crust descends into the mantle
Hot mantle plumes transport heat toward the
surface
Deep-layer model
Earth’s heat causes these layers of convection to
slowly swell and shrink in complex patterns
Some material from the lower layer flows upward
as mantle plumes
Plate movement
We still have a lot to learn
We do know this:
The unequal distribution of heat within the Earth causes
the thermal convection in the mantle that ultimately
drives plate motion
Measuring plate motion
 Using space-age technology to directly measure the
relative motion of plates
 Very Long Baseline Interferometry (VLBI)
 large numbers of time difference measurements from distant sources
observed with a global network of antennas over a period of time
 Global Positioning System (GPS)
Figure 15.25
A possible view of the world
50 million years from now
• Areas west of the San Andreas Fault slide northward past the North
American plate
• Africa collides with Eurasia, closing the Mediterranean and initiating
mountain building
• Australia and new Guinea are on a collision course with Asia
Figure 15.28