Theory of Plate Tectonics

Lecture 4

ReviewWegener - continental drift hypothesis

Continental puzzle Fossil recordsMatching mountain rangesAncient climate

Main objection to Wegener's proposal Inability to provide a mechanism

Theory of Plate tectonics4 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 boundariesDivergent

• Oceanic and continentalConvergent

• Oceanic-continental Continental volcanic arcs

• Oceanic-oceanic Volcanic island arcs

• Continental-continental Mountain ranges - Himalayas

Transform • Aid in crustal movement

Theory of Plate Tectonics-Paleomagnetism

Paleomagnetism is probably the most persuasive evidence supporting the theory of plate tectonics

Theory of Plate tectonics -Evidence from ocean drilling

Seafloor spreadingSome 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

Figure 15.26

• 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

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 convectionCold oceanic crust descends into the mantleHot mantle plumes transport heat toward the surface

Deep-layer modelEarth’s heat causes these layers of convection to slowly swell and shrink in complex patternsSome material from the lower layer flows upward as mantle plumes

Plate movementWe still have a lot to learnWe 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

Figure 15.25

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)

A possible view of the world 50 million years from now

Figure 15.28

• 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