Plate Tectonics I. The Discovery of Plate Tectonics II. A Mosaic of Plates III. Types of Plate Boundaries IV. How Plates Move
I. The Discovery of Plate Tectonics A. Continental Drift (Alfred Wegener) Proposed largescale movement of the continents
Evidence: 1. Puzzle fit of continents to form Supercontinents a. Suess (1900)- Gondwana b. Wegener (1915)- Pangea
More Evidence 2. Similar rock ages 3. Similar geologic structures such as mountain ranges 4. Fossil Evidence from the Mesosaurus 5. Climate Evidence such as glacial deposits **Wegener s theory did not gain much support because he could not explain how the tectonic plates move
B. Seafloor Spreading- Henry Hess) 1. Convection currents move plates around 2. Mantle source of heat and convection 3. proposed new and recycled seafloor 4. Evidence from fathometers on submarines during WWII around the Mid-Atlantic Ridge showed the seafloor is not completely flat **There was no evidence for this theory, until
C. Magnetic Tape Recording 1. Magnetic Reversals a. Switching strength to the south b. Preserved in lava (iron points towards stronger pole) c. Age can be dated and if the seafloor is spreading from a center, should be mirror images on each side
Magnetic Tape Recording (continued) 2. Magnetic anomaly a. Normal-positive anomaly b. Reverse-negative anomaly
Mid-ocean ridge Million years ago (Ma) 4.0 3.0 2.0 Ocean crust today
Magnetic mapping can measure the rate of seafloor spreading An oceanic survey over the Reykjanes Ridge, part of the Mid-Atlantic Ridge southwest of Iceland, showed an oscillating pattern of magnetic field strength. This figure illustrates how scientists worked out the explanation of this pattern. A sensitive magnetometer records magnetic anomalies, alternating bands of high and low magnetism. Iceland Mid- Atlantic Ridge Symmetrical bands on both sides. Why?
Looking at pole reversals preserved in the ocean floor provides the evidence needed to support seafloor spreading. Seafloor spreading provides the mechanism for how the continents move! (Wegener)
II. A Mosaic of Plates A. Entire Earth s surface is made up of lithospheric plates B. Both oceanic crust & continental crust C. Geologic activities occur at plate boundaries: 1. Earthquakes 2. Volcanoes 3. Rifts 4. Folding 5. Faulting
III. Types of Plate Boundaries A. Divergent Boundaries 1. Continental plate separation 2. Oceanic plate separation-mid Ocean Ridges spreading centers
Divergent Boundaries Continental Plate Separation East African Rift Valley
Divergent Boundaries Continental Plate Separation Parallel valleys; volcanoes and earthquakes. East African Rift Valley
Divergent Boundaries Oceanic Plate Separation Mid- Atlantic Ridge
Divergent Boundaries Oceanic Plate Separation Volcanoes and earthquakes concentrate. Mid- Atlantic Ridge
III. Types of Plate Boundaries B. Convergent Boundaries - Conserve Earth s surface area 1. Ocean-ocean convergence 2. Ocean-continent convergence 3. Continent-continent convergence
Convergent Boundaries Ocean-Ocean Convergence Mariana Islands Marianas Trench
Convergent Boundaries Ocean-Ocean Convergence Deep-sea trench; volcanic island arc. Mariana Islands Marianas Trench
Convergent Boundaries Ocean-Continent Convergence Andes Mountains Peru-Chile Trench South American Plate
Convergent Boundaries Ocean-Continent Convergence A volcanic belt of mountains forms. Andes Mountains Peru-Chile Trench South American Plate
*when oceanic crust is involved in convergence the process of subduction occurs, where the more dense plate is pushed under the least dense plate
Convergent Boundaries Continent-Continent Convergence Himalaya Tibetan Plateau Main thrust fault Eurasian Plate
Convergent Boundaries Continent-Continent Convergence Crust crumbles, creating high mountains and a wide plateau. Himalaya Tibetan Plateau Main thrust fault Eurasian Plate
III. Types of Plate Boundaries C. Transform-Fault Boundaries 1. Plates slide past one another 2. Fracture with relative displacement
Transform-Fault Boundaries Mid-Ocean Ridge Transform Fault
Transform-Fault Boundaries Mid-Ocean Ridge Transform Fault Spreading centers offset.
Transform-Fault Boundaries Continental Transform Fault
Transform-Fault Boundaries Continental Transform Fault Offset continental crust.
As plates move past each other...
As plates move past each other... creek beds are offset
As plates move past each other... creek beds are offset San Francisco Los Angeles http://www.youtube.com/watch?v=ryrxa GY1dmE
IV. How Plates Move A. Driving Forces 1. Mantle convection (heating of a fluid)- warm matter rises, cool matter sinks 2. Gravitational pull
IV. How Plates Move B. Plate Recyling 1. New lithosphere occurs at ridges 2. Old lithosphere is destroyed at subduction zones 3. Recycling of heat energy within upper and lower mantle creates seismic waves
IV. How Plates Move C. Convection Currents 1. Explain the movement of lithospheric plates 2. Create Mantle plumes (upwelling and collection of magma) that fuel a. Hot spots b. Volcanoes http://www.youtube.com/watch?v=nybt NFN3NBo
Whole-mantle convection Upper mantle 700 km Lower mantle 2900 km Outer core
Whole-mantle convection Mantle Outer core Inner core Upper mantle 700 km Lower mantle 2900 km Outer core
Stratified convection Boundary near 700 km separates the two convection systems.