Plate Tectonics The Discovery of Plate Tectonics A Mosaic of Plates Types of Plate Boundaries “How” Plates Move

A. Continental Drift (Alfred Wegener) I. The Discovery of Plate Tectonics A. Continental Drift (Alfred Wegener) Proposed large-scale movement of the continents

Evidence: 1. “Puzzle fit” of continents to form “Supercontinents” a. Suess (1900)-Gondwana b. Wegner (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) Convection currents move plates around Mantle source of heat and convection 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 Switching strength to the south Preserved in lava (iron points towards stronger pole) Age can be dated and if the seafloor is spreading from a center, should be mirror images on each side

Magnetic “Tape Recording” (continued) Magnetic anomaly Normal-positive anomaly Reverse-negative anomaly

Mid-ocean ridge Million years ago (Ma) 4.0 3.0 2.0 Ocean crust today 5.0 3.3 2.5 0.7 0.7 2.5 3.3 5.0 million years old

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 Mid-Atlantic Ridge High intensity Symmetrical bands on both sides. Why? Low intensity

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 Entire Earth’s surface is made up of lithospheric plates Both oceanic crust & continental crust C. Geologic activities occur at plate boundaries: 1. Earthquakes 2. Volcanoes 3. Rifts 4. Folding 5. Faulting

A. Divergent Boundaries III. Types of Plate Boundaries A. Divergent Boundaries Continental plate separation Oceanic plate separation-Mid Ocean Ridges “spreading centers”

Divergent Boundaries Continental Plate Separation East African Rift Valley Somali Subplate African Plate

Parallel valleys; volcanoes and earthquakes. Divergent Boundaries Continental Plate Separation Parallel valleys; volcanoes and earthquakes. East African Rift Valley Somali Subplate African Plate

Divergent Boundaries Oceanic Plate Separation Mid- Atlantic Ridge North American Plate Eurasian Plate

Volcanoes and earthquakes concentrate. Divergent Boundaries Oceanic Plate Separation Volcanoes and earthquakes concentrate. Mid- Atlantic Ridge North American Plate Eurasian Plate

B. Convergent Boundaries 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 Philippine Plate Pacific Plate

Deep-sea trench; volcanic island arc. Convergent Boundaries Ocean-Ocean Convergence Deep-sea trench; volcanic island arc. Mariana Islands Marianas Trench Philippine Plate Pacific Plate

Convergent Boundaries Ocean-Continent Convergence Andes Mountains Peru-Chile Trench South American Plate Nazca Plate

Convergent Boundaries Ocean-Continent Convergence A volcanic belt of mountains forms. Andes Mountains Peru-Chile Trench South American Plate Nazca 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 Tibetan Plateau Himalaya Main thrust fault Indian-Australian Plate Eurasian Plate

Convergent Boundaries Continent-Continent Convergence Crust crumbles, creating high mountains and a wide plateau. Tibetan Plateau Himalaya Main thrust fault Indian-Australian Plate Eurasian Plate

C. Transform-Fault Boundaries III. Types of Plate Boundaries C. Transform-Fault Boundaries Plates slide past one another Fracture with relative displacement

Transform-Fault Boundaries Mid-Ocean Ridge Transform Fault Eurasian Plate North American Plate

Spreading centers offset. Transform-Fault Boundaries Mid-Ocean Ridge Transform Fault Spreading centers offset. Eurasian Plate North American Plate

Transform-Fault Boundaries Continental Transform Fault Pacific Plate North American Plate

Offset continental crust. Transform-Fault Boundaries Continental Transform Fault Offset continental crust. Pacific Plate North American Plate

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 San Andreas fault Los Angeles http://www.youtube.com/watch?v=ryrXAGY1dmE

IV. “How” Plates Move A. Driving Forces Mantle convection (heating of a fluid)- warm matter rises, cool matter sinks Gravitational pull

IV. “How” Plates Move B. Plate Recyling New lithosphere occurs at ridges Old lithosphere occurs at subduction zones ‘Recycling’ of heat energy within upper and lower mantle creates seismic waves

IV. “How” Plates Move C. Convection Currents Explain the movement of lithospheric plates Create Mantle plumes (upwelling and collection of magma) that fuel Hot spots Volcanoes http://www.youtube.com/watch?v=NYbTNFN3NBo

Whole-mantle convection Upper mantle 700 km Lower mantle 2900 km Outer core

Whole-mantle convection Upper mantle Outer core Inner core 700 km Lower mantle 2900 km Outer core

Stratified convection Boundary near 700 km separates the two convection systems.