Slide 1: Plate Tectonics
Chapter 9
Slide 2: Geologic Time
• Human time is mere seconds when compared to the time frame over which geologic processes have been operating to develop landforms
Slide 3: Geologic Time
Slide 4: Earth Science Theories
• Uniformitarianism
– “The present is the key to the past.”
• Plasticity
– It was once believed that Earth’s crust was hard and brittle and could not bend – Not true! Earth’s crust CAN bend, much like a tough plastic, before breaking
• Next:
– Isostacy – Continental Drift – Plate Tectonics
Slide 5: Isostacy
• Addition or removal of crustal material causes a sinking or rebounding of crust as Earth tries to maintain equilibrium
– Add or remove continental mass and the crust will sink or rise to accommodate the added/removed weight • Examples: a glacier growing or remelting, crust eroding off the surface, sediment deposits, water bodies on land, esp. those created by dams
Slide 6: Alfred Wegener and His Continental Drift Theory
• German meteorologist, 1920s
“The present continents were originally connected as one enormous landmass that has broken up and drifted apart over the last few 100 million years. The drifting continues….”
• Pangaea (Gk. “whole land”)
Slide 7: Wegener’s Lines of Evidence
• • • • Similar geology (rocks and rock structures)… …petrology (rock chemistry), …paleontology (fossilized plants and animals), …matching glacial features (U-shaped valleys, glacial deposits, etc.) on continents separated by oceans • …continent shapes that seem to fit together, • …patterns in the locations of volcanoes, • …even animal migration patterns
– South America/South Africa, Madagascar/India, Australia/Antarctica
Slide 10: …but no one bought it.
The c rust is too rigid! Yeah, why don’t we see the crust ripping apart right now?
What do you mean, “The cont inents ar e floating???”
What a knucklehead.
And hey, what’s the power source driving these movements of all the land masses, anyway???
Slide 11: Then along came Oceanographer Harry Hess in the 1960s…
Slide 14: And the evidence began to mount…
• Military seafloor mapping: Seafloor geology—
structure, chemistry, and age
– Oceanic crust: only 100 m.y.o – Continental crust: 4.1 b.y.o.
• • • • •
Core sampling Seafloor sediment Rigid Earth folks retired—paradigm shift to plasticity Convection currents as mechanism/power source Geologists, geophysicists, seismologists, oceanographers, physicists, and paleontologists all agree the theory fits the evidence gathered within their respective fields
Slide 15: The Theory of Plate Tectonics
Tectonic (crustal) plates • Pulling apart (spreading/diverging) • Slamming together and sinking (subducting/converging) • Sliding laterally (sideways)
Slide 16: Divergent Plate Boundaries
• Spreading centers
– Crust pulling apart, magma rising to the surface
Slide 17: Convergent Plate Boundaries
• Subduction zones
– Crust being forced together – Lightest material rises (mountain-building) while the heaviest stuff sinks (pushed back into the mantle) – Remelting (mostly from friction) creates volcanoes – Intense, deep-focus earthquakes
Slide 19: Three Types of Subduction Zones
1. Continental crust meets oceanic crust
– Oceanic crust sinks – Big trench offshore – Volcanoes on the continental margin – Big earthquakes (potential for tsunamis)
Slide 20: Continental-Oceanic Subduction
Slide 21: Three Types of Subduction Zones
2. Oceanic crust meets oceanic crust
– The older and colder crust will probably sink – Big earthquakes and volcanic islands (called “island arcs”) – Deep ocean trench – Potential for tsunamis
Slide 22: Oceanic-Oceanic Subduction
Slide 23: Three Types of Subduction Zones
3. Continental crust meets continental crust
– Too light to subduct – Mountain-building – Big earthquakes – Little if any volcanism (mostly intrusive)
Slide 24: Continental-Continental Subduction
Slide 25: Transform Fault Boundaries
• Tectonic plates slide past one another
– Earthquakes are less intense than subduction – No volcanoes – Little or no mountain-building
Slide 26: “Hot spots”
• Also called magma plumes • Generally occur some distance from any other type of plate boundary • Unrelated to convergent, divergent, or transform boundaries • Anomalous (odd) “balloons” of rising magma
– Hot spot stays in one position as the moving, island-covered crustal plate rides away from it
Slide 29: Accreted Terrains
• A moving continent may pick up new land material as lighter (felsic) material scrapes off of a subducting plate
Slide 30: Accreted Terrains
• A moving continent may pick up new land material as lighter (felsic) material scrapes off of a subducting plate
