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Slide 1: Plate Tectonics Chapter 8 1
Slide 2: Rigid Earth Theory • It was once believed that Earth’s crust was hard and brittle and could not bend • Plasticity – We now know that Earth’s crust can bend (like a tough plastic) before breaking 2
Slide 3: Isostacy • “The maintenance of hydrostatic equilibrium in the crust” – hydrostatics—branch of physics related to the pressure and equilibrium of liquids (hydro) • statics—bodies not active; at rest; in equilibrium; as opposed to dynamics 3
Slide 4: Isostacy • Addition or removal of crustal material causes a sinking or rebounding of crust – Add or remove continental mass and the crust will sink or rise to accommodate the added/removed weight • a glacier growing or remelting, crust eroding off the surface, sediment deposits, water bodies on land, esp. those created by dams 4
Slide 5: 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”) 5
Slide 6: 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 Ex.: S. America/Africa, Madagascar/India, Australia/Antarctica 6
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Slide 8: …but no one bought it. The crust is too rigid! So why don’t we see the crust ripping apart right now? What do you mean, “The continents are floating???” What a knucklehead. And hey, what’s the power source driving these movements of all the land masses, anyway??? 8
Slide 9: Then along came Oceanographer Harry Hess in the 1960s…
Slide 12: The evidence continued 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 13: The Theory of Plate Tectonics Tectonic (crustal) plates • Pulling apart (spreading/diverging) • Slamming together and sinking (subducting/converging) • Sliding laterally (sideways) 13
Slide 14: Divergent Plate Boundaries • Spreading centers – Crust pulling apart, magma rising to the surface 14
Slide 15: 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 15
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Slide 17: 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) 17
Slide 18: Continental-Oceanic Subduction 18
Slide 19: 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 19
Slide 20: Oceanic-Oceanic Subduction 20
Slide 21: 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) 21
Slide 22: Continental-Continental Subduction 22
Slide 23: Transform Fault Boundaries • Tectonic plates slide past one another – Earthquakes are less intense than subduction – No volcanoes – Little or no mountain-building 23
Slide 24: “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 24
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Slide 27: Accreted Terranes • A moving continent may pick up new land material as lighter (felsic) material scrapes off of a subducting plate 27
Slide 28: Accreted Terranes • A moving continent may pick up new land material as lighter (felsic) material scrapes off of a subducting plate 28
Slide 29: Craton • These terranes were added to the original material first formed from magma that rose out of Earth’s earliest crust – Craton--the name given to these ancient protocontinents cratons 29
Slide 30: Continental Shields • More magma material was added to the cratons, forming continents. – Continental shields: Where the earliest continental material still exists intact and is exposed at the surface. 30
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Slide 32: Topography • Right from the very beginning, the crust was affected by stresses and strains that caused crustal deformations • Over time, the crust has continued to be folded, faulted, broken, eroded and further built upon, creating the topography, the ups and downs of land relief, that we see today 32