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25 Cards in this Set
- Front
- Back
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who came up with the idea of plate tectonics and when
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alfred wegener in 1915 wrote "origin of continents"
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continental drift
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hypothesis that all the continents were once one large continent
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evidence in support of continental drift
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1. continents look like they'd fit together
2. fossils - geographically restricted organisms such as mesosaurus and glassopteris found on continents with large water bodies inbetween 3. rock type and structural similaritiy 4. paleoclimate evidence - a whole bunch of stuff such as seafloor spreading paleomagnetism etc. |
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sea floor spreading - who proposed the hypothesis?
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harry hess 1961 - Suggested that new oceanic crust is produced at the crests of mid-ocean ridges and then moves away from the axis
Old ocean floor is then consumed at deep-ocean trenches |
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Sea Floor Spreading
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1. The average thickness of the sediment in the deep sea today is only 1.3 km.
Modeling suggests this only represents about 260 my of sediment accumulation. 2. Ridges tend to be in the center of ocean basins, have high rates of upward heat flow, a central furrow, and volcanoes. Mid ocean ridges are where new ocean crust is being formed. 3. Guyots (flat topped sea mounts) on the deep ocean floor. Fossils on top of guyots proved they were once near sea level. There are two few guyots if the ocean basins are permanent (at least by modern volcanic production rates). Guyots are the eroded remnants of volcanic islands that sank due to thermal contraction. |
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Name the three (main?) compositional layers then describe them visually
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Crust
Oceanic Basalt 7-10 km thick Continental Granite 25-40 km thick Mantle Peridotite ultramafic Core Iron & Nickel |
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describe the 5 mechanical layers
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Lithosphere
rigid Asthenosphere plastic Mesosphere Increasing rigidity Outer Core liquid Inner Core rigid |
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4.5ish things to know about density and gravity
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Gravity pulls objects down
Buoyant force pushes up in fluids Object will sink if it is denser than the fluid it displaces Floating object: displaced fluid mass is equal to the object mass Gravity-buoyant force are equal |
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isostacy
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Water levels change throughout geologic history
Water volume change due to climate change? Dutton – Land rises and falls Crust “floating” on fluid materials (isostasy – Greek for equal standing) Sink with loading (lava flows, glaciers, etc) Rise with unloading (erosion of material, etc) |
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Isostatic Equilibrium
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Buoyant force = gravitational force
Equilibrium line: essentially water line Hbelow=Htotal(ρobject/ρfluid) Habove=Htotal(1-(ρobject/ρfluid)) |
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Global Isostasy
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Continents: 0.84 km above SL
Ocean basins: 3.69 km below SL Continents: Granite Ocean basins: Basalt Mantle: Peridotite |
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proposed mechanisms to move crusts
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Wegener’s proposed mechanisms to move continents:
Tidal influence of the moon may be strong enough to move the continents in a westward motion Larger & sturdier continents broke through oceanic crust, like icebreakers cut through Arthur Holmes (1928) Proposed that convection currents (currents driven by caused by heat rising, cooled stuff sinking) operating within the mantle were responsible for propelling continents across the globe |
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what happened in 1968?
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1968 - the hypotheses of Continental Drift (Wegener - 1915) & Sea-Floor Spreading (Hess - early 1960’s) combine to form the Theory of Plate Tectonics
Theory of Plate Tectonics - Earth’s outer, rigid lithosphere consists of ~ 20 individual segments (plates) Plates are composed of both oceanic & continental crust Plates are rigid Most active geologic features (earthquakes, volcanoes, mountains) occur at plate boundaries |
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three types of plate boundaries
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convergent, divergent, transform
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Divergent Plate Boundary summary and example plus formation
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Plates move apart (rift)
Results from: tensional stress Makes new room for magma to rise New crust is formed Ex. Mid-Atlantic Ridge formation: Example: Rifting of a continent: Into 2 separate continents Ocean forms between Mid-ocean ridge - area where new crust is being formed |
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convergent plate boundary summary example and formation and subtypes
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Plates move together
Results in collision (mountain chain) Result from: compressional stress Crust is either destroyed (1 plate can be consumed back into mantle) or deformed (mountain chain) 3 sub-types: Ocean-Ocean Ocean-Continent Continent-Continent |
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Ocean-Ocean & Ocean-Continent collisions associated with:
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Trenches
Volcanism Subduction zone |
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subduction zone
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the process by which one oceanic plate bends & sinks down into the asthenosphere beneath another
denser plate sinks |
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Continent-Continent Convergent Plate Boundary
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Oceanic plate subducts beneath continental plate, until 2 continental plates come together
Results in: Double thickening of the continental crust High mountains Fold & thrust belt (Mountain Chain) Ex: Himalayas & Appalachian Mtns (ancient) |
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transform plate boundary
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Plates slide past each other
Result from: shear stress Associated with mid-ocean ridges to compensate for 3-D of globe Also stand alone as a plate boundary Ex. San Andreas Fault |
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plate boundaries, action/stress, crust
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divergent = split/tension/newly formed
convergent = collision/compression/destroyed crust transform=sliding/sheer |
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hot spots
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Hot Spot - volcanism generated from heat source in the mantle
Plates move over stationary heat source (a hot spot) and leaves a “trail” Use trails to determine the direction & rate of plate motion ~ 40 hot spots worldwide Ex: Hawaii, Iceland, & Yellowstone |
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wilson cycle
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A cycle of opening & closing of ocean basins
J. Tuzo Wilson 1966 6 stages 500 m.y. cycles |
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why do rocks melt?
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1. temp up (advection)
2. pressure down (rise) 3. solidus down (flux added) |
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role of metamorphism and flux
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Mid-ocean ridges (MOR):
The warm basalt crust at MORs, in the presence of water, is metamorphosed to zeolite, greenschist and amphibolite. Down going plate: The metamorphosed basalt undergoes dehydration reactions, water is driven from the subducting slab causing melting in the mantle wedge. The slab RARELY, if ever, melts. Overriding plate: heat from mantle magma and crustal thickening metamorphoses and melts the crust. |
