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25 Cards in this Set

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who came up with the idea of plate tectonics and when
alfred wegener in 1915 wrote "origin of continents"
continental drift
hypothesis that all the continents were once one large continent
evidence in support of continental drift
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.
sea floor spreading - who proposed the hypothesis?
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
Sea Floor Spreading
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.
Name the three (main?) compositional layers then describe them visually
7-10 km thick
25-40 km thick
Iron & Nickel
describe the 5 mechanical layers
Increasing rigidity
Outer Core
Inner Core
4.5ish things to know about density and gravity
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
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)
Isostatic Equilibrium
Buoyant force = gravitational force
Equilibrium line: essentially water line
Global Isostasy
Continents: 0.84 km above SL
Ocean basins: 3.69 km below SL
Continents: Granite
Ocean basins: Basalt
Mantle: Peridotite
proposed mechanisms to move crusts
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
what happened in 1968?
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
three types of plate boundaries
convergent, divergent, transform
Divergent Plate Boundary summary and example plus formation
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
convergent plate boundary summary example and formation and subtypes
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 collisions associated with:
Subduction zone
subduction zone
the process by which one oceanic plate bends & sinks down into the asthenosphere beneath another

denser plate sinks
Continent-Continent Convergent Plate Boundary
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)
transform plate boundary
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
plate boundaries, action/stress, crust
divergent = split/tension/newly formed
convergent = collision/compression/destroyed crust

hot spots
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
wilson cycle
A cycle of opening & closing of ocean basins
J. Tuzo Wilson 1966
6 stages
500 m.y. cycles
why do rocks melt?
1. temp up (advection)
2. pressure down (rise)
3. solidus down (flux added)
role of metamorphism and flux
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.