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Plate Tectonics and the Ocean Floor (Layers of the Earth (Crust - outer…
Plate Tectonics and the Ocean Floor
Layers of the Earth
Crust - outer most layer of the Earth
Continental crust - thick, low density, old and never subducts
Ocean crust - thin, high density, young and always subducts
Lithosphere - includes the top layer of the mantle and the crust
Mantle - second layer of the Earth; thickest layer
Lithosphere - includes the top layer of the mantle and the crust
Asthenosphere - near the top of the mantle; convection currents in magma move lithospheric plates
Outer core - third layer of the Earth; made of liquid iron and nickel
Inner core - fourth layer of the Earth; made of solid iron and nickel
Solid due to intense pressure
Theories of moving continents and ocean floor
Continental drift
Alfred Wegener
States that the continents where once together (Pangaea) and are constantly moving apart
Evidence
Continents look like they fit together
Matching fossils and rock layers on separate continents
Coal, from swamp plants, found in Antarctica
Evidence of glaciers in Africa
Seafloor spreading
Harry Hess and Robert Dietz
States ocean floor is being formed at mid-ocean ridges and moving towards continents where it is recycled back into mantle
Evidence
Alternating bands of magnetism match either side of mid-ocean ridges
Seafloor gets older as move towards continents and away from mid-ocean ridges
Sediments show layering of organisms from warmer water under colder water organisms
Plate tectonic theory
States tectonic plates of continental and oceanic crust are moving due to convection currents in the mantle
Combines continental drift and seafloor spreading
John Wilson
Plate movements
Transform plate boundary
Plates slide pass each other
Crust is not created or destroyed
Divergent plate boundary
Plates move away from each other
Crust is created
Locations of mid-ocean ridges and rift valleys
Convergent plate boundary
Plates move towards each other
Crust is destroyed
Oceanic-oceanic convergent plate boundary
Oldest oceanic plate subducts
Location of ocean trenches and island arcs
Continental-oceanic convergent plate boundary
Oceanic plate subducts
Location of ocean trenches and volcanic coastal mountain ranges
Continental-continental convergent plate boundary
Neither plate subducts; forced upwards to form mountain ranges
Hotspots
Located somewhere away from the edges of tectonic plates
Volcanic island chains form over a magma plume pushing through the mantle
Age of islands, youngest to oldest, shows the direct the plate is moving
Mapping the ocean floor
SONAR - speed of sound through water TIMES the time sound ping takes to travel from boat to the ocean floor and back DIVIDED by two.
Satellites - can map the entire ocean floor, but the detail is not as good as sonar
Parts of the ocean floor
Continental margin - generally shallow near the continents
Edges of the ocean near the continents
Continental shelf - shallow, fairly flat underwater edge of the continent
Continental slope - steep drop from the continental shelf to the ocean floor
Continental rise - a pile of sediment that is sometimes found at the bottom of the continental slope
Submarine canyon - valley carved into the continental shelf and slope by an avalanche of sediment called a turbidity current
Abyssal fan - wedge of sediment deposited at the base of a submarine canyon
Types of continental margins
Passive continental margins
Near coasts with a divergent plate boundary in the middle of the ocean as the closest plate boundary
Have wide continental shelves and continental rises, few seamounts, and have smooth abyssal plains
Little to no earthquake or volcanic activity
Examples: Coasts along the Atlantic Ocean, like Virginia
Active continental margins
Coastline near or along convergent or transform plate boundaries
Have narrow continental shelves, narrow or missing continental rises, often the site of trenches
Have a lot of earthquake and volcanic activity
Examples: Coasts along the Pacific Ocean; the Ring of Fire
Ocean basin - generally deep ocean floor
Abyssal plain - flat ocean floor
Seamount - active or inactive underwater volcano (mountain)
Guyot - inactive, flat-topped, underwater volcano
Once was an inland that sank under the waves after weather and erosion worn down its top
Trench - deep depression in the ocean floor near subduction zones (convergent plate boundaries)
Island arc - chain of volcanic islands alongside trenches near subduction zones (convergent plate boundaries)
Ocean ridges - two chains of mountains on either side of a divergent plate boundary
Rift valley - valley between mountain chains at the divergent plate boundaries