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

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

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

Sediments show layering of organisms from warmer water under colder water organisms

John Wilson