Oceanography Chapter 2: Plate Tectonics

Who first proposed the theory of plate tectonics?

Alfred Wegener 

What was the evidence for plate tectonics when the theory was first proposed?

the distribution of fossils across different continents and the fit of and geological similarities between continental borders, such as signs of glacier activity in now tropic regions

What was inherently incorrect about Wegener’s theory?

he proposed that the continental land masses themselves moved over time across the oceans 

What further evidence for plate tectonics arose after Wegener’s death?

magnetic paths align together when moving the continents together, the magnetic poles ‘wander’ overtime as indicated by the presence of magnetite in igneous rock (which tells us generally where the magnetic fields were when the rocks cooled)

What causes the tectonic plates to move?

mantle convection pushes the plates when molten mantle is released and cooled, and pulls older plates back into the mantle; seafloor spreading and subduction

Where does the creation of new plate material occur?

mid-ocean ridges

Where does the destruction of old plate material occur?

trenches or subduction zones

What are the three plate boundaries called?

divergent, convergent, and transform

Divergent boundary

tectonic plates move away from each other, where new sea floor is created; two types of plate activity at ocean rises and ocean ridges

Divergent activity at ocean rises

seafloor spreads quickly, low seismic activity, broad shallow rift valleys

Divergent activity at ocean ridges

seafloor spreads slowly, higher strength seismic activity, deep rift valleys

Convergent plates

plates move towards and either collide or one subducts under the other, destroying old ocean floor

Transform plates

plates that slide away from each other

Characteristics indicative of a divergent plate boundary

young seafloor, mid-ocean ridges, shallow earthquake activity

Characteristics indicative of a convergent plate

old seafloor, deep depth, volcanic activity near oceanic crust, mountains near continental crust, volcanic arc on land if oceanic-continental, and volcanic arc as islands if oceanic-oceanic 

Characteristics indicative of a transform plate

fault lines, shallow but powerful seismic activity, lack of volcanic activity, lack of seafloor being created or destroyed 

How do volcanic features occur far away from plate boundaries?

natural hot spots

Seamount

volcano below sea level with a cone top

Guyot

volcano below seawater with a flat top; the peaks used to stick up above sea level, but were eroded by wind and waves until flat and below water

Passive margin features

wide continental shelf, continental shelf break, gentle continental slope(s)

Active margin features

narrow continental shelf, no continental rise, steep slope(s), ocean trenches

Continental shelf

part of the continent that is underwater that extends from the shoreline to the shelf break, shallow

Continental slope

extends from the shelf break downward towards the seafloor, considered the boundary between the continental and oceanic regions

Continental rise

boundary between the continental slope and seafloor, made up of accumulated sediments from the contientnal shelf

Submarine canyons

underwater canyons caused by erosion from rivers outflowing into the continental shelf and rise

Abyssal plain

the seafloor, covered with thick sediments that cover small features

Ocean ridges

volcanic mountain ranges where seafloor spreading occurs

Ridge crest

highest peaks of the range in the ocean ridges

Rift valley

area of the ocesan ridge where motlen rock is exposed as plates pull apart from each other

Ocean trenches

the deepest parts of the ocean, always close to a continent or an oceanic vnic island chain

Order of seafloor features

Continental shelf → continental slope → continental rise→ Abyssal plain→ Ocean trenches