EESA06H3 Study Guide - South American Plate, Continental Crust, Seafloor Spreading

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24 Apr 2012
What are plate tectonics?
Tectonicsthe study of the origin and arrangement of the broad
structural features of the earth’s surface including folds, faults,
mountain belts, continents, and earthquake belts
Plate Tectonicsthe Earth’s surface is divided into a few large, think
plates that move slowly and change in size
Plate boundaries – plates move away from one another, past one
another, or towards one another
Eight large plates/smaller plates – makes up the outer shell of the Earth
(the crust and upper part of the mantle)
Plate tectonic theory developed by:
Continental drift – continents move freely over the Earth’s surface,
changing their positions relative to one another
Sea floor spreading
A hypothesis that the sea floor forms at the crest of mid-oceanic
ridges, then moves horizontally away from the ridge crest toward
an oceanic trench.
Two sides of the ridge are moving in opposite directions
How did plate tectonics theory evolve?
The Early Case for Continental Drift
Alfred Wegener
Continental drift
Examined continents that have identical late Paleozoic rocks and
Pangeacontinents form a giant supercontinent
Laurasia –northern supercontinent (North America)
Gondwanaland – southern supercontinent
Paleoclimatology – study of ancient climate
Examined ancient sedimentary rocks he discovered that
paleoclimatic reconstructions suggested polar positions very
different to those at present
Skepticism about Continental Drift
Believes that the less dense continents drifted through oceanic
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crust, crumpling up mountain ranges on their leading edges as
they pushed against the oceanic crust (DIAGRAM PAGE 25)
Renewed Interest in Continental Drift
Study of the Sea Floor
oSamples of rocks and sediments can be taken from the sea
floor by:
1. Rock Dredge – open steel container dragged over
the ocean bottom at the end of a cable
2. Corera weighted steel pipe dropped vertically
into the mud and sand of the ocean floor
3. Sea-floor drilling – drilling a hole in the deep sea
oSubmersibles – small research submarines for geologists
oSingle-bean echo sounder – measures water depth and
draws profiles of submarine topography
1. A sound is sent downward from the ship and bounces
off the sea floor and returns to the ship
2. Water depth = amount of time interval
oSidescan sonarmeasures the intensity of sound
reflected back to the tow vehicle from the ocean floor and
provides detailed images of the sea floor and information
about sediments and bedforms
Geophysical Research EXPLAIN (PAGE 28)
oPolar wandering
Magnetic poles are located close to the geographic
Magnetic poles move from year to year
oMagnetism of old rocks can be measured to determine the
direction and strength of the magnetic field in the past
oPaleomagnetism – study of ancient magnetic fields
Recent Evidence for Continental Drift
Refined rock matches between now-separated continents (EX.
South America and Africa)
Rocks are similar in type, structure, fossils, age, etc.
What is sea-floor spreading?
Harry Hess
The Sea floor might be moving too like continental drift
Sea floor spreading – the sea floor moves away from the mid-oceanic
ridge as a result of mantle convection
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Subduction – the sliding of the sea floor beneath a continent or island
Convection – is a circulation pattern driven by the rising of hot material
and the sinking of cold material
Hot materials have a lower density so it rises
Cold materials have a higher density so it sinks
Andesitic volcanism – it is produced by the sea floor moving downward
into the mantle along a subduction zone. This interaction between the
moving sea-floor rock and the stationary rock causes these volcanoes
and even earthquakes.
Formed on the edge of a continent or an island arc
How old is the sea floor?
Young sea floor is being formed by basalt eruptions at the ridge
Basalt is then carried sideways by convection and is subducted
into the mantle at an oceanic trench
Old sea floor is being destroyed and new sea floor is being
What are plates and how do they move?
Continental Drift and Sea floor spreading Formed Plate tectonic
Plate – is a large, mobile slab of rock that is part of Earth’s surface
Made up of sea floor, continental or oceanic rock
Lithosphere – rigid outer shell of earth that includes rocks from the
crust and the uppermost mantle
Continental lithosphere is thicker
Asthenosphere – below the lithosphere, which is a zone of low-seismic-
wave velocity that behaves plastically because of increased temperature
and pressure
Allows the plates to move
Below this is mantle rock
Plates made up of sea floor – subducted down into the mantle forming
oceanic trenches
Plate leading edge made up of continental rock –NOT subduct because
continental rock is less dense than oceanic rock
Three types of plate boundaries:
1. Divergent plate boundary plate boundary moving APART
2. Convergent plate boundary – plates moving TOWARDS each
3. Transform plate boundary move HORIZONTALLY past each
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