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Chapter 2

Chapter 2 Notes


Department
Environmental Science
Course Code
EESA06H3
Professor
Nick Eyles
Chapter
2

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Chapter 2
PLATE TECTONICS
What is Plate Tectonics?
-tectonics: the study of the origin and arrangement of the broad structural features of the
Earths surface, including not only folds and faults, but mountain ranges, continents, &
earthquake belts
-plate tectonics: the earths surface is divided into several large, thick plates that move
slowly and change in size; intense geologic activity occurs at plate boundaries
-concept was developed in the late 1960s by combining continental drift & sea floor
spreading theories
-continental drift: idea that continents move freely over earths surface, changing their
positions relative to each other
-sea floor spreading: idea that the sea floor forms at the crest of mid- oceanic ridges, then
moves horizontally away from the ridge crest toward an oceanic trench
How Did the Plate Tectonics Theory Evolve?
The Early Case for Continental Drift
-ideas been around for 130 years
-Alfred Wegener
-noted in early 1900s that SA, Africa, India, Antarctica, & Australia all had almost identical
Paleozoic rocks & fossils
Scepticism About Continental Drift
-people didnt believe Wegener at first because they thought that rocks were much too
strong to be able to move about and crumple mountains, etc
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-the driving mechanism he proposed was a combination of centrifugal forces that causes
tides
-therefore in the northern hemisphere, they didnt believe him because they thought the
force wasnt strong enough
-more support in the southern hemisphere where the rocks were matching other places
Renewed Interest in Continental Drift
-because of 1) study of the sea floor & 2) geo research in relation to rock magnetism
Study of the Sea Floor
-oceans cover more than 70% of earth
-rock dredge: rocks are taken from sea floor by being broken off by a dredge, an open steel
container dragged along
-corer: used to sample sediments from the sea floor
-single- beam echo sounder: measures water depth & draws profiles of submarine
topography
-sidescan sonar: measures the intensity of sound reflected back to the tow vehicle from the
ocean floor and provides detailed images of the sea floor & info about bedforms & sediments
-seismic reflection profiler: works on pretty much same as above but uses a louder noise @
lower frequency
Geophysical Research
-inclination of the magnetic alignment preserved in the magnetite minerals in the lava
flows can be used to determine the paleolatitude at which the flow formed
-paleomagnetism: study of ancient magnetic fields
-polar wanderings are now used to reconstruct continental movement over time
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Recent Evidence for Continental Drift
-greatest evidence came from large rock formations that match each other on different
continents
-like SA & Africa
History of Continental Positions
-Pangea split up 200 million years ago
-continents have been in motion for the past 2 billion years
What is Sea Floor Spreading?
-Harold Hess
-he thought that the sea floor was spreading too, even though Wegener thought that the sea
floor stayed till while the continents moved about through them
-1962 sea floor spreading: the sea floor moves away from the mid- oceanic ridge as a result
of mantle convection
-spreading axis (spreading centre): the ridge crest, with sea floor moving away from it on
either side
-subduction: the sliding of the sea floor beneath a continent or island arc
-convection: circulation pattern driven by the rising of hot material & the sinking of cold
material
-hot material has a lower density so it rises, and cold material has a higher density so it
sinks
-Benioff zones of earthquakes: associated with trenches
-andesitic volcanism: forms volcanoes either on the edge of a continent or along an island
arc
How Old is the Sea Floor?
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