Earth Sciences 1022A/B Lecture 12: Earth Science Lecture February 25 2016
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7 Aug 2016
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FEBRUARY 25 2016 LECTURE
PLATE TECTONICS
Continental Drift and Paleomagnetism
Magnetic studies of seafloor following World War Two
Paleomagnetism
oMagnetic minerals in basalt point to the existing magnetic pole at the time of rock
crystallization
Basalt is like a fossil compass to find paleo poles
Polar Wandering
oPole positions for North America and Europe over the last 500 years may show
diverging plates
Seafloor Spreading
oSeafloor gradually moves from ocean ridges to ocean trenches
Geomagnetic Reversals
oEarth’s magnetic poles have reversed many times
The compass would work backwards
oPaleomagnetism revealed ocean floors are striped mirror images
Alternating normal and reversed polarity
Canadian Lawrence Morley discovered that stripes get older away
from oceanic ridges as the sea floor moves
oSea floor spreading is faster in some oceans
Average is about 5 centimetres per year
Rigid Plates
oCanadian J. Tuzo Wilson discovered that Earth’s lithosphere is divided into
several plates that gradually move relative to one another
Plate Tectonic
Lithosphere comprises of seven major plates
oTheir interactions cause earthquakes, volcanoes, and mountain building
Mainly occurs at plate boundaries
Divergent
oWhere plates move apart at oceanic ridges and new sea floor is created or on
continents at rift valleys located above magma plumes
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