EAS100 Lecture 7: EAS 201 - Plates, Boundaries, and Continental Shifts

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EAS 201: Week 3
January 21, 2019
Alfred Wegener and concept of Continental Drift.
o 1911 looking at the inherent “jigsaw fit” of all the continents together in one
piece.
o Looked at glacial deposits across varying locations, also fossil distributions.
o Initially (1930) this idea was rejected; by 1950s/60s, idea was revived via the
study of the strength of rocks, Earth’s magnetism & oceans.
Magnetic Lines of Force:
o During WWII, navies of world start focusing studies on magnetic fields
(magnetism) to detect submarines travelling underwater.
Paleomagnetism:
o Basalts & gabbros are rich in ferromagnesian materials.
o Most Fe-bearing minerals are weakly magnetic at surface temperatures, while at
high temperatures, they lose their magnetism.
o Magmas aren’t magnetic, but as they cool below the “Curie Point”/solidify, the
FE-bearing minerals tend to align along magnetic field running N-S.
They then retain their paleomagnetism unless they’re re-heated.
o Magnetic north has changed throughout geological time, through process known
as magnetic reversal”.
Initially discovered by scientist in Japan.
The flipping’ doesnt have a direct pattern, it happens more randomly.
Seafloor spreading rates vary from about 0.5cm per year to 20cm per year.
o On average about 3 square km of new seafloor is formed each year.
Plate Tectonics:
o 1968 topographic maps of seafloor show a number of interesting features.
o There are ridges running N-S in both Atlantic & Pacific oceans.
o Along the margins of some continents there are trenches several km’s deep.
o The other continents are bounded by shallow water shelves.
o The distribution of earthquakes & volcanoes is also concentrated in belts,
directly corresponding to edges of different plates.
o These patterns arise due to the nature of the Earth’s subsurface, in which the
rigid shell of the lithosphere is broken up into pieces, or plates”.
Each plate moves as a distinct rigid unit, riding on the asthenosphere,
which is also in motion.
Earth’s crust and upper most mantle compose the lithosphere.
o Beneath lithosphere is plastic layer known as asthenosphere.
o Lithospheric plates can move over the plastic layer; plates carry continents.
Transform-Fault Boundaries plates slide horizontally past each other.
o Also, convergent & divergent boundaries.
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