Continental Drift
The earth in motion
A few contributors
Early days
• Wegner: Noticed the fit between South America and
West Africa.
• Made measurements from Greenland that he thought
showed continental drift in action; the rate he proposed
was clearly wrong…
• Pointed out geological continuities between North
America and Europe, some real (formations that link up–
the Appalachians, for instance) and some imagined
(glacial morraines that he thought also linked up).
• Proposed that the ‘plasticity’ of the mantle could allow
the continents to move slowly through the ocean beds.
Jeffrey’s challenge
• The geophysics just doesn’t work– continents
can’t sail through the ocean bedrock.
• Some mistakes in Wegner’s work also undercut
his case– some elements of which were very
strong.
• The case for biogeography– Wegner & later du
Toit worked this up: Land bridges were ruled out
on fundamental grounds of isostacy. But
paleontological evidence made it very clear that
Gondwanaland (with Laurasia
above)
Gondwanaland fossils
The Southern Influence
• From the 1930’s into the 1950’s Several
geologists from the southern hemisphere,
including Alexander du Toit (from South Africa)
continued work on continental drift while
European and North American geologists
regarded it as absurd and indefensible.
• Their evidence showed very convincingly that
the southern continents had once been united.
• But the rest of the geological world was not
ready to accept such a radical idea.
Post War Geology
• After WWII the U.S. Navy donated and loaned
many surplus ships for oceanographic work.
• Sonar, developed for military purposes, turned
out to be very useful for mapping features on the
sea floor; other new technologies, including
drilling rigs, were also put to work.
• The Ocean of Truth, by William Menard, is a
wonderful book about the geological work that
these oceanographers did, mapping the sea
floor and revealing the world-girdling network of
mid-ocean ridges.
More details
• Deep ocean trenches were also found in many places.
• Relations between these features and the distribution of
volcanoes and earthquakes were identified.
• Meanwhile, an entirely new kind of evidence was being
developed: paleomagnetism.
• As volcanic rocks cool and solidify, magnetic minerals in
the rocks align with the earth’s magnetic field.
• Investigation of volcanic rocks’ magnetic orientation and
age thus provided a way of identifying whether and how
the earth’s magnetic field had changed over time.
Two important points
• The north magnetic pole had clearly wandered over time.
• More surprisingly, the path it seemed to have followed
was different in Europe than in North America.
• The two paths could be turned into one consistent path,
but only by slowly closing the Atlantic Ocean as older
and older rocks were compared.
• Further, the results also showed that, at long and varying
intervals, the direction of the earth’s magnetic field had
been reversed—at times, the earth’s field had its ‘north
pole’ in the south and vice versa.
Midocean Ridges
Harry Hess
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In 1960 Princeton geologist Harry Hess provided an imaginative bit of
‘geopoetry,’ as he called it.
Hess proposed that the earth's mantle is really a giant convection system.
Like hot air in a room material heated by radioactive elements in the earth's
interior slowly rises out of a relatively fluid layer of the earth's mantle called
the asthenosphere.
This lava surfaces at the mid-ocean ridges causing higher water
temperatures. As it flows away from the ridges, it hardens into a more rigid
layer called the lithosphere. When new lava oozes out, it attaches itself to
the older lithosphere and continues to move laterally from the mid-ocean
ridges.
Millions of years and thousands of miles later, the lithosphere plunges back
into the earth, carrying its sediment with it and forming deep ocean trenches
found at the edge of continents.
Hess's theory provided a workable mechanism needed by continental-drift
theorists. It was the sea floor itself that moved. Like giant conveyor belts,
the ocean bottoms transported the earth's huge land masses on top of them
and spread them apart.
The smoking gun
• The mid-ocean ridges were obviously
fundamental to the geography of ocean basins.
• Magnetic profiles of the orientation of the ‘frozen’
field of igneous rock moving out from a ridge
were measured by Vine and Matthews.
• They showed alternating stripes, indicating a
simple pattern of spreading from the ridge, with
rock closest to the ridge oriented in line with the
present field, followed by a stripe oriented
against the present field, and so on as the profile
moved out from the centre of the ridge.
Tuzo Wilson
• Born on October 24, 1908, in Ottawa, Ontario
• Studied geophysics at the University of Toronto, the first
student in Canada to take such a course
• Did graduate work at Cambridge and Princeton, received
a doctorate in geology in 1936
• During the 1960's he refined and championed the theory
of plate tectonics, which was then held in disrepute
• Introduced the idea of "hot spots" which remain
stationary under the moving plates and produce chains
of islands like Hawaii and Japan
• First to identify "transform faults" which link trenches
(where the plates collide) and rifts (where the plates pull
apart)
Transform faults interrupt a ridge
Theory and fact
• Once again, a theory proposed on the tenuous basis of a
few striking facts became an established fact in geology.
• The relations between far-distant phenomena– mountain
chains, formations, fossils (some parts of Newfoundland
have European trilobite fossils) provide dramatic
evidence of the connections that have been broken up,
torn asunder, rearranged and rejoined over the history of
the earth.
• We can even measure the expansion of the Atlantic
ocean (it’s continuing at about the rate that our
fingernails grow).
• Ocean crust is younger near ridges, and steadily older
as we approach trenches. No ocean crust has yet been
dated to before the Jurassic.
Hot Spots
Drift over time