Late 20th Century Tests of the
Continental Drift Hypothesis
3 – Continental “Fit” Revisited
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What to look for:
• The true continental edges (at the continental slope) fit
together much better than could have been determined in
the 1600’s.
• The rotational movement of convexly curved continents
across the globe requires that some parts of the pieces
move faster than others
• Transform faults allow this to happen.
• If the continents used to be connected, but aren’t now, then
the have necessarily moved.
Recall that even in
the 17th century
people were
noticing that the
coasts of some
continents “fit
together”.
The fit was,
apparently, far from
perfect though.
Part of the problem
was the quality of
the maps, but there
were two additional
ones that couldn’t
really be addressed
until the 20th
century.
This question arose
in the 1960’s: How
good is the fit?
• Even the best of map projections introduces
considerable distortion into both the sizes and shapes of
things on the map. Things closer to the equator are
fairly accurate, but toward the poles things look much
bigger than they actually are. Look at a wall map. If
Greenland were actually bigger than North America we’d
call it a continent, not an island.
• The coast of a continent is not its edge. The edge is out
at the continental slope, where the shelf ends and depth
drops down to abyssal (oceanic) depth.
Correcting for these problems must
involve a 3-D geometric model (rather
than a flat map), better detail of lat/long,
and a concept of movement of convex
pieces across the spherical surface
rather than flat ones on a flat
surface.
If a convex piece moves across
a spherical surface it does
so around a “pole of rotation”.
The pole and latitude lines
shown here do not represent
the Earth’s pole and latitude
grid, but the pole and latitude
of rotation of one piece (South
America) away from another
(Africa), which is assumed to be
stationary. We will return to why
Africa is the one set stationary.
(Modified from Bullard et al, 1965 and McKenzie, 2006)
If we now rotate (NOT “slide”)
South America toward Africa
along the latitudes of rotation,
they meet like this.
The coasts of the continents
don’t seem to match any better
than they did on the Cabot
map in 1544, but the
continental shelves fit
very well indeed. The
accuracy of shelf fit for
all continents using this
method is estimated
at better than 95%.
Shelf edges can erode by
mass wasting and other
processes, or they may grow by
reef and other sedimentation, so
a certain amount of mismatch is
actually expected.
(Modified from Bullard et al, 1965 and McKenzie, 2006)
ORIGINAL VECTORS FROM
THE PREVIOUS PAGE
Equator of
Rotation
If there really has been rotational motion of South America away from Africa, then some parts of the
South American piece must have moved farther than others. Minimum motion would have been at
the poles of rotation and maximum motion at the equator of rotation.
Because all the motion happened over the same time span, this means that some parts of the piece
(near the equator of rotation) moved faster than other parts (near the poles of rotation).
If the piece is brittle rock, how could this have been done? If you push a piece of plywood across a
floor, can you make some parts go faster than other parts?
A solid piece of plywood
will necessarily all move
at the same rate.
But if it is cut or broken into shorter
segments, these can all move at
different rates to different distances!
The strike-slip transform faults
that cross and offset the ridges
are the mechanism that
accommodates the differential
rates of movement of a spherical
segment of lithosphere (a “plate”)
across the spherical surface of
Earth.
Take-home Message
• The true continental edges (at the continental slope) fit
together much better than could have been determined in
the 1600’s.
• The rotational movement of convexly curved continents
across the globe requires that some parts of the pieces
move faster than others
• Transform faults allow this to happen.
• If the continents used to be connected, but aren’t now, then
the have necessarily moved.