Continental Drift 1620-1915
A hypothesis to explain a set of observations
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What to look for:
• As early as there were maps to allow it, people noticed that the edges of
some continents look as though they would fit together across an ocean.
• In 1915 Alfred Wegner assembled an impressive array of evidence for
continental movements:
–
Not only do the edges of the continents match, the rocks on them, geologic structures on them, even
biogeographic provinces seem to continue across the intervening ocean.
–
Directions of glacial movement also make better sense in most cases if another continent originally lay
beside the one with the glacial deposits.
–
The continental edges have been affected by what seems to be lateral, compressional mountain-building
stresses. Their centers are unaffected.
–
The present latitudes and/or orientations of many continents cannot explain indicators of paleoclimates
(rocks and fossils) and paleowind directions.
CONTINENTAL EDGE “FIT”
The Cabot Map
published 1544 by
Sebastian Cabot.
This was the first
reasonably
accurate map of
the coasts of the
Americas.
By the early 1600’s
a number of
people (for
example Francis
Bacon in 1620) had
noticed and
commented on an
interesting pattern.
The similarities in shape
of these coasts was
quite striking.
Given the distortion
inherent in mapping a
spherical globe onto a
flat piece of paper and
the inaccuracy in
determining latitude
and longitude while
mapping a coast in the
1500’s, the fit is quite
good.
People presumably
assumed that more
precise mapping
techniques would make
it even better.
Little more thought was given to continental movements before the 19th century.
People remembered that the continental edges seem to fit across the Atlantic, but
geology really didn’t exist as a science and nobody could think of a way to test any
hypotheses about why the edges might be so similar.
In the 19th century, however, lots of things happened. By 1815 a new way to think
about geologic time had been discovered by William Smith in England. By the 1850’s
it had been well tested and found to work exceptionally well.
In consequence, people began recognizing, mapping, and studying such structures as
folds and faults, and to consider their implications about the forces that formed them.
By the 1880’s it was also well established that something beneath the land was
capable of slow gradual flow, and that the land could subside or be uplifted by the
behavior of this “asthenosphere”. (This is Greek for “sphere of no strength”).
There was some sniping between two schools of thought: one that thought structures
require lateral forces (continental collisions) and one that thought up and down on the
asthenosphere could explain them perfectly well.
STRUCTURAL GEOLOGY
View toward Pacific Ocean
To Mediterranean Sea
Gently dipping away
Almost vertical
To Atlantic Ocean
To Gibraltar (Mediterranean)
To Atlantic Ocean
How to get great hunks of crust to move across
ocean basins was a dilemma for which nobody
could see a solution. By 1900 the consensus
opinion was that it is impossible. Therefore the
continents are stationary.
Period.
As is often the case with a consensus, this one
turned out to be wrong. But not yet.
In 1915 the German
meteorologist Alfred
Wegner published a book
named “The Origin of
Continents and Oceans”.
This book outlined a
complex hypothesis to
explain a number of
observations about
Earth’s geography.
Including the apparent fit
of certain of the
continental edges). He
proposed that the
continents had originally
been connected into a
large landmass that he
named “Pangaea” (Greek
for “all land”), which has
since rifted into the
pieces we now know as
continents.
Wegner’s work was constructed in the form of a theory.
He, in fact, judged that some of the observations tested
the idea and supported it, and so he thought of it as a
theory in the truest sense of the word.
However, because the prevailing opinion of the day
was that the continents could not move, most of his
contemporaries thought of it more as a hypothesis, and
proceeded to test it to death.
It is interesting to notice that Wegner’s “theory”, whether
or not it actually was one, had all the necessary
components for a scientific theory:
1) Explanatory power for a number of unrelated
observations.
2) Additional observations which the idea predicts, and
which it explains after it has been constructed.
3) A proposed physical mechanism for causing the
operation of the model.
As we will see, it was this third component of the hypothesis that caused Wegner so much trouble
and (wrongly) led to its complete rejection.
GEOLOGIC MATCHES
TR - J basalt flows
P - TR redbeds
C – P coal meas.
C tillite
pre-D ig/met rocks
TR - J basalt flows
TJb
P - TR redbeds
PTrb
C – P coal meas.
CPcm
C tillite
Ctl
pre-D ig/met rocks
IG
Africa
S. America
TJb
PTrb
CPcm
Ctl
Madagascar
Sri Lanka
IG
India
Antarctica
Australia
Pennsylvanian fold
belts and coal
measures
Fold belt formed during
Carboniferous collision
PALEOWIND & PALEOCLIMATE
Glaciers cannot form in water. If they did, they would have to stay in the water.
They could not move onto land.
In the first place, sea water can only freeze to a modest depth because the ice soon insulates the underlying
water and doesn’t allow it to cool any farther. When sea-ice forms in the Arctic Ocean in winter it is only a few
meters thick. Even if it doesn’t thaw appreciably the next summer, it does not get thicker the following year.
Even supposing that a glacier-like thickness of ice could form in the water, ~90% of it would be submerged and
so its center of mass would be far below sea level. Gravity could not push that center of mass uphill to get it
onto land, no matter how gentle the slope.
Glaciers can and do form on land. Even though the lithosphere beneath them
subsides somewhat, the bulk of the ice always remains above sea level. The ice then
flows, slowly, from its thickest part to its edges.
Because the shore is always the lowest part of a landmass, and because ice calves (breaks off) from the ends of
glaciers into the sea (and thus form icebergs), continental glaciers should always move from a landmass toward
the ocean, not the other way.
Striations are abrasion grooves left by
pebbles and rocks embedded in the base of
a glacier. They typically become narrower
and shallower down-flow, thereby indicating
the direction of movement.
These striations form the “front porch” of
the Leeman Brook Lean-to on the
Appalachian Trail near Monson, ME.
Though it is evident that the ice either
moved toward or away from the
photographer, which of those two directions
is difficult to determine from the
photograph. The bedrock is relatively soft
slate and so the striations are very long and
consistent.
The Pleistocene glacier that created these
should have been moving roughly
southward. The photograph was made
looking almost directly northward, so the
motion was toward the observer.
(The picture looks great on my i-pod. It
didn’t translate well into jpeg format,
unfortunately.)
Directions of ice motion (interpreted from striations) beneath late Paleozoic glacial sediments in the southern
continents is indicated on this map. Notice that those in South America, India, and Australia suggest ice moved onto
the continent from the ocean.
The Direction of ice
motion in Africa is as
expected
The direction in South
America makes no
sense at all.
(Also in India and
Australia.)
Remember from earlier that these
two continents look like they would
fit together. India, Antarctica,
Australia, and various small bits of
Asia look like they would fit as well.
Africa
S. America
India
For this and lots of other reasons, Wegner proposed
that they were, in fact, joined for a long period of
time, throughout the Paleozoic and into the early
Mesozoic Eras. He called the large landmass they
formed a “supercontinent” and named it
“Gondwanaland”.
Antarctica
Australia
There is also a strictly climatic enigma created by these glaciers. Their deposits are not at present only near a pole
(where we’d expect) but also within the tropics (where we certainly would not expect them).
The existence of glaciers in
polar Antarctica seems
reasonable
Tropic of Cancer
Equator
Tropic of Capricorn
Tropical glaciers
make no sense.
One possible solution is to assume that the entire world was colder in the Permian.
Freezing
Cold?
If so, one should expect indicators of cold climate in Permian rocks of Eurasia and North America as
well. The fact that we do not is a problem for this particular hypothesis.
In fact, the relevant observations suggest just the opposite. Immediately beside the latitude of the
Indian glacial tills we find limestone and reefs (tropical deposits) and just beyond those, evaporites
(hot temperate deposits).
Temperate Desert
Permian Reefs
Permian Evaporites
Tropical Heat
Permian Reefs
Glaciers
Freezing
Cold
Glaciers
For many time periods we see the same pattern:
indicators of “paleoclimate” don’t make sense in
the latitude where they are presently found.
This type of observation is one we will see again
and again: the way the world looks and behaves
now cannot explain the way it was during the (Fill
in the blank) Period.
Make sure you realize that the principle of
uniformitarianism underlies all such observations!
Indicators of
POLAR CLIMATES:
Indicators of
TEMPERATE CLIMATES:
Indicators of
TROPICAL CLIMATES:
Tillite (glacial deposits)
Coal (cool temperate swamps
– see next slides)
Limestone
Fossils of cold-tolerant
organisms. (Tundra
plants or certain kinds
of birds, for example)
Very low overall
biological diversity.
Evaporites (warm temperate
deserts – see next slides)
Fossils of frost-tolerant
organisms. (deciduous
plants or warm blooded
animals, for example)
Higher overall biological
diversity.
Fossils of frost-intolerant
organisms. (evergreen
broadleaf plants, corals,
or cold-blooded animals,
for example)
Exceptionally high
overall biological
diversity.
DESERT
TAIGA/RAIN FOREST
DESERT
Ascending air masses (“lows” -Equator and ~60° N and S) cool
down, driving their humidity to
condense. This creates Earth’s
rainy zones.
The now dry air
spreads in both
directions as
upper-level winds,
cooling further by
heat loss to space.
RAIN FOREST
DESERT
TAIGA
DESERT
(The underlying wind diagram will be discussed in the next set of slides).
The descending air masses
(“Highs” -- 30° and 90° N and S)
are warmed as they sink. Their
already very low absolute
humidity thereby becomes a
nearly non-existent relative
humidity. This creates Earth’s
major desert zones.
Air circulates vertically in the
atmosphere because of differential
heating of the equator and poles by
the sun. Equatorial air is always
warmer than polar air because more
solar energy is absorbed by Earth at
the equator.
Warm air at the equator rises
(and cools as a consequence)
to feed the upper air currents.
Meanwhile, cold air sinks at
the poles and returns across
the Earth (as surface winds)
to the equator, to replace the
rising air there. This cyclic
system is called convection and
we will see it again and again.
IF the world did not rotate, this
would be a simple two cell system
and the surface winds would always
blow directly south in the northern
hemisphere and directly north in the
southern.
In fact, they hardly ever blow those
directions.
Earth’s rotation creates a
phenomenon called the Coriolis
deflection such
that moving objects (air
molecules and Atoms,
for example) seem
to follow curved rather than
straight paths. The apparent
curvature is rightward in the
northern hemisphere and
leftward in the southern.
Because of these complications,
three cells exits in each
hemisphere rather than
one and winds often blow
roughly eastward or
westward rather than north
or south.
Because the wind direction is
reliably predictable at a given
latitude they are called
prevailing winds.
From equator to pole the
prevailing winds are: the trades
(easterly), westerlies, and polar
easterlies.
Present latitude of Tennessee
Present prevailing wind direction
Volcanic ash beds in Ordovician deposits of North America are thickest in NE Tennessee and SW Virginia
and thin to the north and west. This implies a southeasterly prevailing wind in the Ordovician.
Currently this part of the world is in the mid-latitude southeasterly wind belt. (This is why out weather
almost always comes from the west).
Again we see that a modern climatic system can’t explain past conditions, and the principle of
uniformitarianism seems to be being “disobeyed”.
There are two ways we could fix this, but either requires that the continent has
moved.
On the one hand, the continent may be in a different latitude from where it
was in the Ordovician. Clearly this requires it to move across latitude. This
hypothesis might also help explain our earlier climatic observations.
On the other hand, the same thing could be accomplished by simply rotating
the continent from an earlier orientation. (In this case, something like 150°!)
Obviously this doesn’t help with our other climatic enigmas.
Of course, both of these things might have happened, each explaining part of
the discrepancy between present and past winds.
What is patently NOT possible is that the wind patterns have changed. These
depend on the behavior of the entire globe, which we have good reason to
think has always behaved as it does now – rotation around a fixed axis tilted
~26° with respect to the plane of revolution around the sun!
Putting off an explanation
of why we chose this
particular setting, imagine
that North America has
both rotated ~80°
counterclockwise and
moved northward from a
low-latitude southern
hemisphere location since
the Ordovician.
In its original Ordovician
position (at left) the
modern wind direction
(southeasterly trades) is
almost exactly what is
needed to blow ash the
way it clearly did blow.
Ordovician latitude of Tennessee
Eternal prevailing wind direction
Ordovician wind
direction in
Tennessee as
indicated by ash
distribution.
The dip direction of the crossbeds in aeolian dunes (like
these Jurassic ones in Zion National Park, Utah) is another
good indicator of paleowind direction.
PALEOWIND
BIOGEOGRAPHY & DIVERSITY
Does it make sense that these land plants and animals migrated between far-flung continents in several climatic
zones?
Glossopteris (a seed fern or “tongue fern”)
Lystrosaurus (a terrestrial reptile)
Mesosaurus (a freshwater lizard-like reptile)
Cynognathus (a terrestrial reptile)
Africa
S. America
Or does it make more sense
that they all lived on a single
landmass and migrated
more-or-less freely across
it?
Madagascar
India
Antarctica
Glossopteris
Lystrosaurus
Australia
Mesosaurus
Cynognathus
Take-home message
• As early as there were maps to allow it, people noticed that the edges of
some continents look as though they would fit together across an ocean.
• In 1915 Alfred Wegner assembled an impressive array of evidence for
continental movements;
– Not only do the edges of the continents match, the rocks on them, geologic
structures on them, even biogeographic provinces seem to continue across the
intervening ocean.
– Directions of glacial movement also make better sense in most cases if another
continent originally lay beside the one with the glacial deposits.
– The continental edges have been affected by what seems to be lateral
compressional mountain-building stresses. Their centers are unaffected.
– The present latitudes and/or orientations of many continents cannot explain
indicators of paleoclimates (rocks and fossils) and paleowind directions.