Download Continental Drift

Plate Tectonics 1
Continental Drift
Unless otherwise noted the artwork and photographs in this slide show are original and © by Burt Carter.
Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin.
Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities.
Other copyrighted material is used under the fair use clause of the copyright law of the United States.
Observation 1
Continental Edge Fit
Mercator’s 1538 map was the first to show the lands on both sides of the Atlantic. It was drawn in this unusual orientation to
handle certain distortions inherent to representing a curved surface (Earth) on a flat piece of paper (map). This always comes with
trade-offs so other distortions arise as a consequence. These didn’t stop people noticing the similarities between the opposing
coastlines, as shown by the red markings.
The Sebastian Cabot map of
1544 was drawn to
emphasize the continuity of
the oceans, but this means
the distortions that Mercator
was trying to minimize are
back in this version.
Nevertheless, the matching
coasts are again pretty
obvious and led Francis
Bacon to comment upon
them and speculate that the
continents had once been
joined.
This was the first suggestion
of a hypothesis of continental
drift. If two things were joined
in the past, but are not now,
then one or both must have
moved.
Illustrating the same thing on a globe fixes the
distortion problem of distortion. In addition the
edges of the continents on these maps is the
continental shelf break, not the coast.
The upper map shows the present positions of
Africa and South America. These have been
chosen as examples, other choices would have
worked the same. Notice that the two arrows,
indicating places that will “match” when the
continents are pushed together, are not the same
length. This is a consequence of motion on a
sphere (Earth) rather than a flat surface (map).
The bottom picture shows the continents in their
hypothetical pre-rift position. The edges fit
beautifully – better than some puzzles I’ve worked.
The pole and latitude lines are not the rotational
poles/latitude of Earth, but of the rotation of the two
pieces on the spherical surface. The disparity in
arrow lengths is because the farther from the pole
of plate rotation a point is, the farther it must move
during the rotation.
Observation 2
Geological Matches on OnceJoined Continents
The Paleozoic rocks of most of the southern continents is virtually identical: pre-Devonian
crystalline rocks are overlain by Carboniferous glacial sediments (tillite) which is overlain by
coal-bearing rocks and finally hematite rich detrital sediments (red-beds).
TR - J basalt flows
TJb
P - TR redbeds
PTrb
C – P coal meas.
CPcm
C tillite
Ctl
pre-D ig/met rocks
IG
This similarity of rocks suggests that all the rocks formed in the “same place”. The likelihood of identical
geologic histories in widely separated continents is staggeringly small, particularly at their present disparate
latitudes. The early-mid Mesozoic basalt flows a the top of the stack indicate rifting, just as basalts in the
ocean ridges do.
TR - J basalt flows
TJb
P - TR redbeds
PTrb
C – P coal meas.
CPcm
C tillite
Ctl
pre-D ig/met rocks
IG
Alfred Wegner proposed
from this and other
observations that all the
present “southern
continents” (including
India, the Arabian
Peninsula, and other
parts of southern Asia)
were once joined as one
“supercontinent” that he
dubbed “Gondwanaland”
(from a region of India).
The Upper Carboniferous
coal measures and
Permian redbeds (but not
the older rocks) are also
found in Europe and
North America,
suggesting an even
bigger supercontinent
formed when ‘we”
collided with
Gondwanaland. This
larger continent is called
“Pangaea” (“all land”).
The collision is the one
that made the
Appalachians.
The Mesozoic rifting
that split the
continents also
created the oceans
between them
(Atlantic and Indian
Oceans) and the
rifting continues
today at the ridges
in those oceans.
Africa
S. America
TJb
PTrb
CPcm
Madagascar
Ctl
Sri Lanka
IG
India
Antarctica
Australia
In the northern continents (Europe and North America) it isn’t just the coal measures that
match, the very structure and landscape of the rocks containing them is the same: the
Appalachians continue across the pond in various parts of Greenland, Europe and Africa!
(There is an “International Appalachian Trail” with segments in Canada, Scotland, and Morocco, with additional trail in other
countries in the works.)
Covered by
Younger Rocks
Pennsylvanian fold
belts and coal
measures
The mountains in these various regions were originally a single chain, quite
likely the biggest the world has ever know.
Fold belt formed during
Carboniferous collision
Observation 3
The Geographic Distribution of Fossils
(Paleobiogeography) Makes no Sense
on a Modern Map.
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)
Or does it make more sense
that they all lived on a single
landmass and migrated
more-or-less freely across it?
Africa
S. America
(Notice that this is again
Gondwanaland.)
The Oceans that presently
serve as barriers to the
movement of animals and
plants (keeping Giraffes in
Africa and Kangaroos in
Australia, for example) are
the oceans, and they did not
form until the
supercontinents Pangaea and
Gondwanaland rifted apart.
Glossopteris
Lystrosaurus
Mesosaurus
Cynognathus
Madagascar
India
Antarctica
Australia
Observation 4
Continental Mountains Were
Formed by Compressional
Stresses
The world’s major terrestrial mountain chains (outlined in red) are all very “linear” in the sense
of having one very long dimension. They are also conspicuously parallel to the edge of the
continent they occupy or are close to the edge of. (Remember that the Urals separate Europe
and Asia.) Many volcanic arcs in the ocean are omitted, but they show a similar thing.
When we study the rocks in the mountains we find that they have been buckled
(“folded”) into contorted shapes. When we do this experimentally we find that
the steeper dip of the beds (the left side of each folds in this case) is on the
side toward which a major push is directed. Furthermore, when the rocks can
take no more stress by bending and the break (fault – shown in red) with the
piece closer to the source of the push riding up the fault (green arrows) as if it
were a ramp.
“PUSH”
Alps (Europe -- France)
Marathon Mts (Appalachians) (west Texas)
View toward Pacific Ocean
(push is toward you)
To Mediterranean Sea
Gently dipping away
Almost vertical
The folds in every mountain
chain suggest that the main
push came from the direction of
the nearest ocean.)
To Atlantic Ocean
To Gibraltar (Mediterranean)
To Atlantic Ocean
Appalachians (West Virginia -- Maryland)
Pyrenees (northern Spain)
PUSH
The faults in these
regions always tell us
the same thing as the
folds: The major
direction of stress that
caused the folds and
faults was from the
oceanward direction.
What can buckle and
break the entire edge
of a continent like this?
Another continent
would be big enough.
To the Atlantic Ocean
The push directions from the previous two slides are as indicated here.
All the mountains in the original version of this picture would reveal the same pattern.
To quote (and extend) the
songwriter Donovan Leitch:
(Read from bottom.)
Then there was a mountain.
then there was no mountain.
Then there was a mountain,
then there was no mountain.
Then there was a mountain,
then there was no mountain.
First there was a mountain,
Observation 5
The Geographic Distribution of Ancient
Climate Indicators (Paleoclimates)
Makes no Sense on a Modern Map.
There are late Paleozoic deposits of glaciers in the Gondwanaland continents. 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 at that time.
Freezing
Cold?
If so, one should expect indicators of cold climate in rocks of the same age in 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 have seen again
and again: the way the world looks and behaves
now cannot explain the way it was during the (Fill
in the blank) Era or 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.
As we saw earlier in
the term, Earth’s
atmosphere is
heated differently at
the equator and the
poles, setting up
convection to move
the heat around.
The Coriolis
deflection on the
moving air masses
means that at any
given latitude we
expect a particular
prevailing wind
direction.
Some volcanic ashe
deposits in the USA
(very well exposed
in Tennessee) allow
us to compare early
Paleozoic wind
directions to modern
ones.
Present latitude of Tennessee
Present prevailing
wind direction
Because the ash beds get thinner and less numerous westward from east
Tennessee and southwestern Virginia we conclude that the volcano that
erupted them was located in that region. The prevailing winds were evidently
blowing northwestward at the time to distribute the ash as we see it.
This is odd because the modern prevailing wind direction is not the same at
all. (In the winter time it is often in exactly the wrong direction!)
From other evidence
we know that North
America was much
farther south at the
time, and oriented
differently.
Tennessee would
have been at around
20°S and rotated
clockwise by 70-80°
as shown here.
In this orientation
and latitude the
prevailing winds are
just right to blow the
ash in the direction it
was actually blown.
The next slide places
this in a global
context.
As we saw earlier in
the term, Earth’s
atmosphere is
heated differently at
the equator and the
poles, setting up
convection to move
the heat around.
The Coriolis
deflection on the
moving air masses
means that at any
given latitude we
expect a particular
prevailing wind
direction.
Some volcanic ashe
deposits in the USA
(very well exposed
in Tennessee) allow
us to compare early
Paleozoic wind
directions to modern
ones.
“Fossil” latitude and orientation of Tennessee
”Fossil” prevailing
wind direction
The observations that contributed to the hypothesis of continental drift were therefore these
(among others we haven’t examined):
1.
Indications that once-joined continents have moved apart:
A) Continental edge-fit
B) Matching geological features on separate continents
C) Paleobiogeographic distributions of fossils.
2. Indications that the edges of continents are often crumpled by “collisions”:
A) Folds and Faults in those mountains
B) Evidence for repeated creation and erosion of sedimentary source areas.
3. Indications that continents have moved across latitude:
A) Various climatic indicators (rocks and fossils) are found in latitudes where they
couldn’t have formed.
B) Wind indicators indicate that prevailing winds used to blow from the “wrong”
direction for the modern latitude where they are found.
The hypothesis of continental drift was not well received, at least in North America, even though
in retrospect it looks like it is very well supported by the observations. The criticisms were of
two sorts: What happened to the oceans, and what could be powerful enough to move such
large objects? It wasn’t until the 1960’s that these were answered.