Plate Tectonics 1

Continental Drift

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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 Once-Joined Continents

TR - J basalt flows P - TR redbeds C – P coal meas. C tillite pre-D ig/met rocks

TJb

PTrb

CPcm

Ctl

IG

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 P - TR redbeds C – P coal meas. C tillite pre-D ig/met rocks

TJb

PTrb

CPcm

Ctl

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.

S. America

Africa

Antarctica

Australia

India

Madagascar Sri Lanka

TJb

PTrb

CPcm

Ctl

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.

Pennsylvanian fold belts and coal

measures

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

Fold belt formed during Carboniferous collision

The mountains in these various regions were originally a single chain, quite likely the biggest the world has ever know.

Observation 3 The Geographic Distribution of Fossils (Paleobiogeography) Makes no Sense

on a Modern Map.

Glossopteris (a seed fern or “tongue fern”) Lystrosaurus (a terrestrial reptile) Mesosaurus (a freshwater lizard-like reptile) Cynognathus (a terrestrial reptile)

Does it make sense that these land plants and animals migrated between far-flung continents in several climatic zones?

S. America

Africa

Antarctica

Australia

India

Madagascar

Or does it make more sense that they all lived on a single

landmass and migrated more-or-less freely across it?

(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

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”

To Mediterranean Sea

Gently dipping away Almost vertical

View toward Pacific Ocean

To Gibraltar (Mediterranean)

To Atlantic Ocean

To Atlantic Ocean

Alps (Europe -- France) Marathon Mts (Appalachians) (west Texas)

Appalachians (West Virginia -- Maryland) Pyrenees (northern Spain)

(push is toward you)

The folds in every mountain chain suggest that the main push came from the direction of the nearest ocean.)

To the Atlantic Ocean

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.

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.

Tropic of Cancer Equator Tropic of Capricorn

The existence of glaciers in polar Antarctica seems reasonable

Tropical glaciers make no sense.

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).

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.

Temperate Desert Tropical Heat Freezing Cold

Permian Reefs

Permian Reefs

Permian Evaporites

Glaciers

Glaciers

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).

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: Tillite (glacial deposits) Fossils of cold-tolerant organisms. (Tundra plants or certain kinds of birds, for example) Very low overall biological diversity.

Indicators of TEMPERATE CLIMATES: Coal (cool temperate swamps – see next slides) Evaporites (warm temperate deserts – see next slides) Fossils of frost-tolerant organisms. (deciduous plants or warm blooded animals, for example) Higher overall biological diversity.

Indicators of TROPICAL CLIMATES: Limestone Fossils of frost-intolerant organisms. (evergreen broadleaf plants, corals, or cold-blooded animals, for example) Exceptionally high overall biological diversity.

Present latitude of Tennessee Present prevailing wind direction

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.

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.

“Fossil” latitude and orientation of Tennessee ”Fossil” prevailing wind direction

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.

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.