Describe Continental Drift including supporting evidence.

Geology textbooks all make a big fuss over two things that are really pretty similar (both having to do with how earth’s geography of continents and oceans changes drastically over time).
We call them–
Continental Drift Hypothesis  and   Plate Tectonic Theory.
But, to be fair,  it really is worth making a distinction!
A hypothesis is an idea, and in this case a very good idea.
But the other is a true theory with loads of supporting evidence and the capacity to make testable predictions.

Alfred Wegener, who lived in the early part of the 20th century, is credited with being one of the first scientists to advocate for continents having been in vastly different positions over the great span of earth history.
In fact, he really did do a superb job of accumulating evidence for drifting continents, but he was hardly the first to notice the jig-saw puzzle type of fit when comparing coastlines of the Americas with Europe and Africa.nn

Antique Maps of the World Map of the World Abraham Ortelius c 1570https://pubs.usgs.gov/gip/dynamic/historical.html?

Here’s the problem though–
Wegener thought that the continents must bash through the ocean crust, kind of like ice-breaker ships blast through polar oceans.

He was RIGHT about the fact that they moved.
He was WRONG about how the moved!

In fact, earth’s continents do nothing of the sort–
Instead, continents approach each other when intervening ocean crust gets shoved underneath (subduction zones), and they separate when new seafloor forms between them along a divergence zone (e.g. the big undersea ridges found in Pacific, Atlantic, and Indian Oceans).

So, Wegener had a good idea (i.e. continents moving over time) but he had no idea how it really happened.
Wegener had what we might distinguish as a “hypothesis” while Plate Tectonics (the work of many scientists in the mid-20th century) was a real fully functioning theory complete with explanations for how and why continents move.

Continental Drift

The Continental Drift Idea

Figure 1. The continents fit together like pieces of a puzzle. This is how they looked 250 million years ago.

Here is a map showing the connectivity of continents suggested by Wegener way back in the early 1900’s.
He was unsure how far back this continental agglomeration might have existed, but he was convinced that it had occurred.

Alfred Wegener proposed that the continents were once united into a single supercontinent named Pangaea, meaning all earth in ancient Greek. He suggested that Pangaea broke up long ago and that the continents then moved to their current positions. He called his hypothesis continental drift.

Evidence for Continental Drift

Besides the way the continents fit together, Wegener and his supporters collected a great deal of evidence for the continental drift hypothesis.

• Identical rocks, of the same type and age, are found on both sides of the Atlantic Ocean. Wegener said the rocks had formed side-by-side and that the land had since moved apart.
• Mountain ranges with the same rock types, structures, and ages are now on opposite sides of the Atlantic Ocean. The Appalachians of the eastern United States and Canada, for example, are just like mountain ranges in eastern Greenland, Ireland, Great Britain, and Norway (figure 2). Wegener concluded that they formed as a single mountain range that was separated as the continents drifted.
  

  Figure 2. The similarities between the Appalachian and the eastern Greenland mountain ranges are evidences for the continental drift hypothesis.

• Ancient fossils of the same species of extinct plants and animals are found in rocks of the same age but are on continents that are now widely separated (figure 3). Wegener proposed that the organisms had lived side by side, but that the lands had moved apart after they were dead and fossilized. He suggested that the organisms would not have been able to travel across the oceans.
  • Fossils of the seed fern Glossopteris were too heavy to be carried so far by wind.
  • Mesosaurus was a swimming reptile but could only swim in fresh water.
  • Cynognathus and Lystrosaurus were land reptiles and were unable to swim.
    

    Figure 3. Wegener used fossil evidence to support his continental drift hypothesis. The fossils of these organisms are found on lands that are now far apart.

• Grooves and rock deposits left by ancient glaciers are found today on different continents very close to the equator. This would indicate that the glaciers either formed in the middle of the ocean and/or covered most of the Earth. Today glaciers only form on land and nearer the poles. Wegener thought that the glaciers were centered over the southern land mass close to the South Pole and the continents moved to their present positions later on.
• Coral reefs and coal-forming swamps are found in tropical and subtropical environments, but ancient coal seams and coral reefs are found in locations where it is much too cold today. Wegener suggested that these creatures were alive in warm climate zones and that the fossils and coal later had drifted to new locations on the continents.

Although Wegener’s evidence was sound, most geologists at the time rejected his hypothesis of continental drift. Why do you think they did not accept continental drift?

Scientists argued that there was no way to explain how solid continents could plow through solid oceanic crust. Wegener’s idea was nearly forgotten until technological advances presented even more evidence that the continents moved and gave scientists the tools to develop a mechanism for Wegener’s drifting continents.

Magnetic Polarity Evidence

Figure 4. Earth’s magnetic field is like a magnet with its north pole near the geographic North Pole and the south pole near the geographic South Pole.

Puzzling new evidence came in the 1950s from studies on the Earth’s magnetic history (figure 4). Scientists used magnetometers, devices capable of measuring the magnetic field intensity, to look at the magnetic properties of rocks in many locations.

Magnetite crystals are like tiny magnets that point to the north magnetic pole as they crystallize from magma. The crystals record both the direction and strength of the magnetic field at the time. The direction is known as the field’s magnetic polarity.

Magnetic Polarity on the Same Continent with Rocks of Different Ages

Geologists noted important things about the magnetic polarity of different aged rocks on the same continent:

• Magnetite crystals in fresh volcanic rocks point to the current magnetic north pole (figure 5) no matter what continent or where on the continent the rocks are located.
  

  Figure 5. Earth’s current north magnetic pole is in northern Canada.

• Older rocks that are the same age and are located on the same continent point to the same location, but that location is not the current north magnetic pole.
• Older rock that are of different ages do not point to the same locations or to the current magnetic north pole.

Figure 6. The location of the north magnetic north pole 80 million years before present (mybp), then 60, 40, 20, and now.

In other words, although the magnetite crystals were pointing to the magnetic north pole, the location of the pole seemed to wander. Scientists were amazed to find that the north magnetic pole changed location through time (figure 6).

There are three possible explanations for this:

1. The continents remained fixed and the north magnetic pole moved.
2. The north magnetic pole stood still and the continents moved.
3. Both the continents and the north pole moved.

Magnetic Polarity on Different Continents with Rocks of the Same Age

Geologists noted that for rocks of the same age but on different continents, the little magnets pointed to different magnetic north poles.

• 400-million-year-old magnetite in Europe pointed to a different north magnetic pole than the same-aged magnetite in North America.
• 250 million years ago, the north poles were also different for the two continents.

The scientists looked again at the three possible explanations. Only one can be correct. If the continents had remained fixed while the north magnetic pole moved, there must have been two separate north poles. Since there is only one north pole today, the only reasonable explanation is that the north magnetic pole has remained fixed but that the continents have moved.

To test this, geologists fitted the continents together as Wegener had done. It worked! There has only been one magnetic north pole and the continents have drifted (figure 7). They named the phenomenon of the magnetic pole that seemed to move but actually did not apparent polar wander.

Figure 7. On the left: The apparent north pole for Europe and North America if the continents were always in their current locations. The two paths merge into one if the continents are allowed to drift.

This evidence for continental drift gave geologists renewed interest in understanding how continents could move about on the planet’s surface.

Summary

• In the early part of the 20th century, scientists began to put together evidence that the continents could move around on Earth’s surface.
• The evidence for continental drift included the fit of the continents; the distribution of ancient fossils, rocks, and mountain ranges; and the locations of ancient climatic zones.
• Although the evidence for continental drift was extremely strong, scientists rejected the idea because no mechanism for how solid continents could move around on the solid earth was developed.
• The discovery of apparent polar wander renewed scientists interest in continental drift.

Wegener and the Continental Drift Hypothesis

Scientists still do not appear to understand sufficiently that all earth sciences must contribute evidence toward unveiling the state of our planet in earlier times, and that the truth of the matter can only be reached by combing all this evidence. . . . It is only by combing the information furnished by all the earth sciences that we can hope to determine ‘truth’ here, that is to say, to find the picture that sets out all the known facts in the best arrangement and that therefore has the highest degree of probability. Further, we have to be prepared always for the possibility that each new discovery, no matter what science furnishes it, may modify the conclusions we draw.

— Alfred L. Wegener, The Origins of Continents and Oceans, first published in 1915.

Wegener put together a tremendous amount of evidence that the continents had been joined. He advocated using scientific evidence to find the “truth.” As his colleague, are you convinced? Let’s explore.

Wegener’s Continental Drift Hypothesis

Figure 8. Alfred Wegener suggested that continental drift occurred as continents cut through the ocean floor, in the same way as this icebreaker plows through sea ice.

Wegener put his idea and his evidence together in his book The Origin of Continents and Oceans, first published in 1915. New editions with additional evidence were published later in the decade. In his book he said that around 300 million years ago the continents had all been joined into a single landmass he called Pangaea, meaning “all earth” in ancient Greek. The supercontinent later broke apart and the continents having been moving into their current positions ever since. He called his hypothesis continental drift.

The Problem with the Hypothesis

Wegener’s idea seemed so outlandish at the time that he was ridiculed by other scientists. What do you think the problem was? To his colleagues, his greatest problem was that he had no plausible mechanism for how the continents could move through the oceans. Based on his polar experiences, Wegener suggested that the continents were like icebreaking ships plowing through ice sheets. The continents moved by centrifugal and tidal forces. As Wegener’s colleague, how would you go about showing whether these forces could move continents? What observations would you expect to see on these continents?

Figure 9. Early hypotheses proposed that centrifugal forces moved continents. This is the same force that moves the swings outward on a spinning carnival ride.

Scientists at the time calculated that centrifugal and tidal forces were too weak to move continents. When one scientist did calculations that assumed that these forces were strong enough to move continents, his result was that if Earth had such strong forces the planet would stop rotating in less than one year. In addition, scientists also thought that the continents that had been plowing through the ocean basins should be much more deformed than they are.

Wegener answered his question of whether Africa and South America had once been joined. But a hypothesis is rarely accepted without a mechanism to drive it. Are you going to support Wegener? A very few scientists did, since his hypothesis elegantly explained the similar fossils and rocks on opposite sides of the ocean, but most did not.

Mantle Convection

Figure 10. Thermal convection occurs as hot rock in the deep mantle rises towards the Earth’s surface. This rock then spreads out and cools, sinking back towards the core, where it can be heated again. This circulation of rock through the mantle creates convection cells.

Wegener had many thoughts regarding what could be the driving force behind continental drift. Another of Wegener’s colleagues, Arthur Holmes, elaborated on Wegener’s idea that there is thermal convection in the mantle.

In a convection cell, material deep beneath the surface is heated so that its density is lowered and it rises. Near the surface it becomes cooler and denser, so it sinks. Holmes thought this could be like a conveyor belt. Where two adjacent convection cells rise to the surface, a continent could break apart with pieces moving in opposite directions. Although this sounds like a great idea, there was no real evidence for it, either.

Alfred Wegener died in 1930 on an expedition on the Greenland icecap. For the most part the continental drift idea was put to rest for a few decades, until technological advances presented even more evidence that the continents moved and gave scientists the tools to develop a mechanism for Wegener’s drifting continents. Since you’re on a virtual field trip, you get to go along with them as well.

Summary

• Alfred Wegener published his idea that the continents had been joined as a single landmass, which he called Pangaea, about 300 million years ago.
• Wegener’s idea was mostly ridiculed, in part because Wegener could not develop a plausible mechanism for continents moving through oceanic crust.
• Calculations showed that his idea about centrifugal and tidal forces powering the continents could not be right.
• Wegener also thought about mantle convection an idea expanded on by Arthur Holmes as the driving force for continental drift. There was no evidence available to support the idea at the time.