In this video we are going to describe continental drift,
an idea that served as a precursor for our modern theory of plate tectonics.
And we'll see just how difficult it can be to get people to shift their perspective on cherished concepts.
Scientists have long recognized that earth began as a hot, molten mass.
About a century ago, one popular theory was that mountains and ocean basins were
caused by vertical motions in the crust as the planet cooled.
It was suggested that contraction of the planet had produced features on the surface that
were analogous to creases and folds of a baked apple.
At the same time, geologists were examining rocks and fossils to make interpretations about how Earth's
physical environments had changed throughout the planet's history.
But despite a growing realization that Earth had changed, everyone seemed to agree on
the fact that the continents and oceans had essentially remain fixed in place over time.
Well, everyone that is but Alfred Wegener. Wegener was a German meteorologist
who did research on Greenland and studied ancient climates.
He collected data from several different sources to generate a new and
controversial hypothesis that came to be known as continental drift.
Wegener suggested that the world's major land masses
had formed a single supercontinent he named Pangaea.
And that rather than moving vertically, Earth was dominated by horizontal motions
as the continents drifted across its surface.
These colorful illustrations of Pangaea were created by our colleagues at geode.net.
They also produced a great animation that shows what
happened when Pangaea started to break up around 200 million years ago.
Geode it makes that look something like this.
Wegener believed that the shapes of most of the continents we know today were
formed when Pangaea broke up and the land masses "drifted" to their current locations.
Wegener had four main lines of evidence in support of continental drift.
These include, the fit of the continents, the match of mountain belts and ancient rocks,
indications of past climates, and the distributions of fossils.
Let's start with the fit of the continents.
Wegener wasn't the first person to notice that the shapes of some continents seemed complementary.
The most obvious match is for the southwest margin of Africa and the northeast coast of South America.
The fit is even better if we use the outline of the shallow continental shelf rather than the edge of the land.
Now we can see that the curved northwestern edge of Africa
seems to match the eastern margin of North America. What about the other continents?
Let's revisit those geode resources. Notice that India Australia and Antarctica
are tucked together on the other side of Africa, not too far from the present-day South Pole.
And Eurasia, that is Europe and Asia, is further north than its current location.
So, the continents fit together.
Let's look at the second item on Wegener's checklist, matching mountain belts and rock types.
Continents on either side of the North Atlantic Ocean have mountain ranges of similar age and structures.
These include mountains such as the Appalachians in North America,
the Atlas Mountains in Africa, and the Highlands in Scotland.
If we take Pangaea from 300 million years ago and superimposed the modern continents
it would look something like this.
Notice that the separate mountain ranges now come together to form a single chain of mountains
that formed when continents collided to create the northern half of Pangaea.
If we take the same template and now look at the distribution of ancient cratons,
representing rocks older than two billion years, we find another match between Africa and South America.
So now we have evidence for Pangaea from two types of data.
What about climate reconstructions?
As you can see from the map,
many of the southern continents were located at high latitudes, close to the South Pole.
Rocks from 300 million years ago in these continents show evidence for the presence
a massive ice sheet, even larger than Antarctica today.
Striations, scratches in the bedrock formed by glaciers, are common in rocks of these regions
and indicate that the ice sheet was moving toward the interiors of many of these continents
from a central high point somewhere to the south.
If the continents had always been in their present location, a truly massive ice sheet would have been
necessary and would have required global-scale glaciation.
However, there is ample evidence that elsewhere in Pangaea conditions were a lot warmer,
forming tropical coal swamps along the Pangaea equator and hot deserts further south.
The same locations today we have very different climates so these patterns
only make sense if the continents were part of Pangaea.
Finally, what about fossils? How can they be used to support the concept of continental drift?
Let's jump forward about 50 million years and examine what was going on in the southern half of Pangaea.
We are going to consider four different fossil species found on multiple continents.
They represent two land dwelling reptiles, another that lived in coastal environments,
and a plant with seeds that were too heavy to be carried great distances by winds.
Just as modern-day crocodiles, wolves, and pigs didn't swim across oceans, the only way to find the
same organisms on different continents was to have a single giant landmass,
so that all the organisms could move easily between locations.
When the continents split apart, the rocks containing the fossils were preserved on different sides of the oceans.
So we've checked all the boxes for Wegener's evidence in support of continental drift.
What do you think?
It seems pretty convincing doesn't it?
But it turned out it wasn't convincing enough for Wegener's contemporaries.
First of all, trying to get people to dramatically change their views about something is very difficult.
Many geologists of Wegener's time refused to accept this paradigm shift in thinking about Earth's continents
and came up with other explanations for Wegener's observations.
But, most importantly, Wegener didn't have a good explanation for what caused the continents to move.
Without a cause for continental motion, his drift hypothesis did not gain wide acceptance.
It would take another 30 years before other scientists made a series of new observations about Earth
that led to the development of the theory of plate tectonics which
confirmed several aspects of Wegener's hypothesis.
Unfortunately, Wegener died on the Greenland ice cap at 50 years old and didn't live to see his ideas verified.
So, today we had two learning objectives.
How confident are you that you can successfully complete these tasks?