School of Earth Sciences

Earth Sciences 1100

Planet Earth: How It Works

Instructor: Ozeas Costa, PhD

Lab Exercise #3

Continental Drift

Earth Sciences 1100 – Planet Earth: How it Works Lab Exercise #3

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Early evidence for “Continental Drift”

Dutch mapmaker Abraham Ortelius postulated in his Thesaurus Geographicus (published in 1596) that the Americas were once joined with Europe and Africa, and then were “… torn away … by earthquakes and floods.” This idea developed after European navigators set out to explore the oceans during the 15th and 16th centuries, and began drawing maps of the coastlines. The more they explored, the more precise their maps became. Other important names who also mentioned the apparent fit between the continents include: Francis Bacon, an English philosopher (considered to be the father of the “scientific method”) in the 17th century; Georges de Buffon, a French naturalist, in the 18th century; and Alexander von Humboldt, a German naturalist, in the 19th century. Although these famous names have pointed out the correspondence between the continents, they did not go much beyond that. The first credible proponent of continental drift was Antonio Snider-Pellegrini, a French cartographer who ascribed the biblical flood to the existence of a single continent that was torn apart to restore the balance of a lopsided Earth. He drew the maps below in 1858. They show his version of how the American and African continents may once have fit together, then later separated.

While these early proponents had only suggested the idea of continental drift, Alfred Wegener was the first to collect actual evidence in support of the idea. Here is what he wrote to his future wife in a letter from December 1910: "Doesn't the east coast of South America fit exactly against the west coast of Africa, as if they had once been joined? This is an idea I will have to pursue.“ The following fall, Wegener came across scientific papers promoting the prevailing theory that Africa and South America had once been connected by a continent-size land bridge that had since sunk into the sea. Over the following years, Wegener collected and synthesized a number of geologic and paleontologic evidence indicating that the present-day continents were once together. on January 6, 1912, Wegener startled a meeting of the Geological Association in Frankfurt with his radical theory. Dismissing the concept of sunken land bridges, he proposed instead a grand vision of drifting continents and widening seas to explain the evolution of Earth's geography. Four nights later, he made a similar presentation to the Society for the Advancement of Natural Science in Marburg, Germany. He had just began a revolution! In this exercise, we will explore some of the evidence he compiled.

Earth Sciences 1100 – Planet Earth: How it Works Lab Exercise #3

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Part 1 - The Puzzle-Piece Argument

As we saw above, many scientists have noticed the apparent snug fit between the continents, particularly between Africa and South America. According to the land bridge hypothesis – which was the accepted version at the time – the continents were connected by large pieces of land that have sunk to form the oceans between the continents. Wegener disputed this idea. He reasoned that, if the oceans formed in areas where continents previously existed, the composition of the ocean floor should be the same as the continents. However, he pointed out that the continents are made of a different, less dense rock (granite) than the volcanic basalt that makes up the deep-sea floor. As such, if their composition is different, they could not have the same origin. Instead of a land bridge connecting the continents, Wegener proposed that the continents were once together, and then floated away from each other, somewhat like icebergs in water.

To explore how well the continents fit with one another (which Wegener called Pangaea, meaning “all land”), you will cut out the continental landmasses from a cardboard copy (provided during your lab session) of the map on page 4 of this tutorial. You will then try to match all landmasses with each other, like a puzzle, and find the position of best fit. Part 2 - The Matching Geology from Neighboring Continents

Wegener also noted that the geology of the rocks on continents that are thousands of miles apart are strikingly similar. On page 5 you will find a map of Africa and South America showing the geology (distribution of Precambrian rocks) in both continents. To explore how the matching geology helped Wegener piece together the separate continents into Pangaea, you will cut out the two continents (Africa and South America) from a cardboard copy (provided during your lab session) of the map on page 5 of this tutorial and try to fit these continents together into a single landmass based on their matching geology. Wegener also observed that mountain ranges are not spread evenly over the Earth. They instead occur in narrow bands, usually at the edge of a continent. Wegener said they formed when the edge of a drifting continent crumpled and folded – as when India hit Asia and formed the Himalayas. The figure on the right shows a reconstruction of the northern part of Pangaea, showing the matching of ages of mountains formed at the same time in eastern North America (the Appalachians), in eastern Greenland, in Norway (the Caledonides), and the Scotland highlands, as well as the southern Appalachians in North America, the Atlas Mountains in Northern Africa, and the Pyrenees in Europe.

Earth Sciences 1100 – Planet Earth: How it Works Lab Exercise #3

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Map for Part 1 – The Puzzle-Piece Argument

Earth Sciences 1100 – Planet Earth: How it Works Lab Exercise #3

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Map for Part 2 – The Matching Geology from Neighboring Continents

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Part 3 - Similar Fossil Record across the Southern Continents

Wegener also documented the fact that there is a remarkable similarity among fossils of plants and animals in the southern continents (Africa, South America, India, Australia, and Antarctica). He found that these communities of plants and animals evolved together until the time that Pangaea split apart, and after that time, they evolved separately. Attached are the maps of these southern continents showing the geographic distribution of major fossils. One of the fossil species used in the maps is that of an ancient tree, the fern Glossopteris. The seeds of Glossopteris were large and heavy and they could not have travelled far on wind or water currents.

Certain animal fossils also display a similar match. The fossil remains of Mesosaurus, a small freshwater reptile from the Permian Period, are found in both South America and South Africa. The types of rocks in which these fossils are found are also very similar. Mesosaurus did swim but it was too small to swim all the way across the Atlantic Ocean. It lived during the early Permian period (286 to 258 million years ago) and its remains are found solely in South Africa and Eastern South America. Another reptile whose fossil remains are found only in South America and Africa is the Cynognathus. Roaming the terrains during the Triassic period (250 to 240 million years ago), the Cynognathus was as large as a modern wolf. As a land-dominant species, like the Mesosaurus, the Cynognathus would not have been capable of migrating across the ocean, if the two continents had always been in their present-day positions. Another long-extinct reptile have fossils spread among multiple southern continents. The Lystrosaurus was an herbivore with a stout build like a pig. It is approximated that it grew up to one meter in length and was relatively dominant on land during the early Triassic period (250 million years ago). Lystrosaurus fossils are only found in Antarctica, India, and South Africa. Similar to its land-dwelling counterparts (Mesosaurus and Cynognathus), the Lystrosaurus would have not had the swimming capability to traverse any ocean.

Earth Sciences 1100 – Planet Earth: How it Works Lab Exercise #3

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To explore how this fossil zoogeography helped Wegener piece together the separate continents into Pangaea, cut out the contour of the continents from a cardboard copy (provided during your lab session) of the map on page 9 of this tutorial and try to fit these continents together based on the matching fossil record. Part 4 - Evidence from Glaciation

Today, glacial deposits formed during the Permo-Carboniferous glaciation (about 300 million years ago) are found in Antarctica, Africa, South America, India and Australia, as indicated in the figure below.

If the continents have not moved, then this would suggest an ice sheet extended from the South Pole to the equator at that time - which is unlikely since regions of Europe and North America at that same time were also close to the equator and had extensive coal and limestone deposits (evidence of a warm and humid climate). If, however, the continents of the southern hemisphere are re-assembled near the South Pole, then the Permo-Carboniferous ice sheet assumes a much more reasonable size, as can be seen in the figure below. The glacial striations (scratches on the bedrock made by blocks of rock embedded in the ice as it moved) show the direction in which the glacier was flowing from a single central point (an ice center).

Earth Sciences 1100 – Planet Earth: How it Works Lab Exercise #3

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Part 5 - The Origin of Continents and Oceans

Wegener eventually concluded that ALL present-day continents were united during the Late Paleozoic and Early Mesozoic Eras. In 1915, he publish his grand synthesis of all the evidence he collected in a paper titled Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans).

The figure on the right is a map based on Wegener’s 1915 paper showing the supercontinent Pangaea, as he believed existed about 300 million years ago, during the Late Carboniferous Period.

Q1. Identify the present-day landmasses in this map (numbered 1-6 in the figure above)?

1 _______________________ 2 ______________________ 3 _______________________

4 _______________________ 5 ______________________ 6 _______________________

Q2. The map also shows a couple of slivers of water within Pangaea (numbered 7-8 in the figure above). These slivers of water between land masses have since grown to become two of our planet’s largest oceans. What are the names of these oceans?

7 ________________________________ 8 ________________________________

Earth Sciences 1100 – Planet Earth: How it Works Lab Exercise #3

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Map for Part 3 – Similar Fossil Record across the Southern Continents

Earth Sciences 1100 – Planet Earth: How it Works Lab Exercise #3

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Part 6 – Paleogeography: The Changing Face of our Planet

Paleogeography is the study of what an environment was like during a certain time in the past. By examining a set of rocks in a number of different locations, all representing the same age, we can begin to reconstruct the ancient environment using the rock types and their fossil content. The rock samples can be collected over a large or small lateral area, and the locations and shapes of land masses and seas can be deduced by knowing which types of rocks and fossils are expected to be found in particular environments.

In the next pages, you will find paleogeographic reconstructions of the shapes and positions of the continents over the last 400 million years. They are, however, out of order. Your job is to analyze each of these maps and PLACE THEM IN THEIR CORRECT ORDER.

Q3. Match the maps on the next pages to the correct age, as indicated below:

Oldest Map: 400 Ma (million years ago) – Early Devonian Period Map # ___________

370 Ma – Late Devonian Period Map # ___________

340 Ma – Mississippian Period Map # ___________

300 Ma – Pennsylvanian Period Map # ___________

280 Ma – Early Permian Period Map # ___________

260 Ma – Late Permian Period Map # ___________

240 Ma – Triassic Period Map # ___________

200 Ma – Early Jurassic Period Map # ___________

175 Ma – Jurassic Period Map # ___________

150 Ma – Late Jurassic Period Map # ___________

90 Ma – Cretaceous Period Map # ___________

Most Recent: 65 Ma – Early Tertiary Period Map # ___________

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Map 1

Map 2

Map 3

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Map 4

Map 5

Map 6

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Map 7

Map 8

Map 9

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Map 10

Map 11

Map 12