Plate tectonics, the study of the movements and interactions of the lithospheric plates, has a history which shows how scientists work by studying one another’s work, arguing, disagreeing, and proposing new hypotheses. It demonstrates the importance of researching a problem carefully and being open to new information. It is a wonderful example of interdisciplinary work among the various branches of science with paleontologists, climatologists, oceanographers, ecologists, biologists, and geologists all working to solve the same mystery; continental drift. It also reveals an important source of global change; changes that are reflected in the climate, landforms, and lifeforms.
We will begin with a brief introduction presenting some of the early history of the concept of continental drift. This will include the first ideas of landbridges connecting the various land masses. Then the work of Wegener and Dutoit will be looked at in more detail. We will use Wegener’s clues to draw and assemble a partial map of Pangaea. Next we will look at some of the clues that have been found in support of the plate tectonic theory. Comparisons of strata on different continents, and looking at the specific kinds of sediments and how they form will tell us some things about climate and about movements in the region involving rifting or colliding of continents. Glaciation clues can tell us not only about climate but also possible alignment of early land masses and their geographic locations. Changes in global climate resulted in changes in the endemic plant and animal forms, all being influenced by the changes in geographic location of the moving continents. Paleomagnetism and the concept of magnetic reversals will be looked at in order to determine the earliest placement of the continents.
My students always respond well to ideas involving animals so I will use both living and fossil species of plants and animals extensively to show the “mystery” of biogeography as it relates to plate movement. We will look at fossils of dinosaurs and early Cenozoic mammals to maintain the interest level.
We will be doing a lot of drawing as we look at maps, draw maps, and draw in clues. Hopefully the students will be able to use some clues to figure out some maps of their own. A final exercise will be to look at some of the unsolved mysteries of animal distribution and try to come up with some of our own ideas. The unit must be very visual since so many of the words we will use are long and threatening. All the representative species will be drawn and colored; the maps will be constructed using continent outline tracings; and posters of geologic eras and periods will be drawn and hung along the wall for constant reference, with representative species listed, drawn, and colored.
This unit is designed for the middle-school age level, to be used in an Earth Science curriculum.
The idea of continental movement has been with us since the middle of the last century. The understanding of the driving force is relatively recent, the 1960’s. The basic concept extends all the way back to early map makers who noticed that the coastlines of Africa and South America had a remarkable similarity. As early as 1858 a French scientist, Antonio Snider-Pelligrini wrote that the Atlantic Ocean had formed when powerful forces broke apart a great continent. However at that time the idea was not given serious consideration by most geologists. Evidence supporting this concept, however, gradually developed from geological and paleontological observations in the continents of the Southern Hemisphere and India.
There is great similarity between the animals of India and Madagascar, two land areas now separated by 2,500 miles of ocean. In order to explain this observation an early idea was that the continents were joined by land bridges of granitic rock that later sank into the ocean’s basaltic crust. This idea also supported the Austrian geologist Eduard Suess’s work with the fossil plant
Glossopteris
which he found to be distributed throughout India, South America, southern Africa, Australia, and Antarctica. He felt that the presence of a land bridge connecting all of these continental areas would explain the distribution of this group of fossil plants over such widely separated locations. He named this land mass of continents and land bridges Gondwanaland (named for a district in India where
Glossopteris
was found in the coal beds.)
1
Between 1872 and 1876 The British H.M.S. Challenger expedition took soundings throughout the Atlantic Ocean which revealed an extensive ridge running north-south down the middle of the ocean floor roughly half-way between the American continents and Europe-Africa. Using these data Frank Taylor in 1908 proposed that the continents had at one time been adjacent but had been pulled apart by tremendous forces, and that the central ridge, the Mid-Atlantic Ridge, was the site of this ancient boundary.
Not long after Taylor’s work was published Alfred Wegener, a German meteorologist, presented his theories in his 1915 book
On the Origin of Continents and Oceans
. He proposed that at one time all the continents were joined into one huge supercontinent which he named Pangaea (Greek for “all the land”) and that at a later date the continents split apart, moving slowly to their present positions on the globe. He felt that the idea of land bridges was wrong because it called for less-dense granitic rock to sink into more-dense basaltic rock which he felt was clearly impossible. And possibly because he was trained in meteorology rather than geology he was able to view the earth’s surface as less immutable than his contemporaries. He searched for analogies and found them in the rift valleys of eastern Africa, an area that is now considered to be a possible site for rifting to become drifting. In matching up coastlines he found that by including the continental shelves the fit was much more accurate, and by doing so large blocks of ancient rock called cratons, the oldest core of a continental land mass, were found to form a contiguous pattern across the boundary of South America and Africa. He also looked at other geologic formations and saw patterns such as the presence of ancient mountains in South Africa which align with the mountains near Buenos Aires in Argentina when the two continents are “fitted” along coastlines. Layers of sandstone, shale, and clay interspersed with coal in both South Africa and Brazil seemed to match in sequence. (This will be discussed more fully later in the paper.) Wegener’s theory was however so radical that he was not given serious consideration by most of the scientific community. But he persisted in his study of the idea finding more and more supporting evidence. In a second and a third edition of his book he included fossil and rock evidence of vastly different climates in the past that could only be explained by a relocation of the particular continent to different latitudes. He also pointed to diamond fields of South Africa and Brazil, coal deposits in Britain and the Appalachians, and a thick band of red sandstone which runs from the Baltic, through Norway, across Britain and Greenland and into North America. Still the acceptance for his theory did not come. In fact he was subjected to ridicule and insult for daring to present his “preposterous” ideas.
Although some of Wegener’s information was off the mark the largest part was accurate,but his theory’s greatest weakness was his explanation of cause. He suggested that the centrifugal force of the spinning planet would tend to force the continents equator-ward and that tidal pull from the sun and moon might cause lateral movement. He did not seem very confident in those mechanisms however as he also stated that the “complete solution of the problem of the driving forces will still be a long time coming.”
2
The first map that the students will construct uses Wegener’s clues to bring the continents together to form Pangaea. Directions are in the Appendix Activity 1. At this time the students should also review some geography by doing Activity 2, also in Appendix I.
While in Europe and North America Wegener’s ideas were being attacked, in the southern hemisphere some of his staunchest supporters were collecting data to support the theory of continental drift. One, a South African named Alexander duToit traveled to Brazil, Uruguay, and Argentina where he found remarkable similarities in fossils and strata to his native land. He also studied the Karroo sequence, expanding on earlier work to show the parallel development of South America, Africa, Antarctica, and India. In the early Permian layers of both South America and South Africa were found the fossil remains of a freshwater reptile called Mesosaurus, and bands of coal were found extending through each of the southern continents which contained fossil remains of a plant group
Glossopteris
. Du-Toit also mapped glacial sediments and striations which pointed to a very different orientation of the southern hemisphere in its geologic past. The presence of ancient mountain remnants running through Australia, Antarctica, South Africa, and South America were also used to reinforce the idea of a supercontinent at least in the southern hemisphere, Suess’s Gondawanland. Living species were also introduced as evidence. Earth worms, very unlikely to be long-distance migrators, were found in soils of all the Gondwanaland continents.
But despite the evidence presented by supporters most of the scientific community still did not accept the theory of continental drift. It was not until thirty years after Wegener’s death that scientists of the 1960’s described sea-floor spreading and plate tectonics which were the undeniable proof of continental drift.
Although the ideas of Wegener and his colleagues were not widely accepted in their lifetimes, the data they collected provides a wonderful set of clues to a grand mystery. By having the students look more carefully at each of the clues and placing this information visually on maps we can help them see the world as a more organized place and how scientists constantly seek to make sense of what they observe.
