Land Forms of the Earth

The earth's surface has traditionally been divided into two parts: ocean basins and the continents which rise above them. Land and sea are very unevenly distributed: the earth can be divided into two hemispheres – one contains four fifths of all the land and the other is nearly nine-tenths covered by water. Nearly three-fourths of all the land surface is underlain by marine rocks. Fossils of marine animals in the rocks of high mountains and inland deserts prove that even many of the continental heartlands were at one time or another drowned beneath the sea.

The World's Main Land Masses. – Most of the land area of the world is located in the Northern Hemisphere. Eurasia and North America, although now separated by Bering Strait, were once a single land mass. All other large land areas, except Antarctica and Australia, are joined to these great land masses. Africa and South America are united with Eurasia and North America, respectively, by narrow necks of land called isthmuses.

Continents. – The great land masses are called continents. Most of them are triangular in shape, pointing toward the south. Tongues of land that extend from continents into bodies of water are called peninsulas. Continents do not end abruptly at the shore of the ocean. In many cases, there is a large area offshore where the ocean water is quite shallow. The land under such shallow water is called a continental shelf. The continental shelves are the submerged edges of the continents. Off most coastlines the bottom slopes gradually (gently) form the shore seaward before passing into a much steeper continental slope. The continental shelf varies greatly in width along different parts of its extent.

Islands. – An island is a body of land entirely surrounded by water. Many islands are submerged mountain ranges. Some, notably in the Pacific Ocean, were created by volcanic action. Volcanoes can be built up beneath the sea until they emerge and become islands. Certain islands are composed of coral.

Mountains. – An elevation above the surface of the earth with a small summit (top) area is called a hill or a mountain. A hill is not so lofty as a mountain and has gentler slopes. Mountain slopes are steeper. Mountains are said to be "old" or "young" according to the length of geologic time that they have been in existence. Old mountains have been acted on by the forces of erosion for so many years that their profiles are smoothed and rounded. Young mountains are jagged in outline and have a rugged relief.

Plateaus. – A plateau is an elevation, which has a large surface area. Some plateaus, like the Plateau of Tibet, are raised far above sea level.

Plains. – Plains are areas of level land. According to location or origin there are several types of plains, each with its own typical characteristics:

1. Coastal plains lying along the shore of the ocean.

2. Interior plains lying in the heart of a continent.

3. Glaciated plains formed by the great glaciers.

4.Flood-plains formed from the alluvium deposited by rivers.

Continental Drift

A casual glance at a map of the world suggests the possibility that at some time in the past the continents were joined together in one or two giant supercontinents. If the margins of the continents are taken to be on their continental slopes at a depth of 3,000 ft, instead of their present sea-level boundaries, the fit between North and South America, Africa, Greenland, and western Europe is remarkably exact. But merely matching up outlines of continents is not by itself sufficient evidence that the continents have migrated around the globe. The first really comprehensive theory of continental drift was proposed early in this century by the German meteorologist Alfred Wegener, who based his argument on biologic and geologic evidence.

Wegener’s Theory

At one time the standard explanation for the similarity of patterns of early life around the world was a series of land bridges linking the continents together. But this meant that the oceans were then separated from one another, so a series of channels had to be devised to permit aquatic plants and animals to pass between the oceans. No really believable scheme of bridges and channels could be devised, and even if one had been, it would still be necessary to account for the disappearance of all traces of them. Wegener was on firm ground when he searched for an alternative to this notion.

What Wegener suggested instead was that originally the continents were all part of a huge landmass he called Pangaea that was surrounded by a single ocean, Panthalassa. Pangaea then began to break up and the continents to slowly drift to their present locations. This model found additional support in geological data regarding prehistoric climates. At one time, South Africa, India, Australia, and part of South America were burdened with great ice sheets, while at the same time a tropical rain forest covered North America, Europe, and China. At various other times, there was sufficient vegetation in Alaska and Antarctica for coal deposits to have resulted, and so currently frigid a place as Baffin Bay was a desert.

Wegener and his followers examined what was known about the climates of the distant past, and tried to arrange the continents in each geologic period so that the glaciers were near the poles and the hot regions were near the equator. The results, in general, were quite convincing, and in some cases startlingly so: deposits of glacial debris and fossil remains of certain distinctive plant species follow each other in the same succession in Argentina, Brazil, South Africa, Antarctica, India, and Australia, for example. A recent discovery of this kind was the identification of a skull of the reptile Lystrosaurus in a sandstone layer in the Alexandra mountain range of Antarctica. This creature, which was about three feet long, flourished long ago in Africa. It is as unlikely that Lystrosaurus swam the 2,700 mi between Africa and Antarctica as it is that a land bridge this long connected them, only to vanish completely later on.

Ex. 1. Here are the answers to some questions on the text. What are the questions?

1. At some time in the past the continents were joined together in one or to giant supercontinents. (In how many supercontinents?)

2. The first comprehensive theory of continental drift was proposed early in this century by the German meteorologist Alfred Wegener. (When? By whom?)

3. Alfred Wegener based his theory on biologic and geologic evidence. (On what?)

4. Wegener suggested that originally the continents were all part of a huge landmass that was surrounded by a single ocean. (What?)

5. At one time South Africa, India, Australia and part of South America were burdened with great ice sheets. (With what?).

6. Wegener and his followers examined what was known about the climates of the distant past. (What?).

7. Wegener tried to arrange the continents in each geologic period. (What?)

8. The results were quite convincing. (What?)

9. In some cases the results were startlingly so: deposits of glacial debris and fossil remains of certain distinctive plant species follow each other in the same succession in Argentina, Brazil, South Africa, Antarctica, India and Australia. (In what countries?).

Ex. 2. Complete the sentences.

1. A casual glance at a map of the world suggests … .

2. The fit between North and South America, Africa, Greenland and Western Europe is … .

3. Merely matching up outlines of continents is not sufficient evidence that … .

4. The standard explanation for similarity of patterns of early life around the world was … .

5. The oceans were then separated … .

6. Wegener called a huge landmass … .

7. Pangaea was surrounded by … .

8. Then Pangaea began to break up and continents began to … .

Ex. 3. Expand on the following:

1. The continents have migrated around the globe.

2. Wegener proposed the first really comprehensive theory of continental drift.

3. According to Wegener originally the continents were all part of a huge landmass.

4. The glaciers were near the poles and the hot regions were near the equator.

Ex. 4. Discuss the following.

1. Wegener suggestion.

2. The evidence for continental drift.

3. The weakness of Wegener’s idea.

4. The attitude of geologists to the idea of continental drift.

Text C

Task. Scan the text and say by what the separation of Pangaea into two supercontinents is supported. Explain why continental drift was not widely accepted until very recently.