TEORI INGSUTAN BENUA DAN LEMPENG TEKTONIK

In the early twentieth century, a young German scientist named Alfred Wegener noticed that the African and South American coastlines on opposite sides of the Atlantic Ocean seemed to fit as if they were adjacent pieces of a jigsaw puzzle (Fig. 4.1). He realized that the apparent fit suggested that the continents had once been joined together and had later separated to form the Atlantic Ocean. Although Wegener was not the first to make this suggestion, he was the first scientist to pursue it with additional research. Studying world maps, Wegener realized that not only did the continents on both sides of the Atlantic fit together, but other continents, when moved properly, also fit like additional pieces of the same jigsaw puzzle (Fig. 4.2). On his map, all the continents together formed one supercontinent that he called Pangea, from the Greek root words for “all lands.” The northern part of Pangea is commonly called Laurasia and the southern part Gondwanaland. Wegener understood that the fit of the continents alone did not prove that a supercontinent had existed. Therefore, he began seeking additional evidence in 1910 and continued work on the project until his death in 1930. 

He mapped the locations of fossils of several species of animals and plants that could neither swim well nor fly. Fossils of the same species are now found in Antarctica, Africa, Australia, South America, and India. Why would the same species be found on continents separated by thousands of kilometers of ocean? When Wegener plotted the same fossil localities on his Pangea map, he found that they all lie in the same region of Pangea (Fig. 3.2). Wegener then suggested that each species had evolved and spread over that part of Pangea rather than mysteriously migrating across thousands of kilometers of open ocean.

Certain types of sedimentary rocks form in specific climatic zones. Glaciers and gravel deposited by glacial ice, for example, form in cold climates and are therefore found at high latitudes and high altitudes. Sandstones that preserve the structures of desert sand dunes form where deserts are common, near latitudes 30º north and south. Coral reefs and coal swamps thrive in near-equatorial tropical climates. Thus, each of these rocks reflect the latitudes at which they formed.

Wegener plotted 300-million-year-old glacial deposits on a map showing the modern distribution of continents (Fig. 3a). The area inside the line shows how large the ice mass would have been if the continents had been in their present positions. Notice that the glacier would have crossed the equator, and glacial deposits would have formed in tropical and subtropical zones. Figure 3b shows the same glacial deposits, and other climate-indicating rocks, plotted on Wegener’s Pangea map. Here the glaciers cluster neatly about the South Pole. The other rocks are also found in logical locations

 

 Figure 3.1 The African and South  American coastlines appear to fit together like adjacent pieces of a jigsaw puzzle. The pink areas show locations of distinctive rock types in South America and Africa

 

Figure 3.2 Geographic distributions of plant and animal fossils indicate that a single supercontinent, called Pangea, existed about 200 million years ago.

 

Wegener also noticed several instances in which an uncommon rock type or a distinctive sequence of rocks on one side of the Atlantic Ocean was identical to rocks on the other side. When he plotted the rocks on a Pangea map, those on the east side of the Atlantic were continuous with their counterparts on the west side (Fig. 3.1). For example, the deformed rocks of the Cape Fold belt of South Africa are similar to rocks found in the Buenos Aires province of Argentina. Plotted on a Pangea map, the two sequences of rocks appear as a single, continuous belt.

Wegener’s concept of a single supercontinent that broke apart to form the modern continents is called the theory of continental drift. The theory of continental drift was so revolutionary that skeptical scientists demanded an explanation of how continents could move. They wanted an explanation of the mechanism of continental drift. Wegener had concentrated on developing evidence that continents had drifted, not on how they moved. Finally, perhaps out of exasperation and as an afterthought to what he considered the important part of his theory, Wegener suggested two alternative possibilities: first, that continents plow their way through oceanic crust, shoving it aside as a ship plows through water; or second, that continental crust slides over oceanic crust. These suggestions turned out to be ill considered.

Physicists immediately proved that both of Wegener’s mechanisms were impossible. Oceanic crust is too strong for continents to plow through it. The attempt would be like trying to push a matchstick boat through heavy tar. The boat, or the continents, would break apart. Furthermore, frictional resistance is too great for continents to slide over oceanic crust These conclusions were quickly adopted by most scientists as proof that Wegener’s theory of continental drift was wrong. Notice, however, that the physicists’ calculations proved only that the mechanism proposed by Wegener was incorrect. They did not disprove, or even consider, the huge mass of evidence indicating that the continents were once joined together.During the 30-year period from about 1930 to 1960, a few geologists supported the continental drift theory, but most ignored it.

Much of the theory of continental drift is similar to plate tectonics theory. Modern evidence indicates that the continents were together much as Wegener had portrayed them in his map of Pangea. Today, most geologists recognize the importance of Wegener’s contributions.