On the Move: Continental Drift and Plate Tectonic By Jim Cornish (1) Have you noticed that South America and Africa look like they could fit together like pieces in a jigsaw puzzle? If you have, you are not alone. It was first observed by the English writer and scientist Sir Francis Bacon in the 17th century. In 1910, German meteorologist Alfred Wegener saw it too and developed a theory to explain how it might have been possible. He postulated that between 200 and 300 million years ago, all seven continents were one giant landmass he called Pangaea meaning all lands." He also suggested that Pangaea had split into pieces and had, over time, drifted apart to f o r m t h e continents as they exist today. (2) During Wagener s day, geologists believed the continents were once connected by land bridges and were stationary. The idea that they were once together, had somehow drifted apart and were still moving, sounded too far-fetched to be believed. (3) Wegener needed evidence to support his theory. Just because it looked like the continents fitted together was not proof that they -1-
Figure 2: The earth as it was 300 million years ago (left) and as it is today (right). The arrows, indicating motion, shows the continents are still moving- apart in some cases and colliding in others. Source: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter16/graphics/plate_tectonics.jpg actually had. Then one day while browsing in a library, he came across a well-known list of plant and animal fossils that were identical even though they were found on opposite sides of the south Atlantic Ocean. But it was not enough to persuade supporters of the land bridge connection to change their minds. (4) Despite the criticism directed at him and his drifting continent theory, Wegener did not give up. To him, the idea made sense. All he needed was more evidence and he found it when he turned his attention to the earth s mountain ranges. (5) In Wegener s day, most geologists believed mountains formed when the planet, once a molten ball, folded up on itself as it shrunk while cooling. Wegener thought his theory offered better explanation. After studying the Appalachian Mountains along the eastern seaboard of North America and the Scottish Highlands in Europe, he discovered that the types and layers of rock on both sides of the Atlantic matched. That would only be possible, Wegener argued, if North America and Europe were once -2-
together and had later drifted apart. (6) Wagener then went to the Arctic. He found fossils of plants and animals embedded in rocks millions of years old. Since tropical organisms could not possibly live in a frigid northern climate, he reasoned that at one time the Arctic landmasses must have been located further south where, like today, the climate is much warmer. Again, he thought of drifting continents as an explanation. were strong enough was discounted. In 1929, Arthur Holmes, a geologist and rare supporter of Wegener s theory, suggested that the earth s thick second layer, the mantle, had currents powerful enough to carry the continents sitting on top of them and compared the process to a giant conveyor belt. Holmes explanation won Wegener many more supporters but It would take thirty more years before this explanation was taken serious. (7) In 1915, Wegener published The Origin of Continents and Oceans, a book that outlined his theory and all the evidence he had collected to support his conclusions. Since his ideas challenged long held beliefs about the geological history of the earth, he found very few supporters. (8) Some of the criticisms directed at Wegener were well founded. While his evidence for drifting continents may have been compelling, he had failed to convincingly explain how it could have happened. He explanation that forces created by the earth s rotation (9) By the 1960's, sciences such as seismology (the study of earthquakes) and volcanology (the study of volcanoes) discovered much more about the nature of the earth s interior than was known in Wegener s time. Some of the discoveries could only be explained by continental drift and so interest in the theory was rekindled. (10) Today, a theory called plate tectonics explains how continental drift works. According to the theory, the earth s surface is fractured just like the shell of a cracked egg. These fractures -3-
outline large plate-like chunks of the earth crust. Number twelve in all they include the continents and/or chunks of the ocean floor. (See Figure 3) These plates are floating on the mantle- drifting apart 1-10 cm Figure 3: Plates of the earth s surface and the location of active volcanoes. Notice that the plate boundaries and most active volcanoes occur very near yearly in some each other. areas and colliding in others. Earthquakes and volcanoes occur more often along the edges of these plates than anywhere else on earth. Mountain ranges such as the Rockies of North America, the 2006, after ten years of measuring, these satellites proved what Wegener believed all along- Europe and North America are moving apart and about 1.5 centimetres per year. Himalayas of Asia and the Alps of Europe are forming where the (12) Although still called a theory and continents are colliding. still being supported by new evidence each year, continental drift has been (11) Proof of drifting continents is coming from special satellites orbiting accepted by virtually all geologists as scientific fact. the earth. Using global positioning technology even minute shifts in the earth s surface can be detected. In -4-
On the Move: Continental Drift and Plate Tectonic By Jim Cornish 1. Vocabulary: Match the definitions below with a word from the paragraph indicated in parenthesis. a. (1) supposed, assumed... b. (2) still, not moving... c. (3) an unproven idea... d. (4) something that supports a theory... e. (5) scientists who study rocks and earth s history... f. (6) a part of, inside... g (7) solution to a problem... h. (8) comments, usually bad ones... i. (9) renewed, revived... j. (10) split into pieces... 2. Using the information from paragraph eight, write a definition for each of the following. Seismologist vulcanologist -5-
Reading Comprehension 1. What evidence did Wegener present to support his theory of continental drift? 2. What was the early reaction to Wegener s theory? 3. How is the earth s surface similar to a cracked egg? 4. What modern technology is providing proof to support the theory of continental drift? Inferential 1. Why do you think the reaction to Wegener s theory gradually changed in favour of his theory? Opinion 1. What do you think of the theory of continental drift? By the Numbers Provide a numerical answer for each of the following. a. the distance North America and Europe move apart yearly... b. the year Wegener first developed his drifting continent theory.. c. the time long ago when the continents were one landmass... d. the year Wegener published his book on drifting continents... e. the year interest in Wegener s theory increased... -6-
Visual Literacy 1. What can you say about the location of active volcanoes and boundaries of the tectonic plates of the earth? 2. How do you think the ring of fire got its name? 3. How have most of the tectonic plates been named? -7-