Learning target: I can explain, with evidence from a passage, why continental drift was set aside in the 1920s and what seafloor evidence turned it into the theory of plate tectonics.
Read the passage once straight through. Read it a second time with a pencil: number the paragraphs 1 to 7 and underline every date and every name. Then answer the questions in complete sentences. When a question asks for evidence, quote or closely paraphrase the passage and give the paragraph number. Two questions need a calculation; show every step.
You will need: This worksheet, Pencil, Scrap paper for the timeline
[1]In 1596 a Flemish mapmaker, Abraham Ortelius, noticed while making an atlas that the coasts on the two sides of the Atlantic looked like the edges of a torn page. He guessed the Americas had been ripped away from Europe and Africa. For three centuries the fit stayed a curiosity. Then, on January 6, 1912, a German scientist named Alfred Wegener told a geology meeting in Frankfurt that the fit was real. The continents, he argued, had once been one landmass, which he named Pangaea, and had slowly drifted apart. His book, The Origin of Continents and Oceans, appeared in 1915.
[2]Wegener was trained in astronomy and worked on weather, not geology, but he gathered clues that other people had kept separate. Fossils of Mesosaurus, a small reptile of shallow water, lay in rocks of the same age in Brazil and southern Africa. The seed plant Glossopteris had left its leaves across South America, Africa, India, Australia and Antarctica. Old mountain belts in eastern North America had the same rock types, in the same order and of the same age, as belts in northwest Africa and in Scotland and Norway. And rocks in India, Africa, Australia and South America carried the scratches and rubble of ancient glaciers, even though those places are warm today. Put the southern lands back together near the South Pole, and all the clues fell into place.
[3]Most geologists were not convinced, and their objection was fair. Wegener could not say what force moved a continent. He suggested the pull of the tides and a slow slide away from the poles, but in 1924 the British physicist Harold Jeffreys showed those forces were far too weak. At a meeting in New York in 1926, many speakers rejected drift. Wegener died in November 1930 on an expedition across the Greenland ice sheet. His idea was not disproved. It was shelved, because an explanation without a mechanism could not be tested.
[4]In 1931 the British geologist Arthur Holmes proposed that heat inside Earth might drive very slow currents in the solid mantle, carrying the continents along. Few took the idea up, because nobody could see the mantle. The test came instead from the ocean floor. Echo sounders improved during the Second World War, and in the 1950s Marie Tharp, working with Bruce Heezen at Columbia University, plotted thousands of depth readings. In 1952 she found a deep valley running down the crest of the Mid-Atlantic Ridge, and their 1957 map showed the ridge winding midway between the continents, following the coasts.
[5]Earthquakes clustered along that valley, as if the crust were being pulled apart. In 1962 the American geologist Harry Hess published an explanation: new seafloor forms at the ridge, moves outward, and sinks back into the mantle at the deep trenches; Robert Dietz had named the process seafloor spreading in 1961. The idea made a sharp prediction. Seafloor rock should get older with distance from the ridge, by the same amount on both sides. In 1963 Fred Vine and Drummond Matthews proposed that magnetic stripes frozen into seafloor rock recorded spreading; by 1966, surveys showed the stripes formed mirror images on either side of ridges, as predicted.
[6]The direct test needed a ship that could drill through kilometers of water into the seafloor. The Glomar Challenger was built for the job in 1967-68. Between December 1968 and January 1969 it drilled 17 holes at 10 sites across the South Atlantic ridge. The oldest sediment resting on the volcanic rock at each site grew steadily older with distance from the ridge, on both sides, at a rate of about 2 centimeters a year. By the end of the 1960s geologists had joined ridges, trenches and earthquake belts into one picture of moving plates: the theory of plate tectonics.
[7]Today satellites measure plate motion directly, and the rates agree with those read from seafloor ages. Wegener's idea waited half a century, not because his evidence was weak, but because science asks two more things of an idea: a mechanism that could work, and a prediction that could fail. Once the seafloor supplied both, continental drift stopped being a hypothesis and became part of a theory.
Before you answer, draw a line across scrap paper and mark these dates from the passage: 1596, 1912, 1915, 1924, 1926, 1930, 1931, 1952, 1957, 1961, 1962, 1963, 1966, 1968. Above each date write the person or event in three words or fewer. Most questions can be answered by finding the right date on your line and rereading the paragraph that holds it.
Imagine that echo sounders and drill ships had existed in 1915. Write a paragraph describing the single measurement Wegener could have ordered that would have done the most to convince the 1926 meeting, and explain what result he would have needed. Then write a second paragraph on whether the discovery of a mechanism (paragraph 4) or the success of a prediction (paragraph 6) mattered more, and defend your choice with the passage.