Big History Project, adapted by Newsela staff
(1) Synopsis: Alfred Wegener produced evidence in 1912 that the continents are in motion. But, geologists rejected his ideas at first. Wegener had no explanation for what forces could move them. Almost 50 years later, Harry Hess confirmed Wegener’s ideas by using the evidence of seafloor spreading to explain what moved the continents.
 
Continental Drift
(2) By 1910, Wegener had noticed on a world map that the east coast of South America fits exactly against the west coast of Africa. It appeared almost as if they had once been joined. He found evidence that it had and, in 1915, published The Origin of Continents and Oceans. In it, he claimed that about 300 million years ago, the continents formed a single mass. He labeled it "Pangaea," a Greek word meaning “whole Earth.”
(3) Wegener was not the first to present the idea of continental drift, as he called it. But he beat everyone else in putting together extensive evidence from several different scientific approaches. He used fossil evidence, such as that of tropical plants found on the Arctic island of Spitzbergen. He found large geographic features that matched, like the Appalachian Mountains in the United States and the Scottish Highlands. He located rock layers called "strata" in South Africa that matched those in Brazil.
(4) Scientists of the time believed that land bridges had once connected the continents, and later sunk into the ocean. Wegener wanted to demolish that claim. He also disputed the theory that mountains formed like wrinkles on the skin of a drying apple. Instead, he claimed that mountains formed when the edges of drifting continents collided and crumpled. For instance, scientists now believe India hit Asia, creating the Himalayas
Seafloor Spreading
(5) During World War II, sounding gear produced new evidence of what the seafloor looked like. The gear, developed in the 1930s, bounced sound waves off the seafloor to determine its depth and features. It became known as Sound Navigation and Ranging (SONAR).
(6) Harry Hess, a geologist from Princeton University, was put in command of an attack transport ship. Hess, then in his late thirties, wanted to continue his scientific investigations even while at war. Ship commanders turned on sounding gear when approaching port or to help navigate a difficult landing. Hess, however, left his ship’s gear on all of the time. What Hess discovered was a big surprise. The bottom of the sea was not smooth as expected. It was, in fact, full of canyons, trenches, and volcanic sea mountains.
(7) By the 1950s, other researchers had found that a huge rift ran along the top of the Mid-Atlantic Ridge. Hess then realized that the Earth’s crust had been moving away on each side of oceanic ridges. The ridges ran down the Atlantic and Pacific oceans and were long and volcanically active. He published his theory in 1962, and it came to be called “seafloor spreading.”
(8) In the early 1960s, dating of ocean-core samples showed that the ocean floor was younger at the Mid-Atlantic Ridge. It became older and older in either direction. This confirmed that the seafloor was truly spreading. Further evidence came along by 1963. Geophysicists realized that Earth’s magnetic field had reversed polarity many times. Each reversal lasted fewer than 200,000 years. Rocks of the same age in the seafloor crust would have taken on the magnetic polarity at the time that that part of the crust formed. Sure enough, surveys of either side of the Mid-Atlantic Ridge found rocks with a pattern of alternating polarity stripes.
(9) By the 1970s, geologists had agreed to use the term “plate tectonics.” They had evidence that continents move. But, they'd also found evidence that so do whole "plates" of the Earth’s crust. A plate might include a continent, parts of a continent, or undersea portions of the crust. Wegener’s idea of continental drift had been taken a step further.
(10) Geologists today understand that the Earth’s surface, or crust, is broken up into eight to 12 large plates and 20 or so smaller ones. These plates move in different directions and at different speeds. Their sizes do not correspond to the landmasses on top of them. For instance, the North American plate is much larger than the North American continent; the plate extends from the western coast of North America to the mid-Atlantic. Iceland is split down the middle. It belongs to two different plates. The continents have come together into one large mass — and then split apart again — more than once. Over the last 500 million years, this may have happened as many as three times.
(11) The force that moves the plates is thought to be convection currents in the mantle under the Earth’s crust. The mantle is solid in the short term. But over longer geologic time, it does flow, though very slowly. Pockets of hot liquid magma ooze up along extensive mountain ridges deep under the water. One runs roughly north-south in the mid-Atlantic and another runs in the mid-Pacific.
(12) Along these ridges are found active volcanoes and hot-water vents. Through these vents pours very hot, mineral-rich water. Astonishing forms of life are supported by the nutrients in the water. These ecosystems are the only ones on Earth who survive in the absence of sunlight.
 
(13) Where the edges of the plates meet, several things may happen. If both plates carry continents, which are lighter than the ocean floor, they may clash head on, causing high mountains to rise. If one plate is heavier, it may go under the other, a process known as “subduction.” The material of the subducted plate returns to the mantle, recycling the Earth’s crust. Or the plates may move sideways, grinding against each other. This grinding produces cracks, or faults, in the plates, as along the California coast. In whatever form the plate edges meet, earthquakes take place.
(14) The European and North American plates are moving apart at the speed a fingernail grows. In a human lifetime, this amounts to about 2 meters (just over 6 feet). Millions of years in the future, parts of California and Mexico will probably drift off to become an island. Most of Africa is pushing northward toward Europe. Eventually it will squeeze out the Mediterranean Sea. When it reaches Europe it will cause high mountains to emerge along the whole southern coast of Europe. The eastern portion of Africa will split off at the Great Rift Valley and float off into the Indian Ocean. In slow geologic time, the Earth’s plates are always moving.
Fact or Fiction
On your paper number 1 – 10. Read each of the statements below. Beside each number put “Fact” if the statement is a fact. If the statement is untrue. Change each “Fiction” statement to make it true. For each “Fact” and “Fiction” statement, write the paragraph number where the correct statement is found.
1. Harry Hess produced evidence in 1912 that the continents are in motion.
2. Earth’s continents once formed one large land mass called Pangaea.
3. Harry Hess was the first scientist to provide evidence that the continents were drifting apart.
4. Matching coastlines, matching rock layers, and fossil similarities were used as evidence to support the theory of seafloor spreading.
5. Wegener claimed that mountains formed when the edges of drifting continents collided and crumpled.
6. Sounding gear used during World War II produced new evidence of what the seafloor looked like. The gear bounced sound waves off the ocean surface to determine its depth and features.
7. The dating of ocean-core samples at the Mid-Atlantic Ridge showed that the ridges became younger and younger in either direction.
8. Geologists today understand that the Earth’s surface, or crust, is broken up into eight to 12 large plates and 20 or so smaller ones.
9. The force that moves the plates is thought to be convection currents in the core under the Earth’s crust.
10. In areas where two plates meet, if one plate is heavier, it may go under the other, a process known as “subduction.”
Short Answer: If crust is added at the mid-ocean ridges, why isn’t Earth getting any larger?
Fact or Fiction
Answer Key
1. Fiction: Alfred Wegener produced evidence in 1912 that the continents are in motion. (P #1)
2. Fact: Earth’s continents once formed one large land mass called Pangaea. (P #2)
3. Fiction: Alfred Wegener was the first scientist to provide evidence that the continents were drifting apart. (P #3)
4. Fiction: Matching coastlines, matching rock layers, and fossil similarities were used as evidence to support the theory of
continental drift. (P #3)
5. Fact: Wegener claimed that mountains formed when the edges of drifting continents collided and crumpled. (P #4)
6. Fiction: Sounding gear used during World War II produced new evidence of what the seafloor looked like. The gear bounced sound waves off the ocean floor to determine its depth and features. (P #5)
7. Fiction: The dating of ocean-core samples at the Mid-Atlantic Ridge showed that the ridges became older and older in either direction. (P #8)
8. Fact: Geologists today understand that the Earth’s surface, or crust, is broken up into eight to 12 large plates and 20 or so
smaller ones. (P#10)
9. Fiction: The force that moves the plates is thought to be convection currents in the mantle under the Earth’s crust. (P #11)
10. Fact: In areas where two plates meet, if one plate is heavier, it may go under the other, a process known as “subduction.” (P #13)
Short Answer: If crust is added at the mid-ocean ridges, why isn’t Earth getting any larger? As crusts is added at the mid-ocean ridges, crust is being melted at subduction zones. The crust moving down into the mantle is heated and then melts.
Student Name ________________________________________ Class Period ______
Probable Passage: Inventors and Scientists: Alfred Wegener and Harry Hess
Key Terms
plate tectonics convection current mantle seafloor spreading subduction zone fault
continental drift continent fossils Pangaea Mid-Atlantic Ridges SONAR
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Separate the key terms into 3 categories.
In two or three sentences, predict what you think you will learn by reading this article.
Was your prediction correct or incorrect? Explain below.
(Complete this section AFTER you have read the article.)
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Inventors and Scientists: