Big History Project
- Introduction to Geology
- Gallery: Geology
- Alfred Wegener and Harry Hess
- Activity: What Do You Know? What Do You Ask?
- Eratosthenes of Cyrene
- Introduction to the Geologic Time Chart
- Activity: Claim Testing – Geology and the Earth’s Formation
- Principles of Geology
- The Universe Through a Pinhole: Hasan Ibn al-Haytham
- Activity: Lesson 4.3 DQ notebook
- Quiz: Our Solar System and Earth
A Meteorologist, a Geologist, and the Theory of Plate Tectonics
Alfred Wegener produced evidence in 1912 that the continents are in motion, but because he could not explain what forces could move them, geologists rejected his ideas. Almost 50 years later Harry Hess confirmed Wegener’s ideas by using the evidence of seafloor spreading to explain what moved continents.
Balloons and Arctic Air
Alfred Lothar Wegener was born in Berlin, the son of a Protestant pastor. He received a PhD in astronomy from the University of Berlin in 1904, but his real love was air balloons. He and his brother, Kurt, set the world’s record in April 1906 for the longest time spent aloft in a balloon — 52 hours.
Later that year Wegener joined an expedition to Greenland to track polar air circulation, which could be done with the help of air balloons. (As well, he had always dreamed of polar exploration.) In 1908 he began to teach at the University of Marburg, and in 1911 he co-wrote The Thermodynamics of the Atmosphere, a textbook that became popular; his fellow author, Vladimir Köppen, was a famous climatologist. Wegener married Köppen’s daughter, Else, two years later.
Wegener was making his mark as a meteorologist, or weatherman. Yet his mind seemed indifferent to the boundaries of academic disciplines. By 1910 he had noticed on a world map that the east coast of South America fits exactly against the west coat of Africa, as if they had once been joined. He looked for further evidence, found it, 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 that he labeled “Pangaea,” a Greek word meaning “whole earth.”
Wegener was not the first to present the idea of continental drift, as he called it, but he was the first to put 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-scale geographic features that matched, like the Appalachian Mountains in the United States and the Scottish Highlands, as well as rock strata in South Africa that matched those in Brazil. He argued against the claim that earlier land bridges between the continents had sunk. He also disputed the theory that mountains formed like wrinkles on the skin of a drying apple, claiming instead that mountains formed when the edges of drifting continents crumpled and folded.
Geologists reacted to Wegener’s ideas with widespread scorn. They knew that his ideas, if accurate, would shake the foundations of their discipline. Wegener was not even a geologist — who was he to overturn their field?
Besides, he couldn’t explain what force could be immense enough to cause the continents to plow through the Earth’s crust like an icebreaker cutting through frozen sheets. At a 1926 international conference in New York, many speakers were sarcastic to the point of insult; Wegener sat smoking his pipe, listening.
In 1924 Wegener accepted a professorship of meteorology and geophysics at the University of Graz in Austria. Six years later he led another expedition to Greenland, this time with government backing, where he would set up yearlong weather-monitoring equipment at three stations on the glacier.
Drifting ice delayed the expedition and the Arctic weather proved a great hardship. In November 1930 Wegener led several dogsled teams carrying supplies to his colleagues at the isolated inland station, which was under-provisioned. After celebrating his 50th birthday at the remote weather station, Wegener and his companion, Rasmus Villumsen, died on their return trip west to the coast.
The idea of continental drift circulated in scientific circles until World War II, when 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 happened that the command of one attack transport ship, the USS Cape Johnson, was given to Harry Hammond Hess, a geologist from Princeton University. Hess, then in his late thirties, wanted to continue his scientific investigations even while at war. So he left his ship’s sounding gear on all of the time, not just when approaching port or navigating a difficult landing.
What Hess discovered was a big surprise. The bottom of the sea was not smooth as expected, but full of canyons, trenches, and volcanic sea mountains. Ocean floor exploration continued, and by the 1950s other researchers had found that a huge rift ran along the top of the Mid-Atlantic Ridge. That enabled Hess to understand his ocean floor profiles in the Pacific. He realized that the Earth’s crust had been moving away on each side of oceanic ridges, down the Atlantic and Pacific oceans, that were long and volcanically active. He published his theory in History of Ocean Basins (1962), and it came to be called “seafloor spreading.”
In the early 1960s, dating of ocean-core samples showed that the ocean floor was younger at the Mid-Atlantic Ridge but progressively older in either direction, confirming the reality of seafloor spreading. Further evidence came along by 1963, as geophysicists realized that Earth’s magnetic field had reversed polarity many times, with each reversal lasting less than 200,000 years. Rocks of the same age in the seafloor crust would have taken on the magnetic polarity prevalent at the time that that part of the crust formed. Sure enough, surveys of either side of the Mid-Atlantic Ridge showed a symmetrical pattern of alternating polarity stripes. That clinched the argument for most geologists.
Unlike Wegener, Harry Hess lived to see his major theory confirmed and accepted. He helped to plan the U.S. space program and died of a heart attack on August 25, 1969, a month after the Apollo 11’s successful mission to bring the first humans to the surface of the Moon.
By the 1970s geologists had agreed to use the term “plate tectonics” for what has become the core paradigm of their discipline. They used the term “plates” because they had found evidence that not just continents move, but so do whole plates of the Earth’s crust. A plate might include a continent, parts of a continent, and/or undersea portions of the crust. Wegener’s idea of continental drift had been developed and refined.
Geologists today understand that the Earth’s surface, or crust, is broken up into 8 to 12 large plates and 20 or so smaller ones. These plates move in different directions and at different speeds and are not directly related to the landmasses on 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, belonging to two different plates.
Over the last 500 million years the continents have come together into one large mass, and then split apart again – possibly as many as three times. Scientists can only guess when the first plates formed and how they behaved further back than that.
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 flows very slowly over geologic timescales. Pockets of hot liquid magma ooze up along extensive mountain ridges deep under the water, one running roughly north-south in the mid-Atlantic and another in the mid-Pacific. Along these ridges are found active volcanoes and hydrothermal (hot-water) vents, also known as “black smokers.” Through these vents pours very hot, mineral-rich water that supports astonishing ecosystems, the only ones on Earth whose immediate energy source is not sunlight. It’s possible that these “vent communities” are where the first living organisms on Earth developed.
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; these fractures are called “transform plate boundaries.” In whatever form the plate edges meet, earthquakes take place; on a global map of earthquake zones, the outlines of the plates are clearly visible.
The European and North American plates are moving apart at the speed a fingernail grows, about two meters (just over six feet) in a human lifetime. 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 and will eventually squeeze out the Mediterranean Sea and 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 geologic time, the Earth’s plates are always moving.
By Cynthia Stokes Brown
For Further Discussion
Some people are surprised that Wegener’s ideas about continental drift were not immediately accepted. Do you think that explanations that can explain most, but not all, of a problem should be taken seriously?
Share your thoughts in the Questions Area below!
Bryson, Bill. A Short History of Nearly Everything. New York: Broadway Books, 2003.
Want to join the conversation?
- My first thought, in response to the prompt, was that "Yeah of course they should be taken seriously, it would be rude and inconsiderate to disregard an idea just because all the dots aren't connected." But then playing Devil's Advocate, I stopped and thought about how important it is that we don't take every idea seriously like that. Evidence is so important. It's one of the major ways we determine what's true or not. It would be nice if everyone helped support everyone else's ideas, but at the same time isn't it beautiful that there are people going out of there way to see if that ISN'T true? One of the things that I love about the scientific community is not only how supportive it is, but how it forces you to explain how you know what you know and there's always at least one person trying to argue against your idea JUST to make sure that your idea actually isn't crazy or rooted in delusion.
So I'm not sure. I think it's important to be supportive, but not coddle the scientific process. I hope we don't discourage people who challenge conventional theories but support them in their quest to find what we're all after in the scientific community. Truth.(13 votes)
- How did his evidence help prove his theory(6 votes)
- Is it possible for the Great Rift Valley, instead of moving apart, to move towards each other?(2 votes)
- I think explanations that can explain most, but not all, of a problem should be entertained until someone is able to confirm it as fact (or fiction). That's why we need people like Harry Hess and Armond Fizeau. Otherwise, we will fall victim to senseless claims by people who aren't qualified to practice in the scientific field.(1 vote)
- Initially, I would've thought that they of course should have taken Wegener's ideas seriously, but then it occurred to me that it maybe should be the other way around. Wegener only had an idea, a completely different thing than a theory. It was quite like a hypothesis. But if it only WAS a hypothesis, scientists should not have taken this seriously, but rather matter-of-factly should have asked questions, make him support his idea. Science is an amazing unit where you NEED to learn about it and ask questions, whether you like it or not, because that's how it works. And when Wegener finally understood that, he went on that expedition to see if he could make people see things his perspective.
In conclusion, I can't decide. I feel like there ISN'T right or wrong in this because people have the opportunity to agree or disagree, I just think that in this world, there are too many perspectives to deal with.
You can comment on this or reply with your own ideas or vote on this comment. This is just one out of many others.(1 vote)
- what are the evidences claiming the seafloor is spreading?(0 votes)
- The evidences of sea floor spreading is the lava cooling underneath the water and the sea is getting bigger(1 vote)
- if volcanoes are formed near tectonic plates what would happen if they move would they get bigger or smaller or no longer be a volcano?(0 votes)
- 3rd Paragraph: Is it simply a matter of randomness that the plates all come together to form pangeas occasionally throughout Earth's history? Or are there other forces at work that cause some kind of attraction between the continents?(0 votes)
- The truth is that the tectonic plates are always moving, and will eventually become another giant supercontinent. Most of it isn't random, but is happening at a slow rate.(2 votes)
- In the section titled
Plate Tectonics, why is the simple term (plate) explained but not the reasoning behind the more complicated term (tectonics)?(2 votes)
- You might notice the third last paragraph discusses the cause of plate motion (mantle convection), is this what you are referring to?(0 votes)
- Do animals eat rocks?(0 votes)
- Yes! Animals such as crocodiles and ostriches are known for swallowing rocks to help crush up food in their stomach and digest it!(1 vote)