Geo-Joint: Continents Drifting Toward Plate Tectonics

Anybody with half a brain idly spinning a globe will notice that the coastlines of eastern South America and western Africa are pretty similar. And that’s just the most obvious coastline match-up, of which there are many. It has always seemed like a logical notion that the two continents were at one time connected, but then there’s that part about the whole Atlantic Ocean being between them, and how in the heck did that happen? The idea that the continents were once attached to one another and then somehow separated, was given a name by Alfred Wegener, a German geophysicist and meteorologist born in 1880. He called it “die Verschiebung der Kontinente,” German for “continental displacement.” The theory itself took on the name “continental drift.” He wrote a book in 1915 entitled Origin of Continents and Oceans, in which he pointed out the coastline patterns, and cited evidence for them having once been in contact. He found studies that showed some sedimentary rock formations in Brazil were the same as those found in Western Africa. Such similarities could also be demonstrated with the fossil records of these places. There had to have been a time when these particular shores were connected, and Wegener presumed that all the continents were once together. For this seminal mega-continent, he coined the term “Pangaea,” meaning “all land” or “all earth.”

Similar fossil finds on continents whose shores are now quite distant gave evidence of their lands having once been connected.

It seemed a good case, given the hard evidence he presented, but the theory lacked a driver. Somehow, the continents breaking off Pangaea had to be pushed by something. Wegener proposed a force he called “Polflucht,” or “pole flight,” in which the continents, once fractured, would be pushed away from the poles and toward the equator by some geologic force. Wegener was theorizing outside of his field of expertise. Those trained in the geological sciences were of a conservative mindset and received his book and theories of Polflucht with high skepticism. Wegener kept publishing new editions of his work, adding more circumstantial evidence all the time, including some showing likely connections between Greenland, where he had done meteorological research, and North America. It was to no avail in convincing the geologists. He was not alone, however, in his efforts. An American geologist, Frank Bursley Taylor, also subscribed to the idea of continental drift, and had done so as early as 1910. By the 1920s, the notion became known as the Taylor-Wegener Theory. Mainstream geologists didn’t oppose the concept of a once-single landmass, but struggled themselves with an explanation for the oceans between the continents. They proposed that subsidence created the ocean basins as we now see them. That didn’t explain away Wegener’s evidence, or the similar coastlines. The argument seemed to be stuck on the conundrum that the evidence seemed so solid, but the process so impossible.

Alfred Wegener, one of the early proponents of continental drift theory.

As with so many other breakthroughs in science, the development of more advanced tools and instrumentation allowed the investigation to move to the next level. In this case, that crucial progress was the improvement in radar sensing during World War II. The new technology, invaluable as a detector of incoming warplanes, was also a priceless research tool for sensing and mapping the ocean floor. This enhanced ability to image what lay under miles of water further revealed something found by German echo-sounding researchers in the 1920s—a long, sinuous ridge running down the middle of the Atlantic Ocean basin. With improved technology, this so-called Mid-Atlantic Ridge structure was seen to continue into the other ocean basins of the world. Finer detail showed there was a 50- to 75-mile-wide canyon running the central length of the ridge. These ridges were cracks in the Earth’s crust, where molten rock from deep below the surface was rising through the seafloor, cooling, and building subsurface mountains. It appeared as though the force of this upwelling could be pushing the two sides of the ridge apart. If this were true, it was a very slow process, fueled by a great many ridgeline eruptions through time.

Matching bands of color indicate similar seafloor age on opposite sides of the spreading ridges. Red is youngest, blue is oldest.

It was known, through research on land, that cooled volcanic magma contains tiny bits of iron which, when in a molten state, align with the magnetic poles of the earth. Different layers of lava, spanning huge periods of time, show opposite polar alignment. This was evidence of the mechanism of polar reversal, which occurs at random intervals. Further research on the mid-ocean ridges involved towing magnetometers back and forth across the ridges to read the magnetic signature of the seafloor. What resulted was a map showing bands of alternating positive and negative charge alignment lying on one side of the central canyon, which was mirrored on the other side. That is, as lava came to the surface during a given period of magnetic orientation, it cooled with its iron bits in one orientation. A subsequent era of magma flow would occur under a reversed polar charge. Its iron would orient oppositely, and in pushing up, split the earlier flow, forcing it both east and west. This process repeated for millions upon millions of years, resulting in a broad array of paired bands of like magnetic signature. At last, a mechanism for continental movement—a spreading center—had been discovered. It moves slowly, perhaps an inch a year on average, but over the course of millions of years, a broad ocean basin can be created. It is theorized that enormous upwelling convection cells in the earth’s magma may be the powerhouses driving the molten matter upward to the spreading centers, but mysteries remain regarding the exact details of the process.

How it works, simplified.

The cracks that were found to crawl all about the earth like seams on a baseball break the the world’s crust into seven major plates (and about seven more minor ones) whose boundaries are either separating, converging, or just sliding past one another. In this theory, appropriately called plate tectonics, the continental land masses float upon the denser, thinner oceanic crust. When converging plate boundaries are both made of continental crust, they collide and build up enormous mountains, such as the Indian and Eurasian plates are doing as they create the Himalaya. If one of the converging plate boundaries is oceanic crust, however, that heavier side will subduct, or dive under, the lighter continental material. And if both sides are oceanic crust, one side or the other will be similarly pushed down into the mantle. In any case, the subducting plate edge will re-melt as it descends deeper into the Earth.
So that’s the way the world recycles its face—bursting forth with new material, pushing it across the surface, and then eating itself in a complex of continuous conveyer-belt rotation. The continents are just along for the ride, as they get transported all over the place by some restless inner engine of our energetic planet. Wegener and Taylor didn’t have all the facts, but they had the right idea, and science kept on marching forward until the story made sense. Stay tuned for more answers to come.

Would you like to keep track of those drifting continents on a colorful wall map? Get yourself a copy of this highly informative National Geographic world map of the plates and all the directions they’re moving, available at Maps.com:

 

CLICK HERE TO VIEW THE MAP!


PHOTO CREDITS:

caption: Alfred Wegener, one of the early proponents of continental drift theory.

source: Wikimedia Commons: unknown (Public domain)

caption: Similar fossil finds on continents whose shores are now quite distant gave evidence of their lands having once been connected.

source: Wikimedia Commons: jmwatson@usgs.gov  (Public domain)

caption: Matching bands of color indicate similar seafloor age on opposite sides of the spreading ridges. Red is youngest, blue is oldest.

source: Wikimedia Commons: NOAA (Public domain)

caption: How it works, simplified.

source: Wikimedia Commons: W. Jacquelyne Kious and Robert I. Tilling – USGS (Public domain)

 

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