Topics Covered in this Lecture:
Slideshow: Plate Tectonics
- The History of Continental Drift
- The Internal Structure of the Earth
- Is the Earth Made of Plates?
- Dance of the Continents
As all of us know, if you make cutouts of Africa and South America, and put them together, they appear to fit perfectly just like a jigsaw puzzle. On this basis, it seems likely that the continents used to be joined together and subsequently have drifted to their present positions. As we have mentioned several times previously, this is precisely what happened. However, for over 100 years, this theory was repeatedly rejected. Just as astronomers in the 18th century finally convinced the world that the earth was not the center of the Universe, so have geologists since the late 1960s finally convinced the world that the continents are moving. That story and more are the subjects of today's lecture.
The History of the Theory of Continental Drift
The theory of continental drift has been around for a long time. Our astronomer friend, Sir Francis Bacon, noted as early as 1620 that the continents seemed to fit together. His contemporary, Galileo Galilei (1564-1642) was even purported to say "Eppur si muove!" -- But it does move!
In 1756, the German Reverend Theodor Lilienthal noted that "the facing coasts of many countries, though separated by the seas, have a congruent shape, so that they would almost fit one another if they stood side by side." Even Benjamin Franklin (1706-1790), who discovered electricity with a kite and a key (among many notable accomplishments), once proposed that our Earth had a fluid core that buoyed up the continents on their rocky shell.
Other noted luminaries who cast their votes for continental drift were the French naturalist, George Buffon (1707-1788), and the German scientist and explorer, Alexander von Humboldt (1769-1859), after whom an ocean current, a college, and a town are named.
In 1858, an Italian geographer, Antonio Snider, drew maps of the first reconstruction of the continents as he thought they should appear prior to separation.
And let us not forget Edward Suess, an Austrian geologist living in the late nineteenth Century (ancestor of the great Dr. Suess?), who coined the name "Gondwanaland", a supercontinent that included all the continents of the southern hemisphere and India; and Laurasia, encompassing all the northern continents (Was his book called "Green Eggs and Gondwanaland?"...I wonder). From 1885 to 1909, he published a series of volumes related to the "theory of separation".
In 1908, Frank Taylor and Howard Baker lined up the mountain ranges on opposite sides of the Atlantic. The rock formations and minerals in the Caledonian mountains of northern Europe match up with the Appalachian mountains of the United States and Canada. Coincidence or continental drift?
The first comprehensive evidence was compiled by another German, Alfred Wegener, a meteorologist who assembled a strong case for the theory of continental drift in his book Die Enstehung der Kontinente und Ozeane, published in 1912, to explain climate changes over the past several hundred years. In this book, Wegener presented compelling arguments, based on paleontology, climatology, geography, and geology. The continents were joined as one great landmass, which he called Pangaea, meaning all lands.
Despite Wegener's best arguments, his theory suffered from one fatal flaw. "How," his colleagues asked, "could a rock solid continent plow through a stone ocean floor. Huh? Answer me that, Dr. Wegener!" Having been born a few thousand years too late, he could not resort to the "Atlas shrugged" mechanism for Earth movement (the "reason" the Greeks gave as the cause of earthquakes), and he was at loss for a plausible explanation As such, Wegener's claims were badly ridiculed, such that in 1930 he escaped to Greenland, where he died in a blizzard. Nonetheless, his ideas remained alive, and for the next 40 years the idea of continental drift was hotly debated.
How else could one explain the distribution of fossils across the continents? Fossils of the great Permian reptile Mesosaurus was found on both sides of the South Atlantic Ocean in the 19th century. No Tarzan when it came to swimming, Mesosaurus probably did not swim across the Atlantic, so paleontologists invented a land bridge to explain his presence on two continents. Still more baffling, British scientists discovered plant fossils only 400 miles from the South Pole -- now had did those get there?! Add to this bit of evidence, ancient coal forests in the Arctic; glacial deposits in the tropics; and desert sands underneath rain forests -- you get the picture.
How about the distribution of living animals? Suess and Wegener pointed out the presence of a land snail, found in Japan, Europe, and the east coast of North America (not the west coast). Did a bird carry it across two oceans, but forget to drop it in California? I think not! When Wegener put together his map of Pangaea and marked it with the distribution of this little snail, he found that the snail's world fit nicely into one closely connected circle. Apparently, the snail's homeland had been ripped apart by the movement of the continents; that is, if you believed that the continents moved.
Sometimes the most obvious things are the hardest to accept, especially if they don't fit our preconceived notion of the world. The theory of continental drift hit its low point in 1957, when the Encyclopedia Brittanica published their opinion that Wegener's Pangaea was "purely fanciful." Bob Ballard, a geophysicist and ocean explorer who discovered the wreck of the Titanic in 1985 and attended graduate school in the 1950s, recalls that "a college professor who taught that the continents moved risked his academic reputation." The Gaia hypothesis is probably held with similar regard today. (Damn the torpedoes! Full speed ahead!)
Yet, throughout the 1940s and 50s, a quiet revolution was occurring in the halls of geoscience. As a result of the development of echo sounders during World War I (to replace the hemp and wire "lead" lines that had been used by survey ships to that points), a very different picture of the ocean floor was emerging. A mountain ridge was discovered in the middle of the Atlantic, and guyots, those curious flat-topped undersea mountains, began to be discovered. How could these features be explained?
In the 1940s, underwater seismic exploration was invented. By throwing crates of dynamite into the ocean, and recording the sound waves that returned from the bottom, scientists could look at the layers of rock beneath the ocean floor. In 1947, two American geophysicists, Maurice Ewing and Bruce Heezen, discovered that the sediments on the ocean floor were far too young and not thick enough given the millions of years over which they should have been accumulating. Another riddle was sprung.
As undersea exploration continued through the 50s and 60s, the accumulated data of thousands of ship crossings began to reveal the true extent of "the mountain in the middle of the Atlantic." In actuality, the "mountain" turned out to be a 46,000-mile ridge that ran in one fashion or another through all the oceans of the world. This ridge came to be known collectively as the Mid-Oceanic Ridge. A different picture of the world was emerging.
Perhaps the most vital evidence came as a result of a World War II device used to detect German submarines. This device, called MAD (Magnetic Airborne Detector) was modified by oceanographers so that it could be towed behind a ship and measure the magnetic fields of the rocks on the sea floor. Though they intended to look for valuable minerals, what they oceanographers found was even more "valuable" and puzzling. Stretching out in a series of zebra-like bands from each side of the Mid-Ocean Ridge were a series of magnetic reversals, i.e. changes in the magnetic direction of the rocks. Some of these bands were directed towards the north (as the magnetic field normally aligns) and some were reversed (where the "north" end of a compass points south). Most interestingly (and puzzling), a comparison of bands on the east side of the ridge with bands on the west side of the ridge revealed that they were "mirror" images; that is, they were exact duplicates of each other extending from the ridge towards the continents.
We will examine this phenomenon in more detail in a few moments, but this symmetry in the magnetic stripes in the ocean floor was just too obvious to be ignored. As one scientist put it, "Earth is trying to tell us something." That something turned out to be sea-floor spreading, as proposed in 1960 by the Princeton geologist and ex-Naval commander Harry H. Hess. Hess reasoned that deep within the Earth, molten material, generated by the natural radioactive decay of rocks, circulated in convection cells, circular movements from the core to the crust and back again. According to his hypothesis, molten material was forced upward until it oozed out at the location of the oceanic ridges and then descended downwards as it cooled near the edges of the continents. As the ocean floor descended, it formed submarine trenches. This hypothesis gained the name of sea-floor spreading, named by another geologist, Richard Dietz, who formed a model of the process at the same time as Hess. Sea-floor spreading was pure speculation at the time Hess and Dietz proposed it. In fact, so uncertain was Hess of its acceptance by the scientific community that he originally called the hypothesis "geopoetry."
This "hypothesis" turned out to be just what another group of scientists needed to explain the magnetic stripe anomalies on the ocean floor. In 1963, two Cambridge geologists, Frederick Vine and Drummond Matthews, reasoned that the "oozing" of molten material at the oceanic ridges represented the source of new ocean floor. As this material cooled, the iron particles aligned themselves with the Earth's magnetic compass, creating a permanent record of the Earth's magnetic field. As magnetic reversals (changes in the direction of the "north" pole, i.e. magnetic north becomes magnetic south) were known to occur, this explanation of the magnetic patterns seemed most plausible. These data were also supported by observations of magnetic reversals in rocks in Europe and North America, as published by Keith Runcorn in the 1960s, who was a proponent of continental movement.
The idea of sea-floor spreading also predicts that the rocks nearest the ridge are younger than the rocks further away, and this is just what scientists found. On either side of oceanic ridges, the age of rocks increased with increasing distance from the ridge. In addition, the parallel bands of rocks on either side not only had the same magnetic properties, but they had the same age as well.
In 1968 and 1969, as part of the Deep Sea Drilling Project and the Ocean Drilling Program, funded by government agencies, deep-sea cores were obtained using the Glomar Challenger, which was designed by oil companies to look for oil deposits deep beneath the ocean floor. Their findings confirmed the age versus distance predictions of sea-floor spreading. No ocean crust older than 180 million years was found. Furthermore, fossils in the core samples from the ocean floor showed a step-by-step increase in age further and further from oceanic ridges. These samples also showed that the thickness of sediments increased with increasing distance from the ridges, further substantiating that the ocean floor was moving outward.
By the end of the 1960s, the theory of continental drift was gaining acceptance. It was clear that the sea floor was spreading outward from ridges of volcanic activity located generally in the middle of the oceanic basins. As the sea floor spread, it undoubtedly took the continents with them. However, the puzzle still remained for many scientists as to what happened to the sea floor at the edge of the continents. We knew it was moving, and we knew it was probably disappearing down submarine trenches, but then what?
The answer, as with many of the clues to continental drift, had been supplied as far back as 1935. At that time, a Japanese seismologist, Kyoo Wadati presented data indicating that earthquakes increased in depth from the Pacific Ocean to the interior of the Asian continent. Another geologist, Hugo Benioff, made the same observation after World War II, but couldn't come up with an explanation so the data were largely ignored.
The final clue finally came as a result of the worldwide ban on aboveground nuclear testing in 1963. As part of the treaty, 125 seismic stations were set up around the world to monitor any unusual nuclear explosions. By examining the seismic recordings, or seismographs, made during underground nuclear tests, scientists could visibly see that the ocean crust was descending in places where submarine trenches occurred. The fact that earthquakes were deeper towards the interior of continents on the side that trenches occurred corroborated the movement of ocean crust back into the interior of the Earth.
By this time the theory of continental drift was gaining many converts. In 1965, another Japanese geologist, Tuzo Wilson, coined the term "plates" to refer to the pieces of continent that moved around. Then, in 1967, two British geophysicists, Dan MacKenzie and R. L. Parker, and an American geophysicist, independently proposed the theory of plate tectonics, meaning plate "construction." The term tectonics is taken from a character in Homer's Iliad, the carpenter Tekton. Two months after Morgan, Xavier Le Pichon published a map indicating the probable locations of the plates.
The theory of plate tectonics was highly revolutionary. It unified Wegener's ideas about continental drift, first proposed in 1912, with Hess's concept of sea-floor spreading. By combining these ideas into one coherent model, all aspects of the formation of the continents, the history of their movements and the present-day structure of the continents and ocean basins could be explained. In fact, the theory of plate tectonics also provided a logical explanation for the occurrence and locations of major earthquakes. Finally, there could be no doubt. After more than 55 years, Wegener's ideas were finally accepted.
The theory of plate tectonics, as it is now known, united many fields of geology and set off a flurry of geophysical research that continues to this day. As seismic and oceanographic techniques are improved, our ability to measure tectonic processes improves. While the theory generally fits much of the available data, the mechanisms and their effects are still only poorly understood. As we probe deeper into our own Earth, and as we explore other planets and compare their geological histories, our understanding of these processes will be considerably improved.
The Interior of the Earth
Now that we have reviewed the historical development of the theory of plate tectonics, let's take a deeper look at the geophysical processes responsible for plate movements.
To begin, we will need to understand a little about the structure of the Earth. The Earth consists of several layers:
The core is made of iron and nickel and can be divided into two parts, a solid interior, the inner core, and a liquid exterior, the outer core. The core comprises slightly more than 15% of the Earth's volume and resides between 2900-5150 km deep (1800-3219 miles). The inner core (3,219 - 3,981 miles) is solid and independent of the mantle. Scientists speculate that the inner core is a solid single crystal of iron, but since no one has been there, it's hard to know for sure. The outer core (1,806 - 3,219 miles) is hot and circulates (much like the ocean) in giant convection cells driven by radioactive decay within the earth. Importantly, the circulation of "fluids" within the outer core create electromagnetic field that give the Earth its magnetic field.
Between the core and the mantle, geophysicists now recognize a layer known as the "D" layer, which rests at a depth of 2,700-2,890 kilometers (1,688 - 1,806 miles). All we know about this layer is that it appears to be chemically distinct from the lower mantle, based on seismic data. Stay tuned to your favorite geophysics research station for late-breaking news on the "D" layer.
The mantle is the region which extends from the outer core nearly all the way to the surface. It comprises about 85% of the Earth's volume. The composition of the mantle is like the rocks we find on Earth, but it undergoes slow convective mixing. While mixing may seem strange for a solid, think of old pane glass windows which get thicker at the bottom (check it out in an old house). The reason for this is because glass actually flows like a liquid (albeit very very slow). Another way to imagine the mantle is to think of toothpaste, which is pretty hard if you bang someone over the head with a tube of it. Still, a gentle squeeze at the end of the tube causes the toothpaste to flow. So it is with materials in the mantle.
The lower mantle is sometimes called the mesosphere. The upper mantle and middle mantle are often referred to as the lithosphere and the asthenosphere. The lithosphere actually sticks to the crust and is fragmented along the surface of the earth into the large thin plates we have been talking about. The asthenosphere is hotter and more plastic, like partially melted rubber, and is the source of lava for volcanoes.
The crust is where we live. The crust is like the thin film that forms on a cup of cooling hot chocolate. These materials are lighter, more brittle -- essentially they "float" on the mantle. One observation that should be very clear is that the crust is very thin! (No deep crust pizza here!) If the Earth was a basketball, the crust would occupy an area equivalent to a thin coat of paint! It is useful to think of the crust (and the continents) as being like icebergs; where the crust sticks up the most (i.e. mountains), it also sticks down the furthest. This balance between the weight of the continental and oceanic crust and the internal forces that cause them to float is known as the principle of isostasy. See your book for a more complete description of this principle.
As alluded to previously, there are two types of crust, the oceanic crust and the continental crust. Oceanic crust is much thinner and younger than continental crust, but it has the property of being heavier because it contains iron silicates along with magnesium. This composition of rocks is referred to as basalt, and the oceanic crust is said to be basaltic. On the other hand, the continental crust is thicker and older, and, being largely composed of aluminum silicates in addition to magnesium. This composition of rocks is more like granite, and the continents are said to be granitic.
Your book also describes how scientists deduce the internal structure of the Earth through a technique called seismic tomography. Briefly, by looking at the patterns of P-waves, or primary waves, and S-waves, or secondary, the internal structure of the Earth can be mapped. Note that P-waves are fast waves, and move like the waves through a slinky when you push one end. P-waves can move through liquids and solids, but their rate of speed depends on what kind of medium they are traveling through (i.e. liquid or solid). S-waves are slower waves, and move up and down in relation to the direction of travel like waves on the ocean, except that S-waves can't move through liquids . Read these pages in your book to get a better idea of the differences between P- and S- waves and how scientists use them to distinguish the characteristics of the mantle and core.
One final feature we should know is the Mohorovicic discontinuity, or Moho. This is the boundary at which the crust and the mantle meet, and it is characterized in seismograms as a place where seismic waves change speed rapidly. Knowledge of the Moho helps scientists to determine the depth of the crust at any given spot and, thus, helps them to understand how the crust forms.
Is the Earth Made of Plates?
Before we begin our study of the forces in the interior of the Earth that drive the movement of the continents and crustal plates, let's familiarize ourselves with where these plates are located and what they are named.
Tectonic plates, sometimes called lithospheric plates, are large, irregularly shaped slabs of oceanic and/or continental crust. Much like a flagstone sidewalk, plates come in many sizes. The largest plates, the Pacific and Antarctic Plate, measure thousands of miles across. Smaller plates, such as the Juan de Fuca Plate off the coast of Washington state, may be only a few hundred miles across.
Oceanic plates and continental plates differ in a fundamental way which gives rise to their "appearance" over the face of the planet. Oceanic plates, being composed of basalt, a relatively iron-rich and heavy rock, tend to sink deeper in the mantle. Continental plates, being composed of granite, which is relatively light, tend to "float" and rise above the oceanic plates. Continental plates also tend to be thicker (up to 100 km) to balance their height (i.e. mountains), whereas oceanic plates tend to be very thin (5 km).
Your book illustrates the major lithospheric plates of the world. Note that there are 13 plates named here. We will add another. The Juan de Fuca Plate occurs along the coast of the state of Washington and has been an area of intense research interest for its hydrothermal vents and possible energy sources. That makes 14 in all. While the names of these plates correspond to the continental regions familiar to us, the continents are just a small part of most plates. Note that the continents may "float" in the middle or off to the side of the plates on which they ride.
The location of the plate boundaries were first identified by plotting all the major earthquakes around the world between 1961-1967. This provides a surface map of the outlines of the major plates. Earthquakes occur along all three types of boundaries, not just where plates collide. This should be obvious if you think about California, where the Pacific Plate and the North American Plate grind against each other.
Because we are all so well trained as children to put together jigsaw puzzles, I'm going to ask you to memorize the names of all these plates. You will see a map of these plates with the names blanked out on your first exam. It is a favorite test question of mine and I require it because it reinforces our knowledge of geography; it instills a better understanding of geophysical processes within the Earth; and I find that students usually get all the answers right if I warn them ahead of time. So be warned!
Question: Can you think of any phenomenon that you have experienced that have the same appearance as plates?
Dance of the Continents
Before we take a look at how the continents moved around, let's look at where they moved from and where they are moving to. As mentioned previously, all of the continents of the world were once assembled into one giant supercontinent named Pangaea. (Quick, what was the name of the ocean that surrounded Pangaea?) As with all theories, scientists are always looking for different ways to apply them. Plate tectonics is no exception. Recent studies of the Earth's oldest rocks have now confirmed that the breakup of the continents didn't start with Pangaea. In fact, Pangaea appears to represent one stage in an endless succession of breakups and consolidations (much like a long-lasting love affair). Whereas a few years ago we were able to start with the continents as they were 200 million years ago (MYA), now we must push the clock back further and tell you what we know about the continents as much as 555 MYA. (Such is the ever-expanding story of science!)
In the pre-Pangaean world, approximately 555 MYA, six continents are recognized: Gondwana (composed of Africa, South America, India, Australia, and Antarctica), Baltica (Scandanavia), Laurussia (North America), Siberia, China, and Kazakhstania. It seems pretty incredible that five of our present day continents used to be one big landmass without distinction, and that Scandanavia was an island, for all intents and purposes. From the looks of it, what we know as the Soviet Union and China (on the same continent today) were pretty chopped up back then.
According to the interpretation of the data that comprise this model of the Paleozoic continents, all these land masses rested near the equator and there were no land masses at the North or South Pole. At 490.- 475 MYA, Baltica and Gondwana began to slide towards the east. Gondwana traveled through the South Pole and turned northward around 400 MYA, causing the north-pointing tips of South America and Africa to completely reverse and point south.
At about 310 MYA, the continents began to draw together, much like ice cubes in the whirlpool of a drain. Shortly thereafter, the continents were assembled as "Pangaea" at about 255 MYA. Scientists conjecture that this process of separation and conglomeration represents a 500-year cycle driven by heat in the interior of the Earth.
The mountains caused by this collision of the continents are still visible today, the most notable being the mountain range that contains Mount Everest, the highest mountain in the world. However, the real bits of evidence that Pangaea was not the first supercontinent came from ancient crustal rocks found in North America. Large chunks of crust, called cratons, are found imbedded in newer continental crust. These cratons may be as much as 3 billion years old. Another ancient crustal formation that makes up parts of Alaska is know as a terrane. Terranes are elongate structures occur along faults and are distinct from cratons.
At 225 MYA to present, the story becomes more familiar (or at least it has been told longer). Pangaea breaks into two continents, called Laurasia and Gondwanaland, and subsequently split up into the six continents we know today: North America, South America, Eurasia, Africa, Australia, and Antarctica. This latter breakup of the continents is the one that has been the most studied, and the one that has lent all of our evidence regarding the theory of plate tectonics. However, as scientific research advances and more evidence is gathered, further refinements to this "dance of the continents" is sure to come.
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