How Continents Move Study Guide (page 2)
One of the most important discoveries of science in the last fifty years is that the Earth's continents move. Not only the continents, but the oceans move, too. In fact, oceans grow and shrink over tens to hundreds of millions of years. How this all works is described by the grand theory of plate tectonics.
In 1912, German scientist Alfred Wegener proposed that continents could move around—that they could "drift." One of Wegener's clues to the movement was the fact that the east coast of South America could fit into the lower half of the west coast of Africa, almost as if they were two pieces of a puzzle that once were joined together.
Wegener also pointed to evidence in South America, Africa, India, and Australia of the previous existence of ice sheets at about the same time, 300 million years ago. This made no sense with the continents in their present positions, because some of these sites are at today's equator. In Wegener's day, his theory was dismissed because no one could think of any mechanism for how objects as gigantic as continents could move across the earth's surface.
In the 1960s, new lines of evidence bore fruit for the idea that continents and oceans can radically shift. The key measurements came from geological research ships that drilled into the floor of the Atlantic Ocean. Scientists who participated in this Ocean Drilling Project hoisted back up to the ship cylindrical cores of the ocean's rocky floor, part of the oceanic crust. Then they analyzed each core for the direction of magnetism in the rock.
In Lesson 4, we saw that Earth's magnetic field, in the geological past, periodically underwent reversals, in which the south magnetic pole became the north magnetic pole, and the north magnetic pole became the south magnetic pole. When magma comes up from deep in the earth—for example, when a volcano erupts— the liquid lava is not magnetized. Then, as the lava cools into rocky minerals, the rock takes on a slight magnetization. This magnetization records the direction of the earth's magnetic field at the time of cooling when the rock was created.
In a line approximately running down the middle of the Atlantic Ocean, from north to south, is a great underwater mountain chain of volcanoes, the Mid-Atlantic Ridge. The ships measured the magnet ism of the volcanic rocks that form on the sea floor on either side of this Mid-Atlantic Ridge. Some rocks showed a magnetic field similar to that of today's Earth. Other rocks showed a reversed field. You can see a diagram of what was found in Figure 5.1.
This diagram of a cross-section of the Mid-Atlantic Ridge shows how the patterns of the magnetic field directions (normal or reversed) are symmetrical on both sides of the ocean floor, which has been spreading away from the ridge for nearly 200 million years. Note how magma (molten rock) comes up from the asthenosphere to create new ocean crust and lithosphere, and how the lithosphere gets thicker as it moves away from the hot ridge.
When maps were drawn of the ancient magnetic field directions in the ocean floor, the maps showed symmetrical stripes on the two sides of the Mid-Atlantic Ridge. Back in deep time, when molten magma emerged from the volcanic ridge, the rock that formed as part of the new ocean crust recorded the direction of the magnetic field. This rock then moved away from the ridge in both directions as new magma erupted from the ridge. That new magma, as it in turn emerged a few million years later and cooled, experienced a reversed magnetic field and so it recorded that direction of Earth's magnetism.
The process continued for many tens of millions of years as Earth's magnetic field was continuously recorded in the stripes in the rocks of the Atlantic Ocean's floor. The resulting pattern has been compared to a tape recorder whose tape has symmetrical patterns on both sides of the Mid-Atlantic Ridge.
Obviously, material had been emerging from and spreading away from the ridge. This was the great discovery of seafloor spreading. The ocean's floor was growing, which meant that the Atlantic Ocean was widening. Earthquakes along the mid-ocean ridges confirmed that the ridges were spreading.
The molten material that flows upward at an ocean ridge to form new ocean crust (which is part of the lithosphere) comes from the hotter, deeper asthenosphere. When ductile asthenosphere cools enough to behave like an elastic solid, it becomes lithosphere. The new lithosphere on both sides of the ridge spreads away from the ridge. In fact, the hot ridge cools and contracts as it spreads the emerging material outward to the sides. As it cools, it becomes denser and floats deeper than the younger, less dense lithosphere at the ridge. The depth of this slab of lithosphere increases as it cools during its move away from the continuously forming ridge.
Other examples of spreading ridges across Earth's oceans include a zone between Antarctica and Australia, Africa and India, and a faster spreading rise in the eastern Pacific Ocean. How fast do the oceans spread? That depends on the location of the spreading zones.
The spreading rate of the Atlantic Ocean is 2 to 4 centimeters per year (about 1 to 2 inches per year). That won't affect the cost of an airplane ticket to Europe. The spreading rate in the eastern Pacific Ocean is much faster, on the average about 10 centimeters per year (or 4 inches per year). These rates are far less than snail paces. But consider the rates as operating over tens of millions of years. South America, Africa, and Antarctica were all joined as recently as about 200 million years ago.
If the Pacific Ocean has an ocean ridge with spreading sea floor and so does the Atlantic, then something major is amiss. All the oceans cannot be growing and also moving the continents around. North America cannot be moving westward and eastward at the same time.
It turns out that the Pacific Ocean is not growing. In fact, the Pacific Ocean is shrinking as the Atlantic Ocean grows. This shrinking occurs despite the fact that the Pacific Ocean has a spreading ridge in its eastern portion. The key is that the Pacific Ocean, unlike the Atlantic Ocean, has what are called subduction zones at its far eastern and western sides.
Subduction zones are places where the ocean's floor dives downward and disappears back into Earth's deep mantle, remelting in the asthenosphere. In the Pacific Ocean, subduction zones occur both along the coast of South America and across Japan. Such places of downward diving of the ocean's floor are another important feature in the grand theory of plate tectonics. Study Figure 5.2 to see how a subduction zone works.
Because the earth is a constant size, it must be true overall that the net creation of ocean floor must balance its destruction in the subduction zones. What happens to the ocean's crust and lithosphere in the subduction zone? Recall from Lesson 4 that the boundary between the lithosphere and asthenosphere is usually about 100 kilometers. That's the depth at which the rock grows hot enough to become plastic, or able to flow like silly putty over very long time periods. The subducting lithosphere—called a subducting slab—goes downward and gets hotter and hotter. Eventually, it rejoins the material of the asthenosphere in the deep mantle.
What drives the subducting ocean slabs down ward? As noted, the moving slabs of lithosphere cool and thicken as they spread away from the hot mid-ocean ridges. Cooler rock is more dense. Eventually, it can become dense enough to start sinking downward. It's like when you are floating on your back on the surface of a pool or lake and you let out the air from your lungs. Your body becomes more dense, and you start to sink.
In the subduction zones, we find the deepest parts of the ocean. For example, at the Mariana Trench in the western Pacific Ocean near the island of Guam, the ocean is more than 11 kilometers deep, almost three times the average depth of the world's oceans. Such depths are created by the subducting slabs, because the ocean's floor is literally going down.
Because new ocean floor is continually being formed and then subducted, the ocean's floor has a limited lifetime. In fact, the average age of the oldest ocean floor is about 100 million years. The Atlantic Ocean, as we said earlier, is almost twice that age, but the Mid-Atlantic Ridge is a particularly slow spreading center.
We can now put the entire story together in the theory of plate tectonics. By the way, the word tectonics comes from the ancient Greek word tekton, for builder or car penter. Thus, the theory of plate tectonics is how the plates build the earth's surface.
What are these plates? The plates are the slabs of lithosphere that float on top of the asthenosphere, like rafts of plywood that completely cover a pool of water. Imagine now that the rafts of plywood have some blocks of foam in their middles or at the edges—those are the continents. Now also imagine that in some places, new wood is being added to the edges of the plywood rafts—those places are the mid-ocean ridges. Now imagine that at some edges, one sheet of plywood is curved downward and is diving underneath another— those are the subduction zones.
The plywood rafts are Earth's tectonic plates. About a half dozen or so are large ones, and quite a few are smaller ones. Note that sometimes, the plates are called continental plates, even though they include the portions of the plates that have huge portions of ocean as well.
The situation of the earth's plates has been likened to the shell of an egg, all cracked into zones. But you never actually see the egg inside. Similarly, with all the motions of the earth's plates described in the section on seafloor spreading and subduction zones, we never see directly into the earth. In the way that the eggshell covers the egg, the plates completely cover the earth's inside. But unlike the static, unmoving pieces of eggshell, the earth's plates, as we have seen, are dynamic. They grow and shrink, like the rafts of plywood imagined in the previous paragraph.
In the theory of plate tectonics, the key parts are the plates themselves. The ocean basin and continents (as parts of the plates) are along for the rides as portions of the motions of the plates. As the Atlantic Ocean grows because the sea floor is spreading at the Mid-Atlantic Ridge, North America moves westward because North America is on the same plate as the western half of the Atlantic Ocean.
Indeed, in the theory of plate tectonics, the edge of two of the plates runs down the north-south middle of the Atlantic Ocean. Thus, you can see that the edges of plates do not necessarily coincide with the edges of oceans or continents. Of course, in some places, there is coincidence between the edge of a plate and the edge of a continent. One example is the western coast of South America. But such a match is not always the case because the eastern edge of the United States is not the plate boundary, which is found in the middle of the Atlantic Ocean, as we have seen.
In the theory of plate tectonics, three different types of edges are possible where one plate meets another. The edges are called margins. We have already seen two different types of margins. The third type will be briefly noted to end this lesson. Here are the three types of margins:
|1.||Divergent margins. These are spreading centers, such as the Mid-Atlantic Ridge, which result in seafloor spreading. Though spreading centers occur in oceans when they are mature, they begin under continents. The material under continents can become so hot that the continents rupture and start to split. An ocean can be born. Some believe that the Red Sea might eventually become a new ocean, because it split about 30 million years ago. Or this divergent margin could just fizzle out as an ocean starts at some other divergent margin on Earth.|
|2.||Convergent margins. Two basic types of convergent margins exist. We have already seen one type: subduction zones. The second type will be described more in the next lesson. This second type is called the collision margin and results in the uplifting of great mountain ranges.|
|3.||Transform margins. In transform margins, plate edges are neither moving apart as in divergent margins nor moving together as in convergent margins. Instead, in transform margins, the two plates are sliding past each other. As they slide, they grind one against the other, causing earth quakes. The most famous transform margin is the San Andreas fault in California, which runs approximately north-south. The city of Los Angles lies on the west side of the transform margin and is moving north. The city of San Francisco lies on the east side of the margin and is moving south. In about ten million years, the two cities will be next to each other.|
Practice problems of this concept can be found at: How Continents Move Practice Questions
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