How Continents Move Study Guide (page 2)

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Updated on Sep 25, 2011

Subduction Zones

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.

Figure 5.2

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.

Continental Plates

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.

Figure 5.3

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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