Plate Tectonics

1. Plate Tectonics The theory of plate tectonics was first proposed by Alfred Wegener in 1910. Wegener noticed that the shorelines of the continents seemed to ‘fit together’ like the pieces of a giant jig saw puzzle. • Wegener’s theory stated: • The continents were once all together in one • place forming a supercontinent, Pangea. • The continents broke apart and drifted to their • present locations. Wegener’s theory was not taken seriously because no one could believe that things as large as continents could move and because Wegener could not propose a mechanism which could explain such motion.

2. Today plate tectonics and continental drift are accepted as facts. Why? Let’s see the evidence! First, there’s Wegener’s original observation. The shorelines of the continents do fit together like the pieces of a puzzle. But that could just be a coincidence. For a theory to be accepted as fact, there must be more.....................

3. If the continents were once together, we ought to be able to find similar kinds of rocks in places that were once together but have since drifted apart. If we believe that area A and area B were once together, then we should find similar rock types in these two locations even though today they are thousands of miles apart. Rocks are a lot like fingerprints. The mineral content and structure of rock outcrops often identify them as unique. Wegener himself investigated this possibility and found that, yes, he was able to correlate rock types from opposite shorelines. Correlating rock types is a second piece of evidence.

4. If we examined rocks from area A and found in them fossils that are also found in area B but nowhere else, this would be powerful evidence that these two areas were once together forming a single habitat. It is important to choose our fossils carefully. Finding fossils of organisms that lived all over the world would mean nothing. But if we could find the same fossils in areas A and B, fossils that exist nowhere else because they had a very limited range, then this is even more powerful evidence that areas A and B were once together. Correlating fossils from opposite shores is a third piece of evidence.

5. Sea Floor Spreading If the continents are moving apart then the seas between them must be getting wider. This is called sea floor spreading. How do we know that this is true? First, a series of under-water mountains called mid-ocean ridges is found throughout the world. These mountains are formed as new sea floor (basalt) is created from magma that wells up from the mantle below.

6. As new sea floor is added it pushes the old sea floor apart. Proof that this occurs can be found by looking at the age of the sea floor itself. New sea floor is found at the mid ocean ridge where material is constantly being added. As distance from the ridge in either direction increases, so does the age of the rock.

7. Because the sea floor is spreading away from the center, rocks which are equidistant but on opposite sides of the ridge are the same age. Rock B is the same age as rock D. Rock A is the same age as rock E. The oldest rocks are found at the edges of the continents.

8. More evidence to support sea floor spreading comes from the study of paleomagnetism, the magnetic properties of ancient rocks. When a magnetometer (a device that detects the magnetic properties of rocks) is towed across the ocean it indicates that there are alternating zones of rock with either normal or reversed polarity. In the green zones bits of magnetic iron in the rocks are pointing to where north is today. They act like countless little magnets trapped in the rock when it solidified from magma. But in the orange zones the bits of iron are pointing in the opposite direction. Why?

9. This is because the earths magnetic field flip-flops on average about every 250,000 years. North becomes south and south becomes north. The rocks of the sea floor record these reversals. In diagram ‘a’ the rock along the ridge emerges during a period of normal polarity. As it cools that polarity is locked into the rock. But as can be seen in diagrams ‘b’ and ‘c’, as new rock emerges along the ridge it pushes the old rock away. The new rock may record reversed or normal polarity depend- on conditions when it formed.

10. The result of these reversals can be seen as alternating bands on the sea floor. These could only have been produced if the sea floor was spreading apart as the earth’s magnetic field reversed itself over long periods of geologic time. So together, studies of the age of the sea floor and of paleomagnetism prove that sea floor spreading has occurred. This is the forth piece of evidence that continental drift is fact, not theory.

11. The Ultimate Proof This is the Laser Geodynamics Satellite (Lageos). It was launched in 1976 and was joined in 1992 by Laegos II. They orbit the earth at an altitude of 3,700 miles and carry no electronic equipment. Laser beams are bounced from one point on the earth, off the satellite, to a second point on the ground. Scientists can then measure the distance between the two points with great accuracy. Geo positioning satellites enable scientists to use GPS devices to make accurate measurements of positions on the earth. These systems show conclusively that the continents are still drifting at a rate of a few centimeters a year.

12. Let’s review the evidence for continental drift................................ • The continents seem to fit together like the pieces of a jig saw • puzzle. 2) Correlation of rocks from opposite shores. 3) Correlation of fossils from opposite shores. 4) Sea floor spreading..... a) The existance of mid ocean ridges (mountain ranges) b) Age of the sea floor..... c) Paleomagnetism 5) Direct satellite measurement of the drifting continents. Continental drift is not a theory. It’s a fact. Wegener was right! But why......?

13. How could continents, giant slabs of rock, possibly move? The key is to understand the internal structure of the earth We live on a very thin, cool crust which averages about 50 km thick under the con- tinents and 10 km under the oceans. Beneath the crust is the mantle. Rock in the mantle is very hot and is under high pressure. Rock here is described as plastic which means that, like silly putty, it can flow very slowly over inimaginably long periods of time.

14. Because temperature increases with depth, the inside of the earth is very hot. Rock near the core is heated and becomes less dense. It slowly rises while cooler rock nearer the surface is more dense and sinks. This forms convection currents just like those in our atmosphere. As these convection currents in the mantle circulate, they cause the continents above them to move. What Wegener had no way of knowing is that the force that is driving plate tectonics is convection currents in the mantle.

15. Let's sum up everything we've learned: 1) The earth's crust is divided into many pieces called tectonic plates which move around over long periods of time. 2) Evidence for crustal movement includes: a) The 'fit' of the continents. b) Correlation of rock types from opposite shores. c) Correlation of fossils from opposite shores. d) Age of the sea floor (youngest near mid-ocean ridges and oldest near the continents). e) Paleomagnetism: zones of oppositely magnetized rock on opposite sides of the mid ocean ridge. f) Direct satellite and GPS measurement of continental movement. 3) The force driving the plates are convection currents (density differences) in the earth's mantle.