Plate Tectonics Unit I: The Observations (3.5 pts)

Transcription

1 T. James Noyes, El Camino College Plate Tectonics Unit I: The Observations (Topic 11A-1) page 1 Name: Section: Plate Tectonics Unit I: The Observations (3.5 pts) Plate Tectonics Plate tectonics is the name given to our modern theory of how the Earth works. It helps us understand how and why earthquakes, volcanoes, mountains, trenches, islands, oil deposits, and much more occur where they do. The basic idea of plate tectonics is that the Earth is covered by a cool, thin, solid outer layer called the lithosphere which floats on a warmer, denser layer which can slowly flow called the mantle. (I like to describe the mantle as semi-solid: it is not solid because it flows and changes shape, but it does not flow as easily as a liquid. Think of something like toothpaste as an example of a semi-solid.) Beneath the mantle there is one more layer: the super-hot Earth s core which is at the center of the Earth. The lithosphere is broken into pieces called plates. The semi-solid rock down in the mantle moves due to heat it gets from the Earth s core. The motion of the mantle pushes the plates away from one another in some places and into one another in other places. Plates separating and colliding causes earthquakes, volcanoes, mountains, trenches, and so on. 1. (a) What is the outer layer of the Earth called? (b) What is the middle layer of the Earth called? (c) What is the deepest layer of the Earth called? 2. (a) Which layer of the Earth is cold and solid? (b) Which layer of the Earth warm and semi-solid? 3. What is a plate? 4. What makes the mantle move? 5. True or false? The mantle rubs against the bottom of plates, causing them to move.

2 T. James Noyes, El Camino College Plate Tectonics Unit I: The Observations (Topic 11A-1) page 2 The Lithosphere and the Crust The lithosphere is made up of two parts, an upper part and a lower part. The upper part of the lithosphere is called the crust. The lower part is made of solidified mantle rock. There two kinds of crust. Continental crust is made of lower-density, lightercolored granite. Oceanic crust is made of higher-density, darker-colored basalt. Thus, continental lithosphere has granite on top and solidified mantle rock on the bottom. Oceanic lithosphere has basalt on top and solidified mantle rock on the bottom. The bedrock (the rock beneath the sediments) of the continents is made of granite and the ocean floor is made of basalt, so continental lithosphere has a lower density than oceanic lithosphere. 6. What is the upper part of the Earth s lithosphere called? 7. What two kinds of rock is the crust made out of? 8. Suppose you have a piece of granite and a piece basalt in your hands. How could you tell which piece of rock is granite and which piece of rock is basalt? 9. Which kind of rock has a lower density, basalt or granite? 10. (a) Does continental lithosphere contain basalt or granite? (b) Does oceanic lithosphere contain basalt or granite? 11. (a) Which has a lower density, continental lithosphere or oceanic lithosphere? (b) Which has a lower density, oceanic lithosphere or the mantle? (Hint: What floats on top?)

3 T. James Noyes, El Camino College Plate Tectonics Unit I: The Observations (Topic 11A-1) page 3 The Observations support the Theory of Plate Tectonics A vast amount of evidence from both the continents and the ocean floor supports the idea that the continents move (continental drift), the ocean floor moves, and that oceans grow and shrink. There are three main ideas that a growing body of evidence began to support that led to the adoption of plate tectonics: continental drift, sea floor spreading at the mid-ocean ridge, and subduction at trenches. Continental drift is the idea that the continents move. Sea floor spreading is the idea that the ocean floor moves away from the mid-ocean ridge, splitting the ridge apart. Lava from inside the Earth comes up into the resulting rift (gap, hole) in the ocean floor to fill in the rift. When the lava comes into contact with cold ocean water, the lava cools and hardens into solid rock and becomes new ocean floor. Subduction is the idea that the ocean floor bends and goes downward into the Earth at trenches in the ocean. As the ocean floor goes down into the Earth, it warms up and melts back into the mantle. Thus, the ocean floor is destroyed at trenches. 12. (a) What is continental drift? (b) What is sea-floor spreading? (c) What is subduction? 13. Where is new ocean floor (new oceanic lithosphere) being created on the ocean floor: on the abyssal plains, at the mid-ocean ridge, or at trenches? 14. Where is ocean floor (oceanic lithosphere) being destroyed on the ocean floor: on the abyssal plains, at the mid-ocean ridge, or at trenches?

4 T. James Noyes, El Camino College Plate Tectonics Unit I: The Observations (Topic 11A-1) page 4 Evidence: The Matching of the Edges of the Continents and of Features along the Edges of the Continents If you examine the coasts of South America and Africa, you can see that they match rather nicely together; the match is even better between their continental shelves. The east coast of North America and northern Africa also match, as do other parts of the world. Not only do the shapes match, but fossils, mineral deposits (e.g., coal, diamonds), and mountain ranges and more! match along the edges of the continents as well. The matching of these features suggests that the continents were once connected to one another and therefore they must have moved to reach their present positions. If the continents were not connected in the past, there would be no reason for the features to match: it would simply be a coincidence. This is possible unlikely things do happen (e.g., someone wins the lottery) but when determining what we believe, we typically favor ideas that are more probable, not less likely. In other words, arguing that these features of the continents are likely to match by chance would be something like saying that if you choose 7 pieces out of jigsaw puzzle box, they are likely to fit together AND produce a nice picture. This is possible, but certainly not something that most of us would expect to happen. In short, it may seem difficult to imagine how or why a continent would move. However, it is also difficult to imagine that all the matching of shorelines, minerals, and fossils would happen chance. It seems like one of these two amazing things must be true if we are to explain what we observe (the evidence). 15. True or false? The edges of the continental shelves match together even better than the edges of the continents. 16. True or false? If one kind of mineral deposit (e.g., coal, diamonds) is found near the coast of one continent, the same mineral deposit is found near the coast of another continents where to two continents coastlines fit together ( match ).

5 T. James Noyes, El Camino College Plate Tectonics Unit I: The Observations (Topic 11A-1) page 5 The Fossil Evidence and Evidence for Climate Conditions on Continents in the Past The fossils in particular are interesting. Currently, the plants and animals on different continents are different from one another. They have evolved to adapt to the present environment (e.g., climate, soil, vegetation, predators, prey, bacteria and viruses that causes illnesses and disease). However, land animal and plant fossils are the same on some continents (e.g., South America and Africa) until a particular point in time, when they begin to start showing evolution in different directions. This suggests that their environment began to change and that they were no long able to interbreed, presumably due the separation of the continents. We also find remains of tropical plants and tropical animals like coral reefs near the Poles, and signs of erosion by glaciers (huge sheets of ice) in the tropics during the same time periods. Fossils of tropical animals at the Poles can be explained if the continent they are on was once near the Equator and moved to the Poles. 17. True or false? We often find the identical species on different continents. For example, chimpanzees, elephants, kangaroos, tigers, zebras are found in the wild (not in zoos) on more than one continent. 18. True or false? Fossil remains of the same species of plants and animals have been found on different continents. For example, fossils of the same species have been found in both South America and Africa. 19. True or false? Fossils of tropical animals and plants have been found at the Poles.

6 T. James Noyes, El Camino College Plate Tectonics Unit I: The Observations (Topic 11A-1) page 6 Evidence from the Ocean Floor Observations from the ocean floor are crucial to understanding how and why the continents move. As we discussed earlier, most ocean trenches lie close to land: either continents or islands. In addition, earthquakes and volcanoes occur near trenches. If the trench occurs next to a continent, the continent has volcanic mountains near the coast (like Mt. St. Helens in Washington, Mt. Fuji in Japan, the Andes in South America, Popocatepetl near Mexico City, Krakatoa near the Indonesian islands of Java and Sumatra, etc.), and if a trench occurs next to an island chain, then the islands are giant volcanoes (e.g., Mariana Islands, Aleutian Islands in Alaska). The mid-ocean ridge is also a site of volcanic activity and earthquakes. In addition, several very different lines of evidence (kinds of sediments, sediment thickness, radiometric dating, seafloor magnetism) all suggest that the ocean floor is very young close to the mid-ocean ridge and gets older and older the farther the ocean floor is from the mid-ocean ridge.

7 T. James Noyes, El Camino College Plate Tectonics Unit I: The Observations (Topic 11A-1) page 7 For example, the layer of sediments on top of the mid-ocean ridge is thinner than the layer of sediments on the abyssal plains on either side of the mid-ocean ridge. This makes sense if new ocean floor is being created at the mid-ocean ridge and accumulates more and more sediments as it moves farther and farther away from the mid-ocean ridge over time. In addition, on abyssal plains a thick layer of calcareous ooze can be found beneath a layer of red clay on the ocean floor in some places. Calcareous ooze accumulates in shallower water, so it should not be found deep on the abyssal plains, but it can accumulate at the mid-ocean ridge and then move to the abyssal plains as the plate it rests on move away from the mid-ocean ridge over time. Note: A young rock is made of material that recently became solid rock. In the case of the volcanic rocks of the sea floor, the sea floor is young if it cooled from lava into solid rock recently. Of course, when geologists use the word recently, they could be discussing that last few millions of years.

8 T. James Noyes, El Camino College Plate Tectonics Unit I: The Observations (Topic 11A-1) page True or false? Ocean trenches are found next to land, either continents or island chains. 21. Where are earthquakes common on the ocean floor: on the abyssal plains, on continental slopes, at the mid-ocean ridge, and/or at trenches? 22. Where are volcanoes common in the ocean or near the ocean: on the abyssal plains, on continental shelves, near the mid-ocean ridge, and/or near trenches? 23. True or false? The layer of sediments on top of the mid-ocean ridge is thicker than the layer of sediments on abyssal plains. In other words, there are more sediments on top of a spot on the mid-ocean ridge than a spot on the abyssal plains. 24. True or false? The abyssal plains are covered by red clay (abyssal clay). In some places, the calcareous ooze is found beneath the layer of red clay even though calcareous ooze only builds up in shallower (less deep) water. 25. Where is younger rock (oceanic lithosphere) found, close to the mid-ocean ridge or far from the mid-ocean ridge?

9 T. James Noyes, El Camino College Plate Tectonics Unit I: The Observations (Topic 11A-1) page 9 Evidence from Rock Magnetism Evidence from rock magnetism provided some of the crucial evidence that made the majority of scientists switch from earlier theories about the Earth to the modern theory of the plate tectonics in the 1960s. When lava cools into solid rock, magnetite particles (little iron-rich minerals) in the lava orient themselves with the Earth s magnetic field. (They behave like little compass needles.) Thus, the rock records information about the direction and strength of the Earth s magnetic field at the time when the lava cooled and hardened into solid rock. Studies of volcanic lava flows on land showed scientists that the Earth s magnetic north pole changes location with time. Scientists monitor the magnetic north pole today. It shifts a couple hundred feet each year. In addition, sometimes the magnetism in rocks points towards places closer to the South Pole: in other words, sometimes the magnetic north pole flips. This happens every million years or so. In fact, the Earth s magnetic field has been weakening for the last few centuries and we are due for a flip: We may actually experience a flip over the next few centuries. The Earth s magnetic field helps protect us from the charged particles (radiation) of the solar wind from the Sun which not only affect background radiation, but disrupt electromagnetic networks (e.g., radio, cell phones, electric power grids), so this could cause some surmountable but expensive problems for future generations. By comparing results from volcanoes on different continents, scientists found that the continents must have shifted location, in agreement with the fossil evidence and other kinds of evidence. This was the only way to get their data from different continents to make sense: either there were multiple magnetic north poles or the continents were moving. Observations of rock magnetism from the ocean floor were the straw-that-broke-the-camel sback, leading scientists to come to a consensus that the theory of plate tectonics is the best explanation for how the Earth works. The rocks of the ocean floor are magnetized pointing north and south in long stripes that run parallel to the mid-ocean ridge and are a mirror images of one another across the ridge (their magnetism points in the same direction and they have the same thickness). This suggests that rocks on either side of the ridge formed at the same time (because they were influenced by the same magnetic field) and then moved outward from the ridge together at the same speed. In addition, the thickness of the stripes on the ocean floor is proportional to the time between the flips of the Earth s magnetic field determined from

10 T. James Noyes, El Camino College Plate Tectonics Unit I: The Observations (Topic 11A-1) page 10 volcanoes on land. In other words, the more time that has passes, the thicker the stripe is, just what you would expect if rock is slowly moving away from the mid-ocean ridge and more rock is being added at the mid-ocean ridge. It is hard to think of another logical explanation. 26. True or false? The Earth s magnetic north pole is located at the (geographic) North Pole. 27. True or false? The Earth s magnetic field flips. In other words, in some time periods the Earth s magnetic north pole is located close to the (geographic) North Pole, and in other time periods the Earth s magnetic north pole is located close to the (geographic) South Pole. 28. True or false? The magnetism of rocks from volcanoes on land that erupted in similar time periods points towards different locations. However, they would all point to the same location if the continents were in different positions in other words, if the continents have moved. 29. True or false? The magnetism of the rocks of the sea floor on one side of the mid-ocean ridge is completely different from the magnetism of the rocks of the sea floor on the other side of the mid-ocean ridge. In other words, their magnetism patterns are completely different.

11 T. James Noyes, El Camino College Plate Tectonics Unit I: The Observations (Topic 11A-1) page 11 The Basics of the Theory of Plate Tectonics The theory of plate tectonics provides the following explanation for the observations discussed previously. According to the theory, the following is occurring: At the mid-ocean ridges, the plates are separating, pulled apart by the motion of convections cells of semi-solid mantle rock beneath the plates. Lava then comes up to fill in the resulting gap and cools into solid rock when it comes into contact with cold ocean water, becoming new, young ocean floor. The ocean floor cools more as it ages (making it contract) and more and more sediments pile on top of it, so it becomes more and more dense with time. As the ocean floor moves away from the mid-ocean ridge (sea-floor spreading), it either pushes a continent, making the continent move (continental drift), or runs into another plate, leading to earthquakes. When two plates collides, both plates cannot occupy the same location, so the plate with the higher density dives into the interior of the Earth, making the sea-floor deeper, creating a trench. (We say that a plate diving into the Earth subducts.) The diving plate begins to melt in the hot mantle, creating hot, low-density magma which works its way back towards the surface, creating volcanoes. 30. Is the ocean floor coming together or moving apart at mid-ocean ridges? 31. Why are there volcanoes at the mid-ocean ridge? 32. Are plates coming together or moving apart at trenches?

12 T. James Noyes, El Camino College Plate Tectonics Unit I: The Observations (Topic 11A-1) page What is the ocean floor doing at trenches? 34. What happens to subducting ocean floor as it dives deeper and deeper into the hot mantle? 35. Why are there volcanoes near trenches?