Lesson #8 (Plate Tectonics)
Worksheet #1 Continental Drift and Seafloor Spreading
In your notebook, please take notes on the following section of the class. Title
it: “Lesson #8: Plate Tectonics”
Continental Drift In the early part of the 20th century, a scientist named Alfred Wegener looked at the world map and noticed that some of the continents appeared as if they could go together like a puzzle. Wegener hypothesized that continents move around on Earth's surface and that they were once joined together as a single supercontinent called Pangea (Figure 6.5). He called his hypothesis continental drift.. However, although Wegener's idea eventually helped to form the theory of plate tectonics, the scientific community did not believe him. Wegener and his supporters collected a great deal of evidence for the continental drift hypothesis. Wegener discovered that 1) identical rocks of the same age could be found on both sides of the Atlantic Ocean. 2) Wegener also matched up mountain ranges that had the same rock types, structures, and ages, but that are now on opposite sides of the Atlantic Ocean. 3) Wegener even found evidence from ancient fossils (Figure 6.6) of the same species of extinct plants and animals in rocks of the same age, but on continents that are now widely separated. Wegener suggested that the continents could not have been in their current positions because the organisms would not have been able to travel across the oceans. The only explanation Wegener could think of to explain what he saw was that the continents at one time been joined together. But because he could not explain what possibly could cause the giant continents to move, his theory was not accepted by the scientist of his time. Analyzing the Seafloor for Evidence of Continental Drift Perhaps surprisingly, it was World War II that gave scientists the tools to find the mechanism (reason) for continental drift that had eluded (escaped) Wegener and his colleagues. During the war, battleships and submarines carried echo sounders to locate enemy submarines (Figure 6.8). However, during the war, the sound waves rarely encountered an enemy submarine, and so most of the sound waves ricocheted off the ocean bottom. Because of this, the battleships were able to make an accurate map of the ocean floor. What was astonishing, was that, whereas most people had thought that the ocean floor was completely flat, the maps showed something completely different. As we know now, there are lots of big mountain ranges Lesson #8 (Plate Tectonics)
Worksheet #1 that extend in a line across the deep oceans. And even more amazing, the mountain ranges are connected as if they were the seams on a baseball. These mountain ranges are named mid­ocean ridges. The mid­ocean ridges and the areas around them rise up high above the deep seafloor (Figure 6.9). Figure 6.9: A modern map of the eastern Pacific and Atlantic Oceans. Darker blue indicates deeper seas. A mid­ocean ridge can be seen running through the center of the Atlantic Ocean. Deep sea trenches are found along the west coast of Central and South America and in the mid­Atlantic east of the southern tip of South America. Isolated mountains and flat featureless regions can also be spotted. Another astonishing feature is the deep sea trenches that are found at the edges of some continents or in the sea near chains of active volcanoes. Scientists have known that the Earth’s magnetic field switches polarity every now and then. But when analyzing the seafloor, scientists found something very interesting. Scientists were surprised to discover that the normal and reversed magnetic polarity of the seafloor creates a pattern of magnetic Lesson #8 (Plate Tectonics)
Worksheet #1 stripes! There is one long stripe with normal polarity, next to one long stripe with reversed polarity and so on across the ocean bottom. Another amazing feature is that the stripes are form mirror images on either side of the mid­ocean ridges. The ridge crest is of normal polarity and there are two stripes of reversed polarity of roughly equal width on each side of the ridge. Further distant are roughly equal stripes of normal polarity, beyond that, roughly equal stripes of reversed polarity, and so on. The magnetic polarity maps also show that the magnetic stripes end abruptly at the edges of continents, which are sometimes lined by a deep sea trench (Figure 6.10). Figure 6.10: Scientists found that magnetic polarity in the seafloor was normal at mid­ocean ridges but reversed in symmetrical patterns away from the ridge center. This normal and reversed pattern continues across the seafloor. The scientists also used geologic dating techniques to find the ages of the rocks that were found with the different magnetic polarities. It turns out that the ages of the rocks increases equally and symmetrically on both sides of the ridge. Scientists also discovered that there are virtually no sediments on the seafloor at the axis, but the sediment layer increases in thickness in both directions away from the ridge axis. This was additional evidence that the youngest rocks are on the ridge axis and that the rocks are older with distance away from the ridge. The scientists were surprised to find that oldest seafloor is less than 180 million years old while the oldest continental crust is around 4 billion years old. They realized that some process was causing seafloor to be created and destroyed in a relatively short time. Explaining the Sea floor Spreading Hypothesis When the scientists noticed that the ages of the rocks increased the further away they were from the mid ocean ridge, they created the seafloor spreading hypothesis to explain it. They suggested that hot mantle from inside the Earth rises up toward the surface at mid­ocean ridges. This hot material is buoyant and causes the ridge to rise, which is one reason that mid­ocean ridges are higher than the rest of the seafloor. The hot magma at the ridge erupts as lava that forms new seafloor. When the lava cools, its magnetic rock crystals take on the current magnetic polarity. The magnetic polarity is locked into place when Lesson #8 (Plate Tectonics)
Worksheet #1 the lava solidifies and the magnetic crystals are trapped in position. As more lava erupts, it pushes the seafloor that is at the ridge horizontally away from ridge axis. This continues as the formation of new seafloor forces older seafloor to move horizontally away from the ridge axis. If the oceanic crust butts up against a continent, it will sink into a deep sea trench and be lost into the mantle. This theory gives a calculated and observed explanation for Wegener's drifting continents. Rather than drifting across the oceans, the continents ride on a conveyor belt of oceanic crust that takes them around the planet’s surface. Evidence for Seafloor Spreading causing the Americas and Eurasia to move One of the fundamental lines of evidence for continental drift is the way the coastlines of continents on both sides of the Atlantic Ocean fit together. So let’s look at how seafloor spreading moves continents in the Atlantic by looking more closely at figure 3 to the left. New oceanic crust is forming at the mid­ocean ridge that runs through the center of the Atlantic Ocean, which is called the Mid­Atlantic Ridge. So new seafloor is forming at the Mid­Atlantic Ridge, which is in opposite direction!
Answer the following questions in complete sentences and define the vocabulary terms in your notebook. Review Questions 1. What are 3 pieces of evidence that Alfred Wegener used to support his theory of Continental Drift. 2. What do scientists use to develop a map of the features of the seafloor. Lesson #8 (Plate Tectonics)
Worksheet #1 3. Why has no ocean crust been located that is older than about 180 million years when the oldest continental crust is about 4 billion years old? 4. Describe why continents move across the ocean floor as if they are on a conveyor belt rather than as if they are drifting as in Wegener's original idea. 5. If you were a paleontologist who studies fossils of very ancient life forms, where would be the best place to look for very old fossils: on land or in the oceans? Why? Critical Thinking Questions 1.
How were the technologies that were developed to fight World War II used by scientists for the development of the sea floor spreading hypothesis? 2.
Why is continental drift referred to as a hypothesis (or idea) and not a theory? 3.
In what two ways did magnetic data lead scientists to understand more about continental drift and plate tectonics? 4.
How does seafloor spreading provide a mechanism (explanation) for continental drift? 5.
Why was Wegener's continental drift idea rejected by the scientific community and why is it accepted today? 6.
How can the locations where ancient fossils are found be used as evidence for continental drift? 7.
Look at a world map. Besides the coast of west Africa and eastern South America, what are some other regions of the world that look as they could be closely fit together? Vocabulary continental drift magnetic polarity mid­ocean ridge seafloor spreading