GLY 108 – Plate Tectonics: The Active Earth Lab 7: Plate Tectonics/Earth History Objective: To understand and identify how the theory of plate tectonics is expressed in the Earth’s topographic features. Introduction: Plate tectonics is probably the most revolutionary concept of geology and has become the central framework to view all of earth’s processes. It is a relatively new theory that gained acceptance in the 1960s because it explains and ties together so many processes on and within the earth. Supporting evidence for this theory is found in: • The fit of the continents, particularly South America and Africa: If you consider the continental margins, the shallow offshore regions underlain by continental crust, as part of the true continents then the fit is even closer. • Similarity in rock types across continents: For example, you can find similar evidence of glaciers, including glacial sediments and landforms in both South America and Africa. There is also parallel evidence of past mountain building in North America, the British Isles, and Scandinavia. However, landforms and mountains are not found on the sea floor between these continents. • Fossil evidence: The best evidence again comes from the Southern Hemisphere, where the fossil Lystrosaurus is found on southern continents. This animal did not swim well enough to cross-oceans, so the continents must have been joined at one time. Even better fossil evidence is the Glossopteris plant, found on southern continents. This plant required a similar climate on all continents, and that climate is not found there today. http://fossil.wikia.com/wiki/Lystrosaurus • Age of the Seafloor: Advances in technology made age dating of the seafloor possible, and a marked trend was found that corresponds to the mid-ocean ridges, the sites of volcanism. The age of the seafloor gets progressively younger toward the ridges. Older seafloor occurs around trenches where earthquakes and volcanism seem to indicate that the crust is sinking, or subducting, back into the interior of the earth. • Paleomagnetic studies: These studies show that the earth’s magnetic field has varied throughout geologic history. Rocks with iron will crystallize with the iron crystals pointing to magnetic north. Studies of old oceanic rocks show how the earth’s magnetic polarity has switched over time. Further, there are symmetrical records of past reversals in polarity on both sides of the mid-ocean ridges. The conceptual framework of plate tectonics is that the earth's crust is broken into plates that move in relation to one another. These plates are slowly drifting across the surface of the globe, driven by convection currents within the mantle deep below. Their shifting accounts for the major geologic activity that occurs at plate borders; this activity includes the creation of oceans, continents, mountains, volcanoes, and earthquakes. The rigid oceanic and continental crusts and the uppermost part of the mantle, jointly called the lithosphere, are broken into seven large plates and 11 or more smaller ones. Continental plates are thicker than oceanic plates because of differences in the thickness of their crust. These plates slide around on the asthenosphere, the soft but solid, putty-like mobile rock that makes up the lower part of the upper mantle. The plates are 100 to 350 kilometers thick and move at a rate of 1 to 12 centimeters per year creating continental drift. The plates may separate, slide past one another, or collide and in this process form the following: • Rift zones, where plates are separating and moving away from each other on continents. • Mid-ocean ridges, where plates are separating and moving away from each other in oceans. • Transformation of fault boundaries, where plates are sliding past each other, such as in the San Andreas Fault. • Trench, earthquake, volcano, and mountain ranges, where an oceanic plate is colliding with a continental plate. As the oceanic plate sinks, or subducts, it forms a trench. Earthquakes occur in conjunction with this movement. As a plate sinks far enough to partially melt, volcanoes form. The pressure of the two plates colliding form mountain ranges. • Trench, earthquake, volcano, and island arcs, where two oceanic plates converge: Here, the difference is that the denser oceanic plate does the subducting, and the mountains and volcanoes that form tend to produce island arcs, such as the Aleutians and the Philippine Islands. • High mountain ranges, earthquakes, and suture zone, where two continental plates collide: No subduction occurs here because continental plates are not dense enough to subduct. The two plates collide and there is “no way to go but up,” as tall mountains form. Earthquakes accompany this pressure, and the suture zone marks the joining line of the two continents. The best modern-day example is the Himalayan Mountains. Today we have a good understanding of how the plates move and how such movements relate to earthquake activity. Most movement occurs along narrow zones between plates; this is where the results of plate-tectonic forces are most evident. There are four types of plate boundaries: • Divergent boundaries – occur along spreading centers where plates are moving away from each other and new crust is created by magma pushing up from the mantle as in the Mid- Atlantic Ridge. • Convergent boundaries – can take several forms. They may involve two continental plates, two oceanic plates or an oceanic plate and a continental plate. If one plate sinks below another plate, this is called subduction. There are several good examples of subduction, where an oceanic plate dips under a continental plate. The oceanic Nazca plate dips under the S. American continental plate. Volcanism, earthquake activity and ocean trenches are often associated with these subduction zones. When two oceanic plates meet, one subducts below the other, and typically forms an ocean trench like the formation of the Mariana’s Trench where the Pacific plate converges with the Philippine plate. When two continental plates meet head-on the crust tends to buckle and is pushed upward or sideways. The collision of India into Asia 50 million years ago caused the Eurasian Plate to crumple up and override the Indian Plate. Over millions of years this collision pushed the Himalayas and the Tibetan Plateau up to their present heights. • Transform boundaries – where crust is neither produced nor destroyed as the plates slide horizontally past each other. Most transform faults are found on the ocean floor. They commonly offset the active spreading ridges, producing zigzagged plate margins. However, a few occur on land such as the San Andreas Fault zone in California. • Plate boundary zones – are broad belts in which boundaries are not well defined and the effects of plate interaction are unclear. In some regions, the boundaries are not well defined because the plate-movement deformation occurring there extends over a broad belt called a plate-boundary zone. One of these zones marks the Mediterranean-Alpine region between the Eurasian and African Plates within which several smaller fragments of plates, microplates, have been recognized. Plate-boundary zones involve at least two large plates and one or more microplates caught up between them. They tend to have complicated geological structures and earthquake patterns. en.wikipedia.org/wiki/Plate_tectonics The figure shows the major plate boundaries. The arrows indicate the type of plate boundary that is present. References: How Does Earth Work? Physical Geology and Process of Science, 2/E Gary Smith and Aurora Pun, Prentice Hall, 2009. and Historical Geology, 6th Edition, Reed Wicander and James S. Monroe, Brooks/Cole, 2009. Questions: Please answer the following questions using the lab packet or the Internet. 1. What type of plate boundary is under the Red Sea? 2. How are oceanic ridges identified? 3. What types of plate boundaries are associated with oceanic trenches? 4. The major mountains of the world are associated with what plate boundaries? 5. Where are the locations of at least three major transform boundaries? 6. What are some similarities and differences between the Caribbean and Scotia plates? 7. Is it possible to have a tectonic plate completely surrounded by convergent boundaries? Why or why not? Does one exist? 8. How is the Juan de Fuca plate related to the volcanism of northern California, Oregon, and Washington? 9. What will happen to Africa in the future? What will it look like? 10. Imagine that you are looking at pictures sent back of a newly discovered planet. You are specifically looking for evidence of plate tectonics. What topographic features would you look for, and why?
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