Ch 4 Plate Tectonics Lab Part 1
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Feb 20, 2024
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Name________________________ 1 “Tec
-no-
world” Plate Tectonics Lab
Part 1 Used by permission from ©2003 Ann Bykerk-Kauffman, Dept. of Geological and Environmental sciences, California State University, Chico. Adapted for use by SAU. Introduction During this two part lab exercise this week and next, we will analyze the plate tectonics of part of a mythical planet called Tec-no-world; a tectonic map of the area is shown on the last page of this lab. Tec-no-world has a plate tectonics system just like ours on Earth, only Tec-no-world is flat instead of spherical. In answering the questions below, you may assume that the movement rates for plates (both relative and absolute) are constant throughout the time between your past and future reconstructions (they were, however, quite different at one time. Sea Floor Magnetic Polarity Tec-no-world has a magnetic field aligned in a north-south direction. The polarity of this field reverses at random intervals through time, just as the one on Earth does. The history of polarity of the magnetic field is well known for the past 150 million years and is shown by the adjacent magnetic polarity time scale. Shading represents normal polarity (the magnetic field points north) while white represents reverse polarity (the magnetic field points south). Detailed aerial and ship-board magnetic surveys have been conducted over the oceans of Tec-no-world. Parts of the sea floor have an extra strong magnetic field while other parts have an extra weak magnetic field. Just as on Earth, the areas with anomalously strong magnetic fields are composed of rocks that are magnetized with the same polarity as the current field while areas with anomalously weak magnetic fields are composed of rocks that are magnetized with the opposite polarity as the current field. Portions of the sea floor with an anomalously strong magnetic field are shaded; portions of the sea floor with an anomalously weak magnetic field are unshaded. Note that the magnetic polarity form a striped pattern on the map of Tec-
no-world. Note also that the pattern in the Elrond Sea is symmetrical while that in the Aragorn Ocean is asymmetrical. This is because there is an active spreading ridge in the Elrond Sea but not in the Aragorn Ocean. The center of symmetry in the Elrond Sea is right along the spreading ridge. Note the time scale of magnetic polarity and sea floor spreading is according to mainstream science and doesn’t reflect the opinions of SAU or the science department. This exercise is intended to demonstrate magnetic polarity and sea floor spreading correlation, but it makes use of the conventional scientific time scale. Figure 1: Magnetic polarity history for Tec-no-world
Name_______________________________ 2 Tectonic Settings Use figure 2 to help you review the various types of tectonic boundaries. Figure 2: The three major types of plate boundaries. Divergent Boundaries Divergent boundaries form where two plates are moving away from each other. They are where new crustal material is formed. Magma flows up from below. When it reaches the surface, it solidifies forming new oceanic crust. Any magnetic material in the magma orients itself with the current magnetic field of the planet. Once solidified, it keeps that magnetic orientation even if the plant’s magnetic field changes. The upwelling magma below a divergent boundary creates a raised surface or ridge. The spreading rate at the ridge is not uniform along the entire boundary causing large transform faults to form. Thus the oceanic ridge or rift will appear as a series of blocky shapes separated by linear features. Convergent Boundaries There are several types of convergent boundaries distinguished on the basis of the type of tectonic plates converging. If one of the plates is an oceanic plate, then the boundary will create a subduction zone where one plate slides beneath the other. Subduction zones are characterized by a line of volcanoes paralleling the boundary on the overlying plate. If both converging plates are oceanic, the resulting line of volcanoes is called an island arc. If one plate is continental and the other oceanic, the oceanic plate is subducted, and the resulting line of volcanoes is called a volcanic arc. Where both converging plates are continental plates, neither is subducted. Instead the two plates buckle and fold to produce tall, nonvolcanic mountains. Transform Boundaries Where two plates slide next to each other, the boundary is called a transform boundary. This type of boundary generally has fewer distinguishing characteristics, but can be noticed by the juxtaposition of differing features that formed far apart but have moved into their present locations.
Name______________________________________ Part A: Preliminary questions
1. List the types of boundaries that fall under the broad category of convergent boundaries. a. _____________________________________________________________ b. _____________________________________________________________ c. _____________________________________________________________ 2. Name or describe the type of mountain building activity associated with each of the three boundaries from the previous question. a. _____________________________________________________________ b. _____________________________________________________________ c. _____________________________________________________________ 3. Name the tectonic plate that Tennessee is on ____________________________________ 4. Name the two plates that boarder the San Andres Fault zone of the western US. a. ___________________________________________________________________ Part B: Analysis of Tec-no-world
A. Sea Floor Magnetic Record 1. On the map on the last page of this lab, highlight the mid-ocean ridge in the Elrond Sea and label it the Hobbit Ridge. 2. Draw a cross section diagram of a mid-ocean ridge and use it to explain the origin of the striped magnetic polarity pattern visible on the map in the Elrond Sea.
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Name_______________________________ 4 2. The pattern of positive and negative magnetic polarity in the Elrond Sea can be correlated with the pattern of normal and reversed magnetism on the magnetic polarity time scale (figure 1). Based on this correlation, how many million years (m.y.) ago did sea floor spreading begin in the Elrond Sea? Spreading Began ____ m.y. ago 3. Using the magnetic polarity pattern in the Elrond sea, determine the 1/2-spreading rate
on the Hobbit Ridge (i.e., determine how fast the Frodo continent is moving away from the Hobbit ridge). Give your answer in mm/y (millimeters per year). Show all your work. Step by step help: 1. use the map scale to determine the distance from the ocean ridge to the ocean edge 2. Use the time from question 3 3. Divide the distance by the time rate = distance
time
4. Your answer is in km/m.y. but the same number can also be recorded as mm/y because 1 km/m.y. = 1 mm/y. (km/m.y. = kilometers per million years) 1/2 spreading rate = ___ mm/y 5. What is the full spreading rate (in mm/y) of the Hobbit Ridge? (i.e., how fast are the Bilbo and Frodo continents moving away from each other)? Show all your work. Full spreading rate = __ mm/y 6. Using the magnetic polarity patterns in the Aragorn Ocean, determine when sea floor spreading started to form the Aragorn Ocean crust (remember, spreading is not actively going on now but it was at one time). Spreading Started __ m.y. ago
Name______________________________________ Part C: Plate Boundaries 1. There are two active plate boundaries on the map. You already highlighted one of them. Highlight the other boundary now.
2. Give one example of each of the following types of plate boundaries on Tec-no-world (Note: the boundary between two plates can vary in type from place to place). Divergent boundaries: Convergent boundaries: Transform boundaries: 3. How many lithospheric plates are there in all?
Part D: Hot-spots and Absolute Movement Direction 1. Notice the Gandalf Islands. They are analogous to the Hawaiian Islands on Earth. They form a linear volcanic chain that originated by the migration of a tectonic plate over a mantle hot spot. Mantle hot spots are assumed to be fixed relative to the center of the planet, so when the plate moves the hot spot stays and the volcanic activity starts on a new portion of the plate. All the Gandalf islands are volcanic in origin but the easternmost and largest island--which is sitting directly over the hot spot--is the only one with active volcanoes. The rocks on the westernmost island have been dated by radiometric methods at 32 m.y. According to these dates, was the sea floor actively spreading when the oldest Gandalf Island formed? Choose the best answer
a. Yes b. No c. Can’t be determined
2. Given that the Elrond Sea is spreading, state how the Rivendell and Misty Mountains might have been related in the past. 3. Suppose Gollum Island is formed by a hot spot. What does this say about the absolute movement of the Hobbit Ridge?
Name_______________________________ 6 Plate Tectonic Map of Tec-No-World
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Name______________________________________