Lab 8 Plate Tectonics-2.docx

“Geoworld” Plate Tectonics Lab 8 Geol 116 Fall 2023 Simplified from exercise © 2003 Ann Bykerk-Kauffman, Dept. of Geol. and Envi. Sci., California State Univ. Chico Purpose: To better understand the principles of the plate tectonics theory by applying them to the past, present and future analysis of a theoretical planet. Introduction During this lab exercise, we will analyze the plate tectonics of part of a mythical planet called Geoworld; a tectonic map of the area is shown on p. 5 Analysis of the Present State of Geoworld . Geoworld has a plate tectonics system just like ours on Earth, only Geoworld is flat instead of spherical. All questions are in bold face. Please put your answers in any blanks provided. A. Sea Floor Magnetic Anomalies Geoworld 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 Geoworld. 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 anomalies form a striped pattern on the map of Geoworld. 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. 1

2 “Geoworld” Plate Tectonics Lab 1. On the map on page 5, highlight the mid-ocean ridge in the Elrond Sea and label it the Hobbit Ridge. 2. Draw a diagram and explain the origin of the striped magnetic anomaly pattern visible in the Elrond Sea. 3. The pattern of positive and negative magnetic anomalies in the Elrond Sea can be correlated with the pattern of normal and reversed magnetism on the magnetic polarity time scale. 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 4. Using the magnetic anomaly 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 your equations and explain where each number came from. Useful Information: 1 cm (10 mm) on the map = 400 km on Geoworld 1 mm/y = 1 km/m.y. (kilometers per million years) distance = rate x time, rate = , time = 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 your equations and explain where each number came from.

4 “Geoworld” Plate Tectonics Lab Transform boundaries 3. How many lithospheric plates are there in all?

“Geoworld” Plate Tectonics Lab 5 C. Transform Faults There are several transform faults in the Elrond Sea. The only parts of these faults that are presently active are the parts between the segments of the Hobbit Ridge. 1. Using arrows, show the sense of motion on each of the active transform faults on the map. 2. In the space below, draw a diagram and explain why the sense of motion on a transform fault is the way it is. Part 2: Analysis of the Future State of Geoworld A. When will the Bilbo Continent and the Sauron Trench first collide? Show your equations and explain where each number came from. Note that, although you computed relative motion across the Hobbit Ridge in Part 1, here you need to know the current rate of movement of the Bilbo Continent toward the Sauron Trench; it turns out this is 50 mm/yr (computed in a part of the exercise that you’re not doing). Time of Collision = ___ m.y. from now B. How wide will the Elrond Sea be at that time? Show your equations and explain where each number came from.

“Geoworld” Plate Tectonics Lab 7 Plate Tectonic Map of Geoworld