General Oceanography Geology 105 Expedition 8 - The Seafloor at Birth

Transcription

1 General Oceanography Geology 105 Expedition 8 - The Seafloor at Birth Name Not attempting to answer questions on expeditions will result in point deductions on course workbook (two or more blank answers will result in a deduction of 2-4 points; a 5 point deduction will be assessed if more than five answers are left blank). 1. Introduction Objective: In this expedition you will learn how to use some basic math, plus a few simplifying assumptions, to calculate the horizontal and vertical movements of the major plates of lithosphere that compose the rigid outer shell of the Earth. The movements of plates influence the shapes of seafloor features, the distribution of the major energy resources on the planet, and result in the tragic earthquakes and tsunamis that caused hundreds of thousands of deaths in the past three years alone. Let s first make sure that you understand three critical features associated with the formation and recycling of the seafloor. What is the lithosphere? What is the asthenosphere? What are the three types of plate boundaries? 1. Plate Motion What is meant by the use of the term "relative plate motion?" General Oceanography, GEOL 105, Spring 2013 Page 93 of

2 What is meant by the term "absolute plate motion?" Do plates move vertically too? (uplift or subsidence) - more on this later 3. Plate Motions and Boundaries a. As we learned in the previous expedition, there are only three types of plate boundaries Divergent Convergent Transform Let s next apply what you have learned on the next page. General Oceanography, GEOL 105, Spring 2013 Page 94 of

3 b. Cut out the triangle below but try to keep the remaining sheet of paper in one piece. First examine the arrow on the triangular-shaped plate A showing the direction of relative plate motion with respect to plate B (the plate surrounding plate A). c. Put the triangle back into the hole and move it in the direction of the arrow on plate A d. Label the types of plate boundaries that surround plate A (convergent, divergent, transform) Plate B Plate A General Oceanography, GEOL 105, Spring 2013 Page 95 of

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5 4. Example 2 Next, in the diagram below, cut out only the black triangle, return the black triangle to the hole and move it in accordance to the following rules: away from divergent, towards convergent, or parallel to transforms) Draw an arrow on Plate B showing the direction of relative plate motion. What type of plate boundary is located along the diagonal line? General Oceanography, GEOL 105, Spring 2013 Page 97 of

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7 5. Review of Plate Boundaries Answering the following questions about in order to describe characteristics of divergent, convergent, and transform plate boundaries: Seafloor Features and Plate Boundaries What is name of seafloor feature at a DIVERGENT plate boundary? What is name of seafloor feature at a CONVERGENT plate boundary? Status of Lithosphere at Plate Boundaries What happens to the lithosphere at a DIVERGENT plate boundary? What happens to the lithosphere at a CONVERGENT plate boundary? What happens to the lithosphere at a TRANSFORM plate boundary? Plate Motions and Plate Boundaries What is direction of plate motion a DIVERGENT plate boundary? What is direction of plate motion a CONVERGENT plate boundary? What is direction of plate motion a TRANSFORM plate boundary? Plate Process and Plate Boundaries What is the name of process that forms the plates at a DIVERGENT plate boundary? What is the name of process that recycles the plates at a CONVERGENT plate boundary? Volcanoes and Plate Boundaries Are there volcanoes at a DIVERGENT plate boundary? Are there volcanoes at a CONVERGENT plate boundary? Are there volcanoes at a TRANSFORM plate boundary? Earthquakes and Plate Boundaries Are there earthquakes along a DIVERGENT plate boundary? Are there earthquakes along a CONVERGENT plate boundary? Are there earthquakes along a TRANSFORM plate boundary? General Oceanography, GEOL 105, Spring 2013 Page 99 of

8 6. Rates of Seafloor Spreading On the diagram below draw a line showing the location of mid-atlantic ridge, which marks the site of seafloor spreading (and therefore seafloor creation) beneath the Atlantic Ocean (remember Africa and South America were once connected and have since split apart) African Plate A South American Plate B Equator South Atlantic Ocean Below you see a profile of the seafloor across the mid-atlantic Ridge (like we examined in one of the previous expedition) between points A and B in the map above. A B Use the distance and age of the oceanic crust at point B to calculate the average half rate of sea floor spreading, which formed the south Atlantic Ocean basins over the past 64 million years (age of oceanic crust beneath arrows under 64 my symbol). Remember rate of motion equals distance divided by time (leave answer in kilometers per million years) What type(s) of data would allow you to determine the age of the oceanic crust, as shown in the profile across a mid-ocean ridge in the diagram above? General Oceanography, GEOL 105, Spring 2013 Page 100 of

9 7. Full Rates of Seafloor Spreading If you were to multiply your "half-rate" answer to the previous problem by 2, you would then have the full rate of seafloor spreading, which is also the relative rate of motion of one plate away from the other plate or, in this example, the motion of the African plate away from South American plate. Clearly we are making a simplistic assumption here.that the amount of plate created on one side is the same as that on the other side of the rift valley.this is not always true. Now you calculate the rate of motion of the African plate away from the South American Plate Next, calculate the rate of motion of the South American Plate away from the African Plate (same but in opposite direction) Once again remember => Rate of motion = distance divided by age Rate=distance/age Yes, it is the same rate of motion (or speed), but in the opposite direction! General Oceanography, GEOL 105, Spring 2013 Page 101 of

10 8. Formation of South Atlantic Basin If you to measure the width of the Atlantic Ocean (see double-headed arrow below) near the equator you would find it to be nearly o of longitude. W 60 o 50 o 40 o 30 o 20 o 10 o 0 o 10 o 20 o E African Plate Equator South American Plate South Atlantic Ocean Knowing that 1 o equals 111 kilometers (km is the symbol), estimate how many years were required for the Atlantic basin to reach its present width at the equator due to sea floor spreading. kilometers = kilometers per degree x number of degrees of longitude This age coincides with the time of the continental breakup of North America from Eurasia, and Africa from South America (remember continental breakup in earlier expedition). 9. Water Depth and Predicted Crustal Age Relationship Based on the graph relating square root of the crustal versus water depth, an equation, in statistics, called a regression equation, can be derived that relates the age of oceanic crust to the water depth of the seafloor D= T 1/2 where D is water depth in meters and T is the age of the ocean crust in millions of years Make up two questions that use this relationship and write and solve them below: General Oceanography, GEOL 105, Spring 2013 Page 102 of

11 10. Life Cycle of Lithosphere Summarize the life cycle of the oceanic lithosphere based on seafloor spreading and subduction. 11. Science Pseudoscience - Required Desire2Learn Discussion This required discussion will address required learning outcome #2 for all Area R course at SJSU, that students must be able to distinguish between science and pseudoscience. Listen to video and audio clips about science and pseudoscience and do some outside research. Take any notes you wish in the space below. General Oceanography, GEOL 105, Spring 2013 Page 103 of

12 Write a short discussion in which you distinguish between science and pseudoscience and post it in Exped. 9 Science versus Pseudoscience Discussion in Desire2Learn. The initial posting should be approximately words long (about sentences). Postings less than 325 words will receive a 5 to 25 point deduction, depending brevity of posting. Exceeding 375 words in posting may result in a 5 point penalty. You will be graded on the quality of your writing and the content of the posting. The students in the class will divided into discussion groups of 6-8 students. Remember that you are required to reply to at least one other member in your group. The required reply to the posting of another student should be approximately words long (about 4 to 5 sentences). The original posting AND subsequent reply MUST BE SEPARATE POSTINGS in the discussion. Also, remember to write in your own words do not plagiarize by copying text from other web sites! 1 minute essay - Summarize what you learned in this expedition - Avoid the "I learned a lot about" syndrome - show some insight Remember to post any questions that you may have at the Exped - Seafloor at Birth? discussion. In order to receive credit for completing this expedition and have access to answer key, students must follow instructions on last webpage of this expedition, which may say to post in a discussion area within Desire2Learn. General Oceanography, GEOL 105, Spring 2013 Page 104 of