Lab2_Desha_Ls (1)

OC 201, Oceanography Lab 2: Marine Geological Evidence for Plate Tectonics In this lab, you will be using GeoMapApp to examine some of the evidence used by marine geologists to develop the theory of plate tectonics. Your GTA will show you how to start GeoMapApp. Remember what that evidence was: A. Bathymetry of the ocean floor that revealed ridges (some with central rifts), trenches, and seamounts. B. Location and depth of earthquakes relative to seafloor bathymetry. C. Age of the ocean floor and how it changed away from the ridges (= rises). D. Magnetic striping of the seafloor with crustal rocks having different polarity. In today’s lab we will explore aspects of B and C. Part 1. Determining Spreading Rates  Under the DataLayers tab select Geophysics , and then Lithospheric Plates , and finally Seafloor Crustal Age (Muller 2008 v3). The Seafloor Crustal Age grid will load and “ Loaded Grids ” and “ Layer Manager ” windows will appear.  In the GeoMapApp window, turn on the Zoom Tool (+) and zoom in to the South Atlantic seafloor between Brazil and Africa by clicking and dragging. Part 1A: In the main window with the seafloor map, turn on the “Distance/Profile Tool” (to the left of the “XYZ” button) in the map window and draw a profile from west to east across the South Atlantic seafloor at a latitude of 20-30 S. Make sure your profile line is perpendicular to the spreading ridge it crosses. When the cursor is released, the profile appears in a new window. You may need to move the Profile window if it covers your map. It should look like a V and be similar to the profile on the next page. Note that you might need to resize the profile box to get the y-axis labels to appear. Spend some time exploring and studying the profile and answer the following questions in the boxes provided. To type in the boxes, position your cursor in the box and click twice. Do not make the font less than 10 pt. 1.) What are the two variables depicted on the chart, and on which axes are they plotted? (2pts) 2.) How many years are represented by each minor interval (step) on the y-axis? (1pt) 3.) What distance is represented by each minor interval (step) on the x-axis? (1pt) Zoom tool Profile tool Y axis: Age, mY X axis: Distance, Km 10 million years 100 km

Run your cursor across the graph in the Profile window and notice that its geographic location is shown as a red dot on the profile in the GeoMapApp window. Notice that the distance along the profile (x-axis) is measured from the point where you started your profile or the end of the continental shelf (which ever comes later) to the point where it ends. And, finally, notice that the latitude, longitude, and age of bedrock are displayed at the top of the profile window for any cursor location on the profile. 4.) At what longitude along the profile are the youngest rocks (not including the continental shelf shown in grey on the map)? (2pt) 5.) An area where two plates are being pushed and pulled apart is called a spreading center (= ridge or rise). Describe how the age of seafloor bedrock changes as you move from the spreading center to the South American and African coastlines. (2pts) 6.) Is the age-distance profile roughly symmetrical on either side of the spreading center? (2pts) 7.) In a sentence or two, describe what is meant by the symmetry of the age profile – what does it tell us about the ages and motion of rocks on either side of the spreading zone? (2pts) 0.8 mY The age of the seafloor starts out older from around 117 mY then going towards the middle the seafloor gets younger, to 0.8 mY, then it begins to rise again when going to the opposite coastline to around 130 mY Almost similar, however, the African coastlines seem to be a bit older Symmetry tells us, that they were formed roughly around the same time, if both ends of the coastline are around the same then they were formed around the same time

2.) Are the rates calculated on either side of the spreading center in this area similar? (2pts) 3.) How could you have known that they were similar by simply looking at the graph? (2pts) 4.) How much wider is the South Atlantic Ocean getting each year? (That is, what is the combined spreading rate?) Enter your answer below. (2pts) 5.) Look carefully at the profile. Has the spreading rate always been the same? (2pts) 6.) Approximately how old is this part of the Atlantic Ocean (hint look at the oldest point of the profile you created)? (1pt) Distance at D – 1000 km Distance at C – 2100 km Distance from C to D = -1100 km Age of seafloor at D – 70 Ma Age of seafloor at C – 20 Ma (70 Ma – 20 Ma) = 50 Ma (-1100 km / 50 Ma) = -22. 0 km / Ma Yes The two graphs look almost identical 0.4 km Roughly 130.9 mY

Part 1.C: 1.) Using the methods described above, determine the spreading rate along a profile drawn from 133 W, 27 S to 90 W, 32 S in the South Pacific Ocean . Remember to calculate the spreading rate on both sides (i.e., west and east) of the profile and report the combined spreading rate. Show your work and record the results below. (7pts) 2.) Write a brief comparison of the spreading rates in the South Atlantic and South East Pacific. How else do these ridges differ? (4pts) Distance at D – 500 km Distance at C – 1800km Distance from C to D = - 1300 km Age of seafloor at D – 23 Ma Age of seafloor at C – 3.8 Ma ( 23 Ma – 3.8Ma) = 19.2 Ma (- 1300km / 19.2 Ma) = - 67.7 km / Ma Distance at A – 2300 km Distance at B– 4000 km Distance from A to B = - 1900 km Age of seafloor at A – 0 Ma Age of seafloor at B – 15 Ma ( 0 Ma – 15 Ma) = -15 Ma ( -1900km / -15Ma) = 126.7 km / Ma Combined spreading rate= -67.7 + 126.7 = 59 km The South Atlantic tends to be spreading a lot slower than the South East Pacific. We can see the comparison rates of South Atlantic going at 0.4 km per year while the south east pacific is 54 km per year.

1.) Change the depth scale to 0-40 km. Describe what you see – in particular what sort of plate boundary are most of these shallow earthquakes occurring at? (1pt) 2.) Now change the magnitude range to be 6-9 and replot. Where are most of these shallow, but large quakes occurring? (1pt) 3.) Lastly, change the depth range to be 60-900 km and the magnitude range to be 3-9. Replot. On what sort of plate boundary do these earthquakes occur? (1pt) Save the completed Word file as follows: “lab2_yourlastname_firstinitial.docx”. Upload your file to appropriate assignment in your Week 2 module on Canvas. Divergent plate boundaries All along the coasts of countries. Again on the divergent, and it looks a lot worse with their being more reds and yellow.