3_plate tectonics-0da22d17-0a71-4c35-b83e-62ac1ed19ad6
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Arizona State University *
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103
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Apr 3, 2024
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1 Lab 3: Plate Tectonics NAME____________________________ Materials: Scissors, tape, colored pencils, ruler, calculator, camera, computer. All other materials (worksheets, videos, etc.) required for this lab are on the 3. Plate Tectonics Intro Page on the course Canvas site. Exercise 1: Reconstructing Pangaea In this exercise you will play the role of Alfred Wegener and reconstruct Pangaea using fossil and paleoclimatic evidence. See Video 1: Continental Drift Hypothesis
for guidance. Instructions: 1.
Print out the continent cutouts and map provided on the last two pages of this lab. 2.
BEFORE YOU CUT ANYTHING look at the color version of this handout and use colored pencils to color in
the fossil bands on your Pangaea cutouts handout (if you didn’t print the handout in color, that is). 3.
Cut out each continent. Your cutouts don’t have to be perfect, but make sure they’re close enough so there isn’t white space overlapping the edges of the continents when you reconstruct Pangaea. 4.
Reconstruct Pangaea by positioning the continents on the map printout provided. Remember that Pangea was existed before the Atlantic Ocean opened. So if it helps, construct current geography o your map and close the Atlantic Ocean. Make sure your: a.
fossil bands match up. b.
mountain ranges match up c.
paleoclimate indicators are placed at approximately the correct latitude (see below). •
coal
= tropical latitudes (~0-30º N & S of the equator) •
salt/gypsum/desert sandstone
= desert latitudes (~30º N & S of the equator) •
glacial
= polar latitudes (~60-90º N & S of the equator) 5.
When you have everything in the correct position tape your continents to the map. 6.
Mostly make sure that your fossil bands match up, your mountain belts are aligned, and the paleoclimate indicators are mostly in the right spot (there are exceptions, so don’t stress too much about having these exactly right). Exercise 2: Age of the Ocean Floor and Divergent Boundaries Here we will investigate how the age of ocean crust varies along the ocean floor and investigate some divergent plate boundaries, where crust is created! For help with Google Earth, watch the Google Earth Instructions
video provided. For help with the concepts, watch Video 2: Seafloor Studies and Seafloor Spreading Hypothesis
and Video 4: Divergent Boundaries
on the course Canvas page. Instructions: •
Start by downloading the Google Earth application on your computer Here's a link: https://www.google.com/earth/versions/
. Make sure you click the "Download Earth Pro on desktop" button near the bottom of the page. •
Download the Lab3_Plate_Tectonics.kmz file to your computer (on Canvas in the 3. Plate Tectonics Intro Page). •
Open Google Earth. •
Click File > Open and then browse to Lab3_Plate_Tectonics.kmz file and open the file. Lab3_Plate_Tectonics will show up in the Places
panel on the left. •
Click the arrow to the left of “Lab3_Plate_Tectonics” to open the folder. MAKE SURE THE BOX BESIDE “Lab3_Plate_Tectonics” IS NOT CHECKED.
2 Everything you need to complete the Google Earth exercises in this lab is contained within this folder. Use the placemarks and overlays in this folder to answer the questions below. a.
Check all three Exercise 2(a) placemarks and double-click on one to fly to a location above the Atlantic Ocean. These placemarks represent locations that were once at the same position (at the Mid-Atlantic Ridge) before seafloor spreading occurred. In other words, these are the points of South America and Africa that were touching! Use the path function in the ruler tool to measure how far these points have moved apart in kilometers. Measure along the east-west trending fracture (marked by an orange arrow) that offsets the ocean floor. b.
Check the box beside “Seafloor Age Map” to display an overlay of seafloor age with a legend. Use this overlay to determine how many millions of years ago the points in question (a) above were together. Be careful that you’re looking at the color of the overlay and NOT the ocean floor. Turn the seafloor age map overlay on an off to identify the color and match the color to the legend. c.
Use the distance you measured in question (a) and the oldest age in your answer for question (b) to calculate the spreading rate of the Mid-Atlantic Ridge in km/Ma. d.
It’s difficult to understand what a mega-annum (one million years) means since humans experience much shorter time scales. Convert your answer for question (c) to cm/year. e.
With the seafloor age map overlay still on, check and double-click “Exercise 2(e) Atlantic” and note the width of the age stripes around the Mid-Atlantic Ridge here. Now check and double-click the “Exercise 2(e) Pacific” and compare to the East Pacific Rise. Which of these spreading ridges is spreading at a faster rate? f.
The Pacific Ocean has trenches on most of its margins while the Atlantic Ocean does not. Knowing this, why might the East Pacific Rise and Mid-Atlantic Ridge have such different spreading rates? (Think about the driving mechanisms of plate tectonics).
3 g.
In general, sea floor age __________ with distance from mid-ocean ridges (increases, decreases, is not related to). h.
Uncheck the box beside “Seafloor Age Map”. Check and double-click the “Exercise 2(h)” placemark. Divergent boundaries begin as continental rift zones, which start as triple junctions (three arms that connect at a center point). The placemark here marks the center point where the three arms meet. Two of the three arms are easily identifiable as they have formed water features, the Gulf of Aden and the Red Sea. To see the names, open the “Layers” Panel on the lower left, then click the box beside “Borders and Labels”. Assuming the third arm occurs 120° from the other two, what country/countries should the third arm cut through? i.
Check the beside “volcanoes of the world” to bring up volcanoes on your map. What evidence should indicate the presence of this third arm? Zoom in and out to see if you can see any. Exercise 3: Paleomagnetic Data and Spreading Rates Paleomagnetic data was used to confirm Harry’s Hess’s seafloor spreading hypothesis and hasten the development of plate tectonic theory. Use the paleomagnetic data available on the course Canvas page to answer the questions below. For more information, watch Video 3: Paleomagnetism and Plate Tectonics Theory
on the course Canvas page. Instructions: •
Download the Juan de Fuca Spreading Ridge file from Canvas. •
To calculate rate you need distance and time. Measure the distance between the Juan de Fuca Ridge (black dashed line) and the oldest crust in the image (furthest blue stripe). Use the 500 km scale provided. •
What’s the time needed for this calculation? The crust forming at the Juan de Fuca Ridge is 0 years old, so our “time” is the time it took for the blue stripe to travel from the ridge to its current position. In other words, it’s the age of the oldest blue stripe! Use the age scale on the lower left. Questions: a.
What is the spreading rate in km/Ma for the Juan de Fuca Ridge? b.
To get a better understanding, convert that to cm/year. c.
Which is spreading faster, the Juan de Fuca Ridge or the Mid-Atlantic Ridge (see your answer for Exercise 2 (c and d).
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4 Exercise 4: Convergent Plate Boundaries Here we will look at the plate boundaries where crust is destroyed, subduction zones! You’ll investigate the effect of how steep the subducting slab is sinking on the distance between deep sea trenches and volcanic arcs. For more information, watch Video 5: Convergent Boundaries
on the course Canvas page. Instructions: •
Open Google Earth and the Lab3_Plate_Tectonics.kmz file using the instructions in Exercise 2. •
Uncheck the “Seafloor Age Map” overlay if it’s checked. Questions: a.
Check and double-click the “Exercise 4(a)” placemark to fly to the Mariana Trench, which marks the boundary between the Philippine Plate to the west and the Pacific Plate to the east. The placemark sits directly on the deep-sea trench, which forms a curved line. Check the box beside “Ex4(a) Volcanoes”. Based on the position of the volcanic arc, which plate is subducting? b.
Do volcanic arcs occur on the overriding slab or subducting slab? c.
What type of convergent plate boundary does the Mariana Trench represent? d.
Check the box beside “Earthquakes”. This will display earthquake positions and depths. Earthquake depth in a subduction zone can tell us the depth of the subducting slab since they occur when the subducting slab fractures as it dives down into the mantle. The depth of the slab increases as it moves down and away from the trench. What is the depth of earthquakes that occur closest to the volcanic arc? The earthquake depth legend shows depth in kilometers (km). e.
While you’re here, use the ruler tool to measure the average distance between the Mariana Trench, and the volcanic arc. Measure from the placemark to the closest volcano and report in kilometers. f.
Check and double-click “Exercise 4(f)” placemark to fly to the Tonga Trench east of Australia. Also click on the box beside “Ex4(f) Volcanoes”. What is the depth range of earthquakes that occur closest to the volcanic arc here? g.
Check and double-click the “Exercise 4(g)” placemark, to fly to the Peru-Chile Trench on the western margin of South America. Make sure the box beside “Ex4(g) Volcanoes” is also checked. What is the range of depth of earthquakes that occur closest to the volcanic arc here?
5 h.
While you’re here, use the ruler tool to measure the average distance between the Peru-Chile Trench and its volcanic arc. Measure from the placemark to the closest volcano and report in km. i.
Recall that subducting slabs need to reach a certain temperature (and, therefore depth) to melt. Once they reach this depth the magma produced rises through the overlying slab and produces a volcanic arc. Earthquakes, however, can occur up to about 670 km all along the subducting slab. Therefore, earthquake depth can indicate the depth at which the slab is melting. Based on your answers in the previous questions, what depth does a slab need to reach in order to start melting? j.
Not all subducting slabs subduct at the same angle, which results in different trench-arc distances. Compare the trench-arc distances you measured in questions (e) and (h) above – they are very different. In the boxes on the next page, sketch cross-sections of subduction zones that illustrate why this might be (think about the depth at which rock melts and the geometry of subduction). Mariana Trench Peru-Chile Trench Exercise 5: Transform Boundaries Here we will look at some examples of transform plate boundaries. Crust is neither created nor destroyed at these boundaries, but they do produce a lot of earthquakes! See Video 6: Transform Boundaries
for more information. Instructions: •
Open Google Earth and the Lab3_Plate_Tectonics.kmz file using the instructions provided in Exercise 2. •
If the Earthquakes box is checked, uncheck it. Questions: a.
Check and double-click the “Exercise 5(a)” placemark. Also check the boxes beside “Mid-Atlantic Ridge” and “Plate Boundary”. What type of plate motion is occurring along this east-west trending fracture (the yellow line) in the ocean crust? Convergence, divergence or transform? Note that the mid-ocean ridge that is running approximate north-south (red, offset lines), which you can see in different positions above and below this east-west trending fracture.
6 b.
Check and double-click the “Exercise 5(b) 1” and “Exercise 5(b) 2” placemarks, which point to two different segments of the Mid-Atlantic Ridge. Which direction (north, south, east or west) has Placemark 1 moved relative to Placemark 2? c.
Check and double-click the “Exercise 5(c)” placemark to fly to a fault in the San Andreas Fault System. Also check the box beside “San Andreas Fault Line”, “North American Plate” and “Pacific Plate”. The fault is represented by a sharp line that runs approximately NW-SE (purple line). There is a riverbed (the light-grey feature) that runs directly across the fault just to the southwest of the yellow pushpin placemark. Use the riverbed to determine what direction the North American Plate is moving relative to the Pacific Plate. Exercise 6: Hawaii Nematath In this exercise we will examine the Hawaiian Island Nematath, which records past movements of the Pacific Plate. A nematath is a line of extinct volcanoes or seamounts that results from the movement of a tectonic plate over a hot spot. See Video 7: Hawaiian Island Nematath and Earth Surface Changes
for more information. Instructions: •
Open Google Earth and the Lab3_Plate_Tectonics.k file using the instructions provided in Exercise 2. •
Check and double-click the “Exercise 6” folder. Questions: a.
Use the ruler tool to measure the distance between the Midway Atoll and Kilauea. You may need to zoom in to see the names of the seamounts (extinct volcanoes). Use this measurement to calculate the average velocity of the Pacific Plate in cm/yr. Note that the age for the Midway Atoll is listed as 27.7+_0.6 Ma. This means it’s 27.7 million years old with a margin of error that is 0.6 million years above and below that. For your calculations, just assume the Midway Atoll is 27.7 million years old. b.
What direction (N, S, E, W, or some combination) is Pacific Plate moving today? What was the original direction (N, S, E, W, or some combination) of motion of the Pacific Plate? c.
What about the shape of the nematath (chain of underwater seamounts) tells you that the Pacific Plate has not always moved in the same direction?
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7 d.
Approximately how many millions of years ago did the Pacific Plate change its direction of motion? Exercise 7: Plate Tectonics in the Solar System For this exercise, we’ll explore the potential for plate tectonics on other planetary bodies. Plate tectonics, as it turns out, is seemingly rare in our solar system. There is one planetary body, however, that appears to have the potential for plate formation and movement. Read the article below about this body: Jupiter’s moon, Europa. https://www.jpl.nasa.gov/news/scientists-find-evidence-of-diving-tectonic-plates-on-
europa a.
What is Europa’s crust comprised of? b.
What about its mantle? What is it thought to be made of? c.
What features appear to be similar to volcanic activity related to subduction?
8 Figures for Exercise 1. Adapted from The Changing Geography of Your Community
, Coordinated Science for the 21
st
Century.
9 Map for Exercise 1.
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