ntroduction
Now that we have a basic understanding of what the theory of plate tectonics is, we can take a look at the multiple data sets that scientists across the Earth sciences used to develop it.
For this lab we will be using mapping software called Google Earth to examine the data sets which include phenomena and measurements from around the world. This is common for geologic science. We often need to look at features that occur on regional or even global scales. Because of this we have found maps to be very useful tools. With the technological revolution our tools have become more precise and even useful because we are able to quickly add information to images such as maps.
There are two parts to this lab, both of which require mastery of techniques outlined in the Pre-lab Tutorial.
Part 1: Building Context
In Part 1, you will be investigating three field sites, using Google Earth to examine Earth’s features in a unique way. Using topographic profiles, which are “side views” of the landscape, you will investigate the depths and elevations in the Atlantic and Pacific Oceans. These profiles cross over plate boundaries that have specific physiographic features such as mid-ocean ridges and deep oceanic trenches. You will analyze earthquake depth data to help illustrate plate boundary processes.
Formation of Mid-Ocean Ridges and Deep Ocean Trenches
According to Plate Tectonics Theory, new ocean crust is created at mid-ocean ridges at divergent plate boundaries. Where two plates pull away from each other, magma rises from below. When the magma cools and hardens it forms new ocean crust in the center of the mid-ocean ridge.
Older ocean crust is destroyed along convergent plate boundaries. When a plate of oceanic crust collides with another plate, the denser ocean crust is thrust downward into Earth. This collision can occur between two ocean plates or between oceanic and continental plates. As the older, denser plate descends, friction causes the plate on top to flex downward, producing linear valleys called deep oceanic trenches. The descending plate triggers melting of the mantle above it, and chains of volcanoes form, called volcanic arcs.
Site 1: Mid-Atlantic Ocean Profile
Open Google Earth with the Plate Tectonics Lab- Lines of Evidence.kmz file loaded download. In order for the graph to come up correctly, it is important to open the layers in the following order. Check the box next to Mid-Atlantic Ocean Profile line under the heading Profile Lines. Then double-click on the text for the Mid-Atlantic Ocean Profile Line. This line crosses the Atlantic Ocean going from North America to Africa. Under the Edit menu and select ‘Show Elevation Profile’. Last, check the box next to Terrain in the preloaded Layers section.
The vertical axis shows elevations and depths. Negative numbers are below sea-level, so the bigger the negative number, the deeper the depth. The horizonal axis shows the distance along the profile. When the mouse is positioned along the profile line the specific depth/elevation information is displayed. Use the mouse to pinpoint the location of the North American and African coasts. Notice where the ocean meets the land, the depth is 0 meters – this is sea-level.
You are now ready to analyze the data using the following discussion questions: (these are on your answer sheet)
In one or two sentences, describe the bathymetry along this profile, where are deep locations, and w
here are shallow locations?
When viewed in profile, mid-ocean ridges have an appearance of a “speed-bump” – the bathymetry goes from deep to shallow, and then back to deep across the center of the mid-ocean ridge. Deep oceanic trenches are narrow valleys that cut far down into the ocean floor. Does the profile show a mid-ocean ridge or a deep oceanic trench?
Turn on the Present Plate Boundaries layer. Different plate boundaries are shown in different colored lines: Divergent – Red, Convergent – Yellow, Transform – Blue. What type of plate boundary is crossing the middle of the profile?
Using the Plate Boundary Map from Module 2, name the two plates that meet along the boundary identified in the previous question.
Turn on the USGS Earthquakes layer – to view the data, be sure to be zoomed in to an Eye Alt of 4000 km or less. Notice the locations of earthquakes. Describe the depth of earthquakes that occur in the vicinity of the plate boundary, are the earthquakes deep (300-800 km), intermediate (70-300 km), and/or shallow (0-70 km)
Site 2: East Pacific Ocean Profile
Uncheck all of the boxes. Check the box next to East Pacific Ocean Profile Line under the heading Profile Lines. Then, double-click on the text for the East Pacific Ocean Profile Line. This line goes from the Pacific Ocean to South America. Under the Edit menu and select ‘Show Elevation Profile’. Last, check the box next to Terrain in the preloaded Layers section.
Position the mouse along the profile and the specific depth/elevation information is displayed. Use the mouse to pinpoint the location of sea-level near the South American coast.
You are now ready to analyze the data using the following discussion questions: (these are on your answer sheet)
In one or two sentences, describe the bathymetry along this profile, where are deep locations, and where are shallow locations?
Does the profile show mid-ocean ridges and/or deep oceanic trenches?
Using the colored lines displayed by the Present Plate Boundaries layer, what type of plate boundaries are crossing the profile? NOTE: There are two plate boundaries to identify.
Using the Plate Boundary Map from Module 2, name the three plates that meet along the boundaries identified in the previous question.
Turn on the USGS Earthquakes layer – to view the data, be sure to be zoomed in to an Eye Alt of 4000 km or less. Describe the depth of earthquakes that occur in the vicinity of the two plate boundaries, are the earthquakes deep (300-800 km), intermediate (70-300 km), and/or shallow (0-70 km)? Describe if the depths of earthquakes are similar or different for the two plate boundaries.
Compare the profiles from the Atlantic Ocean and the Pacific Ocean. In two or three sentences, develop a hypothesis to explain why the profiles are different.
Site 3: Tonga Island, South West Pacific Ocean Profile
Uncheck all of the boxes. Check the box next to Tonga Island Profile Line under the heading Profile Lines. Double-click on the text for the Tonga Island Profile Line. This line goes across a region of the southwest Pacific Ocean, north of New Zealand. Under the Edit menu and select ‘Show Elevation Profile’. Last, check the box next to Terrain in the preloaded Layers section.
The elevation profile shows elevations and depths on the vertical axis, and the distance along the profile line along the horizontal axis. As you slide the mouse along the elevation profile, small maroon-colored boxes appear which give the depth and elevation values, as well as, distance for various positions along the profile line.
You are now ready to analyze the data using the following discussion questions: (these are on your answer sheet)
Find Tonga Island in the profile. How do you know where to find it? How many kilometers is it from the western edge of the profile?
In one or two sentences, describe the bathymetry along this profile. Where are deep locations, and where are shallow locations?
Does the profile show a mid-ocean ridge or a deep oceanic trench?
What type of plate boundary is represented by the feature that identified in the previous question?
Using the Plate Boundary Map from Module 2, name the two plates that meet along the boundary identified in the previous question.
Turn on the USGS Earthquakes layer – to view the data, be sure to be zoomed in to an Eye Alt of 4000 km or less. Describe the depth of earthquakes that occur in the vicinity of the plate boundary, are the earthquakes deep (300-800 km), intermediate (70-300 km), and/or shallow (0-70 km)?
Graphing Data
You are going to need to plot the earthquake depth data relative to distance along the Tonga Island Profile. We are doing this to get a graphic representation of the geometry of the plate boundary at depth. What does this earthquake data tell us about the way in which these plates are moving relative to each other?
Use the graph and questions on the answer sheet.
Part 2: Calculations of Plate Motion
Geologists use calculations similar to these “back of the envelope” versions in their analysis of datasets. These calculations help us to develop a spatial and conceptual understanding of the story the data is telling.
We are going to be examining the speed of seafloor spreading at two different mid-ocean ridges, one in the Atlantic Ocean and one in the Pacific Ocean. Remember that speed is calculated as distance per time.
Mid Atlantic Profile
Open Google Earth with the Plate Tectonics Lab- Lines of Evidence.kmz file loaded. Check the box and double-click on the Mid-Atlantic Ocean Profile Line under the heading Profile Lines. Adjust the tilt so that the view is directly overhead (under the View tab, select Reset, then Tilt). Turn on the Ocean Age layer.
First, examine the age of the ocean crust scale bar and on your answer sheet describe the general patterns you see by answering questions on your answer sheet.
Next, using the Mid-Atlantic Ocean Profile Line, locate the two lines labeled 90 Ma (Ma means Millions of years before present). These locations are where the seafloor age has been measured to be 90 million years old. Click on the Ruler button. Using the mouse, click where the 90 Ma line intersects the profile line on one side of the Mid-Atlantic Ridge and then click again where the 90 Ma line intersects the line on the other side of the ridge. In the Ruler pop-up menu, under the ‘Line’ heading, use the pull-down menu to change the units to kilometers. The distance is given as ‘Map Length’.
Now we will use the distance measured and the age of the ocean crust to calculate speed and express our answers in several different units for clarity.
East Pacific Ocean Profile
Check the box and double-click on the East Pacific Ocean Profile Line under the heading Profile Lines. Adjust the tilt so that the view is directly overhead (under the View tab, select Reset, then Tilt). Locate the two lines labeled 30 Ma. These locations are where the seafloor age has been measured to be 30 Million years old.
First, use the ruler tool to measure the distance between the two lines labeled 30Ma.
Calculate speed using the distance between the ridge and the 30Ma lines.
Describe the speed in mm/yr
Compare and contrast this profile with the Atlantic Ocean profile
Observe, analyze and hypothesize why there is no seafloor 90Ma in the Pacific off the Coast of South America.
Part 1: Building Context (25 points)In one or two sentences, describe the bathymetry along this profile, where are deep locations, and where are shallow locations?1. In one or two sentences, describe the bathymetry along this profile, where are deep locations, and where are shallow locations?Type Answer Here2. When viewed in profile, mid-ocean ridges have an appearance of a “speed-bump” – the bathymetry goes from deep to shallow, and then back to deep across the center of the mid-ocean ridge. Deep oceanic trenches are narrow valleys that cut far down into the ocean floor. Does the profile show a mid-ocean ridge or a deep oceanic trench?Type Answer Here
3. Turn on the Present Plate Boundaries layer. Different plate boundaries are shown in different colored lines: Divergent – Red, Convergent –
Written by Adrienne Sorenson MSc. Central Washington University Geology Department 2020Yellow, Transform – Blue. What type of plate boundary is crossing the middle of the profile?Type Answer Here4. Using the Plate Boundary Map from Module 2, name the two plates that meet along the boundary identified in the previous question. Type Answer Here5. Turn on the USGS Earthquakes layer – to view the data, be sure to be zoomed in to an Eye Alt of 4000 km or less. Notice the locations of earthquakes. Describe the depth of earthquakes that occur in the vicinity of the plate boundary, are the earthquakes deep (300-800 km), intermediate (70-300 km), and/or shallow (0-70 km)?Type Answer Here6. In one or two sentences, describe the bathymetry along this profile, where are deep locations, and where are shallow locations? Type Answer Here7. Does the profile show mid-ocean ridges and/or deep oceanic trenches?Type Answer Here8. Using the colored lines displayed by the Present Plate Boundaries layer, what type of plate boundaries are crossing the profile? NOTE: There are two plate boundaries to identify.Type Answer Here9. Using the Plate Boundary Map from Module 2, name the three plates that meet along the boundaries identified in the previous question. Type Answer Here10. Turn on the USGS Earthquakes layer – to view the data, be sure to be zoomed in to an Eye Alt of 4000 km or less. Describe the depth of earthquakes that occur in the vicinity of the two plate boundaries, are the earthquakes deep (300-800 km), intermediate (70-300 km), and/or shallow (0-70 km)? Describe if the depths of earthquakes are similar or different for the two plate boundaries. 1. Compare the profiles from the Atlantic Ocean and the Pacific Ocean. In two or three sentences, develop a hypothesis to explain why the profiles are different.Type Answer Here12. Find Tonga Island in the profile. How do you know where to find it? How many kilometers is it from the western edge of the profile? Type Answer Here13. In one or two sentences, describe the bathymetry along this profile. Where are deep locations, and where are shallow locations?Type Answer Here14. Does the profile show a mid-ocean ridge or a deep oceanic trench?Type Answer Here15. What type of plate boundary is represented by the feature that identified in the previous question?Type Answer Here16. Using the Plate Boundary Map from Module 2, name the two plates that meet along the boundary identified in the previous question.Type Answer Here17. Turn on the USGS Earthquakes layer – to view the data, be sure to be zoomed in to an Eye Alt of 4000 km or less. Describe the depth of earthquakes that occur in the vicinity of the plate boundary, are the earthquakes deep (300-800 km), intermediate (70-300 km), and/or shallow (0-70 km)?Type Answer Here18. Use the graph below to plot earthquake depth data relative to the distance along the profile. Each color represents a range of depths. For simplicity plot the earthquake along the line representing that range. For example, for an earthquake shown in purple, the depth range is 300-500 km and should be plotted on the horizontal line labeled 300-500. On the horizontal axis, label the location of the trench. 19. Describe the distribution of earthquakes relative to the position of the trench. Do the depths of earthquakes remain the same, get shallower or deeper? Type Answer Here20. Why do you think the earthquakes happen at these locations and depths? What does the earthquake depth data indicate about the position of the subducting plate?Type Answer Here21. Turn off the USGS Earthquakes layer and turn on the Volcanoes layer. Describe the distribution of volcanoes relative to the position of the plate boundary and trench.Type Answer HerePart 2: Calculations (15 Points)1. Examine the Age of Ocean Crust scale bar (HINT: zoom out so the black background of space is behind the scale bar.)
Written by Adrienne Sorenson MSc. Central Washington University Geology Department 2020a. What is the pattern in seafloor age as you cross the Mid-Atlantic Ridge? Type answer here:b. What age is the oldest seafloor, and what color is used to show this age? Type answer here:c. What age is the youngest seafloor, and what color is used to show this age?Type answer here:2. Using the Mid-Atlantic Ocean Profile Line, locate the two lines labeled 90 Ma (Ma means Millions of years before present). These locations are where the seafloor age has been measured to be 90 million years old. Click on the Ruler button. Using the mouse, click where the 90 Ma line intersects the profile line on one side of the Mid-Atlantic Ridge and then click again where the 90 Ma line intersects the line on the other side of the ridge. In the Ruler pop-up menu, under the ‘Line’ heading, use the pull-down menu to change the units to kilometers. The distance is given as ‘Map Length’. a. What is the distance between the two 90 Ma isochrons in kilometers?Type answer here:3. The distance measured in the previous question is the amount the Atlantic Ocean has grown in the past 90 Ma. Geologists call this process of growth seafloor spreading. Use the distance and the age of the seafloor to calculate the speed that the Mid-Atlantic Ridge is spreading. Show all of your work, including the units (km/Ma).????????????????????=????????????????????????????????????????????????Type answer here:4. Geologists use a helpful relationship convert speed from units of km/Ma, (kilometers per millions of years) into something easier to conceptualize, mm/yr (millimeters per year). The speed unit of mm/yr is easier to think about because we all know what a year feels like!Since there are a million millimeters in a kilometer, and there are a million years in a Ma, the relationship is: .1 ????????/????????= 1 ????????/????????
Written by Adrienne Sorenson MSc. Central Washington University Geology Department 2020Therefore, whatever speed you have in km/Ma units will be the samein the units of mm/yr. For example, if you had a speed of 20 km/Ma, this would be the same as 20 mm/yr. a. What is the speed of spreading in the Mid-Atlantic Ocean, expressed as mm/yr (millimeters per year)?Type answer here:5. Use the Ruler tool to measure the distance between the two 30 Ma lines. Give the distance in kilometers.Type answer here:6. Use the distance the seafloor has spread from the ridge, and the age of the seafloor to calculate the speed that the East Pacific Rise is spreading. Show all work, including units.Type answer here:????????????????????=????????????????????????????????????????????????7. What is the speed of spreading in the southeastern Pacific Ocean, expressed as mm/yr (millimeters per year)?Type answer here:8. In the Atlantic Ocean does the distance that the seafloor grew from the ridge appear to be the same on both sides? Explain. What about in the Pacific Ocean?Type answer here:9. Provide an explanation for why there is no 90 Ma seafloor in the Pacific, off the coast of South America.Type answer here:10.How does the speed of spreading compare between the Atlantic and the Pacific Oceans? Are the rates of spreading the same? If not, in which ocean is the rate of spreading faster or slower?Type answer here:11.Looking back on your topographic profiles from Module 3, can you see a topographic difference between the mid-ocean ridge in the Atlantic Ocean and in the Pacific Ocean? Type answer here:
Comments from Customer
I provided all data that I had.
