Plate tectonics is a unifying framework for understanding the dynamic geology of the Earth. The theory posits that the outermost layers of the Earth (the crust and uppermost mantle) make up the brittle lithosphere of the Earth. The lithosphere is broken up into a number of thin plates, which move on top of the asthenosphere (middle mantle). The asthenosphere is solid, but flows plastically over geologic time scales.
This assessment guides you through an examination of patterns on Earth—the topography of the earth’s surface above sea level and the distribution of earthquakes and volcanic rock ages. You’ll then use geologic data to determine long-term average plate motions.
To do this, you’ll use the program Google Earth, and Google Earth layers compiled from various sources. Please answer EACH questions, they are divided by subsections. Remember to access the tips and hints at the bottom of the page.
Getting started with Google Earth
On your computer, install the latest version of Google Earth
- Once installed, open Google Earth, under the Tools/Options/3D View/ menu choose the “Decimal Degrees” and Meters Kilometers” options and makes sure the “Show Terrain” box is checked.
- Open the View menu. Go ahead and experiment with the options, but in general you should just have the Tool Bar, Side Bar and Status Bar checked. Also on the View menu, hover over Navigation and you will see several options for the compass arrow and slide bars in the upper right corner of the Google Earth screen. “Automatically” is a good choice as it leaves a ghost of the image visible until you hover over it.
- Load the DynamicEarth.kmz file into GE. You should be able to double-click on the filename and it will open within GE. Or, you can download the file onto your computer, and open it in GE by using File/Open and navigating to the file. The original file info comes from https://serc.carleton.edu/sp/library/google_earth/examples/49004.html
- Once the DynamicEarth.kmz is loaded, click and drag to move it from “Temporary Places” to “My Places.” Then save “My Places” by clicking File/Save/Save My Places. DynamicEarth.kmz will now be available every time you open GE on this particular computer. When you exit, GE should save “My Places” for the next time.
- But you should manually save “My Places” whenever you make significant changes to it, as GE does not auto save during a session.
- With an active Internet connection, you now have an interactive view of the earth! Take some time to explore the Earth with Google Earth and figure out how the navigation works using the keyboard, your touch pad and your mouse. For example: Zoom in and out, move N, S, E, W, grab and spin the globe, etc. The resolution will change as you zoom. Clicking on the “N” of the navigation compass reorients the view so north is “up.”
- At top left, “search” (and fly to) any place of interest. Zoom in and click on the “street view” icon (orange stick figure under the compass at top right) to explore an area as if you were on foot
- Zoom in to see individual buildings, roads, cars, etc. (Find the crew team and motorboat on Lake Carnegie)
- Go 3D – zoom into a significant topographic feature (e.g. Mount Everest, the Grand Canyon, Niagara Falls). Hold the Shift key down and tilt the terrain using the Up/Down arrows to tilt the terrain, and spin the terrain using the Right/Left buttons. Do the same thing for topographic features on the ocean floor. Note that under Tools/Options/3D View you can increase the vertical exaggeration by up to 3x. This is useful to emphasize subtle features, but is pretty scary when you look at the Grand Canyon that way!
- On the Google Earth tool bar, click the clock-with-an-arrow icon to explore historical imagery in an area of interest (views through time of the Princeton campus, for example)
- By clicking and dragging, you can move things that you have found and want to save, from the “Search” menu into “My Places.” You can also re-organize “My Places” by adding and deleting items, changing the order of things, making subfolders, etc.
- Explore the built-in items under the Layers menu at bottom left, and Dynamic Earth layers in your Places menu.
- Expand and contract the folders and subfolders, turn various items on and off, etc. For example, with the Dynamic Earth/Volcanoes of the World layer displayed, left-clicking on a volcano brings up an information box about it.
Basic Requirements (assignment criteria):
- Expand the “Seismicity” layer item and click “on” the “Twenty years of large earthquakes” layer to show the epicenters of large earthquakes (those with magnitudes [latex]\geq[/latex] 6.0) during a 20-year period.
- Describe any patterns you see in the distribution of earthquake epicenters over the Earth’s surface. You will need to move around the Earth to explore the different locations—do they form lines, arcs, circles or clusters? Are patterns connected or disconnected?
- The different colors refer to the depths of the earthquakes. What color are the shallowest earthquakes? The deepest?
- Look closely at and around the Earth’s ridges (Mid-Ocean Ridge in middle Atlantic) and trenches (southeast Pacific). The earthquake depth patterns associated with these features are different.
- Complete the chart below. Using the earthquake depths as evidence, is the Earth’s lithosphere thicker in the vicinity of ridges or in the vicinity of trenches? Justify your answer.
|In the vicinity of ridges.
(Scan 1500km or so on either side)
|In the vicinity of trenches.
(Scan ~1500 or so km on either side)
Describe the depth or range of depths of earthquakes, and the distribution (symmetric or asymmetric?)
Is there any pattern to the depth distribution?
Volcanoes & Volcanic Patterns
Leaving the earthquake layer on, click on the “Volcanoes of the World” layer. Describe the relationship between the locations of most active volcanoes and locations of earthquakes. What types of volcanoes are typically found along plate boundaries as opposed to interplate volcanoes? What are some common hazards associated with volcanic eruptions?
- Unclick all the layers, and then click on the “plate boundary model” layer (click the box to show it and then click the + or arrow to expand the legend). This shows plate boundaries and the names of major plates.
- Find the boundary between the African and South American plates
- Where is this plate boundary, relative to the coastlines of Africa and South America?
- Now click the other layers on and off so that you can see relationships between plate boundaries and these features. If you did not have the “plate boundary layer” available to you, how could you determine where this plate boundary was? Be sure to consider topography as well as the earthquake and volcano layers. List several ways and be specific.
- What type of deformation is occurring as a result of this boundary, if any? What type of structures or features would you expect to see, be specific?
- Travel westward across the South American plate to its boundary with the Nazca plate
- Where is this plate boundary, relative to South America?
- If you did not have the “plate boundary layer” available to you, how could you determine where this plate boundary was? List several ways and be specific.
- What type of deformation is occurring as a result of this boundary, if any?
- What type of structures or features would you expect to see, be specific?
- Motion across the mid-Atlantic ridge: the South American plate vs. the African plate
- How many years does each colored band represent? _______________________
- On average, continental crust is 2 billion years old; the oldest rocks are 3.8 billion years old, and some of the grains in those rocks are even older.
- What is the age of the oldest seafloor? _______________________________
- On average, which is oldest – the continents or the ocean basins? _________________
- Find the South American plate, the African plate, and the Mid-Atlantic Ridge that marks the boundary between them. What happens to the age of the seafloor as distance increases away from the Mid-Atlantic Ridge?
- Is crust being created or destroyed at this plate boundary (and other spreading ridges)? ___________
- Is this plate boundary divergent, convergent, or transform? ________________
- Focus on the northern Atlantic Ocean, near the east coast of the US and the northwest coast of Africa. How long ago did the northern Atlantic Ocean begin to open up or start spreading? Describe your reasoning.
- Did the northern Atlantic Ocean basin start opening at the same time as the southern Atlantic Ocean basin (the area between to the south end of South America and Southern Africa)? How much older or younger is the northern Atlantic basin than the southern Atlantic basin? Describe your reasoning.
- Clear all layers except the “Seafloor Age” layer. From the Mid-Atlantic ridge, choose either the South American plate side or the African plate side. Click on “Tools” and then “Ruler” to click and measure the distance from the MOR of the various ages of the oceanic crust. Use Excel (or other resource) to make a graph of cumulative distance away from the plate boundary (y-axis) vs. age (x-axis). Format the graph appropriate with title, axes, etc. Have Excel fit a trend line/best fit line from your data. NOTE: each color corresponds to a different age. Use the legend or key provided by Google Earth to determine the age. Copy, save and add your graph to your responses under #26.
- Describe the motion of your chosen plate relative to the Mid-Atlantic ridge, based on this seafloor age data—the direction of motion, the average speed (slope of the best fit line) and whether or not speed and direction has been constant over time.
- Compare these results to independent data from the Tristan da Cunha Volcanic Island/Seamount chain on the African plate off the southwest coast of Africa as follows:
- To access this data, expand the “Volcanic Chains” layer on GE, then expand the “Atlantic Ocean Chains” layer, and then click to display “Tristan da Cunha”. You may need to click off Sea Floor age layer. They are green dots off the southwest coast of Africa.
- These islands and seamounts are volcanic edifices built up on older seafloor, formed by eruption of magma from relatively stationary sources (“hot spots”) underneath the moving plates. The numbers are the radiometric ages in millions of years of volcanic rocks collected from each island/seamount.
- Does data from the Tristan da Cunha Volcanic Island/Seamount chain support the plate motion you deduced from the sea floor age data? Explain.
- If you prefer you can choose to compare your results to data from the New England Seamount chain off the northeast coast of North America—these are on the North American plate on west side of the mid-Atlantic ridge. Does this data support the African plate motion deduced above? Explain. NOTE: You do NOT have to do both but you are free to do so for verification.
- Apply what you have learned—the Pacific Plate
- Turn your attention to the Pacific plate. Note that the Pacific Ocean is comprised of several plates; we want to focus on the very large Pacific plate (not the Nazca plate, or Cocos plate, or Philippine plate or other plates.) The Pacific plate is “born” underwater at the East Pacific rise, the spreading ridge west of South America. It is being destroyed at convergent boundaries around its northern, western, and southern boundaries. NOTE: No instructions are provided for using the different layers. You should be familiar with them by now. If needed you can go back and review.
- Is the East Pacific Rise spreading faster or slower than the mid-Atlantic Ridge and how can you tell – without doing any calculations? Has the rate been constant over time?
- Make and print out a graph (similar to what you did in question 24) for movement of the Pacific plate away from the East Pacific Rise. Remember to save it under #31.
- Describe the motion of the Pacific plate relative to the East Pacific Rise, based on this seafloor age data – direction of motion, average speed and whether or not speed and direction has been constant over time.
- Consider the Volcanic Chains in the Pacific Ocean (Hawaiian/Emperor, Louisville, and Easter Island). Do these data support the movement you deduce for the Pacific Plate? Explain.
- What does the bend in the Hawaiian/Emperor chain indicate happened about 50 million years ago to the direction of Pacific plate movement?
- Based on the evidence of plate motion, explain how supercontinents form and how they rift apart.
This section requires some critical thinking as you will NOT be using Google Earth for this short section. Thinking back over this activity and what you have learned about plate tectonics and plate motion, consider other effects it has on the Earth. Include a brief discussion on the effects plate tectonics has on crustal deformation. Make sure you include stress, strain, folds, faults and any other features you can think of. Discuss the types of forces needed to create the features, the time it would take and so on.
Continental Drift versus Plate Tectonics
- Describe the relationship between continental drift and plate tectonics.
- What evidence was used to support continental drift?
- What evidence was used to support plate tectonics?
- Do you agree or disagree with the evidence cited above?
- What are the three types of plate boundaries?
- What are the key features associated with each boundary?
The Role of Plate Tectonics
- Review the overall activity, what role does plate tectonics play in modern geology? (This includes everything from rock types to earthquakes and volcanoes to mountain building, climate change to evolution of life.)
- Save your completed assessment, make sure you name is on the document and you have included the necessary charts and graphs.
Tips and Hints can be found here.
This assessment is adapted from “Using Google Earth to Explore Plate Tectonics” by Laurel Goodell, originally found here.