Measuring Relative Plate Motion

In this topic, we'll examine the implications of global plate motions for geologic processes at a number of the world's plate boundaries. In this instance, we are less interested in the 'absolute plate motion' (the motion of a plate with respect to the Earth's deep interior) than the 'relative plate motion' (the relative movement that takes place along an individual plate boundary). For it is this relative motion that controls the amount and type of earthquake activity, the presence and style of volcanism, and the geologic processes that take place along the plate boundary and in neighboring areas of the adjacent plates.

Activity 1

Clear the previous settings: Click on the World Map button. Then select the Face of the Earth & Relief map, No Features, and No Plate Velocities. Click on Make Changes.
Base map: Face of the Earth & Relief should be selected.
Feature(s): Select Tectonic Plates.
Velocities: Select No-Net-Rotation, then click on Model. Click on Make Changes.

First, we will use the 'absolute' motion of each of the plates to calculate the 'relative' plate motion at selected plate boundaries. It is this relative plate motion that governs the interactions - the types of earthquakes, volcanoes, and geologic deformation - occurring along individual plate boundaries.

To do this, you will need to calculate the difference between the two 'absolute' plate motion vectors. (Click for information on how to subtract two vectors.) Do the calculation for the following plate boundaries and enter the results in Table 4, the Plate Motion Rate (Estimated from Absolute Vectors) column. Use the zoom utility to make your measurements on the zoomed-in maps of each of the areas described below:

  1. North America - Pacific (San Andreas Fault)
  2. North America - Africa (Mid-Atlantic Ridge)
  3. Pacific - Eurasia (Japan)
  4. India - Eurasia (Himalayan Mountains)

Answer the following questions:

  1. How do these predicted relative motions compare with the orientation of the plate boundary (parallel, perpendicular, oblique)?
  2. What type of plate boundary would you expect based on these relative plate motions for each of the boundaries listed above?
  3. How do the expected plate motions compare with the type of plate motion denoted by the colors on the plate boundary map? (Remember that the key is in the legend pop-up window.)

Activity 2

Next, we can test the predictions by changing the reference frame to that of either one of the two plates facing the plate boundary in question. Let's use North America as an example.

Feature(s): Click on No Features, then the Make Changes button.
World map: Click on the World Map button to return to the global map. Now select North America by clicking anywhere on the North American continent.
Velocities: Select the North American plate reference frame, then click on the Make Changes button.

Note how the velocity vectors shown on the map have changed orientation and scale. Also note how the vectors for sites within the North American plate have dropped to near zero - indicating that most points within the plate are not moving with respect to the plate itself. (Note that we said 'most' but not 'all.') Take a careful look at the model velocities in the westernmost part of North America (if necessary, zoom in on this area). Can you identify any areas where the velocities with respect to North America are not zero? If so, describe the pattern of velocities that you observe and consider a possible explanation for such 'intraplate' motions of the Earth's crust (motions taking place within a tectonic plate).

Now, estimate the rate and direction of relative plate motion at the boundaries marking the junction of the North American plate with the Pacific Plate (at the San Andreas Fault in California) and note these numbers in Table 4 (the Plate Motion Rate/Direction, Neighboring Plate Fixed columns). Using the same reference frame, estimate the motion of Africa with respect to North America and enter the estimate in the second row of Table 4.

Then repeat the procedure by switching to the Eurasian plate reference frame and estimating the following:

Do either of the two estimates of relative plate motion agree? What might explain the discrepancies between the two sets of measurements? Is there a way that your first estimate of relative plate motion might be improved?

Given the direction of relative plate motion observed at each of the four plate boundaries, can you predict the type of relative motion (convergent, divergent, transform) taking place at each of the plate boundaries? If so, enter the result in the Expected Type column of Table 4.

Given the type of plate motion that you predicted at the four plate boundaries, what type of faults (normal, reverse, or strike-slip) would you expect to observe?

Activity 3

World map: Return to global map by clicking on the World Map button. Now select Eurasia by clicking anywhere in the Eurasian continent.
Velocities: Select the Eurasian reference frame and model. Then click on the Make Changes button.

The vectors describing relative motion are symmetrical. To use an analogy, imagine two unmarked police cars driving past one another, one going east, the other going west at 70 mph. Each is clocking the other's speed with a radar gun. The cop in car A measures the speed of car B as 140 mph to the east. Similarly, the cop in car B measures the speed of car A as 140 mph to the west. The same is true with tectonic plates. If a geologist in South America measures the motion of the African Plate, he or she might note that it is moving east at a rate of 35 mm/year; an observer in Africa would measure South America moving to the west at the same rate.

To test this notion, compare the motion of India with respect to Eurasia (examine the vector representing the motion of northern India with the Eurasian reference frame), then compute the motion of Eurasia with respect to India by selecting the Indian plate reference frame.

How would India appear to be moving to an observer standing in Siberia (central Eurasia)? How would Eurasia appear to be moving for an observer in New Delhi (northern India)? How do these velocities compare? In each case, what type of plate motion do they predict for the Himalayan Mountains? What type of fault motion would they predict?

Activity 4

World map: Return to global map by clicking on the World Map button. Now select North America by clicking anywhere in the North America continent.
Velocities: Select the North American reference frame, then click on the Make Changes button.

Note that the relative plate motion occurring between plates depends strongly upon where along the plate boundary you are making observations. For instance, use the model vectors to estimate the motion of the Pacific Plate with respect to the North American plate at two very distant locations: San Francisco, California (you've already done this!) and Anchorage, Alaska.

What direction does the Pacific Plate appear to be moving to an observer standing in San Francisco? For an observer in Anchorage? How do the rates compare? What type of plate motion would you expect at these two sites based on the directions of relative plate velocity?

Activity 5

Settings: Same (don't change anything)

Now we'll compare our measurements of observed plate motion with those predicted using the global plate motion model NUVEL-1A. To do so, we'll use another kind of plate motion calculator, this one developed by University of Tokyo geophysicist Kensaku Tamaki. This one computes the predicted relative motion expected for any point on the Earth's surface, based on the NUVEL-1A global plate motion model. Click to access the relative plate motion calculator.

To use the calculator, you must enter the latitude and longitude of a point along the plate boundary (use the numbers from Table 3), and the two plates on either side of the plate boundary. Note that one of the two plates is considered 'fixed,' meaning that the velocities of the 'moving' plate will be calculated in the frame of reference of the 'fixed' plate. When you press the "Execute Calculation" button, you will get, among other things, an estimate of the plate velocity (in cm/yr) and direction (in degrees, measured clockwise from North).

Use the calculator to predict relative plate motion rates at each of the four plate boundaries, and enter the results in the Plate Motion Rate (Estimated from Global Plate Model) column of Table 4. Note that you must convert the units from cm/yr to mm/yr in order to compare all of your plate motion estimates.

How well do the observed (i.e., surface velocity model) and predicted (i.e., global plate motion model) plate motion estimates agree? What reasons can you think of that might explain the discrepancies between the two sets of measurements?

Activity 6

Finally, we'll do an interesting comparison. How well do the rates of present-day plate motion that we've measured using GPS and other surveying techniques compare with those estimated over the millions of years of geologic time? To do this, we'll look at the relative motion between North America and Africa. Hopefully, you've discovered that North America and Africa are currently moving apart from one another. The result of this plate divergence is the creation of new lithospheric plates by the process of sea-floor spreading. While the process of plate motion is slow (measured in millimeters per year), this divergence between plates results in the creation of tens of kilometers of new oceanic crust every million years. (Note that 1 mm/year = 1 km/million years.)

What's more, this process of sea-floor spreading is sufficiently fast that the Earth's entire sea-floor (two-thirds of the planet's surface area) is replaced every 200 million years or so - a relatively short time when compared with the Earth's 4.6 billion year history.

Now, here's the trick. We can use one of our other base maps, Ocean Floor Age, to estimate the amount of oceanic crust that has been created over the course of recent geologic time. Make the following changes:

Base map: Click on World Map. Then select the Ocean Floor Age base map
Feature(s): Select No Features
Velocities: Select No Plate Velocities; click on Make Changes

The bright colors on this base map signify the age of the sea floor, determined from a combination of measurements of radiometric ages of rocks dredged from the sea floor, ages of fossils from oceanic sediments, and magnetic field measurements of the oceanic crust. (Access the Testing the Sea-Floor Spreading Hypothesis website for an explanation of sea-floor spreading and measurement of the age of oceanic crust.)

If, as the sea-floor spreading hypothesis contends, oceanic crust is created symmetrically on both sides of the mid-Atlantic Ridge, then we can measure the age of the sea floor on both sides of the mid-ocean ridge and the distance across the 'newly created' ocean to calculate the average velocity of sea-floor spreading. (Recall that velocity = distance ÷ time).

Look at the color of the sea floor furthest from the mid-Atlantic Ridge (closest to the margins of the North American and African continents) and use the legend (in the pop-up window) to calculate sea-floor age. Then measure the approximate distance across this stretch of sea floor using a ruler, noting that 1° of latitude = 111 km. Use the distance and age of the sea floor to estimate the average velocity. (Remember that velocity = distance time.) You'll need to convert the units, now measured in km/my, into the more standard plate velocity rates of mm/yr.

How well do the present-day and observed plate motion rates agree? What might explain the discrepancies between the two sets of measurements?