Movements of the Earth's Plates in the 'Absolute' Reference Frame

In this topic, we'll begin using the measurements of plate motion obtained through Global Positioning System (GPS) measurements and explore the relation between movements of the surface of the Earth and the world's plate boundaries.

We'll be using a special reference frame, referred to as the no-net-rotation (NNR) reference frame. NNR is the motion of each plate with respect to the weighted average of all of the world's plate velocities. It's sometimes referred to as absolute plate motion, since it appears to approximately represent the motions of the surface plates with respect to the Earth's deep interior. It closely resembles the apparent motion of the world's plates with respect to hot spot volcanoes.

Activity 1

Clear all 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: Be sure that Face of the Earth & Relief is selected.
Feature(s): Select Tectonic Plates to add the detailed plate boundary classification to the map.
Velocities: Select No-Net-Rotation, then the Model velocity field. (The Model button may not appear until you've selected No-Net-Rotation.) Click on Make Changes.

Note that the tasks in this topic assume some basic knowledge of vectors and the manipulation of vectors by vector addition and subtraction. For additional information, see the background section on vector analysis. In the near future, background information will also be available on frames of reference.

Using the estimated surface velocities indicated by the blue velocity vector arrows, describe the overall pattern of motions of the surface of the Earth. Answer the following questions:

  1. How does the pattern of velocity vectors relate to the distribution of plates? Do motions vary significantly within the plates? Across plate boundaries?
  2. Which of the world's plates appears to have the highest 'absolute' plate velocity'? The lowest?
  3. What is the approximate average motion of the Australian plate in this reference frame? Of the South American plate?

Activity 2

Base map: Same (Face of the Earth & Relief)
Feature(s): Same (Tectonic Plates)
Velocities: Same (No-Net-Rotation and Model)

Now we'll estimate the plate motion for each of the world's major plates. Open Table 1 and enter the following for each of the major plates (if you haven't done so already).

Now, using the velocity vectors shown in the global map, estimate (by eye) the average motion of each of the 13 major plates. Enter the estimated rate in Table 1. Note that to fully describe the velocity, you will need to specify two quantities, the rate (usually measured in units of mm/yr), and the direction or azimuth (usually estimated in degrees measured clockwise from north). The rate can be estimated by comparison with the 50 mm/yr reference arrow shown in the velocity legend. The azimuth can be measured by using a protractor placed against the screen or printout.

Based on your measurements of absolute plate motion, answer the following questions:

  1. What is the measured velocity of the North American plate near Chicago? (Note: Click on the map to zoom in for a more blown-up view; use the Zoom Out button to move out.)
  2. What is the measured velocity of the Pacific plate near Hawaii?
  3. Is there any general pattern of the rates of plate motion for dominantly continental vs. dominantly oceanic plates?
  4. How does the direction of motion of the Pacific plate compare with the configuration of plate boundaries that surround it? (Hint: How does the plate move with respect to the convergent and divergent plate boundaries that form its borders?)

Activity 3

Finally, let's compare the observed surface motions with those predicted by a global plate motion model.

A special kind of calculator, developed by scientists at the UNAVCO Consortium, computes the predicted motion for any point on the Earth's surface, based on a widely used global plate motion model, called the "NUVEL-1A" plate model. Click to access the global plate motion calculator.

Base map: Same (Face of the Earth & Relief)
Feature(s): Select No Features to clear any previous selections, then click on Make Changes. Now select Political Lat/Long to display a latitude and longitude grid.
Velocities: Clear the velocities by clicking on No Plate Velocities. Click on Make Changes.
Zoom: Use zoom utility to examine detailed maps as necessary. (To zoom in, click directly on the map; to zoom out, click on the Zoom Out button.)

Choose four locations around the world and estimate their locations in terms of latitude and longitude. (This is for practice.)

Now estimate latitude/longitude coordinates at the following four points by interpolating between grid points on the map. Use the zoom utility to make your measurements on the zoomed-in maps of each of the following continents. Enter the results in the first two columns of Table 3.

  1. Chicago, Illinois (North America)
  2. Honolulu, Hawaii (Pacific)
  3. Johannesburg (South Africa)
  4. Sydney (Australia)

Use the plate motion calculator to predict the motion at these four locations and enter the results in the Plate Motion Rate (Predicted) column of Table 3. Now estimate the rate and direction of the observed plate motion from the global plate model.

How well do the predicted and observed plate motion rates agree? What might explain the discrepancies between the two sets of measurements? Is it possible that BOTH are correct?