GEOLOGY 108: Crises of the Earth

Homework 1

Use a Plate Motion Calculator on the WWW to determine the velocity of the Pacific Plate in San Francisco (the west side of San Francisco and the San Andreas Fault) with respect to the North American Plate (the east side of San Francisco and the San Andreas Fault). Use the NUVEL-1 plate motion model (developed by Rice Professor Richard Gordon and colleagues). How far will the Pacific Plate move with respect to the North American Plate in 1 million years? In what direction will it move (N, NW, W, SW, or etc.)? (Note that velocity units of mm/year are equivalent to km/Ma; Ma = one million years).

An alternative Plate Motion Calculator is available in case the first one is not working for some reason; this version allows use of either the NUVEL-1 or an updated NUVEL-1A model to calculate relative plate motions, and also absolute motions of individual plates.

To use the Plate Motion Calculators to answer these questions, you will first need to find the latitude and longitude of San Francisco. See if you can find this information on the web. If you get stuck, you may refer to the Latitude and Longitude in the US web site (incidentally, provided by the Bahai faith to allow its members to determine the direction to face for religious worship).

For the coordinates of San Francisco, according to the NUVEL-1 model, the Pacific plate moves past the North American plate with a velocity of about 48.l mm/year or 48 km/million years) with a NNW azimuth of about 326.5 degrees (or 146.5°, depending on the order in which the plates are entered; these results differ by 180° and are equivalent). In one million years west San Francisco would be offset roughly 48 km in this direction (NNW) relative to east San Francisco.

Next, calculate relative plate motions (velocity and direction) for the following plate boundary locations:

  1. East Pacific Rise (Pacific and Nazca plates) at 15° S lat., 110° W long.
  2. Mid-Atlantic Ridge (North America and African plates) at 20° N lat., 45° W long.
  3. Aleutian trench (Pacific and North American plates) at 55° N lat., 150° W long.

In each case, describe briefly the type of boundary (what is happening) and how your results might correlate with your observations from the in-class plate boundary exercise. In one million years, how much new ocean crust will be created at the two spreading centers and how much of the Pacific plate will be consumed at the Aleutian trench?

A view of the 'rigid' tectonic plates, showing surface topography

Some other helpful resources include the Plate Boundary and Satellite Gravity maps used in the in-class exercise. (All these maps can be reviewed from the WWW using the Plate Boundary Exercise site.)

-----LAST MODIFIED: 2/2/99
-----BY: Leeman & Sawyer