Plates in Motion

Topic:
Earth science

Type of Web activity:
How fast is the plate under your feet moving?

Materials / Software needed:
Web browser

Grade Level:
9-12

Time involved:
One class period

Created:
July 2000

This site developed and maintained by Deborah Hunt and Eric Muller .

3601 Lyon St.
San Francisco, CA 94123

©2000 The Exploratorium

You'll need your geographic coordinates. If you don't know your latitude and longitude, start with the Coordinates Lesson and then return here.

As an example, lets start with my coordinates at the Exploratorium. 37N 48' 10" and 122W 26' 54"

First I need to convert these coordinates to decimal degrees.
There are 60 seconds in a minute, so the seconds can be incorporated into the minutes this way:

Latitude = 37N 48+10/60" = 37N 48.17
Longitude = 122W 26+54/60" = 122W 26.9"

There are 60 minutes in a degree, so we can incorporate the minutes into the degrees this way:

Latitude = 37+48.17/60 = 37.80N
Longitude = 122+26.9/60 = 122.45W

Make this calculation on your coordinates now.

Plates in Motion: We know that the Earth's outer shell is broken into 14 major plates that are moving about. Geophysicists can estimate how fast each plate is moving with respect to the earth as a whole and also how each plate is moving with respect to any other plate.

Kensaku Tamaki of the Ocean Research Institute, University of Tokyo ( tamaki@ori.u-tokyo.ac.jp ) has created a very helpful web site that allows one to find the plate motion velocity of any point on the Earth and the relative motion between any two plates. Go to his site now in a second window.

Absolute Plate Motion: Click on "Present-day Absolute Plate Motion" on Kensaku's page. In the "Name of Plate" box, select the plate where you are located. If you are in the US or Canada, you're on the North American plate. Enter your latitude and longitude and then press the "Execute calculation" button. (Remember that western longitudes and southern latitudes must be entered a negative numbers.)

For the Exploratorium, I found:

Velocity = 2.29 cm per year
Direction = 240.65 degree

The direction is measured clockwise from north. For example, a northward movement has a direction of 0 degrees, eastward motion 90 degrees, southward motion 180 degrees, and westward motion 270 degrees. 241 degrees mean the Exploratorium is moving west-southwest.

Since we are near the San Andreas fault, which forms the boundary between the North American plate and the Pacific plate, lets also calculate the absolute plate motion of the Pacific plate at the location of the Exploratorium. Filling in the numbers yields:

Velocity = 5.50 cm per year
Direction = 301.89 degree

If you know vector algebra you can compute the relative motion of the Pacific plate with respect to the North American plate from these two absolute velocity vectors. I computed:

Relative velocity = 4.8 cm per year
Direction = 326 degrees

Relative Plate Motion: We can also calculate the relative plate motion from Ke nsaku's site. If you live near a plate boundary, return to his main calculation site and select "Present-day Relative Plate Motion (NUVEL-1A Model)." I entered North America for the relatively fixed plate and Pacific for the relatively moving plate. Enter the name of the plate you are located on and the name of the plate that is just across the plate boundary you are near. For the Exploratorium, I found:

Velocity = 4.60 cm
Direction = 326.58 degree

This is very close to the relative velocity I computed above from the absolute plate velocity vectors.