Expert: Dr Thomas Bell - 7/4/2009

Question
Hello Dr Bell. I would like to know if modern GPS has accurately measured the rate of separation of Europe and America, and if so, what the rate is?

Answer
Glenn,

I've done a little research on your question and the answers come as a complete surprise to me.  One of the satisfying benefits of answering people's questions about geology is motivation for me to learn even more.

First, the velocity of tectonic plates is difficult to measure for many reasons but certainly the main one is their speed.  This is a very slow process and our time scale to measure it is constrained.  To get a good idea of plate velocity, we need to make sequential measurements over many years.  Another complication is the three dimensional scope of the measurement.  Since plate movement is actually a rotation of a 'skin' on a shape that approaches a sphere (we call Earth's planetary shape a geoid) all measurements have three components, northing, easting, and elevation.

I was quite surprised to discover that geodisists (scientist who measure the dimensions of Earth) use radio telescopes to measure plate velocity!  The technique they use is called VLBI (very long baseline interferometry).  For over thirty years, a group of geodisists have been measuring the distance between a worldwide array of up to twenty radio telescopes to determine the rate of separation between North American and Europe, displacement along the San Andreas fault, and many other plate movements.

VLBI is elegant in its conception and extremely accurate.  The distance calculation is based on the time lag at each station of a simultaneous measurement of a single radio source at a great distance from Earth.  This is all new information to me and the real experts do a great job of explaining the technique and some of their results here:

http://www.haystack.mit.edu/edu/pcr/Data/pdf/Introduction%20to%20plate%20tectoni...