It is said that "tectonic plates move about as fast as your fingernail grows." How fast is that?

I've made two "lines" in my thumbnail with a metal nail file. I'll measure the movement of these lines to find the rate of growth.                                       

Day 0

Day 8

Day 15

Day 24

Day 30

Day 38

Day 49

Day 60

Day 83

An estimate of the convergence along the Cascadia subduction zone in Oregon ranges from 1.3 in the south to 3.1 cm/year in the north.  See:

The least-square slope is 0.10 mm/day, or 3.7 cm/year. 

It's interesting that the growth appears to vary with time, but this may be due to measurement errors.

Graph shows data for May 14 - August 3, 2006.

Over a period of 500 days, my fingernail growth has remained quite steady at 0.1 mm/day, or 3.7 cm/year.


The San Francisco Exploratorium newsletter recently pointed out:

"...that your fingernails grow at the rate of about one nanometer per second? A nanometer (nm) is one-billionth of a meter—about the width of an average molecule. For comparison, a human DNA molecule is about 2.5 nanometers wide, while a strand of human hair can be 50,000 to 100,000 nm in diameter."

If we divide 3.7 cm/year by the number of seconds in a year (Pi * 10^7 s/year) and convert to nm by multiplying by 10^7 nm/cm, it turns out that my thumbnail is growing 1.1 nm/s.  Very interesting.

The following is modified from an E-mail message I wrote in 1997:

I have difficulty with the very slow processes of geology acting over very long time intervals, both within my own mind and in trying to convey these concepts to others. For plate motions, a helpful comparison is that the relative motions are similar to the rate at which a fingernail grows.

 Indeed, an analogy that just occurred to me now, is that as a fingernail moves forward, new nail is being created along one edge and destroyed (by the nail clipper) at the other. On the average, oceanic plates are created by volcanism at oceanic spreading centers at the same rate that they are consumed by subduction at oceanic trenches.

How slow is this motion? This is where long intervals of time come in. I like to suggest visualizing a football player in the year zero. She (sports where well integrated then) catches a kickoff at her own zero yard line and now has to run 100 yards to make a touchdown.  If she runs at the speed of a typical drifting plate (4 to 5 cm/a), she will score in the year 2000. The ability to outrun, and presumably outlive, the opposing team must also be presumed.

At a museum or school, one might label a crack in the floor "spreading center." A year later, paint a 3-cm wide band on either side of the crack and label it as the material created by plate motion during that year due to a 6-cm/a spreading rate. Add two 3-cm wide bands each year, so that after four years the floor would have this pattern, where the oceanic spreading center is marked by a v:

|  |  |  | v |  |  |  |