[cossatot]

I forked this and put plate tectonic velocity vectors (in an earth-centered, earth-fixed coordinate system) in place of wind vectors but didn't have the .js knowledge or time to fix all the annotations and menu stuff. I'm hoping to get to that this fall (but I've been saying that for a year or two...):

http://earth-analysis.com/vels/public/

[elihu]

That's a very good idea. Well done.

[sevenless]

Nice! Lucky the annotations aren't accurate, I wouldn't like to have 68mph plate tectonics... :)

Interesting that there seem to be the plate tectonic versions of cyclones, like off Baja California.

Also, is the absence of data at the poles due to a coordinate system limitation?

[cossatot]

The velocities are in mm/yr. 68 mph plate tectonics would definitely be terrifying; that's an order of magnitude faster than faults move during an earthquake.

There are two phenomena that look cyclonic.

The larger ones have to do with motion on a sphere: Euler showed that any velocity vector on a sphere can be represented as a rotation about a point (called an Euler pole); tectonic plates move basically rigidly, so the whole things rotate around a point. When that point is very close to (or within) the plate, the rotation is quite visible. You can see this in the Antarctic plate, halfway between Antarctica and Madagascar.

The second is an artifact of the vector visualization that Beccario used. The atmosphere is a continuum, so particles move smoothly between one location and another (more or less). But tectonic plates have rigid boundaries, and dive under each other or slide rigidly past one another. But the visualization algorithm doesn't know about these boundaries, so it creates particle paths that cross the boundaries and appear to swirl around. This probably happens to some degree in the mantle below the plates, but not really at the surface, with minor exceptions that I won't get into now unless people really want to nerd out on microplate rotation.

I'm not sure about the data gap at the poles. That's probably a real gap in the dataset I used; I haven't looked into it.

[effie]

The point where velocity field vanishes seems to be a good example of the hairy ball theorem; even if the motion on the surface is not a rotation, if the field is continuous there has to be at least one point on the surface where velocity is zero.

https://en.wikipedia.org/wiki/Hairy_ball_theorem