Chris: How fast does a tectonic plate move? That’s actually a somewhat tricky question, because rigid plates on a sphere cannot translate (all points moving in the same direction at the same speed) – instead they move by rotation about a point on the Earth’s surface known as an Euler pole. This leads to velocity gradients across the plate, with points further away from the rotation pole moving faster than points closer to it. For example, this figure indicates that the central region of the Pacific plate is moving 2-4 times as fast as its northern and southern edges (speeds in mm/yr). How do we turn this range of numbers into a ‘speed’ for the entire plate?