Q.How do you think the rate of plate movement is determined?

The rate of plate movement is determined by measuring how far a plate has moved over a known span of time, then dividing that distance by the time elapsed. The key is getting an accurate age for the rocks or magnetic stripes that record the movement.

Two main methods are used. First, radiometric dating of rocks on the ocean floor gives the age of the crust at different distances from the mid-ocean ridge. Since new crust forms at the ridge and moves outward, older crust is found farther away. Second, magnetic reversal patterns in the seafloor act as a tape recorder: as magma cools at the ridge, it locks in the direction of Earth's magnetic field at that time. By matching these stripes to the known timescale of magnetic reversals, you get the age of each stripe. …

Think of the Earth’s surface as a cracked eggshell, with each piece drifting slowly. You cannot feel it, but the plates move at about the speed your fingernails grow. To measure something that slow, you need a long ruler and a very patient clock. Geologists use three main methods, each with its own strengths and limits.

The most direct method is satellite technology. GPS stations fixed on continents send signals to satellites, and by tracking tiny changes in their positions year after year, scientists can calculate exactly how fast a plate is moving today. This gives the present rate, in centimetres per year, and it is precise enough to see the Atlantic widening by about two to three centimetres annually. But GPS only shows the last few decades — a blink in geological time.

For longer-term rates, scientists turn to the ocean floor. As magma rises at mid-ocean ridges, it cools and records the Earth’s magnetic field. The field flips polarity every few hundred thousand years, leaving a striped pattern of normal and reversed magnetism on either side of the ridge. By dating these magnetic reversals (using known ages of rock samples) and measuring the distance of each stripe from the ridge, you get a simple calculation: distance divided by time equals rate. This method reveals average rates over millions of years, and it is why we know the Pacific plate moves faster than the Atlantic plates.

A third method uses volcanic hotspots. A hotspot is a fixed plume of magma rising from deep in the mantle, punching through a moving plate to create a chain of volcanoes. The Hawaiian Islands are the classic example: the oldest island is far from the hotspot, the youngest is directly above it. If you know the age of each island (from radiometric dating of its rocks) and the distance between them, you can compute the plate’s speed over tens of millions of years. This gives a long-term average, smoothing out any short-term wobbles. …