How do scientists use hotspot volcanism to tell the direction and speed
of a plate?

Since many hotspots are stationary in the mantle, geologists can use some hotspot chains to tell the direction and the speed a plate is moving. Although most hot spots occur far from plate boundaries, they offer a way to measure plate movement. This is because a hot spot generally stays in one place while the tectonic plate above it. At a hot spot, the heat from the plume partly melts some of the rock in the tectonic plate above it.