The plates are divided by seismically-active boundaries. The type of boundary depends on the motion occurring between the plates at that location. The plates move away from each other as new oceanic crust is created. Hotspots are places where molten rock from the earth's mantle is erupting at the surface. Hawaii and Iceland are hotspots.
Some scientists think the hotspots move relative to the earth.
Other scientists think the hotspots are fixed to the earth. Hotspots leave their marks on the plates as the plates move across them. This feature was made by the Hawaiian hotspot as the Pacific plate moved over it. In the northwest Pacific you can see a whole series of seamount chains that were formed by hotspots. There are various theories on what the driving forces are for plate tectonics, including 'slab pull' and the hotspot theories.