One of the reasons for this is that the trails seem to have remained in the same position over very long periods of geological time (they appear to persist for periods of approximately 200 million years), whereas patterns of mantle convection currents seem to be a lot more erratic. .
As the trail rises towards the base of the lithosphere, the reduction in pressure allows partial melting of the mantle material within the trail to form basaltic magma. The magma melts its way through the oceanic crust and erupts onto the ocean floor to build up an active volcanic island. As the plate carries on moving over the trail, the original island is carried away from the magma source and becomes extinct, which means that it has died and no longer burns. The plate acts as a conveyor belt so as the old island is carried away; a new volcanic island is formed in its place above the hotspot. This process builds up a chain of islands, with the age of each increasing with distance away from the currently active island. As the old islands are carried away from the hotspot, they subside (sink down into the crust) and are eroded by the sea, so that many of the older islands are now under the sea surface and form guyots.
Guyots are flat-topped seamounts that were once tall enough to approach or penetrate the sea surface. Generally they are confined to the west-central Pacific. The flat top suggests that they were eroded by wave action when they were near sea level. Their plateau like tops eventually sank too deep for wave erosion to continue wearing them down. Like the more abundant seamounts, most guyots were formed near spreading centers and transported outward and downward as the seafloor moves away from a spreading center and cools.
The Pacific Ocean shows a number of sub-parallel chains of volcanic islands that run diagonally across the Pacific plate. Isotopic dating examines elements that are chemically identical with each other but have different nuclear mass, and in all cases the lavas making up the volcanic islands show that the age of the islands increases from South East to North West.
