kw: oceanography, geology
Now that Google™ Earth has better topography information for the sea floor, I decided to have a look at the "poster child for sea-floor motions", the Hawaii-Midway-Emperor chain of islands and seamounts. Look here for the latest version, which has new tools for ocean floor exploration.
The outlined islands to the right comprise Hawaii. The label shows the location of the Midway island group, and the Emperor Seamounts form the rest of the chain. The chain ends at the Aleutian Trench at the top of the image.
Here is what is happening. There is a "hot spot", anchored in Earth's mantle, over which the Pacific Plate is moving at a rate of a few centimeters yearly. The top of the hot spot is just east of the Big Island of Hawaii, under the newest island, Loihi, which hasn't made it to the surface yet. With its peak (crater) about a kilometer down, Loihi is more than 2/3 of the way up and growing with each undersea eruption. The active volcanoes on the Big Island, mainly Kilauea, are still fed by the hot spot, but waning.
Midway Atoll, a ring of coral islands collectively called "Midway Islands", is about halfway from Hawaii to the kink in the chain. It was atop the hot spot and the active volcanic site more than twenty million years ago. Prior to about fifty million years ago, the Pacific Plate was moving nearly due North. The Emperor Seamounts north of the kink record this motion. The oldest seamount, which is on the brink of vanishing into the Aleutian Trench, is about 120 million years old. Its flattened top is more than a kilometer beneath the surface now. It, like all the seamounts, was an atoll once, but the ocean floor of which it is a part has sunk further into the earth as it has aged and become colder and denser. Between 50 and 45 million years ago, the ocean floor changed direction, so now it is moving about northwest or west-by-northwest.
This is the eventual fate of the Hawaiian Islands over the next few million years: to erode down to the ocean surface, then as the ocean floor slowly sinks under them, to develop coral caps and become atolls, but finally to sink beneath the waves, and after 100+ million years, to be subducted into the Japan Trench, assuming the current motion continues without another kink. They have several thousand kilometers to go, at the rate of a meter every eleven years, or 90 km per million years.