Scattered across the sea floor are many submarine ridges, seamounts and plateaus (Figure 2.4). These are not obviously related to plate tectonics, as are the mid-ocean ridges and deep ocean trenches. Some of these edifices reach above sea level to form islands. In Chapter 3 I recounted how Tuzo Wilson built on the observations of Darwin and Dana, who had discerned an apparent age progression along island chains in the Pacific, from volcanically active islands through eroded islands to atolls marking where an island had been eroded to sea level. The classic example is the Hawaiian island chain, shown in Figure 7.1. As well as the islands, there is a long chain of seamounts extending to the northwest. There is also a broad swell in the sea floor around the Hawaiian chain. These observations give us important information about processes in the mantle.