Sonar turned flat-topped seamounts into clues about moving plates
A ship can map the ocean floor by sending sound pulses downward and timing their echoes. The delay reveals how far away the bottom is; repeated measurements trace underwater ridges, valleys, and isolated mountains that ships cannot see from the surface. Some of those mountains had an unexpected shape: broad, unusually level tops, far below the reach of waves.
These structures, called guyots, are volcanic mountains whose summits were flattened by erosion. When a volcano first rose above the sea, waves wore down its exposed summit, much as they erode a coastal headland. Later, the flat-topped island sank beneath the water, preserving its old shoreline as a submerged surface.
The flat top records a journey
Oceanic crust forms at mid-ocean ridges, where hot material rises and new seafloor is created. As that crust moves away from the ridge, it cools, thickens, and gradually sinks. A volcanic island built on it can therefore be carried across the ocean while its supporting seafloor descends. Once submerged, its summit is no longer exposed to waves, so the ancient flattening remains visible in sonar maps.
A guyot is not a direct marker of where new crust formed, but it records what happened afterward: an island stood at sea level, then moved and sank with the ocean floor. Mapped alongside ridges and other seafloor features, guyots helped scientists read the ocean basin as a changing landscape rather than a fixed one. A smooth, level summit thousands of feet underwater is thus a preserved trace of an island’s former position at the surface.
