A seamount is a an under mountain which does not emerge from the sea and is therefore not an island. They are usually under water volcanoes and can form long chains of which string out across the ocean floor.
Hawaii has an active volcanic seamount, Loihi, which will be the newest island in Hawaii. The Hawaiian islands are the emergent parts of a 3600m mile long chain of seamounts and atolls called the Hawaii-Emporer Seamount chain.The chain has 80 identified volcanoes.The Hawaiian islands are formed by a hot spot ( mantle plume) which pierces the crust above as the Pacific plate moves over the top. As the volcanoes move away from the hot spot they become inactive and erode beneath the waves forming seamounts.