Studies show a clear "flow" of species from older to newer islands. There are also cases of colonization back to older islands, and skipping of islands, but these are much less frequent. By potassium/argon radioactive dating, the present islands date from 0.4 million years ago (mya) (Mauna Kea) to 10mya (Necker). The oldest member of the Hawaiian archipelago still above the sea is Kure Atoll, which can be dated to 30 mya. The archipelago itself, produced by the Pacific plate moving over a hot spot, has existed for far longer, at least into the Cretaceous. The Hawaiian islands plus former islands which are now beneath the sea make up the Hawaiian–Emperor seamount chain; and many of the underwater mountains are guyots.