 Midway was formed roughly 28 million years ago when the seabed underneath it was over the same hotspot from which the Island of Hawaii is now being formed.In fact, Midway was once a shield volcano, perhaps as large as the island of Lana'i.As the volcano piled up lava flows building the island, its weight depressed the crust and the island slowly subsided over a period of millions of years, a process known as isostatic adjustment.