Midway began as a volcanic island, created over the hot spot in the earth's crust that now supplies the Island of Hawai‘i with its lava. As the Pacific plate marched to the northwest, the forces of wind, water and changing sea level eroded the island until it disappeared beneath the ocean surface. A fringing reef, made largely of the calcareous skeletons of coral and coralline algae, formed around the island's edge, creating an atoll. As the island eroded, the reef continued to grow. Today, the basalt that was once Midway is more than 500' below the ocean surface. One day, Midway Atoll will also vanish beneath the waves.
The movement of coral sand within the atoll created the three islands and the wind and water erosion continue to change the shape and size of these islands. Before the first sailing ship crossed Midway's horizon, the islands were wind-blown sandy dunes, covered with native shrubs and grasses. Slow growing, sun-loving plants such as naupaka (Scaevola), bunch grass (Eragrostis) and puncture vine (Tribulus) thrived in the harsh, salty environment. The trees on Sand Island were derived from plantings in the last century.
Midway lies near the most northern limit of coral growth. Although coral diversity is less than in more tropical climates, some species (e.g. Pocillopora, Porites) are abundant. Deep chasms, caves and corridors in the reef create habitat for a wide variety of fish, several of which are unique to Midway.