Midway Atoll - Geography and Geology

Geography and Geology

Midway Atoll is part of a chain of volcanic islands, atolls, and seamounts extending from Hawai'i up to the tip of the Aleutian Islands and known as the Hawaii-Emperor chain. It consists of a ring-shaped barrier reef and several sand islets. The two significant pieces of land, Sand Island and Eastern Island, are habitat for millions of seabirds. The island sizes are shown in the table above. According to other sources, Sand Island measures 1,250 acres (510 ha) in area and the lagoon within the fringing rim of coral reef 9,900 acres (4,000 ha). The atoll, which has a small population (approximately 60 in 2009, but no indigenous inhabitants), is designated an insular area under the authority of the U.S. Department of the Interior. Midway was formed roughly 28 million years ago when the seabed underneath it was over the same hotspot from which the Island of Hawai'i 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. As the island subsided, a coral reef around the former volcanic island was able to maintain itself near sea level by growing upwards. That reef is now over 516 ft (160 m) thick (in the lagoon, 1,261 feet (384 m), comprised mostly post-Miocene limestones with a layer of upper Miocene (Tertiary g) sediments and lower Miocene (Tertiary e) limestones at the bottom overlying the basalts. What remains today is a shallow water atoll about 6 miles (10 km) across.

The atoll has some 20 miles (32 km) of roads, 4.8 miles (7.8 km) of pipelines, one port on Sand Island (World Port Index Nr. 56328, MIDWAY ISLAND), and one active runway (rwy 06/24, around 8,000 feet (2,400 m) long). As of 2004, Henderson Field airfield at Midway Atoll has been designated as an emergency diversion airport for aircraft flying under ETOPS rules. Although the FWS closed all airport operations on November 22, 2004, public access to the island was restored beginning March 2008.

Eastern Island Airstrip is a disused airfield in use by U.S. forces during the Battle of Midway, June 4–6, 1942. It is mostly constructed of Perforated Steel Plate sectional matting, built by the U.S. Navy Seabees.

Unique among the Hawaiian islands, Midway observes UTC-11 (also known as Samoa Time), eleven hours behind Coordinated Universal Time and one hour behind the state of Hawaiʻi.