The state of Hawaii is made up of an island chain that extends for about 1,600 mi between the island of Hawaii in the southeast and Kure Island in the northwest. The state has a total area of 6,459 sq mi, including 36 sq mi of inland water. It is the fourth smallest state. The mean elevation is about 3,030 ft.

Nearly all of the state’s total area is accounted for by eight main islands, which are from east to west Hawaii, Maui, Kahoolawe, Lanai, Molokai, Oahu, Kauai, and Niihau. Northwestward from the main islands extends a long string of islets, coral reefs, and shoals. The largest of these is Laysan, which covers only 1,000 acres. These landforms are either uninhabited or are sparsely populated by people staffing government facilities.

The state of Hawaii is not coextensive with the geographical unit called the Hawaiian Islands, or Hawaiian Chain. The inhabited Midway Islands, in the northwest, are not part of the state but are administered by the U.S. government as a separate dependency. The atoll of Palmyra, an island southwest of the main islands, was part of the Territory of Hawaii but was specifically excluded from the state when statehood was achieved in 1959. It remains a U.S. territory.

The Hawaiian Islands and the many seamounts to the northwest represent the exposed peaks and submerged mountains of a great chain of extinct, dormant, or active volcanoes. This chain has been forming for many millions of years as vast outpourings of lava issue from a relatively fixed vent or “hot spot” of volcanic activity on the deep ocean floor. This hot spot is believed to have remained in its present general position for many millions of years. The large tectonic plate (see Plate Tectonics) that forms the floor of much of the Pacific Ocean appears to be moving slowly in a northwesterly direction at a rate of about 4 in a year. Lava flows pouring out of this vent over long periods of geological time have built a series of broad, gently sloping volcanoes. Each has subsequently migrated to the northwest along with the slowly moving tectonic plate. Eventually becoming distant from the hot spot, the volcanoes become dormant and then extinct. Over long periods of time the volcanoes submerge into the sea as their great mass causes them to sink back into the crust, leaving no volcanic rock above sea level. Over time, coral growth produces first fringe and then barrier reefs, and the tops of the sinking volcanoes become completely covered with coral (see Coral Reef). In this process atolls, such as Laysan, Midway, and Kure, have formed at the northwest end of the Hawaiian Archipelago. Atolls are characterized by a large lagoon surrounded by a barrier reef which protects one or more small, low lying, sandy islets.

The still active, and therefore younger, volcanoes of Hawaii today are technically those that have erupted since written records have been kept. These active volcanoes include one on the island of Maui, three on the island of Hawaii, and a recently discovered submarine volcano, Loihi, about 20 mi southeast of the island of Hawaii.

Above the surface of the ocean the lava and limestone rock has been subjected to erosion, and today the islands of Hawaii reflect the intensity and duration of these forces of erosion. The oldest islands, in the northwest, have been worn down to sea level and are now represented only by low atolls and coral reefs that rest on the submerged remnants of volcanoes. Farther southeastward are tiny lava islets. The southeastern end of the island chain is geologically the most recent section and includes the eight main islands. The island of Hawaii, the most recent of all, is the highest and largest island and, compared with the other main islands, has not been heavily eroded since it is still in the formative stage.

On the other, older main islands the long dormant volcanoes have been heavily eroded and the mountain ranges are characterized by steep slopes and numerous sharp ridges. As the process of erosion continues, all the main islands are being slowly worn down.

The volcanoes of the Hawaiian Islands are all so-called shield volcanoes, or lava domes. Unlike the volcanoes of Alaska and South America, those of Hawaii were not created by very explosive eruptions. Formed mostly by lava flows, they are great rounded mountain masses, rather than steep-sided cones. Mauna Kea, dormant for centuries, is the highest mountain in the state. It rises to 13,796 ft above sea level, and its summit is dotted with cinder cones formed by fire fountains ejecting millions of small pieces of volcanic cinder and ash.

Lava occurs on all the main islands in either of two basic forms, pahoehoe and aa. Pahoehoe is a smooth, ropelike form of lava with small holes formed by gas escaping as it cooled. Aa is a rougher and more pitted kind of lava, formed when the flow of escaping gas is less regular and of greater intensity.

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Official Website for the State of Hawaii