The Hawaiian archipelago is a string of islands and reefs, 3,300 kilometers long, that forms a broad arc in the mid-Pacific. The archipelago begins in the east with the island of Hawaii and ends almost at the international date line with a small speck in the ocean called Kure Atoll. Only the easternmost 650 kilometers of the state contains islands of any size, as well as almost all of the state's population. It is this portion that is usually considered as the actual "Hawaii."
The volcanoes of the Hawaiian Ridge have formed as a plate of the Earth's crust beneath the Pacific Ocean moves northward and westward relative to an area of anomalously high temperature, called a hot spot, in the Earth's mantle. As a volcano moves northwestward away from the hot spot, eruptions become less frequent, and a new volcano begins to form above the hot spot. Many of the younger volcanoes have grown above sea level, forming islands. As islands age, they erode and subside, eventually becoming atolls and then seamounts.
The Hawaiian islands are the exposed parts of the Hawaiian Ridge, which is a large volcanic mountain range on the sea floor. Most of the Hawaiian Ridge is below sea level. The State of Hawaii consists of a group of 132 islands, reefs, and shoals that extend for more than 1,500 miles from southeast to northwest across the central Pacific Ocean between about 155 and 179 degrees west longitude and about 19 to 28 degrees north latitude. The main inhabited islands are at the southeastern end of the group.
The Hawaiian islands are geologically youngest in the southeast and oldest in the northwest. The eight largest islands near the southeastern end of the group account for practically all of the 6,426-square-mile land area of the State. The eight islands and their approximate size, in square miles, from southeast to northwest are Hawaii, 4,021; Maui, 728; Kahoolawe, 45; Lanai, 141; Molokai, 259; Oahu, 603; Kauai, 553; and Niihau, 71.
The biodiversity of the native plants and animals that live in the Hawaiian islands is breathtaking. On the northern and eastern sides of the islands, where the trade winds drop their moisture on upsloping hills and mountainsides, luxuriant rainforests contain a profusion of species of trees, shrubs, birds, and insects. On the southern and western sides of the islands—often just a few miles away—open landscapes of grasses and shrubs receive just a few inches of rain a year. On the flanks of the high volcanoes of Maui and the Big Island of Hawaii, scattered bushes and hardy flowering plants endure frequent frosts and even an occasional snowfall. In the reefs surrounding the islands, hundreds of species of tropical fish swim amid colorful beds of coral.
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