It is believed that the islands began to develop from a fissure in the ocean floor about 25,000,000 years ago during the Miocene period. Lava outpourings have dominated volcanic activity and have formed a chain of shield-shaped domes, eroded over time into their present shapes. Current active Hawaiian volcanoes are characterized by fissure eruptions, generally preceded by local earthquakes. The leading contributors to personal income in Hawaii are government spending and tourism. These industries have eclipsed sugarcane and pineapple production and processing, the mainstays of the islands' economy from about 1900 until the post-World War II period.
Most government spending is by the U.S. armed forces, which maintain several large bases in Hawaii.
Agriculture Hawaii's agricultural sector is relatively small and is dominated by sugarcane and pineapple production. Hawaii is the United States' leading producer of sugarcane, and about 10 million tons are harvested yearly. Most sugarcane is produced on large, highly mechanized plantations located on Hawaii, Maui, Oahu, and Kauai islands. Hawaii is also the nation's leading producer of pineapples, grown on immense plantations covering most of Lanai and parts of Oahu and Maui. In a social system similar to medieval European feudalism, there was a hereditary ruling class, called the alii nui, and below it were the common people, primarily fishermen and farmers.