Maui, one of the younger islands in the Hawaii chain, began as two separate volcanoes on the ocean floor. Time and again, eon after eon, they erupted, and thin new sheets of lava spread upon the old, building and building, until the volcano heads emerged from the sea.

Lava, alluvium and wind-blown ash, eventually joined the two by an isthmus or valley, forming Maui, "The Valley Isle." Finally, Haleakala, the larger eastern volcano, reached its greatest height, 12,000 feet (3,600 meters) above the ocean - some 30,000 feet (9,100 meters) from its base on the ocean floor.

For a time, volcanic activity ceased, and erosion dominated. The great mountain was high enough to trap the moisture-laden northeast tradewinds. Rain fell and streams began to cut channels down its slopes. Two such streams eroding their way headward created large amphitheater-like depressions near the summit.

Ultimately these two valleys met, creating a long erosional "crater." At the same time a series of ice age submergences and emergences of the shoreline occurred; the final submergence formed the four islands of Lanai, Molokai, Kahoolawe, and Maui.

When volcanic activity resumed near the summit, lava poured down the stream valleys, nearly filling them. More recently, cinders, ash, volcanic bombs, and spatter were blown from numerous young vents in the "crater" forming multicolored symmetrical cones as high as 600 feet (180 meters). Thus this ware-carved basin became partially filled with lava and cinder cones, and it came to resemble a true volcanic crater.