Modern geology indicates that the Hawaiian Islands are situated near the middle of the Pacific Plate, one of a dozen thin, rigid structures covering our planet like the cracked shell of an egg. Though adjoining each other, these plates are in constant slow motion, the Pacific Plate moving northwestward several centimeters per year. Scattered around the world are many weak areas in the earth's crust where magma slowly wells upward to the surface as a 'plume'. Here volcanoes and volcanic islands, such as Maui, are born. This constant northwestward movement of the Pacific Plate over a local volcanic 'hot spot', or plume, has produced a series of islands one after another in assembly line fashion. The result is a chain of volcanic islands stretching from the island of Hawaii along a southeast/northwest line for 4,050 kilometers (2,500 miles) toward Japan.
Maui, one of the younger islands in this 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, wind-blown ash, and alluvium eventually joined the two by an isthmus or valley, forming Maui, 'The Valley Isle'. Finally, Haleakala, the larger eastern volcano, reached its greatest height, 3,600 meters (12,000 feet) above the ocean - some 9,100 meters (30,000 feet) 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 the numerous young vents in the 'crater' forming multi-colored symmetrical cones as high as 180 meters (600 feet). Thus this water-carved basin became partially filled with lava and cinder cones, and it came to resemble a true volcanic crater. Several hundred years have passed since the last volcanic activity occurred within the crater. This stillness in Maui is attributed by modern geology to the constant northwestward movement of the pacific Plate.
NPS National Parks Service