Geology & Climate
The Hawaiian Islands formed one by one as the Earth’s crust moved northwest over a stationary “hot spot”. At the hot spot, magma (molten rock) rose from deep within the Earth and exploded into the sea. When magma reaches the Earth’s surface, it is called lava, which hardens into land mass.
The Northwestern Hawaiian Islands are older than the main Hawaiian Islands, and Big Island is the youngest island of all. Kaua‘i is estimated to be 5 million years old, O‘ahu to be 3 million, Maui to be 1 million, and Big Island to be half a million.
Because the Big Island is closest to the hot spot, its volcanoes are still active. As the Earth’s crust continues to move, a new undersea volcano, Lo‘ihi, is growing into an island off Big Island’s southeast coast.
Hawai‘i is a hot spot in more ways than one. Air temperatures rarely drop below 65˚F in Oct.-April and hover near 90˚F in May-Sept. The surface waters of the sanctuary measure 70-80˚F. The deepest waters--around 600 ft.--can be as cold as 40˚F.
Some places are wetter than others: Kaua‘i’s Mount Wai‘ale‘ale is one of the wettest spots on earth with an annual rainfall of approximately 450 inches. The sanctuary’s headquarters in Kihei, Maui receives about 15 inches of rain a year. In general, the average annual rainfall over sanctuary waters is 15-60 inches.
On the main Hawaiian Islands, trade winds blow northeasterly eight days out of 10. The large-scale surface current patterns around the Islands generally go from east to west.