Geography and Geology

The Hawaiian Islands

Hawaii is a string of 137 islands in the north central Pacific Ocean, forming the bottom of a chain of islands stretching off across the Pacific in a north-westerly direction for 2,600km starting from the 'Big Island' of Hawaii, then past the other major islands of Maui, Kahoolawe, Lanai, Molokai, Oahu, Kauai, Niihau, and then onward all the way to Midway and Kure Atoll.      

'Hotspots', ridges and chains

The Hawaiian Islands themselves are the youngest islands in an immense, mostly submarine, mountain chain composed of more than 80 volcanoes, that joins with the Emperor Seamount Chain, and extends across the seafloor for 6,000km up to the Aleutian Trench off Alaska. 

This chain of Hawaiian islands has been formed by the Pacific Plate moving over a deep, stationary 'hotspot' in the mantle of earth.  Heat from this hotspot has produced a persistent source of magma by partly melting the overriding Pacific Plate.  This magma, which is lighter than the surrounding solid rock, then rises through the mantle and crust to erupt onto the seafloor forming an active seamount (a submarine volcano).  Over time, countless eruptions cause the seamount to grow until it finally emerges above sea level to form an island volcano.  Continuing plate movement eventually carries the new island beyond the hotspot, cutting it off from the magma source, and the volcanism ceases.  The Hawaiian Islands are drifting north-westward at around 10cm (4 inches) a year.  It has taken Kure Atoll, the most north-westerly of the Hawaiian Island chain, around 40 million years to reach its current position. 

As one island volcano becomes extinct and drifts away, another develops over the hotspot behind it, and the cycle is repeated.  This process of volcano growth and death, over the past 70 million years, has left a long trail of volcanic islands and the Emperor seamount chain across the Pacific Ocean floor.  The island of Hawaii is just the latest to be formed by this one hotspot.

As the Pacific Plate continues to move west-northwest, the island of Hawaii will itself be carried beyond the hotspot, setting the stage for the formation of a new volcanic island in its place.  In fact, this process may be under way already.  The Loihi Seamount, an active submarine volcano, is forming about 35km off the southern coast of Hawaii and has already risen about 3km above the ocean floor to within 1km of the ocean surface.  Assuming Loihi continues to grow, it will become the next island in the Hawaiian chain. 

Impact on reef formation

These geological processes mean that the age of the islands increases as you travel along the chain away from Hawaii, the newest, towards Midway.  As the movement of the Pacific plate pushes these islands north-westwards into increasingly cooler waters they also begin to slowly sink back towards the ocean floor.  These factors, together with Hawaii's geographic isolation, all impact on the growth and development of coral reefs in and around these islands and shoals, and you will find more on this in the Reef Ecology section of this site.