Hawaii Geologic Map

Basically, all of the islands of the Hawaiian chain are less than 10 million years old, with the Big Island the youngest and the oldest being Nihoa (which is part of the islands but not part of the state), off the map to the northwest. The map color refers to the composition of the lava, not its age. The magenta and blue colors represent basalt and the brown and green (just a smidgen on Maui) are rocks higher in silica.

All of these islands are the product of a single source of hot material rising from the mantle—a hotspot. Whether that hotspot is a deep-seated plume of mantle material or a slow-growing crack in the Pacific plate is still being discussed. To the southeast of Hawaii island is a seamount named Loihi. Over the next hundred thousand years or so, it will emerge as Hawaii's newest island. The voluminous basaltic lavas build very large shield volcanoes with gently sloping flanks.

Most of the islands have irregular shapes, not like the round volcanoes you find on continents. This is because their sides tend to collapse in gigantic landslides, leaving chunks the size of cities scattered around the deep seafloor near Hawaii. If such a landslide happened today it would be devastating to the islands and, thanks to tsunamis, the entire coast of the Pacific Ocean.

I've prepared two larger versions of this map that include the explanation at 2100x1400 pixels (250 KB) and 5000x3300 pixels (1 MB).

More about Hawaii Geology

More Hawaii resources on About.com:
Hawaii Travel
Hawaii Maps
Hawaii Geography, State Symbols & Facts
Hawaii Campgrounds
Hawaii Farmers Markets
Hawaii Fishing
Hawaii National Parks
Hawaii Archaeology