Physical Feature: High Islands

Gardner Pinnacles is the last high island in the Hawaiian Archipelago. Photo Credit: Andy Collins

The Hawaiian Archipelago is composed of "high" islands, and low-lying islands and atolls. "High islands" generally refer to islands where the basalt rock from volcanic formation is still above the ocean's surface. Low islands, by contrast, are islands composed of sedimented material, coral rubble, or uplifted coral reefs. In the Northwestern Hawaiian Islands, the last high island in the chain is Gardner Pinnacles. Beyond Gardner, basalt cannot be observed above the surface, and remnant basalt structures are covered by coral reef skeletons. At Midway and Kure these "coral caps" are thicker than 1000 feet. If you consider that the islands are eroding and collapsing back into the sea after they leave the hot spot, currently beneath the Island of Hawaiʻi, you will notice an anomaly with the partially submerged atoll of French Frigate Shoals (FFS). It does not appear to make sense that Gardner is still above the surface, whereas most of FFS is submerged. This is likely a by-product of the bending of Earth's crust beneath the tremendous weight of a Hawaiian shield volcano. The weight of new volcanic matter causes a ripple in the Earth's crust that uplifts adjacent volcanic islands, like a wave. It is likely that the island that once was Gardner was lifted up by the formation of FFS approximately 12 million years ago and this is evident in Gardner's basalt rock still being visible today.