Sea Arches

A sea arch on the coast at sunrise
Hōlei Sea Arch (NPS Photo/J. Wei)

A temporary display of nature’s handiwork

Sea arches, such as Hōlei Sea Arch at the end of Chain of Craters Road, can be found along a wide swath of the southern coastline of Hawai’i. They are remnants of once-continuous sea cliffs, where lava has previously met the sea and added to the size of the island.

The moment that new lava cooled and turned to rock, however, it began to erode. Immediately, the pulverizing powers of wind and water began to pound and fracture the cliffs. The seas on the southern edge of Hawai’i are known for being rough and having a violent surf. Ever so slowly, these eroding waves break apart and wear away the rock, sculpting sea caves into the cliff.

As the waves pound into these new sea caves, the water hits the back wall of the cave and refracts. The force of the water bouncing off the back of then begins to erode the cave’s headland, over time, forming a sea arch.

However, the forces of erosion don’t stop once this beautiful sculpture is created. Given enough time, the force of the waves will cause the remaining bridge to collapse, leaving behind a pile of rocks known as a sea stack.

Though they may appear mighty and permanent, individual sea arches are an ephemeral feature of the island’s constantly shifting coastline.
 
Triptych of the stages of sea arch formation
Stages of sea arch formation, as seen in various places along the Puna coast. A cliff (left), a sea arch (center), and the sea stack that remains after collapse (right).
 

Last updated: March 31, 2020

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