Hawaiʻi Coral Reefs

Blue Holes and Their Origin

This block diagram of the South Molokaʻi reef tract portrays the distinctive shape of blue holes. Both the erosional and constructional processes may have affected the development of blue holes.

Large holes in coral reefs, known as "blue holes" are common features in the South Molokaʻi reef tract. Nowhere are they more fully developed than around Kamalō. Locally, blues holes can reach depths of 80 feet. The origin of blue holes is enigmatic.

One idea suggests that blue holes are mostly erosional in their formation. After a drop in sea-level during the late Pleistocene (last ice age), surface and groundwater may have dissolved older reef limestone to form sink holes. Subsequent rise of sea level during the Holocene has since flooded these cavern-like holes.

Another idea also recognizes the effect of sea-level fall during the last ice age, but in this model blue holes are interpreted as constructional features. Initially, streams cut across the older, exposed reef during the Pleistocene. Holocene sea-level rise drowned the narrow stream cuts, and recent reef growth has bridged across and partially filled channels.

Another credible possibility is that both constructional and erosional forces have played a role in the origin of blue holes.

The series of block diagrams below illustrates the contructional model of blue-hole formation.

Schematic diagram showing formation of a blue hole, step 1.

1. Reef Platform Established (5e Isotope Stage ~ 120 kya?).

Schematic diagram showing formation of a blue hole, step 2.

2. Reef Platform Exposed and Channel cut into Platform (Intermittently, ~120 to 5 kya)

Schematic diagram showing formation of a blue hole, step 3.

3. Reef Re-established (~7 kya?)

Schematic diagram showing formation of a blue hole, step 4.

4. Channel Overgrown Locally by Coral Colonies (Present Day)

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