A 165-Million-Year Journey

Jordan Vinson, for the Kwajalein Hourglass

If you recently moved to Kwajalein Atoll, you now live and work amid one of the planet’s largest lagoons, encircled by one of the planet’s largest coral atolls. Lying at the heart of the Ralik Chain (the western, or “sunset,” chain) of the Marshall Islands, Kwajalein Atoll sits at a point roughly equidistant to Honolulu, Japan and northeast Australia. It’s at least a 2,000-mile swim to the nearest significant landmass, which in our case happens to be Australia. You’re at eight degrees and some change, or about 500 miles north of the equator.

The atoll itself, like all 29 atolls comprising the RMI, is what remains of a shield volcano that once extended from the top of the Pacific Plate at the floor of the ocean up through the surface of the ocean. Born out of volcanic eruptions occurring thousands of feet below the Pacific Plate between 165 and 76 million years ago, this seamount grew and grew. Finally, when it peaked out of the surface of the ocean, it became an island, reaching the first transformative stage of atoll formation.

Exactly how far it extended above the water’s surface is hard to say. Did this island look like a mountain, like Mauna Kea in Hawaii, which tips the charts in the Pacific Ocean at more than 13,000 feet in height? Or did it resemble current-day Kosrae or Pohnpei, both of which reach just over 2,000 feet in height?

“We really just do not know,” says Dr. Curt Storlazzi, a research geologist and oceanography with the USGS’ Pacific Coastal and Marine Science Center. “Because the base of the atoll is 50-80 kilometers across, it could have been really big. We just don’t really know.”

Regardless of how high the island stuck out of the water in its pre-atoll state, it provided a large base close to the water’s surface for coral to latch onto and grow. That surface proximity is important. It is one of the most essential requirements for coral growth, providing cozy, nutrient-rich real estate close enough to the water’s surface to allow the coral animals to pull in energy from the sun, sustain themselves, grow and reproduce.

When did coral reefs first begin forming around this volcanic island? Dr. James Hein, a long-time USGS scientist specializing in marine mineral deposits, does have an answer: roughly 56 million years ago. From that point onward, individual coral colonies began to form along the perimeter of the island and grow into ever-greater colonies, eventually merging together around the island perimeter into a contiguous mass of coral animals, which we call a fringing reef. This is the second major transformational stage in atoll formation. According to the National Oceanic and Atmospheric Administration, it can take 10,000 years for a fringing reef to form around an island. If those conditions remain favorable, the reef will continue expanding throughout the next 100,000 years, the agency states.

Contrasting with the expansion of the reef along its perimeter, the island landmass eventually began to undergo its own transformation. As the Pacific Plate slowly crawled across the Earth’s mantle, the position of the large Kwajalein volcanic island became more and more distant from its source of volcanism (aka a hot spot). It was this hot spot that had spewed the magma out of the planet’s mantle, through the lithosphere and into the ocean to form the volcanic seamount and island, says Storlazzi. The greater the distance from the hot spot, the more the local region of the Pacific Plate sank. And the more it sank, the more the volcanic island subsided downward and receded away from the reef around it.

As this large island subsided, a veritable moat of seawater filled the growing gap between the landmass and the reef, forming a barrier reef, the third major transformative step in the atoll formation process.

Fast forward until the landmass subsides completely below the ocean, and you’re left with a thin necklace of coral reef. This is an atoll, the final step in the process.

An obvious question does come up at this point: If the volcanic landmass subsides over time, why hasn’t the coral reef subsided along with it, pulling it below the ocean’s surface?

Actually, it has—sort of. What one has to remember is that a coral reef is vastly different from a volcanic seamount, chiefly in the fact that the reef is a conglomeration of living lifeforms. In an island’s case, if geologic forces pull it below the water or erode it away to the surface, that landmass has no choice but to go along for the ride. A reef, on the other hand, consists of millions and millions of coral animals, which may reproduce and continually grow, building on top of one another in an attempt to remain at the surface where the sun’s energy is strongest. Because it’s a living entity, the coral reef making up Kwajalein Atoll was able to react to its changing depth in the water over time. The rocks, sand and other stuff making up the volcanic island simply could not.

Currently, the seamount formerly comprising this island reaches a height that is only about 200 feet below sea level. Atop it lie layers of very old, dead coral colonies, which have subsided below about 160 feet, the greatest depth at which coral typically can survive. Atop those dead colonies lie the living coral colonies that have yet to subside below the rough 160-feet kill point. As the seamount continues to subside, bringing the coral along with it, new coral colonies continue to grow near the ocean surface, piling up on top of dead reef sections subsiding below. The process continues today.

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