Coral Reefs: Indicators of an Old Earth

Viewed from the air, Pacific coral reefs generally appear as circular islands called atolls. They have a shallow lagoon in the middle and the open ocean lies toward the outside. Other features include: (1) a steep slope towards the ocean outside that descends into the depths; (2) a flat reef-platform between the island and the steep slope; (3) faster-growing corals on the windward, outer side; (4) storm-broken pieces of coral on the windward side, many of which have fallen partway down the slope and become fused to the reef; (5) slower-growing corals on the leeward (down-wind) side of the island.

The reefs are built by living organisms, primarily corals. The corals contain green algae in their interiors that provide oxygen the corals need to live. The corals, in turn, provide protection for the algae - a mutually supportive relationship that is called a symbiosis. The algae produce the oxygen by photosynthesis, so they need sunlight. This requirement limits reef-building coral to the upper 65 feet or so of water where sufficient light exists for photosynthesis. Of course, the dead carbonate "skeletons" of the coral can continue to exist at much greater depths.

The most reasonable explanation for coral growth begins with a volcano. Volcanoes can build themselves thousands of feet upward from the ocean floor, and some of them will grow tall enough to break through the surface of the water. During periods of volcanic inactivity, corals and lime-secreting algae colonize the areas just below the shoreline around the volcano. The corals and algae cement themselves together with lime as they grow, thereby constructing a circular reef around the volcano. Eventually the volcanic peak erodes to sea level. Further, as a result of tectonic activity, the volcano slowly sinks into the ocean depths. If the rate of sinking is slow enough, the reef-building can keep pace and continue constructing the reef. In this way a reef can be built that is several thousand feet tall, even though living corals can only survive in the upper layers of the ocean. Deep sea drilling at several atolls in the Pacific has confirmed this theory of reef growth, revealing volcanic rock below the corals.

We will here concentrate on one atoll - Eniwetok - as an example of how we can determine the age of a reef. This reef was thoroughly investigated by deep core drillings in preparation for its use as a test-site for a hydrogen bomb explosion. This atoll is roughly circular with all the standard characteristics of a growing reef. It rests upon an extinct volcano, as expected, and the volcano rises about two miles above the ocean floor. The reef itself is 4,610 feet tall. Examination of the material from the bore holes reveals that this is a normal reef that formed from the cementing together of corals and lime-secreting algae. (This algae is different from the kind that lives within the corals.) In addition, three unconformities (discontinuities in the growth of the reef) were located at depths of 300, 1000, and 2780 feet. These unconformities contain pollen from seed-bearing shrubs and trees, which indicates there were periods when the reef surface was above sea level (and so no coral growth at the surface) which lasted long enough for land plants to colonize the surface.

With this information we are ready to calculate the age of the Eniwetok reef. All we need to do is divide the height of the reef by the rate at which it grew. This calculation is rather like finding how long it would take to travel a certain distance. The time is calculated by dividing the distance to be travelled by the speed or rate of travel. For example, if one is to travel 150 miles and one's average rate of speed is 50 miles per hour, then the trip will take 150/50 = 3 hours to make the trip, not allowing for stops along the way. Just think of the height of the coral as the distance travelled and the rate of coral growth as the speed.

Research indicates that maximum rates of reef growth are about 8 millimeters per year, determined by examining the present growth rates of numerous reefs in the vicinity of Eniwetok. Admittedly, one may question whether the growth rate wasn't perhaps faster for this particular reef, but there are limits to how fast corals can grow. Growing biological systems obey strict physical and chemical laws relating to metabolism, reproduction, and intake of nutrients. This last item is particularly important because the rate of growth of coral depends on the amount of dissolved calcium carbonate it can extract from the seawater. Calcium carbonate, though, is rather insoluble, so there is not a large concentration of it in ocean water. Thereby reef growth is limited to a fraction of an inch per year.

Thus 8 millimeters per year cannot be far from the actual growth rate of the Eniwetok corals. Using this value, the age of the reef is calculated by dividing 4,610 feet by 8 millimeters (about .3 inch) per year, which is about 175,000 years. But this is a minimum age since we have not taken into account the time periods (represented by the unconformities mentioned above) when the reef was not growing. Nor have we taken into account the time necessary to form the volcanic base on which the reef grew.

Recently, further calculations for the rate of reef growth have been based on the concentration of dissolved calcium carbonate in seawater and upon the rate at which corals can absorb it and manufacture their shells. This rate turns out to be only about 5 millimeters per year, which means that the Eniwetok reef is more like 280,000 years old, not counting pauses in growth. A similar analysis for the much larger Grand Bahama Reef reveals an age of 790,000 years. And again, this is a minimum age, since that reef also contains numerous unconformities.

Young-earth creationists, of course, object to these great ages. They attempt to find alternative explanations for the formation of reefs. One idea is that the reefs formed as calcium carbonate precipitated out of the waters of Noah's Flood. This is nonsense, however, because precipitation involves dissolved calcium carbonate. How can one explain that the calcium carbonate managed to precipitate in the form of a reef, complete with the five characteristic features mentioned above, and the presence of corals that look just like those that were once alive? Besides, the insolubility of calcium carbonate is such that all the ocean waters of the world could not hold enough to construct past and present reefs in a one-year flood.

Another young-earth proposal is that reefs were formed by the piling up of lime during the time of Noah's Flood. But if this were true, how did the raging flood waters just happen to pile up the lime in a structure that has all the appearance of having been a growing entity? And why are the reefs free from the mud, clay, and other debris invariably present in flood waters? Finally, how were the waters able to pile up the reef material only on the upper slopes of ancient volcanoes? This would be the last place we would expect waters to deposit their debris, especially on those that are thousands of feet tall. For all these reasons, the proposal that reefs were piled up by flood waters lacks any credibility.

Finally, one can find statements in young-earth literature that corals can grow as fast as five centimeters per year. This is true for unusual and isolated corals, not the ones that construct large reefs. One also needs to keep in mind that although some individual corals may grow this quickly, the reef as a whole grows much more slowly because such faster growing corals are easily broken by storm waves. In addition, reefs are constantly being degraded by storm breakage, weathering, and dissolution of calcium carbonate back into the ocean water. These competitive processes prevent the reef from growing faster than the rates cited earlier.

In conclusion, the only rational interpretation for the presence of very tall reefs in deep ocean water is that these reefs grew over long periods of time by the natural processes discussed above. As such, reefs are indicators of ages on earth that are far greater than the 10,000 or so years allowed by young-earth creationists.

Dr. Perry G. Phillips

Materials for this tract were developed from:

Press, F. and R. Siever. 1982. Earth. 3rd ed. W. H. Freeman. San Francisco.

Wonderly, D. E. 1983. "Coral Reefs and Related Carbonate Structures as Indicators of Great Age." IBRI Research Report 16. Interdisciplinary Biblical Research Institute. Hatfield, Pennsylvania.