The Age of Coral Reefs

All of the pages within this page are linked-to in AGE OF THE EARTH - SCIENTIFIC EVIDENCE where you can find longer pages with more in-depth treatments, and many additional resources about age-science.  This page, assembled by Craig Rusbult, is an "overflow" from a collection of pages with Examples of Old-Earth Evidence.

• Coral Reefs: Indicators of an Old Earth
by Perry G. Phillips

      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 th e 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.

• Tidal Slowdown, Coral Growth, and the Age of the Earth
by Perry G. Phillips

      The earth is slowing down.  It is not rotating as fast today as it was in the past.  This slowdown is caused by the tides which the moon raises on the earth.  However, before we explain this slowdown, we need to discuss how tides are formed.
      All physical objects attract one another through the force of gravity.  This force depends upon the mass of each object and upon the distance between the objects.  The closer they are to each other, the more strongly they attract.  In the case of the moon and the earth, the moon pulls the ocean water on the nearer side of the earth more strongly than it pulls the rest of the earth, so the water forms a bulge.  The opposite occurs on the far side of the earth.  Here the water experiences the least pull because it is farthest from the moon.  The rest of the earth is pulled away from the water, forming another bulge of water pointing away from the moon, though the bulge is not quite as large as the first one.  The moon holds these bulges (more or less) in place while the earth rotates beneath them.  The net effect is the tides we experience at the seashore.
      Tides slow down the earth's rotation speed because of friction between the earth and the water under which it rotates.  The effect is very small - a slowdown rate of about 0.0002 seconds per day per year.  This means that as each year goes by, each day of the year lasts 2 hundred-thousandths of a second longer.  Thus the length of the day changes by about 20 seconds every million years.
      Since the earth's rotation is slowing down, it took less time in the past for the earth to rotate on its axis than it does today.  But the time for one complete rotation is the length of a day.  So if the days were shorter in the past, then there were once more days in the year than there are now.  This, of course, assumes that the length of the year has not changed.  This is a reasonable assumption, since the year is the time (measured in unchanging units) it takes for the earth to go once around the sun, and there is no known mechanism to make any measurable changes in this period over a few billion years.
      If we assume that the rate of slowing of the earth's rotation has been constant, we can calculate the number of days in a year at various times in the past (Hayward, 1985, p. 95).  Suppose we want to know how many days made up a year in the Devonian period, estimated to have been some 400 million years ago.  Each day was 20 sec shorter per million years x 400 million years = 8,000 seconds shorter.  This means each day was only 21.8 hours long then, as opposed to 24 hours per day now.  Since a year is 8799 hours long (24 hours/day x 365.25 days/year, using modern-length days) and this length has not changed, we can calculate the number of ancient days in a Devonian year by dividing 8766 hours/year by 21.8 hours/day, to get about 400 days/year.  A similar calculation for the Pennsylvanian period, beginning about 280 million years ago, gives 22.4 hours/day, or 390 days in the Pennsylvanian year.
      The reason for choosing the Devonian and Pennsylvanian periods is that we can check to see if these calculations correspond to reality.  In certain modern corals and shellfish, we find growth-bands that indicate yearly, monthly, and even daily growth, rather like the annual rings that trees produce.  By counting these bands, we can determine how long a particular coral or shellfish lived just as we can for a tree by counting its rings.  We can also see that there are about thirty daily bands per month and about 365 daily bands per year for modern corals and shellfish.  But careful analysis of the growth-bands of fossil corals and shellfish from the Devonian and Pennsylvanian has confirmed that years in these periods contained more days than years do now, and that the number of days per year for both these periods is remarkably close to the values calculated above.
      This correlation between theory and observation is striking.  After all, three different modes of dating are used here, and they all correlate with each other.  The fossils are dated by the rock layers in which they are found, which dating ultimately depends on radiometric methods (decay rates of radioactive elements).  The growth bands in the fossils are biological in origin, depending on the response of the organism to daily, monthly and yearly changes in environment (light, weather, and temperature).  The earth's slowdown is an astronomical phenomenon.  The three processes upon which the dates depend - radioactivity, biological growth, and tidal friction - are independent processes, yet all three combine to form a coherent, natural picture of what is happening. 
      This is surely not mere coincidence.  The Lord himself seems to be providing sincere seekers and careful investigators with valid evidence that the earth is old.

• Coral and the Moon
by Don Lindsay

      The Moon causes tides.  Tides make the Earth slightly asymmetrical, and one result is that the Earth's rotational energy is slowly being stolen by the Moon.  We spin more slowly: and the Moon rises to a higher, slower orbit.
      This was worked out mathematically in the 1800's.  Today, however, it has been measured.
      One consequence is that in the future, there will be fewer days in a year.  And in the past, there would have been more.
      Modern corals deposit a single, very thin layer of lime once a day.  It is possible to count these diurnal (day-night) growth lines.  You can also count annual growth.  So, given the right piece of coral, you can measure how many days there are in a year.
      These measures can equally well be done on fossilized coral.  For example, coral from the Pennsylvanian rockbeds have about 387 daily layers per year.  Coral from the Devonian rockbeds have about 400 daily layers per year.  In the Cambrian, a year was 412 days.  One Precambrian stromatolite gave 435 days per year.
      With bivalves, you can count days and lunar months.  Recent bivalves give 29.5 days per lunar month; Pennsylvanian give 30.2; Devonian give 30.5.
      If you care, there ore a lot more details about coral.  There's a reading list at the rear of that, and the topic is covered in Strahler, and the Creationist Daniel Wonderly has written about it.  But there are also some broader issues.
      For one, do all these numbers increase, as one goes to supposedly older and older layers of the "geologic column"?  The answer is yes.
      For another, are these numbers the same, if one takes corals from different continents, but in the "same" rock layer?  The answer is yes.
      For a third, do these numbers agree with the theoretical numbers that the astronomers had in hand?  In order to tell, we need to use radioactive dating techniques, to get dates for the various rocks. So, the comparison is somewhat a test of radioactive dating.

• How long does a coral reef take to grow?

by Answers in Genesis (? - no author is listed), paper in Creation 14(1):14–15, December 1991

      Australia’s beautiful Great Barrier Reef is the world’s longest coral reef.  It extends from near Papua New Guinea down Australia’s east coast for about 2,000 kilometres.
      In a previous Creation magazine (Vol. 8 No. 1), we showed that using measured growth rates at the mouth of the Burdekin River, the Great Barrier Reef could have formed in the time since Abraham lived.
      However, the Great Barrier Reef, in spite of its huge area, is not the thickest known reef. This distinction probably belongs to Eniwetok Atoll in the Marshall Islands. This is a living reef resting on an extinct volcano cone which comes up about three kilometres (two miles) from the ocean floor. Drilling revealed about 1,400 metres (4,600 feet) of reef material. At least two writers have attacked the young age position using the argument that this coral atoll must have taken a very long time to form—they estimate 138,000 and 176,000 years respectively as the minimum age for Eniwetok.
      Ariel Roth of the Geoscience Research Institute has commented on the fact that estimates of net reef growth rates vary from 0.8 millimetres per year to 80 millimetres per year, whereas actual measurements based on soundings at depth are many times these estimates. Roth suggests a number of reasons for this difference.
      The main one is that measurements made at the surface will show lower rates of growth because of exposure to air at low tides and intense ultraviolet light. Lack of light will of course kill a reef—no live coral growth takes place below about 50 metres under the surface. Hence thick atolls such as Eniwetok require the ocean floor to sink as the coral builds. As the coral is lowered, faster growth is possible than that which we measure at the surface.
      There are complex factors which both add to the growth of a reef and take away from it. For instance, attack by certain organisms and wave destruction will contribute to a decline in reef size. On the other hand, a growing reef can trap sediments as they are moved along by currents, thus adding to its thickness. Storms can dramatically add to the thickness of a reef by bringing in coral from other areas.
      For example, in 1972, Cyclone Bebe ‘constructed’ a rampart of coral rubble 3.5 metres high, 37 metres wide and 18 kilometres long in a few hours.
      Given all the above, it seems reasonable to rely on the actual figures reported from depth-sounding measurements for coral reef growth rates, rather than calculations trying to take all these other factors into account. Such reef growth rates have been reported as high as 414 millimetres per year in the Celebes. At such a rate, the entire thickness of the Eniwetok Atoll could have been formed in less than 3,500 years.
In addition, actual experiments indicate that the rate of coral growth can be nearly doubled by increasing the temperature five degrees Celsius (remember that Eniwetok sits on a now-extinct volcano), or increasing the carbonate content of sea water.
      To maintain that Eniwetok Atoll could have formed in the time-span since the Flood recorded in Genesis is not at all inconsistent with real-world evidence.

• Reefs and Young-Earth Creationism

      Eniwetok Atoll as a Post-Flood Reef
      The Eniwetok atoll is a reef in the Marshall Islands. The US detonated hydrogen bombs there in the 1960's. Drill cores show that this reef is about 4600ft thick, and rests atop the surface of a submerged volcanic seamount. The entire thing is composed of corals, calcerous algae, foraminifera, echinoderms, oysters and so forth, which are cemented together. In terms of texture and composition, this reefs is very similar to buried reef strucures in the fossil record. How quickly could it form? Could it form in the 4500 year post-flood period?
      Doing research on the Eniwetok atoll, I decided to visit some creationist websites and see how they explained these structures in terms of an earth less than 10,000 years old. According to the "Stand for Jesus" web page, the oldest living coral reef is only 4200 years old. This claim can be found on dozens of creationist web pages, and is evidently widely accepted as an accurate estimate. Answers in Genesis makes the same claim: ". . . reef growth rates have been reported as high as 414 millimetres per year in the Celebes. At such a rate, the entire thickness of the Eniwetok Atoll could have been formed in less than 3,500 years. To maintain that Eniwetok Atoll could have formed in the time-span since the Flood recorded in Genesis is not at all inconsistent with real-world evidence." I could not find any articles at ICR addressing the issue.

      The Evidence for Rapid Reef-Building
      Let's examine the "real-world evidence" cited for this conclusion. Every one of the creationist web pages I found seem to be relying, directly or indirectly, on two papers, one by Arthur Chadwick, and an Origins paper by A. A. Roth. These papers list various estimates of reef growth rates from a variety of methods. Most of the estimates cited by Chadwick and Roth give long ages for the growth of a 1,400m coral reef. However, both authors include a single anamalously high estimate rate of 414mm(!)/yr. These estimates were based on "soundings" done in the early 1930's. They cite only a single source for this astounding rate, a 1932 paper by J. Verstelle, 'The Growth Rate at Various Depths of Coral Reefs in the Dutch East-Indian Archipelago', Treubia 14:117-126, 1932.
      Virtually all of the other estimates in Chadwick's paper yielded rates of reef growth of 0.8-30mm or so, requiring many thousands or even millions of years to form a reef 1400m thick. For instance, Hubbard et al. (1990), estimated growth rates of 0.7 tp 3.3mm per year. Davies and Hopley (1983) estimated a *maximum* of 20mm/yr. Smith and Kinsey (1976) listed rates of 2-5mm/yr. Smith and Harrison (1977) listed rates of 0.8-1.1mm/yr, and so on. Many additional studies indicate Holocene reef growth histories on the order of 1-15mm/yr, with the upper range only being attained in reefs dominated by the fast-growing Acropora corals (e.g. Aronson et al., 1998; Hubbard, 2001). While Chadwick's paper included many reasonable estimates, his readers predictably siezed upon the one rate reported by Verstelle over 60(!) years ago, ignoring a massive body of more recent research on the subject.
      Another odd thing is that both Roth and Chadwick's papers also included estimates of the growth rates of *individual corals,* and they showed that even most individual corals cannot grow nearly that fast (i.e. ~400mm/yr)! Most studies document maximum *coral* growth rates of only 10-50mm per year.

      How Fast Can Reefs Really Grow?
      By far the most contentious isse here is the rate at which reefs can grow. Studies of reef growth in the modern Pacific show that even under ideal conditions, the growth of the actual reefs is only on the order of 8-10mm a year (see below). Note that individual corals can grow a bit faster than this, but this cannot be used to estimate the growth rate of the *reef* itself, since the reef is not one giant coral, but is largely composed of billions of coral fragments that are broken by waves and cemented to the growing mass (see below).
      So, assuming an average 10mm per yr growth rate, the Eniwetok Reef would require 140,000 years to grow to its present thickness. And this assumes no compaction, no destruction by storms, no temporal breaks in growth, continuous optimal growth rates, and adequate subsidence rates. All of these assumptions are entirely unreasonable, and thus any estimate based on extrapolation of optimal reef growth rates is clearly a minimum.
      For instance, we know that there are at least 3 major weathered unconformities within the Eniwetok, at 300ft, 1000ft, and 2,800ft depth. These unconformities not only show the type calcite cementation which develops on exposed reef surfaces, they are also extremely enriched in pollen, most of which appears to be from Mangrove trees. Mangrove trees are growing on many exposed reefs in the Pacific today. In some cases, the pollen is so abundant that there are an estimated 10,000 or more pollen grains per gram (Leopold, E. B., 1969, "Bikini and Nearby Atolls, Marshall Islands, Miocene Pollen and Spore Flora of Eniwetok Atoll, Marshall Islands," U. S. Geological Survey Professional Paper 260-II , U. S. Government Printing Office, 53 pp). This shows that at each of these unconformities, the reef surface remained exposed for an extended period of time, although exactly how long is not known.

      Subsidence as a Limiting Factor of Reef Growth
      And there is one more simple reason why such high estimates assumed by AIG and others are entirely unreasonable. The reason is this -- the net growth of the reef can only be as fast as the net subsidence of the seamount or platform on which it is growing. This is a limiting factor. Thus, even if a reef could grow at, say, 3cm per year rather than around 1cm or less as virtually all of the empirical estimates show, the reef can still only grow to the surface of the water. Where rates of subsidence of seamounts can be measured, it is only a few mm per year. Subsidence rates have been estimated with high precision for the Hawaiian Islands, which are similar in most respects to the submerged seamount atop which the Eniwetok atoll rests. These islands are subsiding at only a few mm per year.
      Carbon dating of drowned reefs on the side of Hawaii show that it has subsided at this slow rate for hundreds of thousands of years. In fact, its a little more interesting than that. You can actually predict the radiometric ages of a drowned coral reef, with considerable accuracy, simply by dividing the depth in mm by the observed subsidence rates in mm per year.
      Radiometric ages of Hawaiian corals compared to ages predicted by extrapolating observed subsidence rate of 2.7mm per year. Judging by the close correlation between predicted age and actual age, the rate of subsidence for the island of Hawaii has remained very close to 2.7mm per year throughout at least the last half-million years.
      But for agument's sake, let's disregard the radiometric dates. Do any YECs have a plausible explanation for the growth of a 4600ft thick reef in 4500 years of post-flood time? And if YECs really think reefs can grow at rates of 100-400mm per year or more, when can we expect to see the research documenting this, and the evidence showing that the required conditions for this were somehow satisfied throughout post-flood times, while not being satisfied anywhere today? Remember, even if we multiply the fastest observed reef growth rates by a factor or 10, and assume *continuous* maximum growth rates, and assumed *no* erosional breaks or storm damage, and assumed that subsidence was somehow *greatly* accelerated, we would *still* need 14,000 years for the growth of Eniwetok.