Coral Clocks

Creationism, in its guises of Intelligent Design and "teach the controversy", has been popping up all over, often in school boards and state legislatures. The ignorance displayed by the the proponents of the various laws, policies and curricula is staggering, and this is most true for the literalist Young Earth Creationists (YEC's) that believe in a six-day creation six thousand or so years ago.

There are numerous lines of evidence pointing to a much older earth and still older universe. Various radiometric dating methods (oft-maligned by YEC's) all give general agreement that the age of the earth is roughly 4.5 billion years old. In the last ten or fifteen years, observations first by the COBE satellite and more recently by the WMAP satellite have pinned the age of the universe to roughly 13.7 billion years.

My favorite line of evidence for an old earth does not actually give a precise age, other than establishing a minimum age of about 500 million years. The reason I like it, I think, is that it brings together two seemingly unrelated things: the moon and coral. What could they possibly have to do with each other? I am glad you asked.

The current consensus is that the moon was formed when an impactor roughly the size of Mars struck a glancing blow on the early Earth. Much of that mass was incorporated into the Earth itself, but a sizeable amount of mass, especially lighter elements (like iron, as compared to nickel) was flung into low earth orbit where much of it eventually coalesced into the moon. While the moon is currently about 250,000 miles from Earth, it would have formed much closer.

If it formed closer to the Earth, why is it farther away now? The answer to that question lies in the ocean. (But no, we are not to the coral yet.) The gravitational interaction between the Earth and the moon produces tides, and the tides slow down the Earth's rotation. But if the Earth's rotation is slowing down, it is losing energy, and energy must be conserved, so where is the energy going? The energy is being transferred to the moon, which slowly moves farther away. This slow retreat has been measured to be (currently) about 3.8 cm per year with a corresponding decrease in the rotational period of the Earth.

Now if the rotation of the Earth is slowing down, and has been for billions of years, then it must have been rotating faster before. On the other hand, the time required for the Earth to revolve around the sun is expected to be essentially constant. That means that the number of rotations (days) per revolution (year) has been going down and was higher in the past. And that is where the corals come in.

Corals grow layer by very thin layer, with identifiable layers produced each day. Monthly and yearly variations are also detectible, and in modern corals (as you would expect) you find 365 daily layers per yearly cycle. But these layers can also be examined in fossilized corals from various time periods. By this point, you should not be surprised to learn that coral fossils (dated using geologic and radiometric techniques) have more daily layers per yearly cycle, and the ratio increases with the age of the fossil. Fossils from the Pennsylvanian period (325 to 280 million years ago) have about 387 daily layers per year. Fossils from the Devonian (408 to 360 million years ago) have about 400 layers. Fossils from the Cambrian (540 to 500 million years ago) have about 412 layers per year. These values are in rough agreement with those calculated based on Earth/moon tidal dynamics, which is more complicated than you might think because the continents get in the way, and worse, the continents move over these timescales.

Cool, huh?