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The Age of the Rocks

October 4, 2012

This is the third post in a series I’m writing about science and Christianity. The others were:

1. How Can A Christian Write Science Fiction?
2. The Age of the Stars

My high school biology course used a Bob Jones textbook that maintained that the Earth was 6000 years old. From this book, I learned that scientists determined how old rock layers were based on the fossils they found there, and then determined how old fossils were from the rock layers they found them in. These foolish scientists didn’t recognize this patently circular reasoning because they were so dead set on disproving the Bible. I also learned that the dating techniques they used were flawed because they were only accurate going back a few thousand years, and that all the mixing of bones and rocks throughout the world could be explained by the Flood.

Even at the time, I found these claims a little suspicious (could scientists really be as dumb as all that?), but I felt obligated to accept them. After all, the alternative was to reject the Bible, right? It wasn’t until I was willing to look into these claims on my own that I realized how far they missed the mark. I still believe that the Bible is the inspired Word of God, and that the text of the Bible must be taken very seriously as the primary source of truth. Before considering what the Bible says, however, let’s look at the facts about rocks.

In 1669, a Catholic bishop named Nicholas Steno became fascinated with the seashells, shark’s teeth, and other artifacts found embedded in rocks on mountaintops throughout the world.  He recognized them for what they were, and established the earliest principles for understanding rock strata as indicators of passing time.  By the eighteenth century, when pioneering geologist James Hutton lived, these artifacts were well-known, but the prevailing view was that they had been left there by the Flood. Hutton knew that fossils are only found in sedimentary rock (the kind that forms from sediment at the bottom of oceans and rivers) and never in igneous rock (the kind of that cools from magma) because animal matter can’t withstand the tremendous heat involved. But Hutton noticed something crucial. In the exposed layers of rocks, the layers of fossil-bearing sedimentary rock were interspersed with layers of igneous rock. How could this have been created by a single flood?  Hutton knew that sedimentation was a slow process, and realized that the layers had been generated one at a time, as oceans rose and fell and shifts in the Earth caused magma to come to the surface and cool. This is Geology 101, and already we have clear evidence that the Earth is ancient.

But science has come a long way since the eighteenth century. Thousands of geologists and paleontologists have studied rock strata in every continent and corner of the world, from the Himalayas to the Arctic to the ocean floor. The history of the Earth is a giant jigsaw puzzle, but they have assembled vast databases of information to assemble that history. Chief among the tools they have to accomplish this is radiometric dating.

Dating is accomplished using igneous rock. When magma cools, it often traps radioactive materials like uranium inside. The concept is simple: If you can consistently know (1) how much radioactive material was there when the rock formed, (2) the rate at which it decays, and (3) how much radioactive material is there now, then you know the age of the rock. #1 can be known by the ratio of parent and daughter isotopes in the rock. #2 is a constant for the type of radioactive material; the half-life of any radioactive material is always the same, so the speed at which it decays is always the same. #3 is, of course, measurable in the rock itself.

But is #2 always the same? Could the rate have been faster when conditions were different thousands of years ago? No, it really couldn’t. For one thing, we can introduce changes in temperature, pressure, chemical environment, magnetic fields, etc., and the half-life of these radioactive isotopes never changes. But couldn’t the half-life have been faster for some other, unknown reason? No, it still couldn’t. We know this because many rocks have multiple radioactive elements trapped inside that decay at different rates, but the measured age based on each element comes out the same. If external conditions altered the half-life, you would find different results.  It’s like solving two equations with two unknowns–only one answer works for both cases.  When this is repeated over thousands of rocks of different ages, using different elements with different half-lives, it leaves little room for scientific uncertainty.

But that’s not all. Radiometric dating methods match the dates determined by completely different methods. Take coral, for instance. When coral grows, it produces both daily growth rings and annual growth rings, so you can tell from a piece of coral how many days are in a year. Modern coral, of course, shows about 365 daily rings for every 1 annual ring. But what about fossils of coral found in rocks dated to be from the Devonian era, about 380 million years ago? Wouldn’t they be the same? They shouldn’t be. The Earth’s rotation is gradually slowing down, a tiny bit each year. 380 million years ago, the Earth would have been spinning faster, completing a day in only 22 hours and getting through 396 days each year, so coral from that era should have 396 daily rings for every 1 annual ring. They do.

When we look at the tremendous volume of geologic and fossil data that researches have assembled, we find a consistent story. Billions of years is not just a big number: this period of time has been mapped into eras, each era claiming a distinctive chapter of the narrative. As the continents move and mountains and seas rise and fall, the climates and wildlife change to match. Although there is much still to discover and understand, this story is currently understood in great detail. For example, eastern Pennsylvania (where I live) was a mountainous region during the Devonian period. The climate was tropical. A set of three large rivers flowed down from this high ground westward towards Pittsburgh. When scientists look at Devonian age rocks in Pennsylvania, they find fossils in keeping with the Devonian period: fish and the very earliest amphibians, but no reptiles or mammals.

This is predictable enough that experienced paleontologists interested studying the early development of limbs can look in exposed Devonian rocks in Pennsylvania (or Alaska or Greenland) and find what they’re looking for. Opponents of this science claim that it has no testable predictions, but that’s not true. Paleontologists regularly pinpoint specific ages of rocks to search for fossils of a specific level of development, and consistently find what they’re looking for.

Could all of this be explained by the Flood? No, it couldn’t. A flood leaves chaos in its wake, not order. When I was young, I was told that complex species coming about through the “random chance” of evolution was as likely as a hurricane passing through a junkyard and assembling a Boeing 747. The analogy is more apt here, however. The only way we would expect a global flood to lay down layer after layer of rock that tells a complex and consistent story about the history of the world is if God specifically and miraculously commanded it to do so. God certainly has the power to do this, but if he did, it means he wrote a complex and beautiful story in the rocks that is consistent with the physical laws of the rest of his world, but is actually a divinely appointed fiction.

I have argued that the evidence for an ancient universe is unavoidable. This leaves us, as Christians, with a problem. If the billions of years we see in the stars and the rocks really happened, can we still consider Genesis the inerrant word of God? If, on the other hand, God just inserted the story in the stars and the rocks to deceive us, then what does that say about the character of God? I plan to deal with both of these objections in future posts: I believe that the Bible is inerrant, and that it’s possible to take the Genesis text seriously without stretching it to suit science or undermining its theology.

Before I do that, however, I need to use my next post to tackle the Theory of Evolution.

Your Inner Fish, by Neil Shubin
Why Evolution is True, by Jerry Coyne
The Seashell on the Mountaintop, by Alan Cutler
The Man Who Found Time, by Jack Repcheck

The Map That Changed the World, by Simon Winchester


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