Introduction

The first lines of evidence to examine are the ages of the universe and of the earth. Darrel Falk calls this “putting creation into a timeframe.”1 Scientists claim that there is strong evidence for an old earth (~4.5 billion years) and an older universe (~14 billion years). This significantly impacts how literally we can interpret Genesis, and it provides a needed prerequisite for evolution: long time spans.

In our cosmology lesson, we saw how the old age for the universe was first determined. Edwin Hubble discovered that the universe was expanding, and by calculating backward when the universe had a vanishing size, the universe is found to be about 14 billion years old. It was proposed that the universe began in a massive explosion called the Big Bang, and this was confirmed later by the discovery of Cosmic Background Radiation.

But what about earth itself? What is the evidence for its old age?

Continue: Non-Radiometric Methods

Non-radiometric Methods for Dating the Earth

The full age of the earth is determined by radiometric methods, looking at nuclear decay in rocks. Before considering this data, we will see how other measurements can take us increasingly far back into the earth’s history.

There are a number of methods that use regular cycles to measure passage of time. For instance, trees form yearly rings in their trunks, so one can determine a tree’s age by counting its rings. The oldest trees on earth, bristlecone pines found in the Sierra Nevadas, are about 6,000 years old. However, the dead trees lying beside them are found to be as old as 11,800 years.2

Similarly, lakebeds accumulate different types of sediment depending on the season (i.e. more minerals in spring, more pollen and plant material in summer and fall). This causes the formation of distinguishable annual layers that can be counted, just like counting tree rings. Scientists have found lakebeds with layers as old as 35,000 years.3

Another example of this is the seasonal ice rings in glaciers. The ice was formed through the accumulation of years of falling snow, and one can distinguish seasonal differences (such as increased dust and larger ice crystals in summer) that allow the age to be determined. Scientists have drilled ice cores deep into the glaciers and found ice that is 123,000 years old in Greenland,4 and as old as 740,000 years in Antarctica.5

From looking at trees we know the earth is at least 11,800 years old. Lakebed sediments show us that it is at least 35,000 years old, and ice cores show that it is at least 740,000 years old. Other methods take us even deeper – climatic effects of Milankovich cycles take us back 30 million years and evidence of magnetic reversals as far as 170 million years.6 But to push back to the beginning of earth’s existence, we must turn to radiometric methods.7 

Continue: Radiometric Methods, Pt. 1

Radiometric Methods, Pt. 1

The actual age of the earth is determined by studying radioactive decay. This is a complex topic, but if you’ll press through the details with me, you can reach a basic understanding.

Before going into the nuts and bolts of radiometric dating, let’s look at Darrel Falk’s simple example.8 Imagine being taken captive and kept in solitary confinement with nothing but the duffel bag you had when you were seized. You want to find some way to keep track of the date, and you happily find two boxes of tissues in your bag. Having counted the tissues – 300 total – you decide that every day you will use one tissue to wipe your hands and face. So, after one day you will have 299 clean tissues and one used one. After 150 days, half the tissues will be clean and half will be used. This basic principle can be applied to date rocks, but first, we need a brief chemistry review.9

All matter is made of elements, such as carbon, oxygen, hydrogen, and nitrogen. These elements can come in different forms known as isotopes, such as the three forms of carbon: 12C, 13C, and 14C. All three forms have identical chemical properties, but they each have a slightly different mass. This difference is revealed on the atomic level: each atom for a given isotope has a certain number of protons and neutrons in its nucleus. 12C has 6 protons and 6 neutrons, while 14C has 6 protons and 8 neutrons. The extra neutrons make 14C unstable, and sometimes the nucleus will decay into a more stable form. One of the eight neutrons will split into a proton and a high-energy electron, and the electron will fly away. Left behind is an atom with a new proton (for a total of seven) and one less neutron (also seven) – a new nitrogen atom.

Although most isotopes of most elements are stable, there are a number of unstable ones that are very useful. The energy released during decay (called radioactivity) can be measured with an instrument called a Geiger counter, and it ultimately allows us to determine the age of rocks.

Continue: Radiometric Methods, Pt. 2

Radiometric Methods, Pt. 2

One useful type of radioactive material found in rocks is uranium. Like 14C, Uranium-235 (235U) decays into lead-207 (207Pb) through a series of radioactive decay events. Uranium-235 is like the clean tissues in the example above, while lead-207 is like the dirty tissues in the example above. As time progresses there will be more lead and less uranium, so we can use the relative amounts of lead and uranium to determine the age of a rock. If the earth were infinitely old, there would be no uranium-235, because it all would have been converted to lead-207. This is not the case – rocks are found with a combination of uranium and lead.

In the captivity example, we used up one tissue each day. Similarly, to date a rock, we need to know how fast the uranium is being converted into lead. For radioactive decay, we speak of a half-life – how long it takes for half of the material to be converted. If the half-life were one hour, and we started with 40 uranium atoms, then after one hour we would have 20 uranium atoms, after two hours we would have 10 uranium atoms, and after three hours we would have 5 uranium atoms left.

The actual half-life of uranium-235 is much larger: 713 million years! This is determined with a Geiger counter that can detect the energy emitted each time a uranium atom decays. One ounce of uranium produces approximately 4 billion decay events every second. This seems like a large number until it is compared with the number that do not decay – in an hour, only one in a trillion uranium atoms will decay. From this one can determine when one in every two uranium atoms will have decayed – 713 million years.10

Continue: Radiometric Methods, Pt. 3

Radiometric Methods, Pt. 3

To date the rock, we also need to know how much uranium and lead the rock had to begin with. If we assume that the rock started out as pure uranium, then dating would be easy: if it now was half-uranium, it would be 713 million years old; if it now was only a quarter uranium, then 1.426 billion years old. But this cannot be assumed; we do not know that all rocks began as pure uranium. 

Remembering our captivity example, if we had started out with some used tissues our calendar would have been in error unless we had taken the initial used tissues into account. Similarly we must figure out the initial composition of a rock to obtain an accurate age. Fortunately, this is fairly easy. There are two isotopes of lead: lead-207 and lead-204. The two isotopes are chemically identical, so when the rock is forming, no preference will be shown – the two types will be incorporated in same relative amounts as are found in the earth’s crust. However, when uranium-235 decays, it only creates lead-207, so one can use the excess lead-207 to determine the initial amount of lead in the rock.11

Now we have the information needed to date the rocks. The dates can be further confirmed and improved by comparing to other radioactive systems. There are about 40 radiometric techniques available for dating rocks, each with different half-lives and precisions.12 Using the uranium/lead system, the age of the earth has been determined to be 4.566 billion years old, with a margin of error of ±2 million years. 13

Continue: Objections to Radiometric Dating

Objections to Radiometric Dating

Two common objections are raised to radiometric dating, primarily by those who believe in a young earth. First, there is concern about inconsistencies in the dates determined by different systems. While there is error intrinsic to the measurement, it is not significant.  Even if the errors reached 10%, this would leave the earth at 4 billion years old instead of 4.6 billion. This is a long way from an earth that is only thousands of years old.14

Second, some suggest that the decay rates have slowed down over time. However, radioactive decay is a nuclear event and very resistant to change. Even in tests at extreme temperature and pressure, no change in decay rate has been observed. Further, all decay rates would have had to change in tandem by an astronomical amount in order for a relatively young rock to appear several million years old.15

Continue: Conclusion

Conclusion

The scientific evidence clearly points to a universe and earth that are billions of years old. Using the red shift, cosmic background radiation, and other methods, astronomers have determined the universe to be ~14 billion years old.  As for the Earth, evidence from tree rings, lakebeds, and ice cores point to an old age of at least tens to hundreds of thousands of years.  However, the true age is revealed by radiometric dating: ~4.5 billion years old.

To get a sense of how long this time period is, imagine making a timeline of earth’s history where each year is 1/1000th of an inch.16 To begin at earth’s creation, our timeline would 67 miles long. The last century, with all of its advances and wars and lives lived, would be thickness of pencil line – a pencil line on a timeline 67 miles long!

This evidence forces us to consider our understanding of Genesis 1 and 2. Interpretations that mandate an earth less than 10,000 years old clearly are not in line with the scientific record. The clearest conclusion is that Genesis should be interpreted to allow for an old earth.17 But this should not scare us. Rather, realizing the immense age of the earth and the universe should give us a greater appreciation for the magnitude of God’s creative work. Because all truth is God’s truth, we need our beliefs about creation to line up with the scientific evidence. And the scientific record is actually quite helpful, because the text by itself does not clearly align to one particular interpretation. But even more this can lead us again to worship. As said in the cosmology lesson, in the antiquity of the universe we glimpse the extravagance of God’s love—that he would put on a fireworks show lasting billions of years in order to create humans who can relate to him and know him.

Next Page: Fossil Record