On Radiometric Dating and the Age of the Earth

We Are Understandably Confused

One scientific analytical technique that seems to thoroughly flummox the non-scientist is radiometric dating.  Being what sometimes appears abstruse and obfuscated in its methods and analysis, biblical creationists on occasion throw their hands up and cite its difficulty as suggesting that evidence of an ancient earth is fabricated.  When working at a summer bible camp in northern Michigan in 2009, I encountered a camp nurse who actually said, "One thing to consider about all this radiometric dating is that the baseline numbers they use in their analysis are arbitrary, made up.  You can't draw any meaningful conclusion from that."  In spite of my insistence, she walked away smugly satisfied that radiometric dating is bunk.

Refer to my first blog post "The Purpose of this Blog" to see how I felt.

You might remember this idea from CHEM 115, but you might not, or maybe you never really understood it.  Here's the skinny on this technique.

Isotopes and the Formation of Carbon-14

Start with carbon dating.  All elements in the universe have what are called isotopes.  An isotope is a version of an element that has the same number of protons, but a different number of neutrons.  Different isotopes have different masses, but they retain the same elemental identity, which is determined by protons.  See the three isotopes of hydrogen below for an example.  The yellow balls are protons (each hydrogen atom only has one), while the orange ones are neutrons (each different isotope has different numbers of neutrons).

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A somewhat rare isotope of carbon is made out of nitrogen in the upper atmosphere.  The stable and most common isotope of carbon is carbon-12: it has 6 protons and 6 neutrons.  Sunlight strikes the common isotope of nitrogen-14, turns one of its protons into a neutron, and now you have a heavier-than-usual carbon atom, with a mass of 14.  Carbon-14 is produced in the atmosphere at a predictable rate, depending on the activity of the sun, which varies somewhat according to the solar cycle.

All living things have the same concentration of C-14 in their bodies as the atmosphere has.  This is because the C-14 is absorbed by plants and algae, which we and other animals consume to constitute the carbon-based materials of our bodies.  When we die, we stop absorbing C-14.

Radioactive Decay and Half-Lives

Follow everything so far?  Because here comes the important part.

Now C-14 is chemically identical to C-12, but it is also slightly radioactive.  That means that a sample of C-14 will radioactively decay (revert back to nitrogen-14 and spit out some electrons and exotic particles called "electron neutrinos" in the process of beta-decay) over a measurable period.  Precisely, exactly half of a sample of C-14 will decay into N-14 over the course of 5730 years.  We know this because when we measure a sample of a known number of C-14 atoms, they emit beta radiation at a measurable rate.  We can extrapolate this measurement of decay rate to find that in 5730 years, said sample will have only 1/2 as many radioactive particles (C-14) as it does today.  Note that C-14 is always radioactively decaying in your body, but it is also be replenished quickly enough that your C-14 levels stay the same throughout your life.

So as soon as an organism dies, it stops replenishing the C-14, and so C-14 concentration drops according to its half-life of 5730 years.  The C-14 decays into nitrogen, which leaves as a gas.

Now we can take a sample and measure how much C-14 is has, either by using a process known as mass spectrometry (which is scary accurate) or simply measuring its radioactivity with a precise Geiger counter (a little less reliable, but still good).

If only 1/2 of the C-14 remains that ought to, then we know that one "half-life" has elapsed, so 5730 years has gone by since the organism died.  If 1/4 remains, then two half-lives, or 11,460 years, have passed.  If 1/8 remains, then three half-lives, or 17,190 years have elapsed, and so on as seen in this chart.  There are some calculations we can do with exponents that allow us to calculate other fractions.  To the right is an excellent little animation that visualizes radioactive decay.  Note that C-14 dating is only good for organic samples that are less than 60,000 years old, otherwise too little C-14 remains to make an accurate measurement.

Use in Rocks

So what if the sample you are testing is from a non-living substance, like a rock?  This gets much, much more complex, but the basic radiometric foundations are the same as those described above.  We can't use C-14 anymore, because it's not found in rocks.  So we have to use a different set of radioactive isotopes.

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The isotopes we use for rock dating come in sets of pairs.  The first of the pair is found in greatest abundance in fresh igneous (volcanic) rocks, the next is found in increasing abundance as the first isotope radioactively decays.  When rocks are brought to what is called closure temperature and melted in the lab, the ratios of these pairs reset in a predictable fashion and the radiometric clock goes back to zero.  This shows that fresh magma will always have a predictable ratio of isotopes.  It also means that rocks can only be dated to the last time they were melted.

If we can compare how much of each isotope there is, we can use the same calculations outlined above to identify how old the rock must be.  Different measurements, if done carefully, usually end up agreeing within about 2% to 5%.  This means that the method is reliable and precise.  The isotope pairs (and a triplet) include, but are not limited to:

• rubidium-strontium (Rb-Sr)
• uranium-lead (U-Pb)
• samarium-neodynium (Sm-Nd)
• argon-argon (Ar-Ar)
• uranium-thorium (U-Th)
• iodine-xenon (I-Xe)
• lanthanum-barium (La-Ba)
• potassium-argon (K-Ar)
• lead-lead (Pb-Pb)
• lutetium-hafnium (Lu-Hf)
• neon-neon (Ne-Ne)
• rhenium-osmium (Re-Os)
• uranium-lead-helium (U-Pb-He)
• uranium-uranium (U-U)

(Note: The pairs that are named the same are different isotopes of the same element; in this case neutrons or photons undergo a decay, so the mass or energy changes, but the number of protons stays the same.  Sometimes, in the other cases, the decay changes a proton, so the identity of the element changes.)

There is a broad list of preconditions that must be considered when trying to precisely peg the age of rocks with this method.  It includes the physical and chemical conditions under which the rock was formed, and it helps scientists choose which of the pairs above they should use for analysis.  Some pairs are better to use on very, very old rocks; while others are more reliable for younger rocks.  Unfortunately, sedimentary and metamorphic rocks can't be reliably radio-dated because of the physical and chemical changes that have taken place in the rock.  Ages of these rocks are usually determined by relation to nearby igneous rocks that can be dated.

How Old are the Oldest Rocks?

The oldest material ever dated by chemists in the lab was a zircon crystal estimated to be 4.404 billion years old, plus or minus 8 million years.  

Image Source

The oldest rocks dated are 4.031 billion years old, plus or minus 3 million.  They're up in the Canadian greenstone belts and some other ancient exposed areas.  This coincides pretty nicely with the age that astrophysicists and geologists estimate Earth to be, around 4.5 billion years.  

The broad acceptance and agreement of these results by the scientific community speaks for itself.  Earth is very, very ancient.