Radiometric dating

In J. R. R. Tolkien’s, The Hobbit, Gollum gives a riddle to Bilbo:

This thing all things devours:

Birds, beasts, trees, flowers;

Gnaws iron, bites steel;

Grinds hard stones to meal;

Slays kings, ruins town,

And beats high mountain down.

If Bilbo couldn’t give the right answer, it would lead to grave consequences, so as Gollum was getting impatient, Bilbo tried to say, “Give me more time; give me more time.” However, he was so scared that all that came out of his mouth was “… time … time” which of course was the correct answer. This riddle exemplifies how time normally results in decay and destruction.

So, why are people so interested in the topic of time? Perhaps the biggest reason is because of evil: how long can I put off dying? how long must I endure this pain? how long is the bad stuff going to last? Humans have a limited lifespan to get things done, so we try to synchronize schedules in order not to waste time. We have a limited endurance for boredom or pain, so before the use of anesthesia, the best surgeons were the quickest. In Bible times the Israelites complained that “The days are prolonged” (Eze 12:22) and the saints under the altar pleaded, “How long, O Lord?” (Rev 6:10). Although some complain that the Lord delays His coming (Mt 24:48; 2Pet 3:3,4,9), tribulations will be shortened or else none will be saved (Mt 24:22).

Perhaps another reason for the interest in time is that humans are bound by time and can’t move around in it as they can in space. The best they have is consciousness with a limited ability for such movement providing memories of past times and hopeful pictures of future times. Seventh-day Adventists show their great interest in time by having it as part of their name, referring to a recent seven-day creation in the past and a soon-coming advent of Christ in the future.

Within this philosophical and theological context of time, the next sections discuss geological time as determined by radiometric dating: (1) how the technique works well and (2) some young-earth creationist responses.

RADIOMETRIC DATING WORKS

Geology observations about the relation between different rocks can give relative ages, but radiometric dating is the primary method for giving absolute ages. The basic concept of radiometric dating is simple. During a certain length of time called a half-life, half of the radioactive parent atoms in a sample decay to stable daughter atoms. The number of both parent and daughter atoms are measured and the higher the daughter/parent ratio, the older the sample. The exact age is easily determined with a simple equation.

Of the many radioactive parent-daughter pairs of atoms, about half a dozen are used for age dating. They can be divided into two types. (a) Organic: Carbon-14 methods can date organic material such as bones, wood, and shells directly. With a half-life of about 5000 years, it is mainly useful for archaeology and not expected to give reliable ages beyond about ten half-lives or about 50,000 years. (b) Inorganic: Rubidium decaying to strontium, potassium decaying to argon, uranium and thorium decaying to lead, and [sometimes] samarium decaying to neodymium are used to date inorganic minerals and rocks. In most cases, these rocks are not fossil-containing sedimentary rocks, but are volcanic, granitic, or metamorphic. These inorganic methods date fossil material only by association and give ages of millions or billions of years.

The resulting ages are based on several assumptions: (a) the decay rate has been constant; (b) the original number of parent and daughter atoms in a sample can be estimated; and (c) the system [bone, wood, mineral, rock, etc] has not lost or gained parent or daughter atoms after the time of the event being dated, except by radioactive decay. In many cases these assumptions seem to be valid, and when they are not it is often obvious. For example, loss or gain of parent or daughter atoms might be expected if the sample shows signs of mechanical breakdown, chemical weathering, or metamorphic high pressures and temperatures.

Following are three of the many reasons that scientists put confidence in radiometric dates:

(1) Decay rates appear to be constant. (a) The strong force that governs nuclear decay rates is several orders of magnitude greater than the electromagnetic force that governs atomic interactions at even the highest temperatures and pressures found on earth. Only at the high temperatures in the interior of stars would the decay rates be expected to change significantly. (b) If the decay rate increased, atoms such as uranium, thorium, and samarium that decay by emitting alpha [helium nuclei] particles would have higher alpha emission energies. The alpha particles would then travel further in the mineral and damage a larger spherical halo in the mineral lattice. Such large halos have been searched for, but no significant examples have been found.

(2) Many types of radioactive atoms are known, but all primordial ones have half-lives greater than 450 million years and none have half-lives less than that. [Any atom found in nature with a shorter half-life, such as carbon-14 or radon, is being produced by on-going nuclear reactions.] The 450 million-year value is significant because it is one tenth the estimated age of the solar system. After ten half-lives the number of parent atoms decreases by a factor of about a thousand, and it is generally accepted that the number of parent atoms remaining in nature would be negligible. The natural occurrence of radioactive atoms thus seems to confirm the age of the solar system.

(3) Often several different parent-daughter pairs give concordant (similar) ages for a given rock or mineral. In one example from Saskatchewan in Canada, radiometric dates were determined for multiple samples from each of four minerals in a bentonite clay that was formed by the weathering of volcanic ash. The rubidium-strontium, potassium-argon, and uranium-lead decay pairs all gave ages of 72.5±0.2 million years.

CREATIONIST RESPONSES

First are several suggestions for young earth creationists studying radiometric dating: (a) work within the scientific community and use caution when making scientific claims; (b) work toward a constructive alternative model, more than just attacking existing models; (c) address the big picture, more than scattered discrete pieces of data; (d) don’t expect quick “silver bullet” proofs for the Bible; and (e) origins activities are one-time events, so don’t expect any origins research to fit a purely naturalistic scientific model.

Using the current understanding of radiometric dating, it is difficult to fit the data into a one-year flood a few thousand years ago; however, creationists have given a number of responses that are here divided into eight categories:

(1) The radiometric age data is bad. – Perhaps it is mainly concordant dates that get published and discordant dates are ignored. Occasionally however, even discordant dates can be found in the literature. For example in a 1969 article, volcanic rock from Mt. Rangitoto in New Zealand gave potassium-argon ages of hundreds of thousands of years, but trees destroyed by the lava gave carbon-14 ages of hundreds of years.

(2) Geochemistry suggests alternate time interpretations for the parent-daughter ratios. – (a) Radiometric ages for some minerals may not give the time of emplacement of the volcanic or granitic rock because the minerals were formed earlier and the emplaced magma was not hot enough to melt them. (b) The radiometric chronometer was not reset to zero by the liquid magma because not all daughter atoms were excluded from the crystallizing mineral or rock. (c) The slope of the straight line isochron through the age data is not due to radioactive decay over time, but to the mixing of two magmas with different compositions.

(3) Decay rates have changed. – A few creationists suggest that a change in the speed of light resulted in changed decay rates. Some creationists note that the decay rates in rocks on earth may vary slightly due to bombardment by solar neutrinos. Other creationists propose speeded-up decay rates to explain rocks containing more than the expected amount of helium, if uranium were decaying at only present-day rates.

(4) Relativity suggests that time is not absolute. – Special relativity finds that time moves at different rates in reference frames traveling at close to the speed of light with respect to each other. General relativity indicates that time moves slower in strong gravitational fields, and this has been suggested as an explanation for an apparent old age for the universe.

(5) Radiometric ages can’t be right because other data suggests shorter ages. – (a) Paraconformities occur in the geologic column where sedimentary layers have a flat contact between them. Even if fossils and radiometric dates associated with the two layers suggest a significant time lapse, the flat contact seems to indicate rapid deposition with no time lapse between layers. (b) Good fossil and soft tissue preservation suggest rapid and recent burial that may not agree with radiometric ages of associated volcanic ash layers. (c) Carbon-14 is sometimes found in wood or coal that is traditionally expected to be much too old to still contain any short half-life carbon-14 atoms.

(6) Radiometric dates should be rejected for philosophical reasons. – (a) Extrapolation from a hundred years of knowledge about radioactive decay to millions of years for the age of the earth is problematic. (b) Geology is an historical science, unlike a laboratory science, and is not repeatable under known conditions. (c) A thousand times more scientists are studying the data from a long-age paradigm than from a short-age paradigm, so one would expect them to have better explanations. (d) Scientific revolutions have occurred in the past and perhaps such a paradigm shift in the future will be able to explain radiometric dates in a short time frame.

(7) An intermediate, partly old / partly young, model could explain the data. – Some have suggested that the inorganic material of the earth is indeed old, but then God came and created life recently. Thus the radiometric ages are valid, but do not actually give the age of the organic material.

(8) Supernatural intervention results in unexpected effects on time perception. – (a) At the end of creation week, Adam and Eve would have appeared as adults although they were only a few hours old. (b) God’s intervention in the Old Testament resulted in time anomalies when Aaron’s rod budded and produced almonds overnight, when Joshua commanded the sun to stand still for a day, and when the sun’s shadow moved backward ten degrees on Hezekiah’s sundial. (c) In the New Testament natural processes were speeded up when Christ turned water to wine, multiplied the loaves and fishes, and healed people.

In conclusion: Radiometric dating has its uncertainties, but often seems to work well. At this point we don’t know how to relate its resulting ages with the Bible record, for we only know in part (I Cor 13:8-12). However, God’s ways are not our ways (Isa 55:8) and with God all things are possible (Mt 19:26; Lk 18:27).

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Ben Clausen

Geoscience Research Institute

July 22, 2013

This entry was posted in Dating and the Age of the Earth, Geology and tagged , , , . Bookmark the permalink.

One Response to Radiometric dating

  1. Richard Rusoke says:

    I think it is good to do Radiometric dating without being religious biased. To be faithful in dating and then see the other side of the version i mean the bible.

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