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Radiometric DatingFrom The New Answers DVD 1. To charge thousands of chemists all over the world with mass incompetence also seems to be beyond the bounds of reason. Animals and fungi get C14 from the plant or animal tissue they eat for food. A TRIP THROUGH TIME: Sr87 ratio is exactly matched by the gain of Sr87 in the strontium-isotope ratio. Teaching about the Temporal Aspects of Geoscience.
An Essay on Radiometric Dating. By Jonathon Woolf http: Radiometric dating methods are the strongest direct evidence that geologists have for the age of the Earth. All these methods point to Earth being very, very old -- several billions of years old. Young-Earth creationists -- that is, creationists who believe that Earth is no more than 10, years old -- are fond of attacking radiometric dating methods as being full of inaccuracies and riddled with sources of error. When I first became interested in the creation-evolution debate, in late , I looked around for sources that clearly and simply explained what radiometric dating is and why young-Earth creationists are driven to discredit it.
I found several good sources, but none that seemed both complete enough to stand alone and simple enough for a non-geologist to understand them. Thus this essay, which is my attempt at producing such a source. Theory of Radiometric Dating.
Common Methods of Radiometric Dating. Possible Sources of Error. Creationist Objections to Radiometric Dating. Independent Checks on Radiometric Dating. Theory of radiometric dating. What is radiometric dating? Simply stated, radiometric dating is a way of determining the age of a sample of material using the decay rates of radio-active nuclides to provide a 'clock. The rules are the same in all cases; the assumptions are different for each method.
To explain those rules, I'll need to talk about some basic atomic physics. There are 90 naturally occurring chemical elements. Elements are identified by their atomic number , the number of protons in the atom's nucleus. All atoms except the simplest, hydrogen- 1, have nuclei made up of protons and neutrons.
Hydrogen-1's nucleus consists of only a single proton. Protons and neutrons together are called nucleons , meaning particles that can appear in the atomic nucleus. A nuclide of an element, also called an isotope of an element, is an atom of that element that has a specific number of nucleons.
Since all atoms of the same element have the same number of protons, different nuclides of an element differ in the number of neutrons they contain. For example, hydrogen-1 and hydrogen-2 are both nuclides of the element hydrogen, but hydrogen-1's nucleus contains only a proton, while hydrogen-2's nucleus contains a proton and a neutron.
Uranium contains 92 protons and neutrons, while uranium contains 92 protons and neutrons. Many nuclides are stable -- they will always remain as they are unless some external force changes them.
Some, however, are unstable -- given time, they will spontaneously undergo one of the several kinds of radioactive decay, changing in the process into another element. There are two common kinds of radioactive decay, alpha decay and beta decay. In alpha decay, the radioactive atom emits an alpha particle.
An alpha particle contains two protons and two neutrons. After emission, it quickly picks up two electrons to balance the two protons, and becomes an electrically neutral helium-4 He4 atom.
When a nuclide emits an alpha particle, its atomic number drops by 2, and its mass number number of nucleons drops by 4. Thus, an atom of U uranium, atomic number 92 emits an alpha particle and becomes an atom of Th thorium, atomic number A beta particle is an electron.
When an atom emits a beta particle, a neutron inside the nucleus is transformed to a proton. The mass number doesn't change, but the atomic number goes up by 1. Thus, an atom of carbon C14 , atomic number 6, emits a beta particle and becomes an atom of nitrogen N14 , atomic number 7.
A third, very rare type of radioactive decay is called electron absorption. In electron absorption, a proton absorbs an electron to become a neutron.
In other words, electron absorption is the exact reverse of beta decay. So an atom of potassium K40 , atomic number 19 can absorb an electron to become an atom of argon Ar40 , atomic number The half-life of a radioactive nuclide is defined as the time it takes half of a sample of the element to decay. A mathematical formula can be used to calculate the half-life from the number of breakdowns per second in a sample of the nuclide.
Some nuclides have very long half-lives, measured in billions or even trillions of years. Others have extremely short half-lives, measured in tenths or hundredths of a second. The decay rate and therefore the half-life are fixed characteristics of a nuclide. Different nuclides of the same element can have substantially different half-lives.
The half-life is a purely statistical measurement. A sample of U ten thousand years old will have precisely the same half-life as one ten billion years old. Obviously, the major question here is "how much of the nuclide was originally present in our sample?
Such cases are useless for radiometric dating. We must know the original quantity of the parent nuclide in order to date our sample radiometrically. Fortunately, there are cases where we can do that. This is the second axiom of radiometric dating. The third and final axiom is that when an atom undergoes radioactive decay, its internal structure and also its chemical behavior change. Losing or gaining atomic number puts the atom in a different row of the periodic table, and elements in different rows behave in different ways.
It may not form the same kinds of compounds. When the number of electrons change, the shell structure changes too. So when an atom decays and changes into an atom of a different element, its shell structure changes and it behaves in a different way chemically. How do these axioms translate into useful science? This section describes several common methods of radiometric dating. To start, let's look at the one which almost everyone has heard of: The element carbon occurs naturally in three nuclides: C12, C13, and C The vast majority of carbon atoms, about About one atom in billion is C The remainder are C Of the three, C12 and C13 are stable.
C14 is radioactive, with a half-life of years. C14 is also formed continuously from N14 nitrogen in the upper reaches of the atmosphere. And since carbon is an essential element in living organisms, C14 appears in all terrestrial landbound living organisms in the same proportions it appears in the atmosphere.
Plants and protists get C14 from the environment. Animals and fungi get C14 from the plant or animal tissue they eat for food. When an organism dies, it stops taking in C If we measure how much C14 there currently is, we can tell how much there was when the organism died, and therefore how much has decayed. When we know how much has decayed, we know how old the sample is.
Many archaeological sites have been dated by applying radiocarbon dating to samples of bone, wood, or cloth found there. Radiocarbon dating depends on several assumptions. One is that the thing being dated is organic in origin. Radiocarbon dating does not work on anything inorganic, like rocks or fossils. Only things that once were alive and now are dead: The second assumption is that the organism in question got its carbon from the atmosphere. A third is that the thing has remained closed to C14 since the organism from which it was created died.
The fourth one is that we know what the concentration of atmospheric C14 was when the organism lived and died. That last one is more important than it sounds. When Professor William Libby developed the C14 dating system in , he assumed that the amount of C14 in the atmosphere was a constant.
A long series of studies of C14 content produced an equally long series of corrective factors that must be taken into account when using C14 dating. So the dates derived from C14 decay had to be revised. One reference on radiometric dating lists an entire array of corrective factors for the change in atmospheric C14 over time.
C14 dating serves as both an illustration of how useful radiometric dating can be, and of the pitfalls that can be found in untested assumptions. U and U are both nuclides of the element uranium. U is well known as the major fissionable nuclide of uranium. It has a half-life of roughly million years. U is more stable, with a half-life of 4. Th is the most common nuclide of the element thorium, and has a half-life of All three of these nuclides are the starting points for what are called radioactive series.
A radioactive series is a sequence of nuclides that form one from another by radioactive decay.