Radiometric Dating: Back to Basicsby Andrew A. SnellingJune 17, 2009Featured In
Radiometric dating is often used to “prove” rocks are millions of years old. Once
you understand the basic science, however, you can see how wrong
assumptions lead to incorrect dates.
Most people think that radioactive dating has proven the earth is billions of years old. After
all, textbooks, media, and museums glibly present ages of millions of
years as fact.
Yet few people know how radiometric dating works or bother to ask what assumptions drive the conclusions. So let’s
take a closer look and see how reliable this dating method really is.
Atoms—Basics We Observe Today Each chemical element, such as carbon and oxygen, consists of atoms. Each atom is thought to be made up of three basic parts.
Radiometric Dating 101 PART 1: Back to Basics PART 2: Problems with the Assumptions
PART 3: Making Sense of the Patterns
This three-part series will help you properly understand radiometric dating,
the assumptions that lead to inaccurate dates, and the clues about what
really happened in the past.
The nucleus contains protons (tiny particles each with a single positive electric charge)
and neutrons (particles without any electric charge). Orbiting around
the nucleus are electrons (tiny particles each with a single negative
electric charge).
The atoms of each element may vary slightly in the numbers of neutrons within their nuclei. These
variations are called isotopes of that element. While the number of
neutrons varies, every atom of any element always has the same number
of protons and electrons.
So, for example, every carbon atom contains six protons and six electrons, but the number of neutrons
in each nucleus can be six, seven, or even eight. Therefore, carbon has
three isotopes (variations), which are specified carbon-12, carbon-13,
and carbon-14 (Figure 1).
Radioactive Decay Some isotopes are radioactive; that is, they are unstable because their
nuclei are too large. To achieve stability, the atom must make
adjustments, particularly in its nucleus. In some cases, the isotopes
eject particles, primarily neutrons and protons. (These are the moving
particles measured by Geiger counters and the like.) The end result is
a stable atom, but of a different chemical element (not carbon) because
the atom now has a different number of protons and electrons.
This process of changing one element (designated as the parent isotope) into
another element (referred to as the daughter isotope) is called
radioactive decay. The parent isotopes that decay are called
radioisotopes.
"Radiometric dating is based on an observable fact of science: unstable atoms will break down over a measurable period of time."
Actually, it isn’t really a decay process in the normal sense of the word, like
the decay of fruit. The daughter atoms are not lesser in quality than
the parent atoms from which they were produced. Both are complete atoms
in every sense of the word.
Geologists regularly use five parent isotopes to date rocks: uranium-238, uranium-235, potassium-40,
rubidium-87, and samarium-147. These parent radioisotopes change into
daughter lead-206, lead-207, argon-40, strontium-87, and neodymium-143
isotopes, respectively. Thus geologists refer to uranium-lead (two
versions), potassium-argon, rubidium-strontium, or samarium-neodymium
dates for rocks. Note that the carbon-14 (or radiocarbon) method is not
used to date rocks because most rocks do not contain carbon.
Chemical Analysis of Rocks Today Geologists can’t use just any old rock for dating. They must find rocks that have
the isotopes listed above, even if these isotopes are present only in
minute amounts. Most often, this is a rock body, or unit, that has
formed from the cooling of molten rock material (called magma).
Examples are granites (formed by cooling under the ground) and basalts
(formed by cooling of lava at the earth’s surface).
The next step is to measure the amount of the parent and daughter isotopes
in a sample of the rock unit. Specially equipped laboratories can do
this with accuracy and precision. So, in general, few people quarrel
with the resulting chemical analyses.
It is the interpretation of these chemical analyses that raises potential
problems. To understand how geologists “read” the age of a rock from
these chemical analyses, let’s use the analogy of an hourglass “clock”
(Figure 2).
In an hourglass, grains of fine sand fall at a steady rate from the top
bowl to the bottom. After one hour, all the sand has fallen into the
bottom bowl. So, after only half an hour, half the sand should be in
the top bowl, and the other half should be in the bottom bowl.
Suppose that a person did not observe when the hourglass was turned over. He
walks into the room when half the sand is in the top bowl, and half the
sand is in the bottom bowl. Most people would assume that the “clock”
started half an hour earlier.
By way of analogy, the sand grains in the top bowl represent atoms of the parent radioisotope
(uranium-238, potassium-40, etc.) (Figure 2). The falling sand
represents radioactive decay, and the sand at the bottom represents the
daughter isotope (lead-206, argon-40, etc).
When a geologist tests a rock sample, he assumes all the daughter atoms were
produced by the decay of the parent since the rock formed. So if he
knows the rate at which the parent decays, he can calculate how long it
took for the daughter (measured in the rock today) to form.
But what if the assumptions are wrong? For example, what if radioactive
material was added to the top bowl or if the decay rate has changed?
Future articles will explore the assumptions that can lead to incorrect
dates and how the Bible’s history helps us make better sense of the
patterns of radioactive “dates” we find in the rocks today.
Dr. Andrew Snelling holds a PhD in geology from the University of Sydney and has worked as
a consultant research geologist to organizations in both Australia and
America. Author of numerous scientific articles, Dr. Snelling is now
the head of the Research Division at Answers in Genesis–USA. 
