82 Evolution and the Fossil Record
even older, with many samples giving ages of 4.5 to 4.6 billion years. Although we have no
earth rocks that old yet, this is not surprising, because the earth has a dynamic crust that is
constantly melting and recycling older material. It is amazing that earth materials as old as
4.4 billion years still survive. The moon and the meteorites, by contrast, have changed very
little since their formation, so it would be expected that they preserve very ancient dates (for
further details, see Dalrymple 2004). The earth, moon, and meteorites apparently formed
at the same time from the primordial solar system, which is why we say the earth is also
4.6 billion years old.
The basic principles of radiometric dating are relatively straightforward and very well
understood. Certain isotopes of elements, such as potassium-40, rubidium-87, uranium-238,
and uranium-235, spontaneously break down or “decay” into atoms of different “daughter”
elements (argon-40, strontium-87, lead-206, and lead-207, respectively, for the “parent”
material just listed) by emitting nuclear radiation (alpha and beta particles and gamma
radiation) plus heat. The rate of this radioactive decay is well known for all these elements
and has been checked and double-checked in the laboratory hundreds of times. Geologists
obtain a fresh sample of the rock, break it down into its component mineral crystals, and
then measure the ratio of parent atoms to daughter atoms within the mineral. That ratio is a
direct mathematical function of the age of the crystal.
Like any other technique in science, there are limitations and pitfalls that have to be
avoided. Because dating is a measure of the time since a crystal cooled and locked in the
radioactive parent atoms, it only works in rocks that cool down from a molten state, known
as igneous rocks (such as granites or lava rocks). Creationists mock scientists over the fact
that we cannot directly date the crystals in sandstone or any other sedimentary rock (the
crystals are recycled from older rocks and have no bearing on the age of the sediment). But
geologists long ago circumvented this problem by finding hundreds of places all over earth
where datable volcanic lava flows or ash falls are interbedded with fossiliferous sediment
or where intruding granitic magma bodies cut across the sedimentary rocks and provide a
minimum age. From settings such as these, the numerical ages of the geologic time scale are
derived, and their precision is now so well resolved that we know of the age of most events
that are millions of years old to the nearest 100,000 years or less.
If the crystal structure has somehow leaked some of its parent or daughter atoms or
allowed new atoms in to contaminate the crystal, the parent/daughter ratio is disturbed and
the date is meaningless. But geologists are always on the lookout for this problem, running
dozens of samples to determine whether their age data are reliable and cross-checking their
dates against other sources for determining age. The newest techniques and machinery are
so precise that a skilled geologist can spot the error in almost any date and quickly reject
those that don’t meet very high standards. Creationists will mention a specific date that
proved to be wrong as evidence that the entire field of geochronology is unreliable, when in
fact it was the geologists themselves who spotted the erroneous date, and quickly rejected
it. As Dalrymple (2004) points out, it’s as if we had a variety of watches and clocks, a few
of which don’t keep accurate time. But that fact doesn’t mean we completely ignore clocks
and watches altogether, as creationists are doing by rejecting all radiometric dating out of
hand. We simply keep checking them against one another to determine which ones are reli-
able and which ones are not.
In another commonly repeated claim, creationists mock geologists over the example
of a living clam that gave a radiocarbon date of many thousands of years old. But this is a