Ikester makes good points. The issue of evolutionist dating techniques really seems to deserve its own thread.

However, there is an easier way to check if a method is reliable. We can see if it gives consistent answers against some other method.For example, Potassium-Argon (K-Ar) dating was tested against the Cenozoic-Era North American Land Mammal ordering. By ordering, I mean that rock layers were given numbers, with bigger numbers at greater depth. Each fossil was given the number of the rock layer it was found in. (Geologists call this stratigraphic order.) Here are the results:

Stratigraphic ................. K-Ar Date .. # Samples
Position ..... Name of Age .. (millions) ... Dated
=============================================
...1 ......... Irvingtonian ....... 1.36 ........... 1
...2 ......... Blancan ............ 1.5 - 3.5 ....... 7
...3 ......... Hemphillian ....... 4.1 -10.0 ...... 8
...4 ......... Claredonian ....... 8.9 -11.7 ..... 16
...5 ......... Barstovian ....... 12.3-15.6 ....... 9
...6 ......... Hemingfordian .. 17.1 ............. 1
...7 ......... Arikareean ....... 21.3-25.6 ...... 4
...8 ......... Orellian ............ --- ............. 0
...9 ......... Chadronian ....... 31.6-37.5 ..... 9
..10 ......... Duchesnean ...... 37.5 ........... 3
..11 ......... Uintan ............ 42.7-45.0 ..... 2
..12 ......... Bridgerian ........ 45.4-49.0 ..... 2
..13 ......... Wasatchian ....... 49.2 ........... 1
..14 ......... Puercan ........... 64.8 ........... 1

The standard geological idea is that "deeper is older". (It's called the Principle of Superposition, and was invented two centuries before Darwin.) In this table, Superposition and K-Ar dating are mutually consistent.

One of the most exciting and important scientific findings in decades was the 1980 discovery that a large asteroid, about 10 kilometers diameter, struck the earth at the end of the Cretaceous Period. The collision threw many tons of debris into the atmosphere and possibly led to the extinction of the dinosaurs and many other life forms. The fallout from this enormous impact, including shocked quartz and high concentrations of the element iridium, has been found in sedimentary rocks at more than 100 locations worldwide at the precise stratigraphic location of the Cretaceous-Tertiary (K-T) boundary (Alvarez and Asaro 1990; Alvarez 1998). We now know that the impact site is located on the Yucatan Peninsula. Measuring the age of this impact event independently of the stratigraphic evidence is an obvious test for radiometric methods, and a number of scientists in laboratories around the world set to work.

In addition to shocked quartz grains and high concentrations of iridium, the K-T impact produced tektites, which are small glass spherules that form from rock that is instantaneously melted by a large impact. ... The results from all of the laboratories were remarkably consistent with the measured ages ranging only from 64.4 to 65.1 Ma (Table 2). Similar tektites were also found in Mexico, and the Berkeley lab found that they were the same age as the Haiti tektites. But the story doesn’t end there.The K-T boundary is recorded in numerous sedimentary beds around the world. The Z-coal, the Ferris coal, and the Nevis coal in Montana and Saskatchewan all occur immediately above the K-T boundary. Numerous thin beds of volcanic ash occur within these coals just centimeters above the K-T boundary, and some of these ash beds contain minerals that can be dated radiometrically. Ash beds from each of these coals have been dated by 40Ar/39Ar, K-Ar, Rb-Sr, and U-Pb methods in several laboratories in the US and Canada. Since both the ash beds and the tektites occur either at or very near the K-T boundary, as determined by diagnostic fossils, the tektites and the ash beds should be very nearly the same age, and they are (Table 2).



There are several important things to note about these results. First, the Cretaceous and Tertiary periods were defined by geologists in the early 1800s. The boundary between these periods (the K-T boundary) is marked by an abrupt change in fossils found in sedimentary rocks worldwide. Its exact location in the stratigraphic column at any locality has nothing to do with radiometric dating — it is located by careful study of the fossils and the rocks that contain them, and nothing more. Second, the radiometric age measurements, 187 of them, were made on 3 different minerals and on glass by 3 distinctly different dating methods (K-Ar and 40Ar/39Ar are technical variations that use the same parent-daughter decay scheme), each involving different elements with different half-lives. Furthermore, the dating was done in 6 different laboratories and the materials were collected from 5 different locations in the Western Hemisphere. And yet the results are the same within analytical error. If radiometric dating didn’t work then such beautifully consistent results would not be possible.

To check, we need one single event which has been dated by several methods. A nice example is the Triassic multiple-impact event, which formed a 4500-kilometer-long chain of huge craters. (There must have been a train of big objects from space, which hit the spinning earth, one by one, across several hours. Much like the way comet Shoemaker-Levy hit Jupiter in 1994.)

Here are the five confirmed craters:

.. Crater ..... Country ... Diameter ... 10^6 yrBP ... Dating Method
------------------------------------------------------------------------------------
Manicouagan ... Canada ..... 100 km ..... 214 ± 1 .... U-Pb on zircons
Saint Martin .... Canada ...... 40 km ..... 219 ± 32 ... Rb/Sr
Rochechouart ... France ...... 25 km ..... 214 ± 8 .... Ar/Ar laser spot fusion
Obolon ........... Ukraine ..... 15 km ..... 215 ± 25 ... stratigraphic
Red Wing ........ USA .......... 9 km ..... 200 ± 25 ... stratigraphic

"Stratigraphic" dating means that the crater itself has not been dated. Instead, the rock strata above and below the crater was dated. (By now, the Red Wing crater is under 1.5 kilometers of sediment.)

Some meteorites, because of their mineralogy, can be dated by more than one radiometric dating technique, which provides scientists with a powerful check of the validity of the results. The results from three meteorites are shown in Table 1. Many more, plus a discussion of the different types of meteorites and their origins, can be found in Dalrymple (1991).



There are 3 important things to know about the ages in Table 1. The first is that each meteorite was dated by more than one laboratory — Allende by 2 laboratories, Guarena by 2 laboratories, and St Severin by four laboratories. This pretty much eliminates any significant laboratory biases or any major analytical mistakes. The second thing is that some of the results have been repeated using the same technique, which is another check against analytical errors. The third is that all three meteorites were dated by more than one method — two methods each for Allende and Guarena, and four methods for St Severin. This is extremely powerful verification of the validity of both the theory and practice of radiometric dating.

In the case of St Severin, for example, we have 4 different natural clocks (actually 5, for the Pb-Pb method involves 2 different radioactive uranium isotopes), each running at a different rate and each using elements that respond to chemical and physical conditions in much different ways. And yet, they all give the same result to within a few percent. Is this a remarkable coincidence? Scientists have concluded that it is not; it is instead a consequence of the fact that radiometric dating actually works and works quite well. Creationists who wants to dispute the conclusion that primitive meteorites, and therefore the solar system, are about 4.5 Ga old certainly have their work cut out for them!

Most of the rocks we have from the moon do not exceed 4.1 billion years. The samples thought to be the oldest are highly pulverized and difficult to date, though there are a few dates extending all the way to 4.4 to 4.5 billion years. Most scientists think that all the bodies in the solar system were created at about the same time. Evidence from the uranium, thorium, and lead isotopes links the Earth's age with that of the meteorites. This would make the Earth 4.5-4.6 billion years old.