100 Interesting Science Facts, page 1

Without resorting to materials that aren't "terrestrial" in nature, we can easily go back to around 4 billion years for rock samples that are native. Here's how it works.

A supernova of some kind creates (by atomic fusion) elements heavier than iron. These elements cannot be created by "regular" fusion as part of stellar nucleosynthesis, but only in the supernova event. Among the trans-iron elements is uranium.

Gas, dust and debris in an area begin to be gravitationally attracted to each other and coalesce. A star and some planets form. One is the third planet from the star. The initial state of the planet is one of molten material. It cools over a long time and rocks form. The core remains molten. The rocks that form begin a "life" based on what is inside them. If they are reclaimed or "remelted" by vulcanism, they have lost their identity and, if set out on the surface of the earth, begin a new "life" based again on what is inside them.

As rocks form, some zirconium gets trapped in the rocks along with some silicon, some oxygen and a bit of uranium. The temperatures at which these rocks form prevents lead from being included. Crystals form. The rocks underwent transformation to become crystals of a mineral called zircon, which is ZrSiO4. But there is some uranium present when things are getting together, and it sneaks into the crystal during formation in the place of some of the zirconium atoms. The uranium is now locked up in a lead-free crystal with zirconium, silicon and oxygen. Zircon is stable, tough and chemical- and weather-resistant. A crystal can hang around for billions of years. Some do.

Uranium has a number of isotopes. All uranium has 92 protons, but the number of neutrons varies. All uranium is unstable; it is radioactive and will eventually decay. What it decays into varies, depending on the starting isotope. Half-lives vary, too. U-238 is the most common isotope of uranium, and it comprises over 99% of the stuff we find. So most of the uranium in that zircon crystal is 238U. The 238U decays with a half-life of 4.46 x 109 years (4.46 billion years). That's a long time. And it's very convenient for us.

The 238U decays (eventually) into lead, specifically 210Pb, which is stable. There are a number of intermediate decay products, but the chain is well understood and half-lives are accurately known. If a sample of 238U is sequestered in a tightly sealed container (like a zirconium crystal), it can be studied there and we can find out how much uranium is there, and, based on what is in there with it (those decay products), we can "look back" and figure out how long it has been in there. Nothing can have happened to this crystal to change the chronology sealed inside if the crystal is intact. This is the heart of uranium-lead dating. Dating things well over 4 billion years old with an accuracy of a few million years is very doable in the lab. It is a painstaking and tedious activity, but it is the bread and butter of some devoted folks. They spend all their lives honing their skills at this - and their skills are razor sharp.

We drag our equipment out into the field looking for zircon crystals. We must find them "in rock" and not lying around on the ground. But we can find some. When we do, we know these little guys have been like they are now for a long time. But how long? Back in the office, we apply our laboratory radiometric dating techniques (micro-beam analysis), and we can date zirconium crystal samples back to nearly 4 billion years with an accuracy of + or - a few million years (not a few tens of millions of years). Easily. And that's how we know the oldest samples of rock found on earth date back to almost 4 billion years. Moon rocks and some material from meteors date back even farther by up to another half a billion years. And that is the source of the 4.54 billion year figure that is so often quoted for the age of the earth.

None of these activities requires Star Trek technology, a magic wand or