Definition of radioisotope dating
The zircon formation may have occurred tens to hundreds of thousands of years before the eruption and deposition.However, when dealing with rocks that are hundreds of millions of year old, the time between zircon formation and eruption really is short in comparison.To see how it works, we'll start at the beginning, using uranium as an example: At left, a zircon crystal in a thin section cut from granite. Crystal structure image adapted fromadapted from Materialscientist CC Attribution-Share Alike 3.0 Unported Tens to hundreds of thousands of years before a major volcanic eruption, magma builds up beneath the surface of the Earth.In the magma, crystals of zirconium silicate (called zircons), as well as other crystals, form.These zircon crystals are tiny just a tenth of a millimeter long but they are the key to uranium-lead dating.If these crystals were pure, they would contain just zirconium, silica, and oxygen; however, uranium happens to have a similar arrangement of outer electrons to zirconium, and so as zircons form, "mistakes" are sometimes made, and uranium is substituted for zirconium.For example, carbon-14 decays into nitrogen-14 and has a half-life of just 5,730 years.
Decay rates are measured in half-lives the amount of time in which half of a radioactive element will decay.
For example, as shown at left below, uranium-235 has a half-life of 704 million years.
That means that in 704 million years, one gram of uranium will be reduced to ½ gram of uranium.
When the eruption occurs, zircons are released in the ash and lava, which then become rocks like rhyolite.
Geologists hunt for these particular sorts of rock to date the volcanic eruption in which the rock formed.