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Dating A Young Earth: Part 2

Updated on February 23, 2015

Igneous Rocks

Igneous rocks are the primary rock used to provide absolute geological dates.  Igneous rocks are formed when melted rock from the mantle, either magma which is melted rock underground, or lava which is melted rock above ground, cools to a solid.  As it cools crystals form and these crystals trap various radioactive isotopes in them.

Radioactive isotopes are types of elements that give off particles causing them to decay into other isotopes and elements.  Assuming that there is a known amount of any element in the rock to be measured and that the isotopes decay at a constant rate it is possible to calculate an age for the rock.  The process of measuring the isotopes involves taking at least one sample of the rock and measuring the radioactive elements in that sample.  A single sample provides a model age, multiple samples (at least 4) provide an isochron age, 3 or more different samples of minerals from the same rock sample will also provide an isochron age.  The most common radioactive isotopes used for dating purposes are Potassium -Argon, Argon-Argon, Rubidium-Strontium, Samarium-Neodymium, and Uranium-Lead.

Carbon-14 which is a commonly known radioactive isotope is not used to date igneous rocks as it is primarily absorbed by organic substances.  It also has a short half-life.  A half-life is the length of time it takes for one-half of the isotope to decay into its daughter isotope, in this case Nitrogen-14.  Carbon-14 has a half-life of approximately 5,700 years.  After 18 half-lives the amount of Carbon-14 left in a substance is too small to measure.  Eighteen half-lives equates to  about 100,000 years, as most igneous rock is thought to be over 1 billion years old there should be no Carbon-14 in it.

The Potassium-Argon and Argon-Argon methods of dating are considered a safe measuring scheme as Argon is an inert gas that will not react with anything to skew the measurements.  Argon also escapes readily from molten rock so it is assumed that any Argon-40 found in rock must be the result of the decay of Potassium-40.  Nevertheless,  Potassium-Argon dating of the ten year old lava dome at Mt. St. Helens gave a whole rock age of 350,000 years and a mineral age of 2.8 million years.  Similar results have been obtained from volcanoes in New Zealand and Hawaii.

Another method of dating rock is fission track analysis.  Fission is the splitting of an atom, in this case Uranium-238.  This is the same thing that happens in a nuclear reactor but only one atom at a time.  The fragments that result from this fission fly apart and leave microscopic tracks in rock crystals.  These tracks can be counted to give an age to the crystal.

As Uranium-238 decays it passes through 13 isotopes before becoming Lead-206.  In this process it gives off neutrons and protons known as an alpha particle.  This alpha particle is the same as a helium nucleus.  These alpha particles can be found within the zircon crystals (not to be confused with cubic zirconia which is an artificial substance).  Helium being a small atom is not trapped in the crystal but can migrate through it and the rate at which it migrates has been experimentally determined.  When the RATE group (a group of creationist scientists) had the helium measured in a number of samples it was determined that only about 6,000 years of helium diffusion had taken place.

While radioisotope measurement is considered a tried and trusted method of dating rocks a review of the literature reveals that there are disturbing discrepancies.  Samples which should give discordant ages don’t, samples which should agree with each other don’t.  Using multiple methods to date rocks often gives discordant ages which cannot be reconciled within the range of error.

It is known that lava can accumulate daughter isotopes from the rocks which it passes through.  This will lead to older ages being assigned to the sample.  Hydrothermal flows can transport many radioisotopes which are used for dating contaminating samples.  Given that many of the discrepancies cannot be explained within the generally accepted theory it is doubtful that radioisotope dating should be given the confidence which it currently has.


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