George Claassen

The Earth is not 6 000 yrs old!

2005-08-12 09:30

When does ignorance become dangerous? This question repeatedly came to mind as I wondered about the sometimes very adamant reactions I am still receiving after last week's column on the discovery of 190 million year old dinosaurus eggs of Massospondylus carinatus in the Eastern Free State of South Africa ("Wake up and smell the eggs").

What is most astonishing is the absolute ignorance of people who still honestly and sincerely believe the Earth is only 6 000 years old. No matter what science says.

I don't even want to venture into the reasons for this chosen ignorance but would rather briefly explain the modern techniques used by scientists to determine the age of rocks, fossils and other matter on Earth.

Scientists use various clocks present in nature to determine the age of matter. Physicists utilise the concept of the half-life of an element found in the soil, rocks or in other forms of matter to describe its age.

Radioactive carbon dating

For many years they used radioactive carbon dating, but it has its limitations as the age of fossils, rocks or meteorite sites older than 40 000 years, cannot be measured in this way.

The half-life of an element such as carbon 14 is 5 370 years. This means half of the radioactive isotopes in carbon 14 would decay in that period of time.

As Carl Zimmer points out in his excellent article, "How old is it?" in National Geographic (September 2001), after another 5 370 years, only a quarter of the carbon 14 would be left.

According to Scientific American's Science Desk Reference, radioactive substances decay exponentially.

That means the time taken for the first 50% of the isotope to decay will be the same as the time taken by the next 25%, and by the 12.5% after that, and so on.

The half-life of the most stable radioactive isotope, tellurium-128 is 1.5 times 10 to the 24th power years.

Carbon 14 dating would have been of no use in determining the age of the Massospondylus eggs because fossils older than 40 000 years have virtually no trace of carbon 14 left.

Other techniques

Other techniques have thus been developed by physicists to calculate the age of organic material, meteorite sites such as the two billion year-old Vredefort Dome in the Northern Free State, declared a World Heritage Site recently, and other meteorite and rock samples found on Earth.

Important dating techniques were developed by the English geologist Arthur Holmes, the American Bertram Boltwood and others.

Boltwood developed the technique "for dating samples of rock from the proportions of lead and uranium isotopes they contain.

Since the radioactive decay of uranium eventually produces lead, with a characteristic timescale, measuring these ratios can reveal the ages of rock," as the scientist John Gribbin writes in Science: A History, 1543-2001.

The following reliable dating methods are commonly used by scientists today:

Isotopic techniques include determining the breakdown of uranium into lead (to establish the age of minerals, 1 million to 4.5 billion years old); rubidium to strontium (also minerals, 60 million to 4.5 billion years old); potasium to argon (minerals, 10 000 to 3 billion years old); uranium series disequilibrium (minerals, shell, bone, for example fossil bones, teeth, and coral, 0 to 400 000 years); carbon 14 (minerals, shell, wood, bone, teeth, water, 0 to 40 000 years).

Radiation exposure techniques include fission tracking (minerals, natural glass, 500 000 to 1 billion years old); thermoluminescence and optical luminescence (minerals, natural glass, 0 to 500 000 years old); and electron spin resonance (minerals, tooth enamel, shell, coral, 1 000 to 1 million years).

Other techniques, according to National Geographic magazine and Scientific American, include geomagnetic polarity timescales (minerals, 780 000 to 200 million years), amino acid racemisation (shells, other bicarbonates, 500 to 300 000 years), obsidian hydration (natural glass, 500 to 200 000 years), dendrochronology (tree rings, 0 to 12 000 years), and lichenometry (lichens, 100 to 9 000 years).

I am, however, not really optimistic that this data will change people from believing what their religious leaders keep telling them, because, as Gribbin puts it, "there are always people reluctant to throw out everything they have been taught in order to espouse a new understanding of the world, no matter how compelling the evidence".

  • George Claassen is the science editor of Die Burger, Cape Town's largest circulation daily newspaper.

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