Saturday, July 30, 2011

Reliability of Radiometric Dating

I have read several blog sites in which there is discussion on the age of the earth and inevitably the topic of the dating of rocks and the reliability of this method and it's data is questioned. Radiometric dating relies on the natural decay of radioactive isotopes. It is a reliable and accurate way to determine the ages of rocks. As with all measurement there is uncertainty in that measurement which can however be quantified. Over the last few decades the advancement in the instrumentation (mass spectrometers) has improved precision of measurement, hence reducing the uncertainty of measurement of rock age. Dates determined by one radiometric scheme can often be verified by independantly determining the age by an alternative radiometric scheme. All of this nonsense about not believing that radiometric dating is accurate stems from the proponents of a "Young Earth" and people feeling threatend by the advancement of science.

Radioactive decay is the spontaneous release of energy in the form of radioactive particles or waves. It results in a decrease over time of the original amount of the radioactive material. A quantity is said to be subject to exponentional decay if it decreases at a rate proportional to its value. Symbolically, this can be expressed as a differential equation where N is the quantity and λ is a positive number called the decay constant: ∆N = - λN/∆ t. The number of decays are represented by ∆N
The short time interval that ∆N occurs is represented by ∆t N is the number of nuclei present λ is the decay constant.

How old is planet earth? Our Solar System formed from a vast cloud of gas and dust 4.65 billion years ago. This age of 4.65 billion years is well established from the decay rates of radioactive elements found in meteorites and rocks from the Moon as well an abundance of evidence from chemistry and physics. As Earth is a dynamic planet in which rocks are continuously being recycled by plate tectonics much of the primordial material from the time of the formation of the Earth is no longer around. If there are any of Earth's primordial rocks left in their original state, they have not yet been found. The oldest rocks found to-date on Earth are those of the Nuvvuagittuq greenstone belt and these give an age of 4.3 billion years. It was not until relatively recently that it has been possible to measure the age of rocks. The early ideas of the age of the Earth date to the ancient Greeks and Romans and it was not until the late 1700s that scientists begun to realise that the Earth was indeed ancient. However, it was not until the discovery of radioactivity and the invention of the mass spectrometer that the quantification of isotopes of various radiometric decay schemes could be performed. In recent decades improvements in detector technology and electronics in mass spectrometers has resulted in an improvement in the precision of analytical data which translates to an increased confidence in the radiometric ages. The use of hyphenated analytical techniques such as laser ablation - mass spectrometry has also allowed the analysis of small sample sizes and individual crystals.

The ancient Greeks and Romans realised that long time spans were required to lay down the thick layers of sediments observed and from this they estimated that the Earth was thousands of years old. But it wasn't until the late 1700s that scientific interest in geological age began when Scottish geologist James Hutton (1726-1797), who observed that sediments built up on landscapes were indeed indicative of an old Earth. Before then, the Bible had provided the only estimate for the age of the world. Bishop James Ussher (1581 – 1656) established the time of “creation” to 6000 years. Using the book of Genesis as a history book, Ussher meticulously examined the genealogy of the Bible and concluded that the date of the creation as the night of Sunday, 23 October 4004 BC (McSween, 1997). Today some biblical scholars, as well as a number of literalist evangelical Christians, believe in a literal interpretation of the Bible calling for a 6000-year-old Earth (Barr, 1984). In 1785 Hutton published ‘Theory of the Earth’ in which he concluded that “slow” processes shape the Earth, mountains arise continuously as a balance against erosion and weathering and the physical and chemical laws that govern nature are uniform. Most geological processes are extremely slow, and evidence for slow change was everywhere; rivers eroded rock and rain inexorably wore away the tops of mountains and the slow movement of glaciers carved out entire valleys. Hutton and other contemporary scientists of the time concluded that the single most important factor why the Earth looks like the way is does was due to time, and lots of it. With this Hutton established the ‘Doctrine of Uniformitarianism: "Present is key to the past". Hutton used fossils to establish relative ages of rocks. There was however a need to determine the absolute age of the Earth. In the late 19th Century this question was first addressed by William Thompson (Lord Kelvin) (1824 - 1907). Kelvin assumed that the Earth was originally molten and calculated a date for the age of the Earth using the then young science of thermodynamics. His calculation was based on the cooling of the Earth through conduction and radiation of heat. Kelvin’s age of Earth was calculated to be about 24-40 million years (ref.). The problem with this view is that the Earth has an internal heat source from radioactive decay – a fact not known to Lord Kelvin at the time of his estimation of the Earths age. At around the same time, John Joly (1857 – 1933) calculated the rate of transfer of salt to the ocean as a means to determine the age of the Earth. The age of Earth by this method was calculated to be 90-100 million years. The main problem with this approach was there was no means to account for recycled salt, salt incorporated into clay minerals and salt deposits. Later work by other scientists used the thickness of total sedimentary record, to determine an age of 500 million years. It was not until the discovery of radioactivity by Henri Becquerel in 1896 that geologists had a tool for determining the age of rocks and ultimately the age of the Earth.

Radiometric dating is no mystery .... the truth is out there.