Geologists claim the earth to be 4.6 billion years old and first life to have appeared about 3.7 billion years ago.  But where do these numbers come from? How can scientists claim rocks, fossils or a bone or tooth to be of a certain age? What methods are used for this dating and are they reliable? In this appendix a brief overview of the most common dating techniques will be discussed. It will show that under certain conditions scientifically reliable dating is possible, however the vast majority of rocks and fossils cannot be accurately dated. And accurate dating of organic material is only possible if these remains are relatively young.

Radiometric Dating

The best-known ways to assign age to an object is through several techniques jointly known as radiometric dating, also called radioisotope dating.

Radiometric dating is based on the fact that some radioactive elements undergo decay to produce new elements. For instance, in the case of uranium-lead dating, uranium 238 (“the parent element”) will eventually decompose to lead 206 (“the daughter”). Or for radiocarbon dating, the carbon 14 isotope will decay to regular carbon in carbon dioxide. By measuring the quantities of the radioactive elements in the object and knowing the rate (speed) by which the decay occurs, one can estimate the age of the object.

Despite of the fact that scientists in general consider these dating methods to be very reliable, there are several critical factors that must be known to be true: [1]

1. The quantity of radioactive elements which were present when the object was formed/buried must be known. This is particularly important for the amount of daughter material. If some of the daughter material would already be present when the rock was formed, radiometric dating would suggest a much too old age.
2. The rate of radioactive decay must have been constant over time. It seems scientifically reasonable to assume that this rate has not changed over the ages, but as mankind has only been measuring radioactive decay for a few decades, there is no certainty that this rate of decay is not changed and/or has not changed over time.
3. The object must have been isolated from outside/external influences. Radioisotope dating assumes a closed system. If either the parent or especially daughter concentrations would have been altered through processes like surface water erosion or ground water leaking, the object could appear much older than its real age.

The above assumptions can be (and are) challenged by various argumentations, and there are examples known of objects for which the exact age is known and they were dated completely wrong by radiometric dating. [2]

All in all, radiometric dating is one of the tools available in the scientific toolkit to help us date objects from the past. However, knowing its limitations can be a valid basis to question its application and it should not be assumed to be more accurate and absolute than it is often made to be.

Dating of Rocks and Fossils

Another significant limitation of radiometric dating, is that only rocks can be dated that are igneous, rocks which were “formed” on the site. This includes rocks like basalt, which is a type of solidified lava. Generally speaking dating is limited to rocks resulting from lava flows. Fossils found in these kind of rocks can therefore be dated using radioisotope techniques.

Sedimentary rocks, such as shale, sandstone, and limestone, cannot be dated through radiometric dating. This because these rocks are formed from pieces of rocks or other material which existed somewhere else. Subsequently these rocks were eroded and deposited by glaciers, rivers or streams at other locations. As these rocks contain previously existing sources, accurate dating is not possible.

Therefore, fossils found in sedimentary rocks cannot be dated with radiometric techniques. Dating of these fossils is usually done through the so called index method. The fossils are compared to other earlier dated reference fossils which are considered unique for certain time periods. By finding an index fossil that is similar to the fossil on the sedimentary rock, the sedimentary rock is dated at the age of the index fossil.

Unless the original index fossil itself was dated using radiometric dating, the index fossil technique has inherent dangers of circular reasoning. Many critics argue the unreliability of the index tables and therefore they question any rocks dated by index fossils.

Dating of Organic Material

The only available method to date organic material, like once living plants and animals, and material such as wood, charcoal, bone, shell and fossils, is Carbon-14 dating. [3]

The half-life of the Carbon-14 isotope is 5,730 years (that means after 5,730 years, half of the original Carbon-14 material has disappeared), therefore this method is best used for organic material less than 10,000 years old and is only reasonably reliable for about 5 times the half-life, so for dating objects to maximum 25,000-30,000 years old.

How does it work? In our atmosphere under the influence of cosmic rays, a small amount of carbon is transformed from C-12 to the C-14 isotope. This C-14, together with oxygen, forms carbon-dioxide, which is breathed by living plants. And as some of these plants are eaten by animals, the isotope enters living organisms, but when these plants/animals die, they stop breathing/eating and therefore no more isotope material is added. By measuring the amount of C-14 still left in the remains of these once living organisms, a fairly accurate estimate can be made for how long ago these remains where formed.

Because radiometric dating methods do not work for organic materials, there are no accurate techniques available to reliably estimate ages for fossils and other organic remains older than approximately 30,000 years.

This identifies an area of great caution. In attempts to estimate the age of modern man and his claimed predecessors, the actual recovered bone fragments or teeth cannot be accurately dated. Therefore the paleontologists scan the area for rocks and date those through radiometric dating or index fossils. The resulting dates are then associated with the recovered remains. Obviously this technique involves the very questionable assumption whether the remains were deposited at the same time as the surrounding rocks were formed and subsequently were never moved. Many would argue this to be more an educated guess than real science, and this explains why the estimated ages of ancient organic fossils seem to change over time.

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