Tuesday, June 02, 2009

Bad Reason #7 Do we know how old fossils are?

This is the seventh article in my series of 10 bad reasons to reject evolution, this post deals with the objection that "They can't possibly know how old all these fossils are".

Since the mid 19th century, palaeontologists have searched the world for fossils. In the past 150 years they have not found a single fossil that Darwin's theory would not have expected, and several that it predicted. New discoveries have filled in the gaps, and shown us in unimaginable detail the shape of the great 'tree of life'. All this work has not led to a single unexpected find, such as a human fossil from the time of the dinosaurs.

So how do we date these finds?

It was discovered in the early 1900s that certain naturally occurring chemicals in rocks were radioactive, this means that they slowly decay or change over time into other chemicals. The rate of change can be measured and is constant for particular substances. Therefore if you know that A decays into B then all you need to do is measure the quantity of B and you can calculate the age of the rock.

So, this objection is based on ignorance, you often hear people saying, "oh but carbon-14 dating isn't reliable for dating fossils" and they would be right. Their error is that most fossils are not actually dated using C14 techniques at all.

Scientists use different chemicals for absolute dating:

The best-known absolute dating technique is carbon-14 dating, which archaeologists prefer to use. However, the half-life of carbon-14 is only 5730 years, so the method cannot be used for materials older than about 70,000 years which is why it is seldom used for older and more important fossils.

Radiometric dating involves the use of isotope series, such as rubidium/strontium, thorium/lead, potassium/argon, argon/argon, or uranium/lead, all of which have very long half-lives, ranging from 0.7 to 48.6 billion years. Subtle differences in the relative proportions of the two isotopes can give good dates for rocks of any age.

Scientists can check their accuracy by using different isotopes and cross checking the results.

The first radiometric dates, generated about 1920, showed that the Earth was hundreds of millions, or billions, of years old. Since then, geologists have made many tens of thousands of radiometric age determinations, and they have refined the earlier estimates. A key point is that it is no longer necessary simply to accept one chemical determination of a rock's age. Age estimates can be cross-tested by using different isotope pairs. Results from different techniques, often measured in rival labs, continually confirm each other, interestingly the range of error can also be calculated, and is more often than not less than 1%.


Elizabeth said...

Even more education from you. thanks for taking all the trouble to put these posts up; we are learning a lot from you.

Steve Borthwick said...

Thanks for your comment E, I'm only an "keen amateur" when it comes to evolution, but I find it relaxing to research and write about it, really tests your understanding.