home1.gif (2214 bytes)

As Old As Time

Professor Ian Plimer

This article appeared in The Skeptic, Vol. 20, No. 1

plimerspic.gif (28328 bytes)

Those Skeptics who are unlucky enough to have to confront the damage being done to science education by the pseudoscientific propaganda perpetrated by fundamentalist creation "scientists" will often have heard the mantra the "radioactive dating is inaccurate". Professor Plimer nails this lie in the following thoughtful article.

When a geologist refers to old rocks, then how old is old and how is time measured?

Geology is the history of nature. A geologist is like a detective who visits the scene of the crime after it has been committed. From the few clues left, the detective pieces together what happened and when. The detective then tries to understand why the crime happened. The geologist arrives at the scene millions to thousands of millions of years after the event. The geologist makes observations and measurements, gathers clues, collects samples and uses sophisticated technology to extract as much information as possible from the samples. He then tries to understand what happened. As with the detective, if there is fresh evidence then the geologist's understanding of the events is modified. Nature can be very fickle and clues are normally concealed; the scene of the events needs to be visited many times and looked at with different eyes, as nature has left us with only a dim and discontinuous record with which to work. Experienced detectives can extract more clues from the scene of a crime than can a lay person. So too with natural science.

Natural science is like a jigsaw puzzle, however, in nature at least half the pieces have been concealed. Furthermore, any place on Earth is just one page in the book of time and to understand it, other pages backwards and forwards in time on different parts of the Earth must be read.

If a geologist wants to understand the history of Earth, then there must be a reading of the rocks by making basic observations and measurements. If there are drill holes or mines, then a three dimensional picture can be put together. Samples need to be collected for testing and the age of rocks needs to be determined by using the clocks in the rocks.

These are important matters, given the frequent misrepresentation of dating techniques being promoted by various groups in pursuit of their own political, commercial and pseudo-religious purposes.

Clocks in the rocks

There are five main methods of accurately determining the age of a rock. They are independent and rely on totally different and discrete processes. A combination of these methods can be used to look at the complete history of planet Earth, ranging from deep time, thousands of millions of years ago, to the present.

The most common method is to use the decay of a radioactive element such as uranium, thorium, potassium, rubidium or carbon. Another method involves measurement of electrons captured in minerals as a result of a long period of bombardment by solar and cosmic radiation. The ever-changing magnetic field of the Earth is also used to determine when magnetic minerals formed, thereby dating the host material. Over time, biological material such as amino acids undergo decay and, by measuring the chemicals in old biological material, it is possible to calculate the length of time the biological material has been undergoing decay. As bones age, nitrogen is lost and fluorine is gained from ground-waters. The use of carbon dating (radioactive decay) combined with amino acid breakdown and bone chemistry change gives three totally independent methods of determining the age of bones.

The last method of age dating is a simple measurement of tidal or seasonal cycles. Tidal cycles are well preserved in some sediments and, not only have these been used to measure time, but they have been used to calculate Earth-Moon rotation and gravitation in former times. In summer, there is far more run-off into glacial lakes and sandy sediments are deposited on the lake floor. In winter, there is little or no run-off and a much thinner muddy layer forms. By counting the doublets of sediment layers, the summer-winter cycles in sediments in glacial lakes can be used to understand ancient climates and to measure the length of time that the glacial lake was active. Dendrochronology involves the measurement of the annual growth rings in trees. Not only can time be measured, but, by using the isotopes of carbon, oxygen and hydrogen, the history of ancient climates can be calculated.

All of these dating methods use independent techniques and technologies, and if, as is often the case, their results support each other, then the level of confidence in their accuracy is greatly enhanced.

Pub Time

The best way to understand how a geologist reads time is to retreat to the bar of your choice for a few scientific experiments using the drink of your choice and an empty glass. This is, of course, not a simple pleasurable drinking session. It is a serious scientific experiment to understand radioactive decay and the laboratory of your choice should be easy to find. Start with a full glass of drink. Slowly pour out half the contents into an empty glass. If you know the speed at which you poured and the amount of liquid in either glass, then you can calculate when you started to pour the drink.

This is what happens with radiometric dating. Physicists have discovered that, over time, radioactive elements will decay into other elements with a different degree of radioactivity, or into stable (non-radioactive) elements. Experiments have shown that each radioactive element decays at a predetermined rate that is consistent over time, and it is this phenomenon that makes radioactive decay an accurate tool for the dating of things of great antiquity. If you have a known amount of a radioactive element, after a certain period half of the atoms in it will have decayed to the lower state, after the same length of time again, half of the remaining atoms will have decayed, and so on. The name given to this measurement parameter of radioactive substances, is their "half-life". Each radioactive element has its own characteristic half-life, and they range from microseconds to many millions of years. It is generally the case that the higher the radioactivity, the shorter the half-life.

A minute amount of uranium 238 (U238) in rocks decomposes to lead 206 (Pb206). It takes 4,680 million years for half the U238 to decompose to Pb206. (Thus the "half-life" of U238 is 4.68 million years.) The rate of change of U238 to Pb206 is known from experiments and evidence from nuclear reactors, so all that has to be measured in the rock is the amount of Pb206 and U238 and the age of formation of the rock can be calculated. U238, the heaviest naturally occurring substance on Earth, has been used for armour-piercing projectiles and as a counterweight in the tail of modern jet aeroplanes.

Like all scientific measurements, this process must be repeated by cross checking. It would be a pity to waste the drinks purchased for the scientific experiment and I'm sure you'll find a use for them. Start mixing your drinks and buy something completely different. Again slowly pour half the drink into an empty glass. If you know your pouring speed and the amount of in either glass, then you can calculate when you started to pour the drink. The same happens in nature. Traces of uranium 235 (U235), the material used in nuclear reactors and bombs, occur naturally in all rocks. It takes 704 million years for half the U235 to decay to lead 207 (Pb207). This figure is very accurately known and no nuclear reactor could work if this figure were wrong. By measuring the amount of Pb207 and U235 in the rock and by using the known rate of decay, the age of the rock can be calculated.

Minerals which contain small amounts uranium and thorium are bombarded by particles as the uranium and thorium decay over time. Bombardment leaves a trail of damage in the mineral crystal called fission tracks. The older the mineral, the more fission tracks. Furthermore, if the mineral undergoes an event of heating after formation, the mineral reorganises and the fission tracks are destroyed. In this way, fission track dating can be used to date events of heating and cooling in rocks.

Again, this should be cross checked, so keep mixing your drinks. Try the same experiment with half the thorium 232 (Th232) in rocks decaying to Pb208 in 14,000 million years. Try it again with half the rubidium 87 (Rb87) decaying to strontium 87 (Sr87) in 48,800 million years. Do it again, this time with different drinks representing half the potassium 40 (K40) decaying to argon 40 (Ar40) in 11,930 million years. Again, try it with Ar40 decomposing to Ar39. By measuring the gas argon in rocks, the proportions of Ar40 and Ar39 can be computed to determine when a rock was heated to above 300 C, another method by which to calculate when rocks were heated and cooled. Mix those drinks again and try half the samarium 147 (Sm147) decaying to neodymium 143 (Nd143) in 106,000 million years and then again with half the rhenium 187 (Re187) decaying to osmium 187 (Os187) in 46,000 million years. Time for another drink, this time to demonstrate the decay of lutetium 176 (Lu176) to hafnium 176 (Hf176).

By the time you have mixed so many drinks you'll be somewhat weather-beaten and would have probably forgotten what you were trying to prove, but you would now have demonstrated numerous independent scientific cross checks in order to get an extremely accurate age of when a rock formed. I'm sure that this knowledge will make you feel much better the next morning.

Such methods can only be used for rocks that were once molten or had been cooked up to very high temperatures. Not only can these methods give the age of rocks, they can also be used to look through time, because many rocks are recycled and inherit characteristics from earlier times. If these techniques are used to date a rock that was once molten, then by looking through time, we can calculate what material was melted, for the sake of an example, mudstone. By looking through time, we can also measure when and where this mudstone formed, how many times it had been cooked up, when it had been cooked up and what the climate was like in the dim distant past.

Other tricks of the trade are that by looking through time, we can calculate when an area was uplifted to form mountains. Minerals form a logbook that records a long sequence of events in history. For example, this technique has been used for very detailed dating of rocks from the Broken Hill district and the latest scientific studies show that there is a very hazy and long history of events. Most of the rocks in the Broken Hill area were formed from volcanic rocks 1,690 million years ago. These volcanic rocks were melted from material which was formed at least 1,740 million years ago and there is some evidence that these 1,690 million year old rocks formed by melting material up to 3,100 million years old.

Time for a few more scientific experiments using glasses of drink. Our planet is constantly bombarded by cosmic rays that form materials such as chlorine 36 (Cl36) in water, beryllium 10 (Be10) on the land surface and carbon 14 (C14) in the atmosphere. Try the simple pleasurable experiments again to demonstrate half the Cl36 decaying to Ar36 in 310,000 years, half the Be10 decaying to boron 10 (B10) in 1.5 million years and half the C14 decaying to nitrogen 14 (N14) in 5,730 years. These materials are used to date more recent events. For example, we can use Cl36 to date how quickly polar ice forms and melts, how quickly lakes, rivers and harbours are filled with silt and the age of ground-waters. Ground-waters in many parts of the world formed when there were warmer wetter climates. Ground-water is actually fossil water and hence it must be used with great care. In the Great Artesian Basin of Australia, ground-water is two million years old. If the water is wasted, we just can't sit around for millions of years waiting for the aquifer to be recharged in future times when we next have a warmer wetter climate.

The northward pushing of Australia under South East Asia carries surface Be10 to a great depth beneath Indonesia. By measuring the Be10 and B10 in modern Indonesian volcanic rocks, we can calculate that bits of Australia started to be melted beneath Indonesia about 50 million years ago and we can show that Australia was initially moving northwards at 1cm per month. This incredibly fast rate of continental drift has now slowed to about 0.5cm per month. Nevertheless, the collision of the Australian continental landmass with South East Asia has resulted in millions of years of catastrophic earthquakes and volcanoes in Indonesia. Furthermore, with the Be10, we can show how quickly Australia was being eroded over the last 20 million years and this gives us a good window into how quickly climate fluctuates from icehouse to greenhouse.

As a result of more than 2,000 nuclear blasts since 1945, minute quantities of radioactive fallout have been spread across planet Earth. On the land, this radioactive fallout resides in the soil. Fallout material such as radioactive caesium 137 (Cs137) are used to monitor and measure post-1945 soil erosion and land degradation. We humans have left a geological mark on the planet which appears as a thin radioactive layer in soils and sediments derived from soil erosion. This will be detectable for many millions of years to come.

Carbon dating is much maligned by those whose agendas are threatened by a truthful representation of the age of earthly things. Atmospheric carbon dioxide contains known relative proportions of two carbon isotopes, radioactive C14 and stable C12. Any living organism (including us) absorbs these isotopes in the same proportions and, on the death of the organism, no more carbon is absorbed. The C14 decays to N14 at a known half-life rate, so the proportion of C14 to C12 found in organic remains gives a method of measuring the time since the death of the organism.

In order to appreciate carbon dating, buy a large drink. Pour half of it into a second glass. Pour half of the remaining drink into a third glass. Again, pour half the remaining drink into a fourth glass. Do this experiment another two times and then see how much drink is left in the first glass. Very little. The same with C14. Half the C14 decomposes after 5,730 years, after another 5,730 years, half again has decomposed. As with the drink, after the original amount of C14 has been halved five times, there is so little of it left that it would be very difficult to measure. This limits the accurate use of C14 dating to less than 40,000 years which, in geological terms, is only yesterday. Material which formed after 1945 has been contaminated by C14 derived from radioactive fallout and hence can not be dated accurately. Carbon dating, like all techniques, has its limitations, but these limitations are well known and taken into account, so dates given by this technique (as with a the others) are always expressed within margins of error.

The experiment has finished so it is now safe to drink every drop. Waste not, want not.

Time and a suntan

Beaches can be used to show another dating method. Anyone who has been sunbaking gets sunburnt and a darker skin. Go down to the beach and have a very good look at all the partially naked bodies, purely as part of a scientific observation, of course. By just looking at a person, we can tell if they have been in the sun for hours, days or weeks. Minerals, especially quartz, also get sunburnt and we can measure how long a mineral has been exposed to sunlight. Quartz exposed to sunlight captures electrons and these are trapped in the mineral. By heating the quartz in the laboratory, it emits light and the amount of light emitted is related to the number of electrons and hence the time that the quartz was exposed to sunlight. This is often used to measure the age of old beaches, campsites and soils which have been exposed to sunlight for a long time.

Magnetic dating

When rocks are heated above 580C, the iron oxide mineral magnetite loses its magnetic properties. When rocks such as lavas cool, the magnetite inherits the Earth's magnetic field at 580C. If we measure the age of the lava, using a method such as potassium-argon dating, and measure the magnetic field of the magnetite crystals in the lava, then we can calculate where on Earth the lava erupted. Using this method, palaeomagnetic dating, we are able to show the history of magnetic reversals, especially around the mid ocean ridges. Furthermore, the position of the Earth's magnetic poles is not the same as the Earth's geographic poles and, over time, it appears that the magnetic poles wander. This apparent polar wandering is not because the position of the magnetic poles changes greatly but because the continents are drifting.

Geology is the history of deep time. The techniques available now can measure when a rock formed, the age and type of the unseen material from which the rock formed, the post-formation history of heating and cooling of the rock and the date when the rock was lifted from depth to the surface. As we have seen, we can measure time very accurately using a great variety of different methods that are crosschecked as part of good housekeeping. The span of time on the Earth since its formation is so vast as to be almost incomprehensible. Given time, then almost anything can occur on Earth, and it has.

Old fashioned common sense

Accurate dating methods were only possible after the discovery of radioactivity. In the 19th Century, although such methods were not available, there was a consensus amongst scientists that the planet was very old. Exactly how old was old was not known.

Until the late 17th Century, most European Christians believed the biblical creation story literally. The first book of the Old Testament outlined a timetable of events for Earth history. In 1650, Archbishop James Ussher used biblical chronology and added up all the lifespans of the descendants of Adam. He calculated that the Earth was created in 4004 B.C. and this was entered as a marginal note in the King James Edition of the Bible in 1701. There it stayed and, despite the scientific advances over the past 350 years, it is still adhered to as a matter of faith by the young Earth creationists.

The Industrial Revolution began in England in the late 18th Century. It included a technological revolution, and as miners tunnelled through rocks to win minerals to feed the new industrial processes, and as engineers built canals to provide transport for raw materials and finished goods, they acquired a great knowledge of rocks through which they tunnelled. Regular sequences of rocks were identified within which there was a regular sequence of fossils of now-extinct animals. For example, the canal engineer William Smith was able to show that distinctive fossils are found in the same sequence of rocks over a very large area of England. Simultaneously in the Paris Basin, Jean Baptiste Lamark, Georges Cuvier and Alexandre Brongniart were able to show that there had been extinctions of life and that there were abrupt changes from marine to terrestrial sequences of rocks.

In the 1788, the Scottish farmer and businessman James Hutton made an observation at Siccar Point on the east coast of Scotland near Edinburgh. This observation changed forever the view of the Earth and showed that the Ussherian age was not consistent with the evidence revealed for all to see. Hutton found a sequence of gently-tilted sandstones which overlay nearly vertical shales and sandstones. The surface between the two sequences is called an unconformity. Hutton deduced a sequence of seven events at Siccar Point:

1. Rivers eroded an ancient landscape, shifting fragments of the bedrock as sediment down to the sea.

2. The material carried by the rivers accumulated at the bottom of the sea to form a sequence of muds, silts and sands which were buried and eventually became horizontal layers of rock.

3. These rock layers were uplifted out of the sea by movements inside the Earth. In the process, they were turned from the horizontal to the vertical, contorted and folded back on themselves.

4. Rivers flowed off the uplifted and contorted rock, wearing down the surface to a flat plain.

5. Subsequently, the flat plain subsided and became the site of accumulation of a new sequence of sands, carried by rivers from high ground elsewhere.

6. Another period of Earth movements uplifted and tilted the new sequence of sediments.

7. Rivers today are again wearing away the uplifted rock, creating the present landscape.

What clearer evidence was needed to show that rocks are a record of deep time? The clearest way to understand geological time is to map an area. Document the rock types, where the intrusions of granite and other igneous rocks occur, where the unconformities occur, where the rocks tilt, where the rocks are broken or folded and plot all these features onto a topographic map or an aerial photograph. Without using radioactive dating or fossils, a logical reconstruction of the order of events shows that the planet could not possible be a few thousands of years old. Unconformities occur throughout the geological sequence on Earth, showing that at one place on Earth erosion was taking place eventually producing an unconformity and, at another place, sedimentation was occurring. The same occurs today. Unconformities are used to reconstruct old mountain chains and to look at the constant recycling of crustal material.

In 1862, William Thomson (later Lord Kelvin) used mathematics to calculate the age of the Earth. He assumed that the heat of the Earth is from the creation of the planet, that the Earth is cooled by conduction and that the Earth's atmosphere has remained at about the same temperature. By using the temperature of a molten basalt (1100C), the thermal properties of rocks, the temperature gradients in deep mines, Kelvin tried to calculate how long the Earth had been cooling. He initially suggested that the age was somewhere between 20 and 400 million years and, with more refined calculations in 1897, he settled on an age of between 20 and 40 million years old. Only a few years later, radioactivity was discovered, and it was shown that Kelvin's assumptions were incorrect and the Earth was billions of years old.

The same common sense can be used today to get crude estimates of the age of the Earth. Measure how long it takes for a few layers of sediment to be deposited, measure the thickness of the rocks preserved in the rock record and then back calculate. Measure the volume of rock removed by erosion in a canyon, measure the rate of sediment flow in the canyon and back calculate. Measure the salt load and amount of water in the Earth's rivers, measure the salinity of the sea and back calculate. This was done by the Irish geologist John Jolly in 1899, and he calculated the age of the Earth at 99 million years.

Measure the volume of a granite intrusion, measure the thermal properties of granite, conduct experiments on the time required for granite to grow large grains, conduct experiments to show the temperature and pressure of granite crystallisation and calculate the time taken for molten granite to cool to solid granite. Whether this experiment is done using the measured hundreds of cubic kilometres of granite or hundreds of thousands of cubic kilometres of folded metamorphic rocks, the answer is the same. The planet is billions of years old.

Young Earth creationists would claim there is a scientific dispute about this matter: either the planet is a few thousand years old or it is billions of years old. They are wrong. There is no scientific dispute, nor are these claims two sides of any sensible question. The first claim is a matter of belief; a belief that is not supported by one scintilla of scientific evidence. The second claim is based entirely on scientific evidence, and this evidence, which comes from so many entirely independent scientific techniques, admits of no compromise. The Earth is many billions of years old.

The only way that the YEC position could be true would be if a preposterous lie had been written in the rocks by a supernatural being in whom they ask us to lodge our faith. If true it would be a misplaced faith.

home1.gif (2214 bytes)