At the beginning of the 20th century, the discovery of the radiometric “clock” revolutionised our understanding of the Earth’s deep history, confirming what geologists had been claiming for decades. Nevertheless, newer and more accurate dating methods posed further problems in themselves. After all, how do we know our Earth is 4.5 billion years old, and not a mere few thousands of years as suggested by the Bible?
Throughout the 19th century, there was no means of dating the geological past with absolute certainty. The age of the Earth was still open to speculation and controversies.
Once the relationship between sequences of strata and successions of characteristic fossils had been established at the beginning of the 19th century, geologists soon discovered that there were very broad changes in the overall fossil content of strata.
The three great geological eras – Paleozoic, Mesozoic and Cenozoic – could each be characterised by distinctive groups of fossils. In some cases, the fossils were so restricted in their distribution through time that they could be used to subdivide divisions of time more discretely.
Ammonites are excellent index fossils, and it is often possible to link the rock layer in which a particular species or genus is found to specific geological time periods. Their fossil shells usually take the form of planispirals, although there were some helically spiralled and non-spiralled forms. The earliest ammonites appear during the Devonian, and the last species died out during the Cretaceous-Paleogene extinction event.
Graptolites are common fossils and have a worldwide distribution. The preservation, quantity and gradual change over a geologic time scale of graptolites allows the fossils to be used to date strata of rocks throughout the World. Graptolites are important index fossils for dating Palaeozoic rocks, as they evolved rapidly with time and formed many different species. British geologists divide the rocks of the Ordovician and Silurian periods into graptolite biozones – each generally less than one million years in duration. At the end of the Ordovician, a worldwide ice age eliminated most graptolites.
The biozonal method is still widely used today in the subdivision of modern marine sediment terrestrial deposits of the recent past. On land, pollen and insect remains are used for dating deposits from the Quaternary ice age.
In 1897, the great physicist Lord Kelvin was one of the first scientists to use physico-chemical principles in calculating the age of the Earth at between 20 and 40 million years! Kelvin assumed that the mass of the Earth had cooled from an initial molten state. His estimate was far lower than had been expected by the geologic community, but Kelvin’s prestige was high, and his figures widely quoted.
Kelvin’s calculations failed to account for the effects of the Earth’s mantle convection and radioactivity – neither of which were known at the time.
Early 20th century, Ernest Rutherford and other scientists realised that radioactive decay could be used to date minerals and rocks. In 1905, Rutherford used the decay of uranium to calculate a mineral’s age at 500 million years old.
This dating method depends on the fact that radioactive decay is random and unaffected by physical or chemical processes, such as surface temperature or pressure variations.
Radiometric dating is based on the amount of time it takes certain radioactive substances to decay. Atoms with identical chemical properties, but different weights, are known as isotopes of an element.
Isotopes decay at a known rate, making it possible to calculate the age of rocks formations. Most minerals are mixtures of isotopes. Radiometric dating measures the proportions of certain chemical isotopes within minerals.
Originally, radiometric dating could only be applied to igneous rocks – formed as a result of the crystallisation of minerals from a molten material as it cooled. Until recently, sedimentary rocks were not suitable candidates for radiometric dating, because the age of a specific grain in sedimentary rock, such as sandstone, is the age at which the mineral formed in its original igneous setting and not when it was locked into the sedimentary deposit. The problem has been to relate radiometric dates from igneous rock to the stratigraphic record, which consists mainly of sedimentary deposits.
Dating a zircon grain from a sediment illustrates the problem with the radiometric method…
Radiometric dating allows rocks to be positioned fairly accurately at their correct time of formation in Earth history. Often interbedded with sedimentary deposits, volcanically-extruded lava is one of the most useful rocks for radiometric dating. The age of lava is closely related to the time at which the sediment above and below it was deposited. This gives an accurate match between radiometric dates and dates obtained from stratigraphic timescale.
The decay of carbon isotopes within living organisms can be used to date organic materials, such as charcoal, or shell and bone.
Unfortunately, the radioactive isotope Carbon-14 decays relatively quickly in geological terms, with half-life of 5,730 years. So the radiocarbon dating method is only really useful for archaeological or sub-fossil materials – like wood, leather, fabric, antlers or bones.
Many igneous rocks and also a range of iron-rich sediments contain magnetic materials. These minerals can become aligned with the Earth’s magnetic field and effectively record that magnetic orientation at the time the rock is formed.
For igneous rocks, the magnetic minerals are locked in their magnetic orientation as the molten rock cools below temperatures of 600°C.
The Earth’s magnetic field reverses sporadically. This gives rise to sequences of rocks that have “normal” polarity and rocks with reversed polarity. The changes in magnetic fields, which have occurred on average every half million years, generate a characteristic pattern over time. A particular pattern can be matched with the timescale of magnetic reversals to determine a relative age, almost like a barcode.
The timescale of magnetic reversal is calibrated using radiometric dating.
As sediments form, tiny particles of magnetised iron included among the sediments grains become aligned with the direction of the Earth’s magnetic field. As the sedimentary rock solidifies, the alignment is preserved. The establishment of magnetic polarity has been particularly useful in dating certain environments – notably giving age estimates for the Miocene terrestrial sediments of the Siwalik region in Northern Pakistan.
Fission Track Dating
Fission track dating is based on the spontaneous fission of the radioactive isotope Uranium-238, which produces microns-long damage trails near the site of the uranium atom within a mineral, such as zircon. Fission track dating is used to verify radiometric dates, and has been particularly useful in dating the volcanic ashes interbedded in the hominid fossil-bearing strata of East Africa.
As the number of trails increases over time, at a rate dependent on the uranium content of the mineral, the age of the mineral can be computed by measuring the density of fission tracks in a mineral and the uranium content.
Electron-spin resonance and thermo-luminescence are two methods that measure the number of electrons caught up in defects within minerals’ atomic structure. These have proved useful for dating a variety of inorganic materials found in anthropological settings, e.g. burnt flints found in ancient hearths.
Some organic chemicals exist in both right-handed and left-handed forms – mirror images of each other.
Amino-acid racemisation is a sophisticated technique, which depends on the slow chemical conversion of the left-handed amino-acids naturally present in living organisms to their right-handed counterparts – a process sensitive to environmental conditions, especially temperature.
When an organism dies, its amino acids are left-handed. But after its death, the amino acids can spontaneously change chirality, flipping from being left-handed to right-handed, and back again. The result of this process is that eventually amino acids will collectively become racemic – each particular amino-acid will have one chirality or another, although after a sufficient amount of time, collectively the amino acids won’t favour one enantiomer over another.
The method has been useful for dating archaeological materials, such as shells and bones, which are not normally suitable for radiocarbon dating methods. However, it does not extend back more than a few hundred years.
Although rarely available, another useful method of dating uses volcanic ash from past eruptions since ash can be distributed over extremely wide areas instantaneously.
Meteorites and asteroid impact events can spread tektites – glassy fragments from rock melted by the heat of the impact – over wide areas in a geological instant…
Many geological features and processes are not directly observable – especially when taking place within the Earth itself – and can only be inferred by the interpretation of data obtained from special kinds of exploration and analysis, such as geophysical methods of gravity or seismic measurement.