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Geological Time Scales

When geologists and earth scientists talk about the age of the planet, different eras and the formation of the planet, they can determine how old certain landforms are. But how do they do this? This article will look at the theories and practices that are used in order to be able to date certain events and ages that took place on the planet long before humans inhabited the surface.

The Lithosphere and Early Theories

The Lithosphere is the only layer of the Earth from which we can gain information about prehistoric events, and is known as the domain of geologists. Although there are some indications that can be gained from the rock records via fossilisation, it is not an accurate and reliable source from which to gather information. Geologists first recognised that the Earth was indeed very old, so the need arose in which to be able to properly date the different periods in order to determine when events occurred in absolute and relative terms. In the year 1654, James Ussher, an Archbishop from Ireland, wrote a paper stating that stated that the world was created on Sunday, October the 23rd in the year 4004bc. He came up with this number by studying the Bible and with probably a lot of help from his imagination also. Scholars were quick to correct this, and a new date was determined, the 26th of October, at precisely 9am, which was an incredibly precise measurement. Due to most scientists and scholars were all within the framework of the Christian church, all of their work was based on the Earth being formed due to a series of catastrophes, eg massive floods 1).

Age Of Earth

It wasn't until the late 1700's when sanity was restored, and a Scottish physician named James Hutton published his theory about the formation and geology of the Earth. By observing the way sediment was moved by water and by looking at water and wave erosion, he determined that this could be the way in which the modern geological features on Earth could have been created. He named his theory 'Uniformitarianism', and if this theory was to be correct, the previous theories by Ushher and other scholars would have to be incorrect. A quick look at something like the rock record also proved that the biblical catastrophe theory could not be remotely valid. The basis theory of Uniformitarianism states that the biological, physical and chemical laws which are present today, must also have been present in the past, and had been influencing the geological processes on Earth for a very, very long time. One of the biggest believers in the theory of Uniformitarianism was English geologist Charles Lyell, who wrote several books and papers on the subject. Charles Darwin was also a strong believer in Uniformitarianism, which supported his theory of evolution due to the large time frames which were required to prove his theories 2).

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James Hutton

Hutton and Lyell were not the only one who started to question the biblical catastrophe theory. In the late 1700's a French Naturalist, Comte de Buffon, using data he gathered from the fossil record, wrote that the Earth would have to be at least 75,000 years old. In 1862 English physicist William Thompson weighed in on the debate, with his experiments indicating the age of Earth to be at least 20-30 million years. Not long after in the late 1800's, Irish chemist John Joly calculated that for the oceans to reach their current salinity, at least 90 million years was required due to the outflow from rivers. It wasn't until the use of radioactive dating that it was realised by scientists that the world was in fact billions of years old, and the current accepted figure is 4.54 billion years 3).

Geological Time Scale

In order to come up with a workable time scale in which to measure different passages of time in the Earth's history, a geological time scale was created. This time scale is measured by the fossil history in the stratigraphic column, which is found in the sedimentary layers in the Lithosphere. The longest geographical time frames are called 'eons' which represent the fundamental stages in the history of Earth. The next longest time frames are 'eras' which are the periods between different extinction events. These times can then be divided into 'periods' or 'ages' which are used to measure the time frames in which it is observed that the fossil record changed significantly. Epochs are the next shortest periods of time which can measure evolutionary changes in certain species, like hominids for example. Before the advent of radioactivity for measuring age and carbon dating, scientists knew n what order all of the eons, eras, ages and epochs occurred, but did not know how long they occurred for. When radiometric dating was discovered, scientists were able to determine an accurate time scale for all of these different time frames. Radiometric dating can determine the age of the element by measuring the rate of decay within the carbon structure 4).

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Fossilisation 5)

Relative Time Scales

Before the advent of radiometric dating, scientists used what is called the relative time scale. This was achieved by being able to put the different time periods into a sequence as to how they were layered on the fossil record. The principle of superposition assumes that each layer in the sedimentary rocks is in sequence, with the layer at the bottom being the oldest and so forth. The sedimentary rocks however were not always in a horizontal formation, and geologists sometimes came across areas which had been distorted due to plate shifting and other geological events. This brought about the principle of cross cutting which states that any formation that offsets or intersects with another layer must be younger than the original layer itself. When the layers are found to have been deposited without interruption, they are called conforming layers. When the layers have been disturbed by factors such as wind or water erosion and there are gaps in the layers, they are called unconforming layers. The Grand Canyon in the United States is a prime example of an area which has unconforming layers. This can often happen due to shifts in plates, which cause an uplift in a section of the layer, exposing it to erosion, thus creating a gap 6).

Absolute Time Dating

In order to determine the correct age of the fossils, scientists discovered a new way to date them, which was by using radioactivity. A fundamental unit when discussing elements is the atom, and consists of a nucleus which contains positive protons and neutrons which are neutral. Surrounding the nucleus is a cloud of negatively charged electrons. When atoms are of the same element but have different numbers of neutrons, they are called isotopes. Not all isotopes are stable, and some are radioactive and break down due to the release of energy and particle matter. The way in which isotopes decay does not change regardless of where on the planet they are located, so the scientists can measure rates of decay in elements that are billions of years old. There are three types of carbon isotopes, carbon 12, 13 and 14. All living organisms absorb the carbon isotopes, and when an organism dies, the carbon 14 isotope decays and turns into nitrogen, and the ration of carbon 14 isotopes compared to carbon 12 isotopes reduces. Once this number has been determined, the correct time scale can be discovered 7).

Whilst radiometric dating is a fairly reliable method, it is not flawless. Not all rocks contain the suitable radioactive nuclei, and some sedimentary rocks may contain fossils that were formed in a different era and deposited into the rock in another time period. High levels of heat can also disturb the decay process, which may give an inaccurate picture, along with the contamination by different isotopes. Despite this, this is still the best method that is available to scientists and under most circumstances is reliable.

Earth Sciences

1) Manutchehr-Danai, M., 2009, 'Geology', Berlin, Germany, Springer Press, ISBN: 978-3-540-69036-8
2) Anderson, O., 2007, 'Charles Lyell, Uniformitarianism and Interpretive Principles', Zygon, vol.42, no.2, p.449
3) Heilbron, J.L., 2003, 'The History of Modern Science', Oxford University Press, ISBN: 9780195112290
4) Celia, M. A., Nordbotten, J. M., 1982, 2012, 'Geological storage of CO: Modeling approaches for large-scale simulation', Hoboken Publishing, New Jersey, USA, ISBN: 9780470889466
6) Loader, S. P., Pisani, D., Cotton, J. A., Gower, D. J., Day, J. J., & Wilkinson, M., 2007, 'Relative time scales reveal multiple origins of parallel disjunct distributions of African Caecilian amphibians', Biology Letters, vol.3, no.5, pp.505-508
7) Cowan, J. J., Thielemann, F. K., & Truran, J. W., 1991, 'Radioactive dating of the elements', Annual Review of Astronomy and Astrophysics, vol.29, no.1, pp.447-497

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