By Fabrício Caxito
No trace of a beginning, no prospect of an end
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“The mind seemed to whirl as it looked so deep into the abyss of time…” So scientist and mathematician John Playfar summed up his perplexity when, in 1788, his friend James Hutton took him to Siccar Point, a cliff in Scotland. The naturalist, chemist, physician and geologist Hutton would occupy a central position in the Scottish Enlightenment of the 18th century, alongside such figures as Joseph Black, Adam Smith, David Hume, James Watt and Benjamin Franklin.
The human being had long wondered about the age of the Earth. Aristotle, on the other hand, when observing that stretches of land ended up becoming sea and vice versa, had interpreted the phenomenon as proof that changes observed in the terrestrial surface indicated an enormous geological time, perhaps infinite. This idea of a cyclical and infinite time, however, soon began to be challenged by Epicureans, contemporaries of Aristotle, who believed in linear time, with a beginning, middle and end.
In the Middle Ages and the Renaissance, several attempts were made to estimate the date of the beginning of the Earth. Archbishop James Ussher was responsible for the most famous of these, from 1658 – starting from the Bible’s backward counting of the generations, he concluded that the world had emerged on October 23, 4004 BC
For those who knew and studied the natural world, however, these numbers were unrealistic. In 1666, Nicolau Steno, anatomist physician at the court of Fernando II de’ Medici, presented an explanation for the calls glossopetrae, or tongue stones, triangular-shaped rocks immersed within other rocks in nature. The explanation for the occurrence of these stones was controversial: Pliny the Elder thought they had fallen from the sky on moonlit nights; Athanasius Kirchner, a contemporary of Steno, spoke of a “lapidation virtue”, which in time would turn all natural things to stone. Steno was the first to present the correct interpretation: the glossopetrae they are in fact fossil shark teeth solidified into new rock. With the advance of field studies and the recognition of several layers with distinct fossil content, scientists considered the estimates of Ussher and other religionists modest: for the development, and even extinction, of all those forms of life, a mere 6,000 years they were a wrong age.
And here comes James Hutton into the picture. In 1875, he presented to the Royal Society of Edinburgh his ideas about the time required for the formation of the earth’s surface, but the reception of his hypotheses was not the warmest. In search of evidence, Hutton decides to undertake a series of field trips to Scotland, during which he discovers several evidences.
At Siccar Point, for example, layers of rock with different dips are separated by a surface that geologists call “discordance”. By dip, we mean the angle at which each layer of rock makes with the Earth’s surface. Hutton was the first to correctly interpret the meaning of this. The lower set of rocks, below the unconformity, would have been horizontally deposited on a bottom of an ocean, lake or other type of sedimentary basin. Afterwards, this set of rocks would need to have been raised above sea level and had its layers tilted due to deformation at the time of uplift, as if they were lifted by a backhoe.
Today we know that this occurs mainly in areas where two tectonic plates meet, forming mountain ranges. After the uplift of the rocks, they begin to be eroded by the action of wind, rain and other weathering agents. Once the mountain is eroded to the base, new sedimentary basin can form on top of it, and new sediment can accumulate horizontally on the surface that marks the erosive line of the mountain range. These sediments that are deposited above the surface can, in turn, also be uplifted later and undergo erosion, thus restarting the cycle. Hutton recognized the enormous significance of disagreements: geological time must have been much longer than previously thought, given the various cycles of deposition, uplift, erosion, deposition, uplift…
And what is the answer to the initial question of Aristotle, the Epicureans, Ussher, Steno and so many others? By dating methods using radioactive decay, we now know that the Earth is actually about 4.5 billion years old. Can you imagine what this number means, or, like Playfar, your mind also seems to “swirl as you look so deep into the abyss of time”?
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Fabrício Caxito is a geology professor and philosophy student at UFMG.
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