By Fabrício Caxito
No trace of a beginning, no prospect of an end
“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 figures such as Joseph Black, Adam Smith, David Hume, James Watt and Benjamin Franklin.
The human being has 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 a linear time, with a beginning, middle and end.
In the Middle Ages and in the Renaissance several attempts were made to estimate the date of the Earth’s beginning. Archbishop James Ussher was responsible for the most famous of these, of – starting from the Bible’s back-counting of generations, he concluded that the world had arisen in the day 23 October 4004 BC
For those who knew and studied the natural world, however , these numbers were fanciful. In 1666, Nicolau Steno, anatomist physician at the court of Fernando II de’ Medici, presented an explanation for the so-called 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 “cutting virtue”, which in time would turn all natural things to stone. Steno was the first to come up with the correct interpretation: the glossopetrae are actually fossil shark teeth solidified in new rocks. 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. In 1788, he presented to the Royal Society of Edinburgh his ideas about the time needed for the earth’s surface to form, but the reception of his hypotheses was not the warmest. In search of evidence, Hutton decides to undertake a series of field trips in Scotland, during which he discovers various evidence.
In 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 rocks are uplifted, 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 should be much longer than previously thought, given the various cycles of deposition, uplift, erosion, deposition, uplift…
And what is it? 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”?
Fabrício Caxito is a professor of geology and a philosophy student at UFMG.
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