longest time in the world

The age of the Earth is 4.54 billion years and it is the longest period in the world. Geologists were the first to realize the vastness of the past and, moreover, they measured it. In doing so, he introduced the concept of deep or geological time, in which everything that is possible on our planet has happened.

geological time hypothesis

One of the main scientific concerns of the founders of geology was to understand and measure time that no one had measured: the time it took for mountains to rise and fall away, the time it took for continents to move, or the time it took for us to grow Formation of the planet.

Since the middle of the 20th century, the development of geochronological methods has allowed us to know these times accurately and currently it is possible to determine the age of any rock, including extraterrestrial ones.

But the idea of ​​geologic time developed long before it could be measured. In fact, this time, sometimes called dark time, was a hypothesis proposed by geologists out of the need to explain observations, which showed that geological processes had been going on continuously since the Earth’s remote origins.

time gap

James Hutton (1726–1797) was one of the first scientists to suspect that geological processes were exceptionally slow.

He found evidence of this one morning in June 1788 when he arrived in a small boat with James Hall (1761–1832) and John Playfair (1748–1819) at Sisker Point, not far from Edinburgh.

At Sikar Point the sea had arranged the exposed strata almost vertically, on which other modern strata rested horizontally. Overwhelmed by the discovery, Hutton understood that what he was seeing with his friends was the result of an extraordinary geological phenomenon: the folding, as if it were the bellows of an accordion, of the oldest strata, their subsequent erosion and Deposition and consolidation of more modern sediments.

It was clear that this sequence of processes took a long time to occur. As his friend Playfair later wrote, in front of those rocks “the mind felt dizzy as it looked so far into the abyss of time.”

Hutton was also aware of something equally important: the formation, transformation, and destruction of rocks must have been repeated many times over Earth’s history, “without giving us any indication of a beginning or the possibility of an end.”

On that wild Scottish coast our current concept of geological time was born.

what do rocks tell us

Geologists must extract information contained in rocks to uncover their history. For this it is necessary to know what the rocks can actually tell us. Geologist Charles Lyell (1797–1875) devoted his life to this question, coming to the conclusion that the geological processes that we can currently observe – such as sedimentation, erosion, volcanic eruptions and earthquakes – are, in essence, the same processes. Which worked in the past.

Lyell’s conclusion allows us to explain what we see in the rocks by resorting to the causes and mechanisms that operate today, something that is often expressed with the phrase: the present is the key to the past. Since it was propounded, this theory has become the basis of all geological research, including research that has begun to be conducted on other planets.

darwin’s mistake

The expansion of geological time was also one of the main concerns of Charles Darwin (1809–1882). Natural selection needs to operate over a very long time for it to be an effective evolutionary mechanism. The validity of his theory depended dramatically on the age of the Earth.

The calculations of the distinguished physicist William Thomson (1824–1907) in 1862, just three years after the publication of origin of speciesdid not ease Darwin’s suffering

Based on his estimate of the rate of Earth’s cooling from its initial incandescent state, Thomson estimated its age to be between 20 and 100 million years. That age of the Earth was too short for species to evolve by natural selection.

Darwin, concerned about how Thomson’s calculations challenged his theory, decided to make his own guess: he calculated the time it would take for the Jurassic and Cretaceous rock formations of the Weald in the south-east of England to erode. of.

For his calculations, Darwin assumed that the sea eroded these structures at a rate of one inch (2.54 cm) per year. Although this corrosion rate value was reasonable, it was only an estimate based on general observations. Darwin concluded that the Veld formations were about 300 million years old (it is believed that the sea had taken time to erode them).

Shortly afterwards Darwin discovered that his calculations were wrong and that he had underestimated the erosive forces. In later editions origin of species He refined his calculations downwards, but failed to provide a concrete value for weld structures. It is currently known that these structures took approximately 66 million years to destroy.

Thomson also made several amendments to his sophisticated calculations, which disappointed Darwin and his colleagues, reducing the age of the Earth even further. The disagreement between geologists and biologists on the one hand and physicists on the other gave rise to a lively controversy.

And the 19th century ended without anyone being able to measure geological time accurately.

watches deep time

At the beginning of the 20th century, the eminent physicist Ernst Rutherford (1871–1937) and his then-protégé Frederick Soddy (1877–1956) had already realized that the radioactive decay of certain elements could be used to date rocks. The idea was simple: if we knew the rate at which one element changes into another (for example, uranium into lead), and we knew the parent element (uranium) and the derived element ( Can measure the relationship between the amount of lead). In a rock, we can estimate its age.

In practice, radiometric dating of rocks is no easy task, and it took decades for scientists to determine the age of the Earth. The most notable advances in the development of rock dating methods were made by chemist Bertram B. Boltwood (1870–1927) and geologist Arthur Holmes (1890–1965), who focused on the decay of uranium into lead and provided the first (and unreliable) measurements of the rocks in the first decades of the 20th century.

deep time

In 1956, geochemist Claire Cameron Patterson (1922–1995) determined the Earth’s age to be an accurate 4,555 million years, well above the recent estimate of 4,540 (+/-45) million years measured with more advanced radiometric dating techniques. Is close. Finally, deep time could be measured.

Currently, several rock dating methods have been developed, almost all based on the decay series of radioactive elements, such as uranium-lead, samarium-neodymium, potassium-argon, and rubidium-strontium. With the ages provided by these methods, and without ignoring the principle “the present is the key to the past”, geologists are gradually uncovering what happened on Earth and other planets during the longest periods of history.