In everyday life, time is measured in hours, minutes and seconds on the dials of our wristwatches. In History, it is measured in years, centuries and millennia, using parchments, clay tablets, papers and other documents with chronological significance. In the Prehistory of Man, as much is done on the basis of utensils and other objects, and one speaks of thousands and, in some cases, millions of years.
The time scale expands when we chronicle the geological past and even more if we go back to the beginnings of the Solar System and the Universe, where the billions of years mark the stages covered with an imprecision that vanishes in this “eternity”.
A billion years more or less, in the beginnings of the matter that we are, represents the same degree of imprecision as a million years more or less in the time of the dinosaurs, of a year or so in the history of old Egypt, or more day – less day, more minute – less minute, in the time we are living, more second-minus second in the chronometers of sports runners.
In the course of our existence, we review, without difficulty, our time, that of our grandparents and even that of history, but it is with effort that we embrace or evoke the vastness of geological time, with figures that only find parallels in the immensity of astronomical distances.
As in History, Geology also needs documents and these are found in rocks, whether they are fossils or some of their minerals containing radioactive isotopes. Among the variables likely to be correlated with time, only two have an irreversible place, since either of these two processes develops in only one direction: biological evolution and natural radioactive decay.
Because it is a story, Geology has in time one of its pillars, being seen there from two different perspectives: that of relative time and that of absolute time.
In RELATIVE TEMPO, one seeks to know if a given event occurred before, after or simultaneously with another, that is, if it was prior, posterior or contemporary. For a long time, the geometric relationships, observable on the ground, between the various outcropping rock bodies, have been used to establish the chronological ordering of the geological events of which they are testimonies. Such an ordering is particularly evident in stratified rocks, in which strata or layers succeed one another in an immediate suggestion of sequence in time.
Such ordering is the same one patented in a pile of papers on a bureaucrat's desk. The relationship between the stacking of rock strata and the course of time caught the attention of the Danish Nicolau Steno, in the XNUMXth century, constituting one of the first fundamental ideas of geology, known as the Superposition Principle, according to which, “in an undeformed stratified sequence , any layer is more modern than the ones underneath and older than the ones that overlay it”.
Evident in light of current knowledge, this principle represents a remarkable advance for the time in which it was enunciated. It relates, for the first time, the stratified rocks with the process of progressive deposition of the sediments that make them up, which corresponds to an idea of succession in time.
As chronological milestones, fossils also, staggered in the evolutionary chain of biodiversity, allow us to approach relative time. With regard to biological evolution, it has long been found, through fossils, that the animal and plant species of the past emerged throughout the Earth's history, remained for more or less long periods, ending, almost always, by extinguishing, not reappearing.
Leonardo da Vinci (1452-1519) was the first to recognize fossils as testimonies of other lives in times past. Until then and even after him, fossils were seen as the whims of nature. It was only in the XNUMXth century that its interpretation as the remains of living beings from the past was definitively established.
Fossils represent links in a chain of increasing complexity. In this understanding, and thanks to the hard work of paleontologists, we know, for example, that layers of sedimentary rocks with trilobite fossils are older (Paleozoic) than those that preserve dinosaur bones (Mesozoic) and that these, in turn , are prior to those that served as a deposit for the mammoths or the australopithecines (Cenozoic), our grandparents.
This reasoning, exemplified here for large time intervals, at the level of geological eras, is currently done for shorter intervals, such as those represented by systems (periods), series (epochs), floors (ages), subfloors and others still more reduced.
The same type of knowledge enables us to consider geologically contemporary all rocks that, in any place, contain the same fossils. Applicable to very many known fossil species, these reasonings have been, since the XNUMXth century, allowing the scaling in time of the set of sequences of sedimentary rocks (and also in metamorphic rocks, in a relatively low degree of intensity, such as that of the Paleozoic series from North to South of Portugal), which contains the essential fossil record of all the biodiversity that preceded us.
From the other perspective, that of ABSOLUTE TIME, which can be quantified, this variable has the sense of duration and, thus, refers to the interval that mediates two events or that which elapsed between one of them and the present moment, that is, its age. One of the most fruitful ways of measuring geological time was born with the discovery of radioactivity by Henri Becquerel, in 1896, and gained substance with the work on the constitution and functioning of the atomic nucleus carried out by Marie and Pierre Curie and many other physicists.
Such advances in science, with reflections on the measurement of time, were wisely used by several researchers, including the English geologist Arthur Holmes, who “was not a Nobel Prize winner because Geology does not figure among the disciplines covered in the respective regulation”.
Routinely performed in many laboratories around the world, isotopic age determinations (based on the natural behavior of some radioactive isotopes) of some minerals (potassium feldspars, muscovite, biotite, among many others) allowed us to frame them in terms of chronology absolute, the great stages of the Earth's history and of Life, many of them, long defined in terms of relative age.
We now know that the Earth was formed approximately 4540 Ma (age still under discussion), that "non-avian dinosaurs" (birds, now accepted as descendants of a certain group of dinosaurs, are thus "avian dinosaurs") made they appeared about 235 Ma ago and that they disappeared, for good, 65 Ma. We know that the Porto granite has 560 Ma, that the Beiras granite has around 300 Ma and Sintra, only 85 Ma. The list of rocks and events that we know the absolute age of is immense and continues to grow.
The monumental work undertaken by paleontologists throughout the XNUMXth and XNUMXth centuries allowed, as has been said, an acceptable scaling in time, based on fossils, and the establishment of finer eras, periods, epochs and other temporal divisions.
Subsequently, thanks to advances in geological knowledge and advances in isotope physics and analysis technologies, we now have a chronostratigraphic scale in which, in ever-improving detail, the temporal divisions, based on fossils, are grouped into time intervals of different hierarchies, quoted by numerical values referring to the adopted geological time unit, that is, the million years, no less than ten thousand centuries, an enormity in the time horizon of our lives, one thousand one hundred and forty times the history of Portugal, but a crumb in Earth time.
(from my book “COME BOLA COLORIDA – The Earth, Heritage of Humanity”, Âncora Editora, Lisbon, 2007).
Author: AM Galopim de Carvalho
Science in the regional press – Ciência Viva
Comments