If the millions and billions of years leave you unmoved, if names like Ordovician (485 to 444 million years before our era), Triassic (-252 to -201 million years) or even Paleoproterozoic (-2, 5 to -1.6 billion years) do not speak to you, this is completely normal.
[Article issu de The Conversation, écrit par Frédéric Simien, Responsable des Éditions, BRGM]
It is very difficult for a human to imagine such durations of time. Even for geologists, who juggle these concepts every day, it is often difficult to get an idea of the chronology of events that have marked the history of our planet.
However, exploring the Earth's past allows us to tell beautiful stories. It is also an opportunity to become aware of the past diversity of the planet's environments in which living things played a primordial role. Moreover, the geological subdivisions of time are based on the different fossil life forms found in the corresponding geological layers.
The turbulent history of life on Earth
The course of events that have occurred over the past 4.5 billion years has not always been a smooth river. Initially, almost no oxygen (current levels were notably made possible by the emergence of cyanobacteria), massive volcanic eruptions, formation of gigantic mountains, mass extinctions, impressive asteroid falls, climate changes and variations in sea level out of all proportion to what we observe.
All of this affected the forms of life that were able to survive on Earth. Living things have seen the appearance of numerous species but also significant biological crises. There have been five major ones just for the fossiliferous times – that is to say the last 540 million years – which nevertheless represent only 12% of the History of our planet since its birth.
It is precisely the different forms of life that make it possible to characterize the different geological periods. Indeed, to determine the succession and duration of these events, geologists set out to study the traces of life fossilized in the sediments. Subsequently, other techniques, notably based on isotopes, made it possible to quantify and refine the geological time scale.
Time and geology: ancient questions
It was Nicolas Sténon (1638-1686) who was the first, in 1669, to formulate the founding principle to define a chronological succession of rocks.
Its principle of superposition states that for undeformed sedimentary rocks, a layer is necessarily older than those which cover it, and conversely, more recent than those on which it rests.
The other major advance in the field was due to William Smith (1769-1839). He showed in 1795 that each geological period had very specific fossils. A surveyor in coal mines located in Wales, he noticed that the succession of layers was similar from one pit to another, and that fossils could serve as a benchmark. This element led him to publish the first geological time scale in 1813.
Subsequently, Georges Cuvier (1769-1832) was the first to foresee rapid changes in species. Without going so far as to imagine mass extinctions on a planetary scale at the time, he proposed, in his Discourse on the Revolutions of the Surface of the Globe published in 1828, that the appearances and disappearances of species were the result of local crises.
The extraction of raw materials and energy, by providing direct access to rocks, subsequently favored the establishment of a time scale and the study of past forms of life.
It was the discovery of a small marsupial in the gypsum (raw material for plaster) of the Butte Montmartre by Georges Cuvier, in 1804, which marked the birth of comparative anatomy, at the origin of the study of fossils.
A tiny fossil to dig the Channel Tunnel in the right place
Knowledge of the different periods, their durations and the ancient species that lived there provides many services on a daily basis. This was used, for example, to guide the digging of the Channel Tunnel.
The level chosen was the Cenomanian blue chalk (Cretaceous), a soft rock that is generally impermeable due to its richness in clay (25%). Gray chalk, the layer just above, contains only 10%, making it less suitable.
The problem is that the transition from one chalk to another is not visible at a glance. The BRGM (national geological service) therefore chose a very small fossil measuring a few hundred micrometers (Rotalipora reicheli) to determine the top of the blue chalk.
Having a short lifespan and being present throughout the Strait of Pas-de-Calais, it represented the ideal marker. It is thanks to this microscopic fossil species that the tunnel was dug in the right place.
This is a classic approach in geology. In order to determine the sequence of different periods, scientists very early set out to find fossils characteristic of certain periods, if possible where the sedimentary record is of the best possible quality, that is to say , where the series of different layers is the most complete. Indeed, it happens that there are periods without deposition and others where the layers have been eroded.
This led to the notion of stratotype, imagined by Alcide d'Orbigny in 1842, which corresponds to the locality where the best sedimentary record is found.
France with many pioneers of geology in the 19th centurye century, forty-four stratotypes were defined in our country, seventeen of which are still used internationally. These subdivisions are known as geological stages. Among those that are still in use, many were defined by Alcide d'Orbigny (1802-1857) and Henri Coquand (1811-1881).
Beyond the academic aspect, the latter's studies made it possible to delimit the vintages of the Cognac vineyard. A fine example of applied geology.
Know the time scale to calmly consider the future
Exploring the history of the Earth encourages us to consider our time on Earth with humility. The XXe century has enabled spectacular advances in knowledge of Earth sciences, with the conceptualization of plate tectonics, dating using isotopes, as well as the discovery of life forms at the bottom of the oceans and in the Earth's mantle.
The XXIe century is one where societal issues directly linked to Earth sciences are very present. Whether it concerns mineral resources or groundwater, natural risks, developments, adaptation to climate change, specialists in Earth sciences are essential each time.
Will we be able to find the necessary resources within an acceptable development framework? Will we be able to plan the necessary adaptations to climate change? Will we be able to predict and mitigate the effects of natural disasters? To answer these questions, it is essential to have a good knowledge of geological times, a valuable and constantly improved tool at the basis of any study.
The study of geology allows us to learn that this is not the first time that the activities of living beings have modified the climate – for example cyanobacteria for oxygen. Better understanding the mechanisms involved in these precedents, their causes and their duration of action is crucial to better understand the complex interactions between Earth, atmosphere, hydrosphere, biosphere and climate.
If there was only one thing to remember, it is that the divisions of geological time were defined according to the different forms of life that we encountered there and that we cannot envisage the world mineral without the living. The biosphere is a major geological player.
Questions remain. Despite scientific advances, the origin of water on Earth is still debated and we still do not know where, when and how life appeared on Earth… The causes of the various mass extinctions are also poorly known and widely discussed. The geological time scale should not undergo major upheavals, but there are still many elements to discover to retrace the history of our planet.
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
