When we talk about time it is easy for us to understand quantities that reach the order of millennia. Things get complicated, however, when it comes to much larger numbers, such as the age of the Earth which is 4,6 billion years old. But why is it so difficult to understand geological times? When were the first measuring scales introduced? To answer these and other questions, we interviewed Leonardo Sagnotti, Director of the INGV Environment Department.
We were often told that ahead of the age of our planet, the interval from the appearance of the first Homo today it is comparable to the blink of an eye. That's it?
Yes, of course it is! It is estimated that the solar system and the Earth are about 4,6 billion years old and if we consider the geological time in which the genus developed Homo this represents a tiny fraction of a very large time interval. Often a very simple analogy is used, that is, the age of the Earth is compared to the duration of a solar year. In this analogy, each hour is equivalent to about half a million years and each day is equivalent to about 12,5 million years. If we consider that our planet was formed on January 10.000, all human civilization of the historical period, which includes the development of livestock and agriculture and has developed in the last 31 years after the end of the last glaciation, takes place in last minute before midnight on December XNUMXst: it really is a blink of an eye! The genre Homo it is older but still appears on December 31st, not before. Continuing with this analogy, the first multicellular life forms would appear only on November 13, the first terrestrial plants on November 24, all reptiles that evolve from amphibians and then give rise to dinosaurs would appear in the month of December (dinosaurs would then disappear only on December 26th). Finally, the Cenozoic, which is the Age of mammals, would take place in the last 5 days of the year. Thanks to geology we know well what we call the Phanerozoic, the Eon of manifest life which takes place over the last 530 million years and in the analogy includes the period between the second half of November and December. Of what came before we know very little.
Why is it often difficult to understand geological time?
It is difficult to deal with geological times because we are talking about large numbers and our mind tends to ensure that when they exceed a certain figure, positively or negatively, in our perception they tend to become equal. It follows that it is no longer clear what comes before and what happens after, just think of the very popular Flintstones cartoon, in which prehistoric men live with dinosaurs. Obviously it has never been like this, between the appearance of the genus Homo and the disappearance of the dinosaurs there are about 63,5 million years, an enormous amount of time! Remaining on the subject of perception, there is a story John F. Kennedy told about a French marshal who asked his gardener to plant a row of trees of a certain variety the following morning. The gardener said he would have done it willingly but warned that that type of tree would take a century to fully develop. At that point the marshal replied “Then plant them today!” If a century is a time that is still understandable for the human mind, when we are dealing with much longer times we lose the ability to mentally depict the measure of the flow of events, despite very slow phenomena that can give rise to transformations on the earth's surface spectacular.
For researchers involved in the study of the phenomena and dynamics of planet Earth, it is essential to have the most accurate control possible over the sequence of events and their times of occurrence and rates of evolution. When did the first geological time scales begin to be developed?
The development of the Geological Time Scale is a very long process. The debate begins with the birth of the stratigraphy which is due to Niccolò Stenone (Italianized name of the Danish cardinal Niels Stensen) who was also a naturalist, geologist and anatomist of the seventeenth century, and laid down the conceptual principles. At that moment a first notion of stratigraphic scale begins to be defined, it is understood that in a succession of sediments the layer below is older than the one above. In the following centuries, the debate arose about how old the earth was. On the one hand there were those who based themselves on the literal interpretation of the facts narrated in the Bible and estimated their age at around 4000 years; on the other hand there were those who, based on geology, stated that our planet must have been much older, however providing estimates that were far from those ascertained today. The turning point came at the beginning of the twentieth century when knowledge of the radioactive decay of some elements found in rocks developed. In this way it was possible to provide their absolute dating through the study of the isotopes of the radioactive elements, which have a half-life which for some elements even reaches the order of billions of years. It is thus that in the twentieth century it is understood that the Earth is more than a billion years old. The estimate we give today, of about 4,4 billion, was defined starting from the age attributed to the most ancient lunar rocks, studying the samples that were brought to earth by the Apollo missions. On planet Earth the oldest rocks found and dated with radiometric methods are about 4,1 billion years old.
What is geochronology about?
Geochronology is the discipline that deals with giving an order and an age to geological phenomena for the definition of a time scale.
We distinguish two possible methods for dating the events recorded in the rocks: one seeks to provide a relative chronology to establish an order of the events recorded in the rock sequences in a before - after succession. On these bases the geological classification of times is organized and the terminology that we have heard many times, think of the Jurassic, the Triassic and so on. This classification does not concern itself with giving a numerical date, which absolute chronological studies do instead, based on the radiometric methods I was talking about earlier which are based on the decay time of the radioactive isotopes which are contained in some rock minerals. In this way it is possible to provide a numerical dating, defined with an interval of uncertainty.
What are chronostratigraphic units and how are they classified?
The chronostratigraphic units refer to the chronological classification of rock sequences and are defined according to their duration in: Eonothems, Erathems, Systems, Series, and Plans. This classification somewhat resembles that of living organisms with systematic categories ordered in a hierarchical sense.
What happens when a type stratum is found and established as a chronostratigraphic unit?
Stratigraphy studies usually look for rocky sequences in which geological time is well marked. When a stratigraphic sequence is found that accurately returns a series of events on the Earth in succession, in terms of physical, chemical and paleontological information, this can be raised to "stratotype” for the formal definition of a certain geological time interval. In this case the stratigraphic sequence is submitted to the examination of the International Commission of Stratigraphy and if the latter recognizes its validity and ability to best represent a certain time interval of the past, it formally establishes the "stratotypes" at a global level
You told me about the International Commission of Stratigraphy. What exactly does it do?
The International Commission of Stratigraphy belongs to theInternational Union of Geological Sciences (IUGS) and is formed by an international group of experts on the reconstruction of geological time. Its objective is to increasingly refine the knowledge of the succession of tempos and to this end it examines the proposals that arrive from the scientific community to establish or refine not only the stratotypes, which as we have said are packages of rocks that best represent a certain geological time interval, but also the stratotypes of the limits that separate two adjacent stratigraphic ages. The International Commission of Stratigraphy therefore defines the so-called Global Stratigraphic Section and Point (GSSP acronym), i.e. global stratigraphic sections and points.
Are there instantaneous geological phenomena of which we can realize?
Yes, normally people have knowledge of important and instantaneous geological events such as earthquakes and volcanic eruptions. A strong earthquake lasts a few minutes maximum while in a volcanic eruption the paroxysmal phase is generally included within a day. These are geological phenomena that can have strong repercussions on the earth's surface and on human societies. Today these phenomena are studied in detail and given their occurrence it is not uncommon in the course of life to experience them and acquire the right perception. Things go differently in the case of very slow phenomena, such as the processes that determine the morphology of the earth's surface. Think of the opening of ocean basins, the formation of mountains as well as the erosion of mountain ranges. These processes occur imperceptibly and constantly during our lives and we do not have a real perception of them. Think of the opening of the Atlantic Ocean which occurs at a similar rate to the growth of our fingernails. It is a matter of a few centimeters per year, an expansion rate which from the Mesozoic to today has meant that all the continents of North and South America have progressively separated from Europe and Africa with the opening of an oceanic basin today thousands of kilometers wide .
To conclude, our Era is the Cenozoic.. but in which Period and in which Epoch are we?
We are in the Cenozoic, the Period is the Quaternary, which includes the last 2,4 million years according to the latest classification, and the Epoch we live in is the Holocene which covers the period of time which has elapsed since the end of the last glaciation. In the Holocene, with a decidedly more stable climate, human civilization as we know it develops: the genus Homo, which existed since the beginning of the Quaternary, arrives in the Holocene with only one species (Homo sapiens) which gives rise to civilizations based on agriculture and livestock. From there, the technological evolution will take place shortly where man learns to use metals, and so on up to the current civilization but this is no longer geology, it is history.
Credits cover image on the newsletter received
United States Geological Survey - Graham, Joseph, Newman, William, and Stacy, John, 2008, The geologic time spiral—A path to the past (ver. 1.1): US