2. Geological Time— An Overview

   

In the previous chapter, we somewhat casually spoke of periods hundreds of millions years ago. But it is not easy to grasp such a overwhelmingly massive concept. Palaeontologists use these very scales to study the ancient life forms. Just like historians categorised the continuum of human history based on socio-political events and patterns (e.g. Italy during renaissance, Victorian England, US during prohibition), geologists and palaeontologists have done the same for geological time based on the rock sequences formed at that time. And by dating these rock sequences we come to know how old they are.

The first to shape the discipline of geology as a science was the Danish Geologist Nicholaus Steno. In 1699, he proposed the principles for relative timing of geological events. They were:

(1) The principle of superposition, in which each rock layer was laid down one upon another,

(2) The principle of original horizontality, in which rock layers were originally laid down horizontally and were inclined only if secondarily tilted, and

(3) The principle of original continuity, in which rock layers were assumed to be continuous over great distances.

These three, taken together with the principle of crosscutting (Charles Lyell, 1830) which states that any feature cutting across another must be the younger of the two, gives the solid scientific foundation for relative geological dating.

However, modern science enables us to determine geological age by absolute dating. This involves studying the radioactive decay of the elements that are in the rocks. Note that the previous method can only tell us which rocks are older than others while the later can give us an idea how old the rocks are. Let us see how it is done.

The method of absolute dating is also known as chronostratigraphy. The layers of rock are usually called strata and chronos (Greek) means “time”. We know that radioactive elements are unstable and constantly decay by means of radiation. Many elements also have such unstable isotopes. Isotopes are variants of an element differing in the number of neutrons.  For example, the element carbon has three isotopes. carbon-12, carbon-13 and carbon-14. A carbon atom has 6 electrons. Therefore the number of neutrons in these isotopes are 6,7 and 8 respectively(the numbers 12, 13 and 14 are called mass numbers. Mass number of an atom is the total number of protons and neutrons in the atom and an atom has equal number of electrons and protons). Of these, carbon-12 and carbon-13 are stable isotopes and carbon-14 is radioactive and therefore undergoes decay and produces a stable “daughter” isotope, which is nitrogen.

carbon-14 –> nitrogen-14 + energy

Now, suppose we found a rock/fossil with such an isotope. If we know

·       The initial amount of parent isotope at the time of rock formation/death of the creature,

·       The residual amount of the parent isotope,

·       The rate of decay of the isotope (which is usually constant)

We can estimate the age of the specimen in years. In this regard, scientists often use the half-life, which is basically the time taken for the isotope to decay 50% of its initial amount, as a marker for these isotopes.

Fig 2.1- radioactive decay

  But, not all isotopes are equally important. Some have a half-life of over billions of years; some have half-life in thousand years (carbon-14 has half-life of about 5,730 years). Therefore choosing the right isotope to analyse is of single most importance; after all, you don’t want to measure your height in light-years or nanometres.

Now we shall look at how the geological time is structured.

The largest unit of time is supereon, composed of eons. Each eon is divided into eras, an era into periods, periods are further divided into epochs and each epoch is composed of ages. This enables us to give a specific “address” to whatever time we are talking about. For example, the dinosaurs belonged in the Mesozoic era of the Phanaerozoic eon, in the later part of the Triassic period and throughout the subsequent periods Jurassic and Cretaceous. 

Fig 2.2- Geological time  

We will learn about these periods, the condition of the earth and of course, we will learn about the dinosaurs. But before that, it is necessary to know a brief sketch of the evolution of life in order to fully understand the dinosaurs. That will be the topic of our next discussion.