Advent of the Anthropocene epoch: Geological time scale, and how it has evolved over time

The findings could change the Earth’s official geological time scale. The AWG is set to present its proposal — to add the Anthropocene on the time scale — in front of different scientific organisations in the following months. Upon their approval, a final decision might come at the 37th International Geological Congress in Busan, South Korea, slated to take place next year.

What is the geological time scale?

The Earth’s geological time scale is a fundamental tool used by geologists, palaeontologists and other scientists to study the planet’s past. It is a system that divides the history of the Earth into discrete intervals of time, based on events, such as the evolution and extinction of different living beings and processes that have occurred.

It is divided into five broad categories: eons, epochs, eras, periods, and ages. As of now, at least officially, we’re in the Phanerozoic eon, Cenozoic era, Quaternary period, Holocene epoch and the Meghalayan age.

Diagram of the geological time scale. (Credit: US Geological Survey General Information/Wikimedia commons)

How has the geological time scale evolved over time?

It has taken hundreds of years to create and evolve the geological time scale as we know it today. The roots of its origin go back to the 1500s and 1600s, when miners became interested in understanding the geological relationships of different rock units, according to a report by the University of California, Berkeley.

The first major breakthrough came in 1669 after Danish scientist Nicolas Steno published the first laws of stratigraphy — the science of interpreting the strata, or layers of rock, in the Earth’s outer surface. He laid out two basic geologic principles: “The first stated that sedimentary rocks (formed on or near the planet’s surface, in contrast to metamorphic and igneous rocks, which are formed deep within the Earth) are laid down in a horizontal manner, and the second stated that younger rock units were deposited on top of older rock units,” the report added.

The second principle essentially means that layers closer to the Earth’s surface must be younger than layers below them. This allowed scientists of the time to conclude that each rock layer represented a specific interval of geologic time.

Further advancements were made when building on Steno’s principles, Italian geologist Giovanni Arduino began to name the layers. By examining the rocks of the southern Alps during the 1750s, he classified the four main layers of the Earth’s crust as Primary (the lowest metamorphic and volcanic layers), Secondary (hard sedimentary rocks), Tertiary (less hardened sedimentary rocks), and Quaternary (the most recently laid rock layer, which is quite soft in comparison to other strata).

But there were some issues with Arduino’s classification and Steno’s principles. First, “because rocks were locally described by colour, texture, or even smell, comparisons between rock sequences of different areas were often not possible”, the University of California report stated. And without a way to compare rock layers of different parts of the world, there couldn’t have been a universal geological time scale. Second, unlike tree-ring dating, in which each ring is equivalent to one year’s growth, rock layers don’t tell the specific length of geologic time, meaning no one layer can convey how long a certain period lasted on the Earth.

These discrepancies were finally dealt with by English surveyor William Smith, who, in the 1800s, observed that fossils could be used to characterise different intervals of time because evolution and extinction are facts of nature. For instance, a rock with a trilobite fossil upon it means that it is Paleozoic in age (541-252 million years ago) and not older or younger as trilobites flourished only in the Paleozoic era.

Moreover, Smith came up with the principle of faunal succession, which stated that fossils are found in the same order under the Earth’s surface from place to place. “Fossil A was always found below Fossil B, which in turn was always found below Fossil C, and so on. By documenting these sequences of fossils, Smith was able to temporally correlate rock layers (or, strata) from place to place (in other words, to establish that rock layers in two different places are equivalent in age based upon the fact that they include the same types of fossils),” the New York-based Paleontological Research Institution said in a report.

This allowed scientists to construct the first rough outlines of the geological time scale. A more evolved and precise time scale appeared with the advent of radiometric dating in the early 1900s, but fossil evidence still plays a crucial role in the division of the timeline.