Jurassic, Pleistocene, Precambrian. The named times in Earth’s history might inspire mental images of dinosaurs, trilobites or other enigmatic animals unlike anything in our modern world.
Labels like these are part of a system scientists use to divide up Earth’s 4.6 billion year history. The biggest divisions are eons which split into eras, which break into epochs, which divide into periods and then all the way down to ages.
Before the ages, naming the rocks
The geologic time scale was not entirely intentional, at least at its start. In the early 1800s, geologists began to create maps and descriptions showing where different types of rocks occurred throughout western Europe.
Some of this was driven by natural curiosity. The Triassic is named because the same three-part layering – carbonate-rich shale on top of fossil-rich limestone on top of red sandstone – was found throughout western Europe. To European scientists, this configuration seemed common enough to warrant a name.
Some labeling emerged from economic motivations. If a particular type of sandstone or limestone or coal proved useful, then people wanted to know where else to put a quarry or mine to find the same rock.
The study of how rocks are layered and organized became formalized as stratigraphy. To assign a name to a particular rock, stratigraphers put criteria in place. There had to be a location where the archetype of that rock could be found. There should be a widespread geographic distribution, as for the Triassic. There might be signature fossils that only occur in that rock, or are not found in younger rocks (suggesting an extinction) or older rocks (telling us when a new species developed).
Names for the divisions of the rock record drew from where those rocks were first or best described – Devonian rocks in Devonshire, Cambrian rocks in Wales (Cambria, as the Romans called the region) – or from obvious characteristics. Cretaceous rocks in Europe are full of fossils that provide a rich source of chalk. Carboniferous rocks around the world include important coal resources.
Rocks near Gubbio, Italy, change in color and texture at the line indicating the Cretaceous-Paleogene extinction event that wiped out the dinosaurs 66 million years ago. A baseball hat shows scale. Robert DeConto and Mark Leckie, UMass Geosciences, CC BY-ND
Rocks equal time
The big mental leap came in connecting rocks with time – those Devonian rocks were formed during what came to be called Devonian time. That’s how geologic time became a convenient shorthand for major events and changes in life’s history on Earth. The Cretaceous is not just chalk. It’s a time when conditions were just right for the seas to be filled with huge populations of plankton – whose bodies sank to the ocean floor and eventually formed chalk when they died.
What began as a system to distinguish different rocks in western Europe has grown into a formalized, sophisticated and systematic way of thinking about life and time and the ways these are recorded in rocks.
The history of Earth’s atmosphere is one example. Invisible chemical proxies created by ancient organisms and preserved in sedimentary rocks record the rises and falls in oxygen and carbon dioxide over the past 600 million years. These coincide with events along the geologic timescale such as major mass extinctions, the evolution of land plants and the assembly and breakup of supercontinents.
Be it fossils or minerals or minute chemical signatures, the stratigraphic records reveals the interplay between life, earth and environment through time.
The official chart of geologic time over Earth’s billions of years. http://www.stratigraphy.org, CC BY-NC-ND
Defining the Meghalayan Age
Scientists still continue to refine the geologic timescale. This summer brought the official naming of a new age: the Meghalayan.
All rocks younger than 4,200 years are now part of the Meghalayan Stage. Time since 4,200 years ago is in the Meghalayan Age. But there is a lot to unpack in these details.