The Geologic Age of the Earth: Eras, Periods, and Time Divisions

The geological age of the Earth should not be equated with the unit of time used in our everyday calendar: it is not a matter of a hundred years, but rather that indefinite span of time during which the remains of particular species of animals and plants were deposited in the rock strata (see also: How life originated in the ancient eras of the Earth).

How is the geological history of the Earth divided?

For the convenience of study, the history of the Earth is divided into large intervals of time, arranged from the largest unit down to the smallest. These divisions nest inside one another, each one a subdivision of the one above it:

• the broadest intervals are called eras;
• eras are subdivided into periods;
• periods are subdivided into epochs;
• epochs are subdivided into ages.

Each unit of time has a matching unit that describes the body of rock laid down during it. The thickness of rock deposited within the Earth corresponds to these time divisions as follows:

• the rock deposited during an era is called a group;
• a period corresponds to a system of deposits;
• an epoch corresponds to a series;
• an age corresponds to a stage.

The chronological sequence of the geological divisions is set out in the following summary table, which lists each era, its constituent periods, and the average duration of those periods in millions of years:

What do the names of the geological eras mean?

The names of the eras are built largely from ancient Greek words that describe the stage of life they represent: "arche" means beginning; "zoe" means life; "proteros" means first; "palaios" means ancient; "mesos" means middle; and "kainos" means new. Combining these roots gives each era a meaning that reflects the development of life on Earth:

• the Archeozoic era is the most ancient — the beginning of life;
• the Proterozoic era is the era of primary life;
• the Paleozoic era is the era of ancient life;
• the Mesozoic era is the era of middle life;
• the Cenozoic era is the era of new life.

The Archeozoic and Proterozoic eras are grouped together into a single interval called the Precambrian, a very long span of time that precedes the Cambrian period, or Cambrian. Alongside the full era names, shorter forms are also in common use:

• Archeozoic,
• Proterozoic,
• Paleozoic,
• Mesozoic,
• Cenozoic.

Where do the names of the geological periods come from?

The origins of the period (system) names are extremely varied — some trace back to obsolete divisions of Earth's history, others to place names or characteristic rocks. The most recent periods in time, the Quaternary and the Tertiary, kept their names from an early nineteenth-century scheme that split the whole history of the Earth into three eras:

• the primary (Paleozoic),
• the secondary (Mesozoic),
• the tertiary (Cenozoic).

A fourth, quaternary (modern) era was later added, but because the last two eras were so short compared with the earlier ones, they were subsequently merged into a single Cenozoic era, while their original labels survived as the names of periods.

The Upper Tertiary period, also called the Neogene, takes its name from the Greek "neos" (new) and "genos" (birth) — literally "new birth", meaning the appearance of new plants and animals. The Lower Tertiary is the same as the Paleogene, which from ancient Greek means "ancient birth", that is, the formation of organic forms more ancient than those of the present day.

Period names often correspond to the places where deposits of a given system were first studied, or they were derived from rocks characteristic of that period — for example, the Cretaceous and the Carboniferous, or Carbon (from the Latin "carbo", meaning coal).

These particular names are not entirely apt, since chalk beds and especially coal deposits occur among the strata of other periods as well, not only within the periods that bear their names. Several other periods take their names from specific regions or ancient peoples:

• the Jurassic period, or Jura, was named after the mountains where deposits of this system were first studied in detail (the French and Swiss Jura);
• the Triassic (from the Greek "trias", meaning trinity) reflects the division of this system's deposits into three parts;
• the Permian was named after the ancient Ugro-Finnic realm of Permia, on whose territory — in the former Perm province — deposits of this system were first discovered.

The deposits of the Cambrian, Ordovician, Silurian, and Devonian systems (the Cambrian, Ordovician, Silurian, and Devonian) were first studied in England, which is where their period names come from:

• the Devonian is named after the county of Devonshire;
• the Silurian is named after the Silures, the ancient inhabitants of the province of Wales;
• the Ordovician is likewise named after an ancient tribe, the Ordovices, neighbours of the Silures;
• the Cambrian is named after Cambria, the ancient name for Wales itself.

The final column of the summary geological table gives the approximate duration of the periods in millions of years. These figures were obtained using the uranium method of determining the age of igneous rocks from the various periods, which measures radioactive decay to date the rock.

How to read a geological map

Careful investigation of where the deposits of one system or another reach the Earth's surface makes it possible to compile geological maps. Although they are based on a geographical outline showing the modern shape of the land, with many rivers marked and the positions of cities noted, they differ sharply from the ordinary geographical maps everyone knows so well.

To someone unversed in geology, these maps are striking for their riot of colour and their intricate, carpet-like patterns. Every colour seems to be present — greens, blues, yellows, oranges, reds, and greys in the most varied shades — along with assorted letters, symbols, and whimsically winding lines. How is one to make sense of them?

There is nothing mysterious about it: each colour on a geological map is linked to the deposits of a particular system. Grey tones of various shades, for instance, correspond to Carboniferous deposits of different ages, distinguishing the sub-divisions of that period:

• the Lower Carboniferous,
• the Middle Carboniferous,
• the Upper Carboniferous series.

Blue colouring marks deposits of Jurassic age, green marks Cretaceous deposits, and yellow marks Tertiary ones. The letter symbols on a map are just as easy to interpret:

• A stands for the Archeozoic and Proterozoic — that is, the Precambrian;
• P stands for the Permian system, or Permian;
• T stands for the Triassic;
• C stands for the Carboniferous, and so on.

A geological map demands strict accuracy from its compilers, because it guides the direction and outcome of a geologist's exploratory work. Once you have familiarised yourself with a geological map, it becomes easy not only to master all its colours and symbols but also to make sense of the tangled complexity of its multicoloured design.

No, the colours are not scattered across our map in disorder and chaos; they obey the great laws of physics and chemistry that govern the world and us,

said Academician A. E. Fersman. The geological map is especially closely associated with the name of the first Soviet president of the Academy of Sciences of the USSR — A. P. Karpinsky (1847–1936), who is rightly called the "father of Russian geology".

Karpinsky compiled the first geological map of Russia in 1892; before that, Russian geologists had to rely on an older map drawn up in the 1840s by the famous English geologist and explorer of Russia, Murchison.

On the maps of the most ancient periods of the Earth's life, known as palaeogeographical maps, A. P. Karpinsky seems to bring the distant past back to life before us — scenes of the eternal alternation of land and sea across the broad expanses of Eurasia.

Karpinsky also made a valuable contribution to the study of rocks and mineral resources. His work is being widely put into practice, and today modern geology, following the paths mapped out by the "father of Russian geology", holds great importance in world science.