Period (geologic time)

I

Introduction

Period (geologic time), unit of time that geologists use to divide the earth’s history. On the geologic time scale, a period is longer than an epoch and shorter than an era. The earth is about 4.5 billion years old. Earth scientists divide its age into shorter blocks of time. The largest of these are eons, of which there are three in the earth’s history. The last eon is formally divided into eras, which are made up of periods. Many periods are divided into epochs. Geological or biological events mark the beginnings and ends of some periods, but some are based on a convenient interval of time determined by radiometric dating (see Dating Methods).

Geologists divide much of the earth’s history into periods. Too little is known about pre-Archean time, from the origin of the earth to 3.8 billion years ago, to divide it into units. The Archean Eon (3.8 to 2.5 billion years before present) is not divided into periods. It marks a time in which the structure of the earth underwent many changes and the first life appeared on the earth. Rocks of the Archean Eon contain some very simple single-cell organisms called prokaryotes and early blue-green algae colonies called stromatolites. During the Proterozoic Eon (2.5 billion to 570 million years before present) the earth was partially covered alternately by shallow seas and ice sheets. Life advanced from the most basic single-celled organisms to plants and animals that resembled some of the species alive today. Pre-Archean time, the Archean Eon, and the Proterozoic Eon make up what is called Precambrian time. The most recent eon of the earth is the Phanerozoic (570 million years before present to the present). During this eon, the earth and life on it gradually changed to their present state.

II

Periods of the Proterozoic Eon

Some scientists divide the Proterozoic Eon into four eras and at least ten periods. These divisions are not universally accepted. The eras defined for the Proterozoic are the Huronian Era (2.5 billion to 2.2 billion years before present), the Animikean Era (2.2 billion to 1.65 billion years before present), the Riphean Era (1.65 billion to 800 million years before present), and the Sinian Era (800 million to 570 million years before present). The four informally recognized periods of the Huronian Era are the Elliot Lake Period, the Hough Lake Period, the Quirke Lake Period, and the Cobalt Period, from oldest to youngest. These periods correspond only to deposits in a region of Canada around Lake Superior and have no definite time correlation. A comparison of rocks of the Elliot Lake and Cobalt periods show that oxygen levels in the atmosphere rose during the Huronian Era. The Hough Lake, Quirke Lake, and Cobalt periods all begin with times of glaciation.

The Animikean Era has only one informally recognized period, called the Gunflint Period, which lasted from about 2.2 billion years before present to about 2 billion years before present. Rocks of the Gunflint Period contain many species of microbes and stromatolites.

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The Riphean Era has three informal periods. The oldest period is the Burzian Period (1.65 billion to 1.35 billion years before present), followed by the Yurmatin Period (1.35 billion to 1.05 billion years before present) and then the Karatau Period (from 1.05 billion to 800 million years before present). All three are named from sedimentary rocks in a section of the southern Ural Mountains in Russia.

The Sinian Era is divided into two informal geologic periods—the Sturtian Period (from 800 million to 610 million years before present) and the Vendian Period (610 million to 570 million years before present). The Sturtian is named from rocks in southern Australia that show two distinct glacial episodes. The Vendian is named from rocks in the southern Ural Mountains. The Vendian Period is divided into two epochs, the Varanger Epoch (about 610 million to 590 million years before present) and the Ediacara Epoch (590 million to 570 million years before present). Rocks from the Ediacara Epoch show the first fossils of complex organisms.

III

Periods of the Phanerozoic Eon

The Phanerozoic Eon is the most recent eon of the earth and is divided into the Paleozoic Era (570 million to 240 million years before present), the Mesozoic Era (240 million to 65 million years before present), and the Cenozoic Era (65 million years before present to the present).

The periods of the Paleozoic Era are the Cambrian Period (570 million to 500 million years before present), the Ordovician Period (500 million to 435 million years before present), the Silurian Period (435 million to 410 million years before present), the Devonian Period (410 million to 360 million years before present), the Carboniferous Period (360 million to 290 million years before present), and the Permian Period (290 million to 240 million years before present). The rocks of the Paleozoic Era contain abundant and diverse fossils, so each period is marked by both geologic and biological events.

The rocks of the Cambrian Period contain many fossils of shelled animals such as trilobites, gastropods, and brachiopods that are not present in earlier rocks. The Ordovician Period is characterized by an abundance of extinct floating marine organisms called graptolites. One of the greatest mass extinctions of the Phanerozoic Eon occurred at the end of the Ordovician Period.

Rocks from the Silurian Period reveal the first evidence of plants and insects on land and the first fossils of fishes with jaws. In the Devonian Period, the first animals with backbones appeared on land. The Devonian was the first period to produce the substantial organic deposits that are used today as energy sources (see Fossil Fuel).

The rocks of the Carboniferous Period contain about one-half of the world’s coal supplies, created by the remains of the vast population of animals and plants of that period. Besides the abundance of terrestrial vegetation, the first winged insects appeared during the Carboniferous.

During the Permian Period all the continents on the earth came together to form one landmass, called Pangaea. The shallow inland seas of the Permian created an environment in which invertebrate marine life flourished. At the end of the period, one of the greatest extinctions in the earth’s history occurred, wiping out most species on the planet.

The Mesozoic Era is composed of the Triassic Period (240 million to 205 million years before present), the Jurassic Period (205 million to 138 million years before present), and the Cretaceous Period (138 million to 65 million years before present). During the Triassic Period, the supercontinent of Pangea began to break apart. Dinosaurs first appeared during the Triassic, as did the earliest mammals.

The continents continued to break apart during the Jurassic Period. Reptiles, including the dinosaurs, flourished, taking over ecological niches on the land, in the sea, and in the air, while mammals remained small and rodentlike. The continents continued to drift toward their present locations during the Cretaceous Period. Another mass extinction, which killed off the large reptiles such as the dinosaurs, occurred near the end of the Cretaceous.

The Cenozoic Era is divided into the Tertiary Period (65 million to 1.6 million years before present) and the Quaternary Period (1.6 million years before present to the present). During the Tertiary Period the continents assumed their current positions. Mammals became the dominant life forms on the planet during this period, and the direct ancestors of humans appeared at the end of the Tertiary. The most recent ice age occurred during the Quaternary Period. The first humans appeared during the Quaternary. The changing climate and melting of the glaciers, possibly combined with hunting by humans, drove many of the large mammals of the early Quaternary to extinction, making way for the animal life on the earth today.