Box Geological Time

Earth is immensely ancient, and yet the history of Earth, and the history of life, have been punctuated by so many crises and dramatic changes that it is possible to find markers that are the same worldwide. This means that geologists can correlate rocks, and establish an agreed chronology of events through time. Geologists began to realize this 200 years ago. At first they saw that particular assemblages of fossils were always found together; they were not scattered randomly through the rocks in different associations. These principles of relative dating, (1) the recognition of repeated fossil assemblages, and (2) their identification as characteristic of particular time units, give a basis for the standard international geological time-scale.

Early in the twentieth century, numerical or absolute dating became possible using the newly discovered property of radioactivity. Some chemical elements exist in an unstable radioactive condition. This means that they decay over time, emitting radioactivity and changing from one elemental form to another. The decay process, in which the parent element changes into the daughter element, may last for a matter of hours, for thousands of years, or for billions of years. It is possible to assess when half the parent has decayed, and the time this takes is called the half-life. Geologists compare the relative amounts of parent and daughter element in particular igneous rocks, rocks formed by crystallization at high temperatures, and they compare the ratios to the known half-lives to establish the absolute, or exact, age in millions of years.

The longest stretch of geological time is the Precambrian, representing most of the history of Earth, from its origin, through its cooling, the origin and early history of life. The last major segment of geological time is the Phanerozoic ('abundant life') eon, the time during which fossils are abundant and document the well-known history of major modern groups, including the vertebrates. The Phanerozoic is subdivided into three eras, the Palaeozoic ('ancient life'), Mesozoic ('middle life'), and Ceno-zoic ('recent life'), and these in turn are divided into periods, such as Cambrian, Ordovician, and Silurian, and epochs, such as Paleocene, Eocene, and Oligocene. The epochs are further divided into ages and zones, based on the distributions of single fossils, or specific assemblages, and zones may represent time intervals of as little as 100,000 years. In practice, rocks are dated in the field by means of fossils, and then numerical ages can be added here and there where there is an appropriate igneous rock band, for example, a layer of volcanic lava.

The geological time-scale, showing the main divisions of geological time, and current numerical age dates, based on the Geological Society of America 1999 time-scale, with dates for the Triassic revised. Full version available at http://www.geosociety.org/science/timescale/ timescl.htm

Eon

Era

Period

(Myr)

Cenozoic Era

Quaternary Period

Holocene Epoch

0.01

Pleistocene Epoch

1.8

Tertiary Period

Pliocene Epoch

5

Miocene Epoch

24

Oligocene Epoch

34

Eocene Epoch

55

N

Paleocene Epoch

n

Mesozoic Era

CL

Cretaceous Period

144

Jurassic Period

200

Triassic Period

251

Palaeozoic Era

Permian Period

290

Carboniferous Period

354

Devonian Period

417

Silurian Period

443

Ordovician Period

495

Cambrian Period

545

Precambrian

4560

centre of the Earth. Where oceanic crust meets continental margins, the sideways movements may continue, hence opening the ocean further, or the oceanic plate may dive down beneath the continental plate, forcing up mountain ranges, such as the Andes. Where continental plates collide, they may move past each other jerkily, as along the San Andreas fault, or they may force into each other, as with the Himalayas, raised by India's continuous movement northwards into the main Asiatic land mass.

Continental drift is critical in the history of the vertebrates. The geography of Earth has never been stable, and it seems that, through time, the continents have amalgamated and divided several times. Most is known about the break-up of Pangaea since the Triassic, but it is possible to make good estimates of continental reconstructions back through the Palaeozoic. Continental drift has affected animal and plant distributions: ranges are sundered at times, and brought together in unpredictable ways. Dinosaurs evolved in a world on one supercontinent, and they could move freely all over Pangaea. By the Cretaceous, however, their movements became restricted, and local, or endemic, faunas are found in South America,Africa and India. During most of the Cenozoic, South America was an island, but 3 million years ago, the Isthmus of Panama was formed, and a great exchange of land animals took place, with profound effects both north and south (see pages 320-2).

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