GEOLOGICAL TIME

Geologists (scientists who study rocks) divide the time since the Earth was formed until today into chunks called periods. During the various periods, different species of animals and plants lived on the Earth. For example, the Cretaceous period, which lasted from 146 million years ago to 65 million years ago, was the final period of the dinosaurs. Some rocks can be given a relative age by identifying the fossils they contain. The date of formation of some rocks can be found by using RADIOMETRIC DATING.

THE FOSSIL RECORD

Over many decades, palaeontologists have built up a database of fossils called the fossil record. This shows when different species of animals and plants lived in the history of the Earth. A fossil can be matched against the fossil record to find the age of the rock it was found in. The information in the fossil record is based on using stratigraphic layers to tell the age of a rock and the characteristic life forms within it. Here are some fossils and the periods they belong to.

QUATERNARY PERIOD

1.8 million years ago (mya) to the present day

MAMMOTH TOOTH

TERTIARY PERIOD

65.5-1.8 mya

CRETACEOUS PERIOD

145.5-65.5 mya

JURASSIC PERIOD

199.6-145.5 mya

ARCHAEOPTERYX (ANCIENT FORM OF BIRD)

TRIASSIC PERIOD

351-199.6 mya

PERMIAN PERIOD

299-251 mya

MESOSAURS (REPTILES THAT LIVED IN WATER)

CARBONIFEROUS PERIOD

359.2-299 mya

FRONDS OF AN EARLY FERN

DEVONIAN PERIOD

416-359.2 mya

JAWLESS FISH

SILURIAN PERIOD

443.7-416 mya

ORDOVICIAN PERIOD

488.3-443.7 mya

CAMBRIAN PERIOD

542-488.3 mya

TRILOBITE

PRECAMBRIAN PERIOD

Before 542 mya

RADIOMETRIC DATING

Radiometric dating is a way of measuring the age of a piece of rock. Igneous rocks contain tiny amounts of radioactive chemicals. As the rocks age, these elements gradually break down into elements that are not radioactive. By knowing the rate at which the elements break down and measuring their level of radioactivity, the age of a rock sample can be calculated.

ARGON-ARGON DATING

In argon-argon dating, the amounts of two forms of the element argon in a rock sample are measured using a device called a mass spectrometer. One form only is produced by radioactive decay as the rock ages, so comparing how much there is of this in relation to the other form of argon reveals the age of the rock.

Copyright © 2007 Dorling Kindersley

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