Exactly how old is the earth? What has happened, what changes have come about, during the millions of years the planet has been in existence? These and similar questions are among the primary concerns of geologists as they attempt to understand the processes that have given our home planet its diverse and multifaceted landscape.

By examining the structure of the rocks and sediments that have formed over the ages scientists can learn a great deal of the story. However, the oldest rocks so far discovered, the Acasta Gneisses in north-west Canada, date back only just over 4 billion years. They, and other ancient rocks that have been found, originated from lava flows and sediments, indicating that other, older geological structures must have been present long before they were formed.

The age of rocks and meteorites is determined by measuring the decay of radioactive isotopes of elements occurring naturally in rocks and minerals. So far no rocks have been discovered that can be said to date back to the origins of the planet.

In the absence of such hard evidence, geologists have estimated the age of the earth from the age of the oldest meteorites so far discovered. They reason that it is likely that the whole of the solar system was formed at the same time, and that it is therefore reasonable to assume that the oldest rocks originating elsewhere in the solar system are not older than the earth.

The oldest meteorite so far discovered fell at Canyon Diablo, Arizona, between 20,000 and 40,000 years ago. The rock of which it is composed, however, dates back some 4.5 billion years, and this is currently science's best estimate of the age of the solar system and, hence, the earth.

A similar story emerges from the rocks brought back by astronauts from the moon. The samples they found range widely in age, but again, the oldest seem to have been formed between 4.4 and 4.5 billion years ago.

Geologic time from the presumed date of the formation of the earth to the present day is measured as a series of eons, eras, periods, epochs and ages, each marking a different stage in the development of the earth. By far the largest chunk of the earth's history is accounted for by the Precambrian eon, which begins 4.5 billion years ago and ends just under 600 million years ago. During this era the earth's atmosphere began to be formed, the tectonic plates that carry what are now the continents arose and started to move, and, by the end of the era, the first primitive animal life had started to evolve.

The second eon - and the one in which we are still living - is the Phanerozoic, which began 543 million years ago. The eon is divided into three eras - the Paleozoic, up to 248 million years ago, the Mesozoic, up to 65 million years ago, and the Cenozoic, our current era. The development of animal life continued during the Paleozoic Era, with living creatures mostly confined to the seas. During the Mezozoic Era the dinosaurs began to evolve, and towards the end of the Cenozoic Era humans became established as the dominant species.