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Geological times

E. Linacre and B. Geerts

11/'97

 

        The history of the earth is divided by geologists into two 'ages' (the Precambrian, until 540 million years ago, or aBP), and the Phanerozoic (since 540 million aBP, when plants appeared on land).

        The Phanerozoic age is divided into three 'eras' (Table 1): the Paleozoic (when shelled animals appeared and left fossils in rocks) until 225 million aBP, the Mesozoic (the Dinosaur era) from 225 - 65 million aBP, and the Cenozoic since 65 million aBP, when mammals appeared.

        The Cenozoic era is divided into two 'periods', the Tertiary (until 2 million aBP) and then the Quaternary, when a series of ice ages (glacials) and interglacials occurred and when homo sapiens appeared.

        The Quaternary period consists of the two 'epochs', the Pleistocene (until 10,000 years ago, at the end of the last ice age) and the Holocene epoch until now.

 

 

Table 1: Geological times of the last 400 million years

Era

Period

Epoch

Million years ago

Upper Palaeozoic

Devonian

 

395-345

Carboniferous

 

345-280

Permian

 

280-225

Mesozoic

Triassic

 

225-190

Jurassic

 

190-139

Cretaceous

 

136-65

Cenozoic

Tertiary

Palaeocene

65-54

Eocene

54-38

Oligocene

38-24

Miocene

26-7

Pliocene

7-2

Quaternary

Pleistocene

2-0.01

Holocene

0.01-present

 

Continents have moved and altered shape during geological times. For instance, Antarctica and Australia were joined until 55 million aBP. Following the breakup, Australia moved equatorward, becoming more arid, and Antarctica moved poleward, becoming colder. An ice cap first appeared on Antarctica about 36 million aBP, and the Antarctic ice sheet has maintained its current volume since 5 million aBP.

The Quartenary is characterised by oscillations of the extent of land ice in the northern hemisphere. Glacial periods were interrupted by brief interglacials about every 100-200 thousand years. The Holocene corresponds to the most recent interglacial. Conditions have been colder than the 20th century during at least 90% of the Quaternary. The latest 160,000 years of the Quaternary consisted of the last full cycle of glacial advance and retreat. The warmest time of the Pleistocene probably occurred at about 120,000 aBP, as shown by oxygen-isotope ratios in sediments of the Pacific Ocean north of Australia, and sea-level records from raised coral reefs of Papua New Guinea (1). Conditions were also more moist, judging from the reduced amount of dust in the ice of that age, in a core from Vostok in Antarctica (2).

References

1.              Sturman, A. and N. Tapper 1996. The Weather and Climate of Australia and New Zealand (Oxford Univ. Press) 476pp.

2.              Kershaw, A.P. and G.C. Nanson 1993. The last full glacial cycle in the Australian region. Global and Planetary Change, 7, 1-9.