E. Linacre and B. Geerts |
11/'97 |
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