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.