The Earth formed about 4.6 billion years ago. By the order of rock layers and the fossils in them, geologists divide this long history into units: the largest is the eon, eons are divided into eras, and eras into periods.
The last 540 million years or so, whose rocks are rich in animal fossils, make up the Phanerozoic Eon, divided into the Paleozoic, Mesozoic and Cenozoic eras. The four billion years before that are together called the Precambrian, divided into the Hadean, Archean and Proterozoic eons. When a page on this site says a rock “dates from the Archean” or “formed in the Mesozoic”, these are the spans it means.
| Time | The Earth then |
|---|---|
| Hadean 4.6–4.0 billion years ago | The newly formed Earth had a scorching surface and was constantly struck by smaller bodies; the Moon also formed at this time. Almost no rocks survive. |
| Archean 4.0–2.5 billion years ago | The first land and oceans appeared, and the simplest single-celled life lived in the sea. The oldest known rock on Earth is the Acasta Gneiss in north-western Canada, about 4 billion years old; zircon grains in the Jack Hills of Australia are older still, about 4.4 billion years, but they are mineral grains rather than whole rocks. In China: Granitic rocks at Dongshan and Baijiafen near Anshan in Liaoning, about 3.8 billion years old, are the oldest known rocks in China and among the oldest in the world; rocks around Qian’an in Hebei are also more than 3.5 billion years old. The ancient rock easiest to see at a scenic area is the “Taishan Complex” of Mount Tai, about 2.5 to 2.7 billion years old; the oldest rocks of Mount Song are also Archean. |
| Proterozoic 2.5 billion–540 million years ago | Oxygen began to build up in the air; organisms with nucleated cells appeared, and multicellular life near the end. There were several ice ages that nearly froze the whole globe. In China: Mount Song has broad expanses of Proterozoic quartzite; strata from all five spans, Archean to Cenozoic, can be seen there – “five generations under one roof”. |
| Paleozoic 540–250 million years ago An age of great expansion for animals and plants. Most of southern China was shallow sea, where thick limestone and sandstone were laid down. | |
| Cambrian 540–490 million years ago | The Cambrian explosion: most animal phyla appeared almost at once, and trilobites filled the seas. Typical fossils: Trilobites; Anomalocaris of the Chengjiang biota. In China: The Chengjiang Fossil Site (early Cambrian). |
| Ordovician 490–440 million years ago | Marine life flourished, including trilobites and nautiloids; an ice age at the end brought a mass extinction. Typical fossils: Graptolites; straight-shelled nautiloids. In China: The limestone at the summit of Mount Qomolangma (Everest), with marine fossils. |
| Silurian 440–420 million years ago | Plants began to move onto land, and fish gained jaws. Typical fossils: Corals and sea lilies; early armoured fish. |
| Devonian 420–360 million years ago | Fish flourished, the first forests grew on land, and four-legged animals appeared near the end. A mass extinction in the Late Devonian (about 372 million years ago) wiped out shallow-sea reefs and many fishes; its cause is debated, often linked to oxygen-starved oceans. Typical fossils: Fishes of many kinds, such as placoderms; the first ferns and trees. |
| Carboniferous 360–300 million years ago | Dense forests covered the land and later turned into great coal seams; reptiles appeared. Typical fossils: Tall scale trees and giant horsetails; dragonfly-like Meganeura with a wingspan of over half a metre. In China: Many coalfields of northern China formed in the Carboniferous and Permian. |
| Permian 300–250 million years ago | The continents joined into one supercontinent; the end of the period saw the largest mass extinction in Earth’s history. It is usually blamed on huge volcanic eruptions in Siberia that caused rapid global warming; more than nine-tenths of marine species disappeared. Typical fossils: Fusulinids; Gigantopteris plants. In China: The limestone of the Stone Forest of Lunan. |
| Mesozoic 250–66 million years ago The age of dinosaurs. In eastern China the crust was very active, and great volumes of magma cooled underground into granite; many inland basins filled with red sandstone and conglomerate. | |
| Triassic 250–200 million years ago | Life slowly recovered after the great extinction; dinosaurs and the first mammals appeared. Most of southern China rose from sea to land. Another mass extinction at its end (about 201 million years ago), usually linked to massive volcanic eruptions, cleared the way for the dinosaurs to dominate the land. Typical fossils: Ammonites; marine reptiles such as Keichousaurus of Guizhou and ichthyosaurs. |
| Jurassic 200–143 million years ago | Dinosaurs flourished and birds appeared. Typical fossils: Dinosaurs (Lufengosaurus, Mamenchisaurus); Archaeopteryx. In China: Lufeng in Yunnan and Zigong in Sichuan have yielded large numbers of Jurassic dinosaur fossils. |
| Cretaceous 143–66 million years ago | Flowering plants appeared; at the end an asteroid struck the Earth and the dinosaurs died out. Typical fossils: Feathered dinosaurs and Confuciusornis of the Jehol biota in western Liaoning; early flowers. In China: The red sandstone and conglomerate of Mount Danxia and Mount Wuyi, the rock of Danxia landforms. |
| Cenozoic 66 million years ago to today Mammals and birds took centre stage, and today’s mountains and rivers largely took shape. | |
| Paleogene 66–23 million years ago | Mammals diversified rapidly; India collided with Asia. Typical fossils: Early mammals, such as primitive horses and early primates. |
| Neogene 23–2.6 million years ago | The Himalayas and the Tibetan Plateau rose sharply, and China’s land gradually took its shape, high in the west and low in the east; grasslands spread. Typical fossils: Three-toed horses (Hipparion), shovel-tusked elephants; apes. In China: The Himalayas and the Tibetan Plateau. |
| Quaternary 2.6 million years ago to today | Ice ages alternated with warm spells, glaciers grew on high mountains, winds piled up the Loess Plateau in the north, and humans appeared. Typical fossils: Woolly mammoth and woolly rhinoceros; early humans such as Peking Man. In China: The volcanoes of Wudalianchi, Tengchong and Heaven Lake of Changbai Mountain (Wudalianchi erupted just over three hundred years ago); the glaciers of Jade Dragon Snow Mountain; the Peking Man Site at Zhoukoudian. The cirques, horn peaks and glacial lakes such as Daye Lake on the summit of Mount Taibai were left by Quaternary glaciers. |
How old a rock is and when a landscape took shape are two different things. The limestone of Guilin and the Stone Forest was laid down on the sea floor hundreds of millions of years ago, but the peaks and pillars were dissolved out slowly by rain only in the Cenozoic, after the rock was lifted above the sea; the sandstone pillars of Wulingyuan and the red cliffs of Mount Danxia were likewise carved by running water in the Cenozoic.
The Earth has gone through several great ice ages, when glaciers covered large areas of land:
- About 2.4 to 2.1 billion years ago, in the early Proterozoic.
- About 720 to 635 million years ago, in the late Proterozoic: ice may have reached the equator – the “Snowball Earth”; the Nantuo tillite in the Three Gorges area was left by its glaciers.
- About 445 million years ago, at the end of the Ordovician, together with a mass extinction.
- About 340 to 260 million years ago, from the Carboniferous into the Permian: ice sheets covered the southern continents of the time.
- The Cenozoic: at the end of the Paleogene (about 34 million years ago) an ice sheet began to form on Antarctica; about 2.6 million years ago the Quaternary ice age began, with glacial and interglacial periods alternating up to the present – we now live in a relatively warm interglacial. Most glacial traces in China’s mountains (the cirques of Mount Taibai, the glaciers of Jade Dragon Snow Mountain) date from the Quaternary.
See also these themes: Karst, Danxia landforms, Volcanoes, Glaciers, Fossils & geological sites.