Geological Time Scale
There's of course no rock that shows all the time sequences in the Earth's history - geologists have constructed that timeline which is put together from many different data from many different rocks.
It is divided into units such as eons, eras, periods, epochs, ages and chrons. Eons are the longest ones, lasting at least a half a million years, while chrons are the shortest ones. Each division has been named, usually after the place on the Earth where the first rocks from that time were found.
The geological time scale is also constantly updated as new discoveries are made. Only quite recently have we got a half descent idea about what happened before Cambrium - the period when the marine life "exploded" and many fossils were suddenly left behind. We have a much more detailed picture of what happened since. Stratigraphy helps geologists to match the rock sequences. This, however, only gives a relative, not definite age. It is only since the mid-1900s when the radioactive dating was developed as we can be quite sure that the dates are fairly accurate.
Precambrian Eon 4600 - 545 mya
The oceans formed, and the first single-celled organisms appeared. Towards the end of Precambrian, simple multi-celled animals evolved. However, they did not have any calcaceous body parts such as vertebrate or shells, to leave fossils behind.
Cambrian Period 545 - 495 mya
The "cambrian explosion" of life - a lot of small marine animals formed, including the first animals with shells, which could easily be preserved as fossils.
Ordovician Peiod 495 - 443 mya
A warm period that caused large polar ice caps to melt. First land plants and first jawless fish appeared. The period ended with a mass extinction.
Silurian Period 443 - 417 mya
The Caledonian Orogeny formed mountains in North America and northern Europe. Animals started to invade land. First vascular plants.
Devonian Period 417 - 354 mya
First insects, and first seed-bearing plants.
Extensive coal deposits formed. First reptiles. Pangaea started to form.
Permian Period 290 - 248 mya
Cold period, which ended with a huge mass extinction - 96% of species were wiped out.
Triassic Period 248 - 205 mya
First mammals, first dinosaurs.
Jurassic Period 205 - 142 mya
Pangaea began to break up. Dinosaurs dominated.
Cretaceous Period 142 - 65 mya
Dinosaurs still dominated although the first predatory mammals had evolved. Dinosaurs went extinct in the end of the period.
Cenozoic Era 65 mya - now
Supercontinents had broken up. Grand Canyon and Himalayas formed. Because dinosaurs were gone, mammals started to dominate. Large mammals appeared, primates and humans developed. First flowering plants.
Sedimentary layers in Grand Canyon. Poster by AllPosters. Click on
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