Earth’s Geological History: Precambrian to Cenozoic Eras
Precambrian Era: From Earth’s Origin to 570 Million Years Ago
Hadean Eon
- Earth separated into three layers.
- The first atmosphere was formed.
- Meteorites continually bombarded the Earth.
- The Moon was formed.
- The first life forms probably existed at the end of the Hadean.
Archean Eon
- Few rocks remain from this eon.
- Almost all of the Earth was an ocean, with only microcontinents.
Proterozoic Eon
- Supercontinents were formed, and lithospheric movement was similar to that of today.
- Meteorite bombardment stopped.
- Tectonic plate movement began.
- Oxygen appeared in the atmosphere.
- Probably, there were supercontinents.
- This was the coldest period.
The Atmosphere
- This was a period of great volcanic activity.
- There was a reducing atmosphere.
- There was no ozone layer.
- The climate was cold and humid.
Life
- The oldest remains of organisms have been found in Archean rocks.
- First evidence of photosynthetic activity.
- Algae produced and released oxygen into the atmosphere.
- First eukaryotic cells.
Paleozoic Era
- This era consists of six periods.
- Animal fossils are abundant.
- Rocks underwent considerable transformations.
- Rodinia began to break apart.
Climate
Periods of warm and glacial climates.
Flora
- Huge variety of multicellular organisms.
- Multicellular algae diversified considerably.
- First plants with conductive tissues.
- Fossils of vascular plants.
- During the later stages of this era, plants with naked seeds appeared.
Fauna
Invertebrates
Mollusks, brachiopods, graptolites, arthropods.
Vertebrates
Fish (the first vertebrates), then amphibians and reptiles.
Mesozoic Era
- Consists of three periods.
- Pangaea broke up into two continents: Gondwana and Laurasia.
- Collisions between some of the formed continents created mountain ranges.
Climate
- Warm during most of the era.
- Humidity increased gradually.
- At the end, the Earth cooled down.
Fauna
- Marine invertebrates are the most characteristic index fossils.
- Fish with bony skeletons were very common.
- The large amphibians disappeared.
- Large expansion of reptiles.
- The first bird fossils with skeletons were found.
- Therapsids had skeletal characteristics very similar to mammals.
- First mammals appeared.
- A mass extinction occurred at the end.
Cenozoic Era
- Two periods.
- The most common materials are sedimentary series, deltaic formations, and neritic formations.
Climate
- The Earth warmed up.
- Seasonal differences became apparent.
- A progressive cooling culminated in the Ice Age.
Flora
- Angiosperms spread out.
- Deciduous forests developed.
- Herbaceous plants started and developed, forming huge prairies.
Fauna
- Ammonites and belemnites disappeared.
- Nautiloid mollusks were predominant.
- Many insects developed, which fed on the pollen produced by flowers.
- Bony fish were spreading and diversifying into different groups.
- Many of the larger amphibians became extinct.
- Anura and Caudata exist today.
- Huge mammals developed.
- Human beings appeared.
Dating Methods
Radioactive Methods
Atoms in some unstable chemical elements lose particles from their nucleus, converting them into other stable elements. This process happens at fixed speeds, so it can be used in dating.
Paleontological Methods
A stratum is the same age as the fossil it contains. A good index fossil for a particular period should have the following characteristics:
- It lived for a short period.
- It was widely distributed geographically.
- It is abundant and easy to recognize.
- It is well-preserved.