Earth's History: A Timeline
The Geologic Time Scale is a system of chronological dating that relates geological strata (rock layers) to time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that have occurred during Earth's history.
Major Divisions of Geologic Time
The largest divisions of time are eons. Earth's history is divided into four eons:
1.Hadean Eon: (Formation of the Earth to ~4.0 billion years ago). A period of intense heat, volcanic activity, and frequent impacts. No rock record from this time survives.
2.Archean Eon: (~4.0 to 2.5 billion years ago). The first continents began to form, and the first prokaryotic life appeared.
3.Proterozoic Eon: (2.5 billion to 541 million years ago). The atmosphere became oxygenated, and the first eukaryotic and multicellular life forms evolved.
4.Phanerozoic Eon: (541 million years ago to the present). This eon is marked by the proliferation of complex animal and plant life.
The Phanerozoic Eon is subdivided into three eras:
Paleozoic Era ('Ancient Life'): The 'Cambrian Explosion' of life, the rise of fish, insects, amphibians, and the formation of the supercontinent Pangaea. Ended with the largest mass extinction in Earth's history.
Mesozoic Era ('Middle Life'): The age of reptiles, dominated by dinosaurs. The first mammals and birds appeared. Pangaea began to break apart.
Cenozoic Era ('Recent Life'): The age of mammals. Following the extinction of the dinosaurs, mammals diversified to fill ecological niches. The rise of humans occurred during this era.
Principles of Relative Dating
Stratigraphy is the study of rock layers. Principles like these allow geologists to determine the relative ages of rock formations:
Law of Superposition: In an undeformed sequence of sedimentary rocks, each layer is older than the one above it and younger than the one below it.
Principle of Original Horizontality: Layers of sediment are originally deposited horizontally under the action of gravity.
Principle of Cross-Cutting Relationships: A geologic feature which cuts another is the younger of the two features.
Index Fossils
An index fossil is a fossil that is used to define and identify geologic periods. A good index fossil must be:
Widespread geographically.
Abundant.
Limited to a short span of geologic time.
Finding a specific index fossil in a rock layer allows geologists to quickly determine the age of that layer.