4 Main Divisions of Geologic Time
• Eon – Largest division of time; there are 4 of them
Hadean, Archean, Proterozoic, Phanerozoic
• Era – Eons are divided into Eras; there are 3 Eras in
the Phanerozoic Eon
Paleozoic, Mesozoic, Cenozoic
• Period – Eras are divided into Periods; there are 3
Periods in the Mesozoic Era
Triassic, Jurassic, Cretaceous
• Epoch – Periods are divided into Epochs; there are 2
Epochs in the Quaternary Period
Pleistocene and Holocene
• We are currently in the Phanerozoic Eon
• The first division of the Phanerozoic Eon is the
Paleozoic Era, and the first division of the
Paleozoic Era is the Cambrian Period.
• Sometimes all of the time before the Cambrian
Period is called the “Precambrian.”
• Many new species formed, becoming more complex!
Reasons for dividing Geologic Time
• Changing environmental conditions
The crust, the atmosphere, the ocean, etc.
• Supercontinent formation
Rodinia, Pannotia, Pangaea, etc.
• New developments in life (new species)
Single-celled, Photosynthetic, Multicellular, Vertebrates,
Fish, Amphibians, Reptiles, Mammals, Fungi, Flowering, etc.
• Mass extinctions
Volcanic eruptions, Meteorite (asteroid-sized) impacts, etc.
Changing Environmental Conditions
New Developments in Life
Evidence for Events in Geologic Time
• Fossils show changes over time
• Plate tectonics
• Certain elements/compounds/sediments in the
• Impact craters
• Core samples of glaciers
• Radiometric Dating
• Oil and coal beds
Layer of Iridium in rock strata!
Drill deep holes into glaciers
• Over 300 km
across when it
• Estimated to
2 billion years
Similar bone structure in many animals
• We are currently in the:
– Phanerozoic Eon
– Cenozoic Era
– Quaternary Period
– Holocene Epoch