4 Main Divisions of Geologic Time Eon Largest

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

The “Precambrian” • 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. ”

Cambrian Explosion • 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

Supercontinent Formation

New Developments in Life

Mass Extinctions

Evidence for Events in Geologic Time • Fossils show changes over time • Plate tectonics • Certain elements/compounds/sediments in the rock record • Impact craters • Core samples of glaciers • Radiometric Dating • Oil and coal beds

Sediment Evidence Layer of Iridium in rock strata! Glacial Evidence Drill deep holes into glaciers

• Vredefort Crater • Near Johannesburg, South Africa • Over 300 km across when it formed • Estimated to be roughly 2 billion years old

Plate Tectonics

Evidence

Similar bone structure in many animals

Present Day • We are currently in the: – Phanerozoic Eon – Cenozoic Era – Quaternary Period – Holocene Epoch