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 Glacial Evidence Layer of Iridium in rock strata!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