Geologic Time

The Geologic Time Scale: During Earth's geologic periods, life emerged from single-celled organisms to vertebrates; amphibians went onto land; land-based plants, dense forests, reptiles, dinosaurs, birds, mammals, and humans appeared. 

Our Planet Earth: If Earth's entire history could be compressed into a single year, modern humans would've appeared just 23 minutes ago! Use this video to introduce your students to the concept of geological time (also called deep time); relative age dating of rock via the principles of stratigraphic superposition, original horizontality, and cross-cutting; absolute age dating by radioactive decay; the chemical elements, heavy and light, that make up the planet; and Earth's three main layers: crust, mantle, and core. 

PBS Eons: Join hosts Hank Green, Kallie Moore, and Blake de Pastino as they take you on a journey through the history of life on Earth. From the dawn of life in the Archaean Eon through the Mesozoic Era — the so-called “Age of Dinosaurs” -- right up to the end of the most recent Ice Age. The evolutionary history of mammals including humans & other modern species is explored with these amazing paleontology experts. 

Geologic Time and and Geologic Column: http://hyperphysics.phy-astr.gsu.edu/hbase/Geophys/geotime.html#c2

Geologic Time Scale: https://www.colorado.edu/cumuseum/sites/default/files/attached-files/geoscale_complete.pdf

• Paleozoic Era: https://www.colorado.edu/cumuseum/sites/default/files/attached-files/large_geo_scale_paleozoic.pdf

• Mesozoic Era: https://www.colorado.edu/cumuseum/sites/default/files/attached-files/3geotimescalemesozoic.pdf

• Cenozoic Era: https://www.colorado.edu/cumuseum/sites/default/files/attached-files/2geotimescalecenozoic.pdf

Geologic Time Scale: http://www.ucmp.berkeley.edu/help/timeform.php

GSA Geological Time Scale: https://www.geosociety.org/documents/gsa/timescale/timescl.pdf

NPS: Fossils Through Geologic Time: https://www.nps.gov/subjects/fossils/fossils-through-geologic-time.htm

USB - Museum of Natural History: Geologic Time Scale: https://www.colorado.edu/cumuseum/programs/schools-and-groups/fossils-classroom/materials-and-resources/geologic-time-scale

Created by Liza Zandonella, Last updated March, 2026