VIII.1 Geologic Time Spiral: A Path to the Past
The Earth is very old—about 4.5 billion years. Most of its history is preserved in the rocks that form the Earth's crust. The rock layers are like pages in a book—they record the events of the past, and some even contain remains of plants and animals. The age of a rock layer (or its “page number”) can be calculated via the molten radioactive elements that provide Earth with an atomic clock. This diagram was designed by Joseph Graham, William Newman, and John Stacy. It shows the evolution of Earth from 4.5 billion years ago to today. Time starts in the lower left corner and spirals upwards. Three billion years ago, organic structures emerged, and two billion years ago, early life forms appeared. The Cambrian explosion about 530 million years ago lead to a proliferation of life forms. Dinosaurs appeared during the Triassic period (230 million years ago) and became the dominant terrestrial vertebrates for 135 million years—from the beginning of the Jurassic (about 200 million years ago) until the end of the Cretaceous period (65.5 million years ago). They went extinct at the close of the Mesozoic era. Homo sapiens appeared 500,000 years ago and modern humans 200,000 years ago. The map is available online at http://pubs.usgs.gov/gip/2008/58.
Graham, Joseph, William Newman, and John Stacy. 2008. Geologic Time Spiral: A Path to the Past. Courtesy of U.S. Geological Survey. In “8th Iteration (2012): Science Maps for Kids,” Places & Spaces: Mapping Science, edited by Katy Börner and Michael J. Stamper. http://scimaps.org
U.S. Geological Survey Geologic Names Committee. 2007. “Divisions of Geologic Time—Major Chronostratigraphic and Geochronologic Units.” U.S. Geological Survey Fact Sheet, series no. 2007-3015. http://pubs.usgs.gov/fs/2007/3015/fs2007-3015.pdf
Newman, William L. 2000. “Geologic Time.” USGS Unnumbered Series General Information Packet. Washington, D.C.: Government Printing Office. http://pubs.usgs.gov/gip/geotime/
Graham, Joseph, William Newman, and John Stacy. 2008. The Geologic Time Spiral: A Path to the Past. USGS General Information Product, series no. 58. http://pubs.usgs.gov/gip/2008/58/
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Acknowledgements: This exhibit is supported by the National Science Foundation under Grant No. IIS-0238261, CHE-0524661, IIS-0534909 and IIS-0715303, the James S. McDonnell Foundation; Thomson Reuters; the Cyberinfrastructure for Network Science Center, University Information Technology Services, and the School of Library and Information Science, all three at Indiana University. Some of the data used to generate the science maps is from the Web of Science by Thomson Reuters and Scopus by Elsevier. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.