Geologic Time

Introduction Nearly 4.6 billion years have passed since Earth’s formation. In that time, life has exploded from a few simple-celled organisms to a great variety of single-celled and multi-celled forms. Scientists know about life’s history from studying the fossil record and rock layers worldwide. This record can be arranged in a geologic time scale.

Organizing Earth’s History The geologic time scale is a timeline that organizes the events in Earth’s history. It reveals that algae, bacteria, and protozoa dominated most of Earth’s history. More complex organisms, such as land plants and fish, evolved only within the last 500 million years. Humans evolved only about 110,000 year ago. If the entire history of earth were squeezed into a single day, humans would not evolve until the last few seconds. 4.6 Billion Years Ago 4 3 2 1 Today Prokaryotes Eukaryotes Humans

Geologic Time Scale The geologic time scale is like a calendar extending from Earth’s formation to the present. The scale is divided into eons, eras, period and epochs. Eon: The largest group: billions of years long Era: mass extinctions mark the boundaries between the eras; hundreds of millions of years long Period: tens of millions of years long Epoch: divisions of the most recent periods; several million years long.

Your Geologic Time Scale The back of your paper is divided into 4 columns In the last or fourth column, list six to ten events in the school year in the order they will happen. For example, you may include a particular soccer game, dance, or event. In the third column, organize those events into larger time periods, such as soccer season, rehearsal week, or whatever you choose. In the second column, organize those time periods into even larger ones. In the first column, organize the time period into the largest group Go back to the 4 columns and label them as eon, era, period, and epoch Conclusion: List in order of smallest to largest. Epoch, Eon, Era and Period. How does putting events into categories help you and geologists see the relationship among events?

Geologic Time Scale in a Calendar Year

Review Why is a time scale used to represent Earth’s history instead of a calendar? When did geologic time begin? When does geologic time end? Put the following words in order of smallest to largest: epoch, era, period, eon.