Geologic Time Scale
One of the most difficult ideas in geology for students to grasp is the immense span of time since the formation of Earth. Human experiences deal in terms of seconds, hours, day, weeks, and months. A year is a long time for most, especially children. A few decades comprise a lifetime. All of recorded human history comprises only a few thousand years. Consequently, humans do not easily conceptualize time periods lasting millions or billions of years, or recognize the enormous consequences of slow processes acting over enormous time spans. Recognizing the difficulties, we have designed several comparisons and activities in the section on Geologic Time to help students begin to explore the almost unimaginable span of time that has passed on Earth.

Geologists divide the history of Earth into periods, eras, and eons. The original divisions were based on groups of characteristic fossils in rocks. Before the discovery of radioactivity, absolute ages were matters of philosophy and indirect measurements. Until the late 1800s most people in western culture thought Earth was only a few thousand years old (many still do). Early geologists, noting rates of sedimentation or deposition of salts in the ocean, argued for ages of tens to hundreds of millions of years. When the radiometric ages were first measured early in the 20th century, the ages of billions of years for the older rocks surprised everyone. The oldest rocks known to date on Earth are found in Canada and are about four billion years old. The commonly cited age of 4.6 billion years for Earth is derived from the radiometric ages of meteorites that formed when the solar system, and presumably Earth, formed from interstellar gas and dust.

Slightly differing dates are often found in different books for the beginnings and endings of geologic time periods. The reason for these differences is that we have difficulty pinning down an exact time based on evidence found in the rocks. We know, for example, that the Cretaceous ended either 66 or 65 million years ago, but the event happened so long ago that our estimate has a margin of error of about 1 million years.

This may be an important point to make to students. For example, we know when each student was born because the hospital recorded the date. However, as we go back to a time when no one was keeping records, we find ourselves searching for evidence from different sources. Fossils are one source of information; the type of rocks in which they are found is another source of information. We can use radioactive elements like uranium or potassium to date the rocks. Exactly how rocks are dated would be a good project for students to undertake.