Focus Questions: How do geologists look at time? How has life changed on the earth over time? Description/Background Information: Time for geologists goes back 4.6 billion years ago to the earth's formation. Geologic time refers to the time that the earth has been around, from its formation to the present. To make sense of this large amount of time, geologists have broken it up into units. The largest of these units are called eras. The Precambrian, which is further divided into the Archean and Proterozoic, represents the earliest of the eras when life on the Earth consisted of very basic forms like bacteria and other single-celled organisms (4.6 billion years ago - 570 million years ago). The Paleozoic era (570 million years - 225 million years ago) represents a time in the Earth's history in which life began to diversify and flourish, producing multi-cellular organisms that included fish, amphibians and reptiles. The Mesozoic era (225 million years ago - 65 million years ago) is known as the age of dinosaurs and is recognized for the evolution and eventual extinction of this group organisms. Birds and mammals first appeared during this era as well. The last era, the Cenozoic (65 million years ago - present), is the era we currently live. This era is identified by the widespread development of mammals, including the human species. Except for the Precambrian, the eras are sub-divided into smaller units of time, referred to as periods. Because more information is known about the Cenozoic, periods within this era are classified into even smaller units of time called epochs.

How have geologists developed this time scale? Direct observations of rocks and principles of relative dating were used to develop the first time scale. Relative ages of rocks is a comparison - how old are different rocks in comparison to other rocks. The principles of relative dating include: Principle of superposition Principle of lateral continuity Principle of original horizontality Principle of cross-cutting relationships Principle of faunal succession Many of these principles are particularly useful in the study of sedimentary rocks. Also important in relative dating were the recognition and understanding of unconformities (gaps in the sedimentary record) and detailed work on fossils.

Placing a number for the age of rocks requires some type of "clock" of a physical process much like our day clock, which is based on the earth's period of revolution or our calendar year that is based on Earth's orbit around the sun. Early attempts at developing a "clock" for the age of the Earth included using rates of sediment deposition and cooling of the Earth, neither of which produced an accurate record of time. In 1905, the physicist Ernest Rutherford proposed that radioactive decay may be a useful avenue to obtain the actual age of a rock. Further developments in understanding radioactivity and the development of techniques to accurately measure the amount of different radioactive isotopes have lead to geologists' ability to determine absolute ages of rocks and therefore a numeric geologic time scale. Vital in this development is the concept of half-life - the time required for one-half of the original number of radioactive elements to decay. This is the "clock" that geologists require to obtain accurate ages for rocks. A detailed review of how isotopes are used to measure the age of a rock can be found at the following web site:

Virtual Age Dating Exercises

Other Resources/Activities Related to Geologic Time:

Dinosaur I: Finding and dating fossils

Time and Change/ Mud Fossils

Time and Change/ Using Radioactive Decay to Determine Geologic Age

Paleontology at the U.S. Geological Survey