Geologic Time

Frederick M. Soster

Department of Geology & Geography

DePauw University

The earth is approximately 4.6 billion years old. This represents an immense span of time that is incomprehensible to humans. Nevertheless, geologists routinely work with rocks that are hundreds of millions of years old and, in some cases, billions of years old. Indeed, the oldest rocks found to date are about 3.8 billion years old and are found in Greenland. Understanding the immensity of geologic time is important because many geologic processes and evolutionary events are so gradual that vast quantities of time are required to effect perceivable changes.

Geologists use the Geologic Time Scale to place events in the history of earth and life in a chronological framework. Historians can place human events in chronological order by refering to a date, or a year, or sometimes even a century in which an event occured. Likewise, geologists can place events in earth history into a similar framework; however, the time frame is very different. Rather than measure time in years, geologists routinely measure geologic time in millions of years. For historians, recorded human history stretches back about 5,000 years or so, when our own species formed complex social structures and learned to write. This amount of time represents 1/10,000th of a percent of geologic time!

The Geologic Time Scale is divided into Eons, Eras, Periods, and Epochs. The four great Eons of geologic time, from oldest to youngest are the Hadean (4.6 - 3.8 billion years ago), the Archean (3.8 - 2.5 billion years ago), the Proterozoic (2.5 - 0.57 billion years ago), and the Phanerozoic (0.57 billion years ago - present). All geologic time prior to the Phanerozoic is often refered to as the "Precambrian." The diagram in the following link shows the Geologic Time Scale. The Phanerozoic Eon is drawn to scale, whereas the Precambrian, because of its immensity, is not. The Precambrian represents about 85% of all Earth history and if the whole geologic time scale were drawn to the same scale used in the link above, it would be almost impossible to show the periods of the Phanerozoic (show me the true scale).

Note that the three Eons in the Precambrian are not further subdivided like the Phanerozoic Eon. There are several reasons for this. The Precambrian interval of earth history is very difficult to study, primarily because of its tremendous age. Many of the original rocks have been destroyed by weathering and erosion, or have been greatly altered by heat and pressure during burial, a process known as metamophism. Because rocks are the basis by which geologists reconstruct earth history, destruction or alteration of the rocks complicates our ability to make reasonable interpretations of past events. Correlation of rock units -- the demonstration of lateral continuity or time equivalency -- is almost impossible. Furthermore, the diversity of life during most of the Precambrian was limited and consisted mostly of microscopic, single-celled organisms, which are difficult to find and even more difficult to study. When multicellular organisms did evolve late in the Precambrian, they were exclusively soft-bodied, lacking hard parts that have a higher likelyhood of being preserved as fossils. Thus, the Precambrian fossil record is very poor, which further hinders our ability to reconstruct the significant evolutionary events during this interval of earth history. Nervertheless, geologists have a reasonable understanding of the timing of major geologic events and significant evolutionary milestones during the Precambrian.

The Phanerozoic Eon is divided into Eras, which are further subdivided into periods (Geologic Time Scale). The three Eras, from youngest to oldest, are the Cenozoic, the Mesozoic, and the Paleozoic. The Geologic Time Scale was developed gradually during the eighteenth and nineteenth centuries by geologists working in Europe. Actually, the periods were established first, based on fossil assemblages, and then later, the Eras and Eons were established.

Fossils played an important role in the development of the Geologic Time Scale. Animals with hard parts appeared suddenly and worldwide about 570 million years ago. Mineralized skeletons are rather easily preserved and ancient life forms began to leave abundant fossilized remains in sedimentary rocks, which geologists refer to as the fossil record. This event brought the Precambrian's Proterozoic Eon to a close and opened the Phanerozoic Eon. The term Phanerozoic is derived from Greek words that mean visible life (Tarbuck and Lutgens, 1996). So sudden and spectacular is the appearance of animals with hard parts that geologists call this event the "Cambrian explosion." The Cambian is the first period of the Phanerozoic and marks the beginning of a well-preserved fossil record.

The fossil record allowed geologists of the eighteenth and nineteenth centuries to subdivide the rock record and to place Phanerozoic rocks into a relative chronological order. These early geologists had no way of knowing the absolute ages of the rocks, but they were able to work out the relative ages by relying on fossil assemblages preserved in the rocks. Geologists could do this using two fundamental principles in geology. The first is known as the principle of superposition, which states that in an undeformed sequence of sedimentary rocks, the oldest rocks are on the bottom and the youngest rocks are on top. As the early geologists examined the fossil record, they recognized that assemblages of fossils succeeded one another in time and that the succession was consistent across widely separated regions.From this recognition grew the principle of faunal succession, which states that fossil organisms succeed one another in geologic time in a regular and determinable order. Using these two principles, geologists began to subdivide the rock record. Moreover, because the principle of faunal succession is valid worldwide, geologists could also establish the age equivalency of rocks in widely seperated regions, a process known as correlation. As geologists recognized distinct assemblages of fossils that were different from those in the rocks above and below, they gave the rock body with the distinctive fossil assemblage a name. These rock bodies became known as systems. Using the principal of superposition, geologists were able to arrange the systems in chronological order, which resulted in a geologic column in which the oldest system was at the bottom and the youngest system was at the top. Thus, the geologic column also represented a relative time scale. The interval of time during which a system was deposited is known as a period. Each period in the Geologic Time Scale is named after a system that contains a unique assemblage of fossils. (Note: North American geologists recognize both a Pennsylvanian period and a Mississippian period, whereas outside North America, geologists combine these two periods into one period called the Carboniferous. Also, the terms Quaternary (0 - 1.8 million years) and Tertiary (1.8 - 66 million years) were originally used to designate the two periods of the Cenozoic Era, but these are now being replaced by the terms Neogene (0 - 24 million years) and Paleogene (24 - 66 million years.))

Epochs are still smaller units of geologic time. The Cenozoic periods are divided into seven epochs that have formal names (show me the Cenozoic epochs). Each of the other periods is divided into three epochs that are informally refered to as early, middle, and late.

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