There are some terms associated with the geologic time scale that are not “officially" recognized as part of it. The best known, and most common, example is Precambrian-the informal name for the eons that came before the current Phanerozoic eon. Although the term Precambrian has no formal status on the geologic time scale, it has been traditionally used as though it did. Hadean is another informal term that is found on some versions of the geologic time scale and is used by some geologists. It refers to the earliest interval (eon) of Earth history-before the oldest known rocks. When the term was coined in 1972, the age of Earth's oldest rocks was about 3.8 billion years. Today, that number stands at slightly greater than 4 billion and, of course, is subject to revision. The name Hadean derives from Hades, Greek for underworld a reference to the "hellish" conditions that pre- vailed on Earth early in its history. Effective communication in the geosciences requires that the geologic time scale consist of standardized divisions and dates. So, who determines which names and dates on the geologic time scale are official ? The organization that largely responsible for maintaining and updating this important information is the International commission on stratigraphy (ICs), which is associated with the International Union of Geological Sciences. Advances in the geosciences require that the scale be periodically updated to include changes in unit names and boundary age estimates. For example, the geologic time scale shown in Figure 11.19 was updated as recently as July 2009. After considerable dialogue among geologists who focus on very recent Earth history, the ICS changed the date for the start of the Quaternary period and the Pleistocene epoch from 1.8 million to 2.6 million years ago. perhaps by the time you read this other changes will have been made. If you were to examine a geologic time scale from just a few years ago, it is quite possible that you would see the Cenozoic era divided into the Tertiary and Quaternary periods. However, on more recent versions the space formerly designated as Tertiary is divided into the Paleogene and Neogene periods. As our understanding of this time span changed, so too did its designation on the geologic time scale. Today, the Tertiary period is considered a "historic" name and is given no official status on the ICS version of the time scale. Many time scales still contain references to the Tertiary period, including Figure 11.19. One reason for this is that a great deal of past (and some current) geological literature uses this name. For those who study historical geology, it is important to realize that the geologic time scale is a dynamic tool that continues to be refined as our knowledge and understanding of Earth history evolves.