What are the time divisions that are used for the geologic time scale and what determineshow they are divided?
A quick Google search found this site.
http://en.wikipedia.org/wiki/Geologic_time_scale
The geologic time scale is a system used by scientists to divide the Earth's history into different time intervals based on major geological and biological events. The time scale is divided into several hierarchical divisions. The main divisions, from largest to smallest, are eons, eras, periods, epochs, and ages.
The divisions in the geologic time scale are primarily determined by significant changes that occurred in the Earth's environment, climate, and life forms over time. These changes are typically associated with major geological events, such as mass extinctions, the formation of new continents, and the evolution of new species. Scientists study rock layers and fossils to help determine when these events took place.
The largest division of time is the eon, which represents the longest periods of Earth's history. The current eon is the Phanerozoic, characterized by the presence of abundant fossil records. The next division is the era, which represents a significant chunk of time within an eon. For example, the Phanerozoic eon is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic.
Each era is further divided into periods, which represent even smaller increments of time. The Paleozoic era, for example, is divided into several periods, including the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. The periods are then subdivided into epochs and ages, representing progressively smaller time intervals.
The exact duration of each division in the geologic time scale is not always precisely defined, as our understanding of Earth's history is continually evolving. As new evidence emerges, scientists may revise the boundaries and durations of these divisions to provide a more accurate representation of geologic time.