How did geologists determine the boundaries between divisions of the geologic time scale? They utilized the Law of Superposition and significant events in Earth's life history, such as mass extinctions. The divisions were not equal; rather, they were based on the distribution of fossils in Earth's strata.

Geologists determined the boundaries between the divisions of the geologic time scale by studying rock layers and their contents. One fundamental principle they used is the Law of Superposition, which states that in an undisturbed sequence of rock layers, the oldest layers are on the bottom and the younger layers are on the top. This principle helps scientists understand the relative ages of rock layers.

Additionally, geologists paid close attention to significant geological and biological events in Earth's history, such as mass extinctions, the appearance of new organisms, and major climatic and environmental changes. These events often leave a distinct signature in the rock record, such as a sudden change in the types of fossils found, or physical changes in the rocks themselves, which can result from a variety of causes including volcanic eruptions or meteorite impacts.

As geologists analyzed the fossil distribution within layers of the Earth's crust (strata), they could identify distinct packages of rock that shared common characteristics, such as similar fossil assemblages, and were separated by these significant events. They used these characteristics to establish the boundaries of the different geologic time periods, eras, and epochs.

It's important to understand that the divisions of the geologic time scale are not equal in length. They vary widely, with some periods being relatively short in geological terms (tens of millions of years) and others being very long (hundreds of millions of years). The scale is structured in a way that reflects significant changes in the Earth system rather than a consistent measure of time.

Extra: The geologic time scale is like a calendar for Earth's history, divided into eons, eras, periods, epochs, and ages. This hierarchical structure helps scientists communicate about the history of the planet and facilitates understanding of the sequence and timing of events that shaped our planet.

An important concept to understand is that the geologic time scale is a result of complex scientific research, which involves several disciplines like paleontology (study of fossils), stratigraphy (study of rock layers), and geochronology (study of the age of rocks). Fossils play a key role in dating rocks because they change over time due to evolution, so certain fossils are characteristic of particular time periods.

Another related concept is the use of radiometric dating techniques that allow geologists to determine the absolute age of rock samples. These techniques rely on the decay of radioactive isotopes within minerals, providing a “clock” that starts when the rock forms.

Finally, it’s important to note that the work of establishing the geologic time scale is ongoing, as new discoveries and methods continue to refine our understanding of Earth's ancient past. This scale is an example of how science is iterative and continuously evolving with new information.