Scientists use significant geological events to mark boundaries in the geologic time scale. These events often cause drastic changes in the Earth's history and are often reflected in the fossil record. Here are some examples:

* Mass extinction events: These events, like the Permian-Triassic extinction, mark the disappearance of a large percentage of species on Earth.

* Changes in Earth's climate: Significant shifts in temperature, ice cover, and sea levels leave traces in rock formations and fossils.

* Major geological events: Events like mountain building, supercontinent formation, and volcanic eruptions have dramatic impacts on the Earth's surface and leave identifiable geological signatures.

* Appearance or disappearance of specific fossils: The evolution and extinction of specific organisms leave clear traces in the fossil record, indicating significant changes in life on Earth.

These events are used to define the major divisions in the geologic time scale, such as eons, eras, periods, and epochs. By studying these events and the geological record, scientists can better understand the history of the Earth and its inhabitants.