Answer:
The boundaries between eons and eras are marked by huge geological events which can be seen in the fossil record. These events are often associated with mass extinctions, major changes in the Earth's climate or geology, or other significant shifts in the evolutionary history of life on Earth. For example, the boundary between the Precambrian eon and the Phanerozoic eon is marked by the appearance of hard-shelled organisms in the fossil record, while the boundary between the Mesozoic era and the Cenozoic era is marked by the mass extinction of the dinosaurs.
Step-by-step explanation:
The geological time scale is divided into several major divisions, including eons, eras, periods, and epochs. These divisions are based on major events in the history of the Earth, such as the appearance or extinction of particular species, changes in climate, or major geological events like volcanic eruptions or asteroid impacts.
The boundaries between eons and eras are particularly significant because they represent major shifts in the evolutionary history of life on Earth. These shifts are often marked by mass extinctions, which occur when a large proportion of the Earth's species go extinct in a relatively short period of time. These extinctions can be caused by a variety of factors, such as changes in climate, geological activity, or the impact of a large asteroid or comet.
The fossil record provides evidence of these major events, allowing scientists to piece together the history of life on Earth and understand how different species have evolved and interacted with each other over time. By studying the fossil record and other geological evidence, scientists can gain insights into the processes that have shaped the Earth and its inhabitants over billions of years.