Why are time periods in the geologic time scale are not even in length?

Question

Responses

They are based on the thickness of the Earth's rock layers.
They are based on the thickness of the Earth's rock layers.

They are based on a different type of calendar used to track Earth’s history.
They are based on a different type of calendar used to track Earth’s history.

They are based on the type of fossils found in rock layers of the Earth.
They are based on the type of fossils found in rock layers of the Earth.

They are based on important events happened in Earth’s history.
They are based on important events happened in Earth’s history.