The largest expanse of time on the geologic scale is the Eon.

Here's how the geologic timescale breaks down:

* Eon: The largest division of geologic time. Examples include the Precambrian Eon (the earliest part of Earth's history) and the Phanerozoic Eon (the current eon, which includes the Paleozoic, Mesozoic, and Cenozoic eras).

* Era: A subdivision of an eon. For example, the Phanerozoic Eon is divided into the Paleozoic, Mesozoic, and Cenozoic eras.

* Period: A subdivision of an era. For example, the Mesozoic Era is divided into the Triassic, Jurassic, and Cretaceous periods.

* Epoch: A subdivision of a period. For example, the Paleogene period of the Cenozoic era is divided into the Paleocene, Eocene, and Oligocene epochs.

So, while eras, periods, and epochs are important subdivisions of geological time, the eon represents the largest and most encompassing division.