Geological Timescale

Geological Timescale

The geological timescale is a 'calendar' of events in the Earth's history. It shows major geological and climactic events, and how these events affected the emergence and disappearance of species over time. In order to help us make sense of this vast amount of time, scientists divided it into smaller units of time. In descending order, these units are: eons, eras and periods.

• An Eon: is defined as a unit of time equal to a billion years.

• An era: is a division of time within an eon but does not have a fixed number of years. The Mesozoic era for example lasted from 252 million years to 66 million years ago.

• A period: typically refers to a subdivision of an era and its length is determined by a system of dating based on examining fossil evidence belonging to a particular era.

The figure below shows one method of representing the geological timescale:

Geological timescale with key events and characteristics shown.

While you will not be expected to remember the names of specific periods, you will be expected to understand the eras and their particular characteristics.

Another visually powerful way of representing Earth's history is via the use of the geological `clock' (see figure below). Human history occupies just 2 million of the \(\text{4 500}\) million year long history of the Earth. On this clock, human existence constitutes less than a minute of the evolutionary history of life on Earth.

"Human history on Earth is a mere second on the clock". In this representation, the two million years' of human history constitute an effect too small to be visible on the timescale.