According to Anguttara ii, 142, there are four periods called incalculable epochs (asankheyya-kappa) within a great aeon or world cycle (maha-kappa).
The duration of each of these epochs cannot be enumerated even by taking hundreds of thousands (lakhs) of years as a unit, hence the name “incalculable aeon”.
These four incalculable epochs are:
(i) Enveloping Epoch – period of destruction or dissolution of the world system.
In the Sun Discourse (Anguttara iv, 99), the Buddha described the destruction of the world by fire that even reaches the realm of Great Brahma.
It commences with the falling of the great rain and terminates with the extinction of flames if the world system is to be dissolved by fire;
or the receding of floods if dissolved by water;
or the cessation of storms if dissolved by air.
An elaborate description of the dissolution of the world by fire, water or the air element is given in the chapter on the recollection of past life in the Visuddhi Magga or Path of Liberation.
(ii) Enveloped Epoch – period when the world system is completely destroyed or in a state of void.
This is the period beginning from the moment of dissolution of the world by fire, water or the air element till the falling of the great rain that heralds the evolution of a new world.
(iii) Developing Epoch – period of evolution. This is the period beginning from the falling of the great rain that heralds the evolution of a new world to the appearance of the sun, moon, stars and planets.
(iv) Developed Epoch – period of continuance after having been reinstated.
This is the period beginning from the appearance of the sun, moon, stars and planets to the falling of the great rain that heralds the dissolution of the world.