Eras are the largest units of geological time. Era are The PreCambrian, The Paleozoic, Mesazoic , and Centazoic. Each Era is divided into periods such as the Cambrian being part of the Paleozoic Era.
The word "into" denotes that an object undergoes a change of state.
Geologists take eras, which are large portions of time, and split them up. The smaller portions are called periods.
So, eras are subdivided into periods.
The word "into" shows that eras become periods once geologists subdivide them.