layers of the earth
Circles within circles, or better spheres with in spheres. Concentric spheres is a good phrasing. A boiled egg has this form and, when sliced in half, it will illustrate this relationship clearly.
"The layers of the earth are like a peanut M&M. There is a thin crust that can change, a melting middle layer and a hard center core with a inner layer and this outer layer just like a peanut.
METAMIA is a free database of analogy and metaphor. Anyone can contribute or search. The subject matter can be anything. Science is popular, but poetry is encouraged. The goal is to integrate our fluid muses with the stark literalism of a relational database. Metamia is like a girdle for your muses, a cognitive girdle.