zonal structure

Zonal structures in geology refer to arrangements of rock units within an igneous body that have contrasting compositions or textures, often in a concentric pattern. These zones can indicate changes that occurred during the rock's formation and growth.12

Examples include:
* Chilled margins: Fine-grained or glassy edges formed when magma cools rapidly against surrounding rock.1
* Orbicular configurations: Alternating light and dark bands in an oval shape found in some diorites and granodiorites.1
* Pegmatites: These can exhibit zonal structures due to variations in fluid composition, sometimes containing unique minerals.1

In the context of Earth's layers, the lithosphere (the rigid outer layer) rests on the asthenosphere, a hotter, more fluid layer of the mantle. This boundary is a zone where tectonic plates move.345