zoning

geology

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July 15, 2026

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In geology, "zoning" refers to arrangements of rock units with contrasting composition or texture within an igneous body, often in a concentric pattern. A common example is chilled margins, which are fine-grained or glassy edges formed when magma cools rapidly against surrounding rock. Other zones can indicate fractional crystallization, helping geologists understand magma differentiation.1

Stratification, on the other hand, is the layering found in most sedimentary rocks and some igneous rocks formed at the surface, like lava flows. These layers, called strata, can vary greatly in thickness and extent. Stratification occurs due to changes in texture, composition, or pauses in deposition. These layers preserve a record of Earth's geological events and can help locate valuable resources like minerals, petroleum, and groundwater.2

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playa
Saline minerals
…of a salt pan are zoned like bathtub rings, with less-soluble sulfates and carbonates at the outer margin and highly soluble sodium chloride (table salt) at the centre. The crystallization of these salts can be compared with the evaporation of brine in a dish. The first precipitates from the evaporating…
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…of a salt pan are zoned like bathtub rings, with less-soluble sulfates and carbonates at the outer margin and highly soluble sodium chloride (table salt) at the centre. The crystallization of these salts can be compared with the evaporation of brine in a dish. The first precipitates from the evaporating…
Read More

feldspar
Chemical composition
…those in many rocks are zoned—i.e., different parts of individual grains have different Ab and An contents. One explanation for zoning in plagioclases formed from magmas can be implied from information known about the Ab-An system. Upon cooling, the first crystals that form from a melt with the composition X…
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…those in many rocks are zoned—i.e., different parts of individual grains have different Ab and An contents. One explanation for zoning in plagioclases formed from magmas can be implied from information known about the Ab-An system. Upon cooling, the first crystals that form from a melt with the composition X…
Read More

igneous rock
Important textural types
Finally, crystal zoning describes faintly to very well-defined geometric arrangements of portions within individual crystals that differ significantly in composition (or some other property) from adjacent portions; most common are successive shells grouped concentrically about the centres of crystals, presumably reflecting shifts in conditions during crystal growth.
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Finally, crystal zoning describes faintly to very well-defined geometric arrangements of portions within individual crystals that differ significantly in composition (or some other property) from adjacent portions; most common are successive shells grouped concentrically about the centres of crystals, presumably reflecting shifts in conditions during crystal growth.
Read More