Layered intrusives may result from

• A. Fractional crystallization
• B. Crystal nucleation
• C. Assimilation
• D. Crystal settling

Answer: (D)

Layered intrusions form when crystals separate from the melt by gravity and accumulate on the chamber floor, creating stacked layers of different minerals (a cumulate texture). As magma cools, dense early-forming crystals such as olivine and pyroxene settle downward, while lighter minerals may crystallize later and form higher layers. This gravity-driven settling builds the distinct stratified structure seen in many large igneous bodies. While fractional crystallization describes chemical changes as crystals are removed from the melt, the pronounced layering in these intrusions specifically comes from the physical sorting of crystals by settling. Crystal nucleation is just the initial formation of crystals, and assimilation involves adding surrounding rock to the melt, which tends to disturb rather than produce layered structures.