In igneous petrology, layered intrusions are most commonly attributed to which process?

• A. Partial melting
• B. Magma mixing
• C. Crystal settling
• D. Fractional crystallization with cumulative segregation

Answer: (C)

Layered intrusions form mainly through the physical separation of crystals from the melt as a crystallizing magma cools and stiffens in a chamber. Early-formed, dense minerals like olivine and pyroxene are heavier than the liquid and begin to settle toward the chamber floor, building up successive layers as more crystals crystallize and settle on top. This gravity-driven crystal settling creates the characteristic stacked bands of different mineral compositions seen in layered intrusions, known as cumulates.

Partial melting would supply melt rather than layered solid, and magma mixing tends to produce mingled textures rather than orderly, discrete layers. Fractional crystallization with cumulative segregation is related, but the most direct and widely observed mechanism for the distinct, vertically arranged layers is the settling of crystals from the crystallizing melt.