Layered intrusions form mostly due to

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

Answer: (A)

Layered intrusions form mainly because dense crystals settle out of the crystallizing magma and accumulate as layers at the chamber floor. As the melt begins to crystallize, early-formed minerals (often olivine and pyroxene-rich) are denser than the remaining melt, so they sink under gravity and pile up, creating a vertical sequence of distinct layers with different mineral compositions. This physical separation by settling produces the characteristic layered structure.

Partial melting would generate magma rather than layers. Magma mixing tends to homogenize composition and erase layering. Fractional crystallization with cumulate segregation is related to how the melt becomes differentiated as crystals are removed, but the defining feature of the layered texture is the direct gravitational settling of crystals into cumulates.