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A body of igneous rock crystallized from magma emplaced beneath the surface. Granite is a common plutonic composition. A pluton's gravity anomaly depends on its geophysical density contrast and geometry, rather than on the rock name alone.

With equal solid and liquid densities, magma entering a dike at constant flux $Q_0$ has $bw=Q_0$: melt added by the walls exactly supplies the increasing cross-sectional storage. If wall heat flux is ${\alpha }_T\rho c_{p}w(T-T_m)$, both walls give $b_t=2{\alpha }_T{c}_{p}w(T-T_m)/L$. Combining this with sensible-heat conservation admits $b=B(t)f(\eta )$, $\eta =2{\alpha }_{T}z/B$, $B^2=4{\alpha }_T{Q}_{0}t/S$. The shape equations are $f(f-\eta f^{\prime })=\vartheta$ and $(S-\eta f){\vartheta }^{\prime }=-(S+\vartheta )\vartheta /f$.