In a magma body, heat loss occurs primarily along the margin. With few exceptions, field evidence and physical models have suggested that most crystallization takes place along the margin during a certain period. In the interior of the body, the melt is generally hotter and chemically less evolved; crystals that settle from the upper part are likely to react with the melt. The high-silica ash flow tuffs erupted instantaneously from zoned magma bodies provide evidence that crystals have sunk and reacted with the surrounding melts. This paper develops a heat and mass transfer model for crystal settling and reequilibration to quantify the trace element behavior in such magma bodies. Previous crystallization models such as fractional and equilibrium crystallization have assumed that a melt with a single composition evolves along a single liquid line of descent with increasing degree of crystallization. In the present model, however, melts of various compositions exist within the body at a given time. In addition, the liquid line of descent itself changes with time. A unique and important feature of the present model is this temporal variation of liquid lines of descent. The chemical evolution expected from crystal settling and reequilibration is consistent with the data from the Bishop Tuff high-silica magma body and might account for some characteristic patterns of chemical zonation observed in high-silica magma bodies.