When a melt undergoes cooling along the liquid line of descent, the results are limited to the production of a homogeneous solid body of intrusive rock, with uniform mineralogy and composition, or a partially differentiated cumulate mass with layers, compositional zones and so on. This behaviour is fairly predictable and easy enough to prove with geochemical investigations. In such cases, a magma chamber will form a close approximation of the ideal Bowen's reaction series. However, most magmatic systems are polyphase events, with several pulses of magmatism. In such a case, the liquid line of descent is interrupted by the injection of a fresh batch of hot, undifferentiated magma. This can cause extreme fractional crystallisation because of three main effects:
* Additional heat provides additional energy to allow more vigorous convection, allows resorption of existing mineral phases back into the melt, and can cause a higher-temperature form of a mineral or other higher-temperature minerals to begin precipitating
* Fresh magma changes the composition of the melt, changing the chemistry of the phases which are being precipitated. For instance, plagioclase conforms to the liquid line of descent by forming initial anorthite which, if removed, changes the equilibrium mineral composition to oligoclase or albite. Replenishment of the magma can see this trend reversed, so that more anorthite is precipitated atop cumulate layers of albite.
* Fresh magma destabilises minerals which are precipitating as solid solution series or on a eutectic; a change in composition and temperature can cause extremely rapid crystallisation of certain mineral phases which are undergoing a eutectic crystallisation phase.