the magma that crystallizes to pegmatite, a rock type commonly enriched in incompatible elements. Bowen's reaction series is important for understanding the idealised sequence of fractional crystallisation of a magma. Magma composition can be determined by processes other than partial melting and fractional crystallization. For instance, magmas commonly interact with rocks they intrude, both by melting those rocks and by reacting with them. Assimilation near the roof of a magma chamber and fractional crystallization near its base can even take place simultaneously. Magmas of different compositions can mix with one another. In rare cases, melts can separate into two immiscible melts of contrasting compositions. Primary magmas When rock melts, the liquid is a primary magma. Primary magmas have not undergone any differentiation and represent the starting composition of a magma. In practice, it is difficult to unambiguously identify primary magmas, though it has been suggested that boninite is a variety of andesite crystallized from a primary magma. The Great Dyke of Zimbabwe has also been interpreted as rock crystallized from a primary magma. The interpretation of leucosomes of migmatites as primary magmas is contradicted by zircon data, which suggests leucosomes are a residue (a cumulate rock) left by extraction of a primary magma. Parental magma When it is impossible to find the primitive or primary magma composition, it is often useful to attempt to identify a parental magma. A parental magma is a magma composition from which the observed range of magma chemistries has been derived by the processes of igneous differentiation. It need not be a primitive melt. For instance, a series of basalt flows are assumed to be related to one another. A composition from which they could reasonably be produced by fractional crystallization is termed a parental magma. Fractional crystallization models would be produced to test the hypothesis that they share a common parental magma. Migration and solidification Magma develops within the mantle or crust where the temperature and pressure conditions favor the molten state. After its formation, magma buoyantly rises toward the Earth's surface, due to its lower density than the source rock. As it migrates through the crust, magma may collect and reside in magma chambers (though recent work suggests that magma may be stored in trans-crustal crystal-rich mush zones rather than dominantly liquid magma chambers ). Magma can remain in a chamber until it either cools and crystallizes to form intrusive rock, it erupts