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As the crystallization process continues, the composition of the melt (liquid portion of a magma, excluding any solid material continually changes.
For example, at the stage when about 50 percent of the magma has solidified, the melt will be greatly depleted in iron, magnesium, and calcium, because these elements are found in the earliest formed minerals.
This left branch is called a discontinuous reaction series because each mineral has a different crystalline structure.
Recall that olivine is composed of a single tetrahedra and that the other minerals in this sequence are composed of single chains, double chains, and sheet structures, respectively.
Lava erupting earlier would come from the top of the magma chamber, and lava erupting later would come from lower down.