St helens dating
The general idea is that many different minerals are formed, which differ from one another in composition, even though they come from the same magma.
The following quote from The Earth: An Introduction to Physical Geology by Tarbuck & Lutgens, pp.
55-57, (1987), gives us an idea of the tremendous complexity of the processes that occur when magma solidifies.
But at the same time, it will be enriched in the elements contained in the later forming minerals, namely sodium and potassium.
Further, the silicon content of the melt becomes enriched toward the latter stages of crystallization.
As the crystallization process continues, the composition of the melt (liquid portion of a magma, excluding any solid material continually changes.