Fractional Crystallization

The chemical composition of igneous rocks from many different igneous provinces displays systematic variation. When the chemical compositions of the rocks are plotted in a diagram, it becomes evident that they apparently define either linear or curved trends. Comparisons of trends for similar rock types have shown that the trends for different provinces are quite similar, neither age nor geographic origin seem to have a major influence on the character of the trends. This consistency suggests that the compositions of igneous rocks are controlled by specific processes that operate under certain circumstances.