1.4 Why Study Petrology? Igneous Rocks

We know that some of the rocks formed within and on the earth have crystallized from hot liquid lava. Formation of these igneous rocks, for example basalt, can be observed at volcanoes. And the liquid lava can be reproduced by heating the solid basalt in a laboratory until it melts. Other rocks formed within the earth, for example granite, display textures and field relations that can be interpreted to show that they were once liquid. But no volcanic rocks have been found that look like granite. In the laboratory, however, granite can be melted and crystallized again under pressure to mimic the original features. Therefore, geologists interpret rocks with certain mineralogy, textures, and/or field relations to be igneous rocks, rocks that have been crystalized from magma (molten rock).

Igneous petrology concerns all the processes involved in the evolution of igneous rocks from melting to crystallization. Important questions about igneous rocks include: What rock melted? Where and when did the melting occur? What happened to the magma after melting, before and during crystallization? When and where did the crystallization occur? Because in most cases petrologists cannot observe these things directly, they must work out the history of an igneous rock by studying it in the field and in the lab. Field relations, hand sample and microscopic textures, whole rock and mineral chemistry are the principle clues to that history. This information is evaluated in the context of the results of experiments simulating igneous processes in the laboratory. Ultimately, answers to the important questions are proposed that satisfy both the rock observations and constraints obtained thorugh laboratory experiments.

To wisely use the information provided by studies of igneous rocks, geologists need to understand some important features about melting and crystallization of magmas that can be explained using equilibrium phase diagrams to represent experimental results. The nature of chemical equilibrium between a solid solution mineral like olivine or plagioclase and magmas of different chemical compositions is also important to consider. Because the crystals that grow from a magma have chemical compositions different from the magma composition, the possible effects of separating crystals from the magma by sinking or floating (fractional crystallization) on the resulting rocks should be understood. Similarly, because the liquids that form as a rock is melting have different chemical compositions than the rock, the effect of completely separating the liquid from a partially melted rock can lead to a range of rock types from the same source.

Igneous rocks collected at the surface of the earth provide some of the best evidence for the workings of the earth beneath the surface, for the chemistry and distribution of rock types, and for the variation of physical conditions. Every geologist who wants to understand those workings needs to undestand and appreciate the petrology of igneous rocks.