Basalt lava and Crust

Figure 00. Ice cubes in a glass of water. A cool, familiar drink, but not a good analog for silic magmas.

Saturation Diagrams

3.1 Overview

By definition, igneous rocks have solidified from a magma, a molten (meaning hot) silicate liquid that may have crystals in it. Creating igneous rocks involves melting of rocks (to make the magma), and crystallization of minerals (to solidify the magma). This article examines melting and crystallization of familiar materials (e.g. water, ice, halite) and uses equilibrium diagrams and the concept of saturation to reveal important features of these processes. Concepts and tools that are discussed include:
  • phase diagrams
  • saturation curves
  • the lever rule
  • eutectic points
3.2 Freezing Water vs. Solidifying Magma

When most people are asked to tell what they know about melting and freezing, they will talk about ice and water. This makes sense because melting of ice and freezing of water are processes people see often, even in tropical climates because of freezers and ice cubes. There are similarities between melting and freezing of pure H2O and melting and freezing of rocks. In both cases, raising the temperature leads to melting and lowering the temperature leads to solidification. And in both cases, energy is consumed during melting (melting is endothermic) and energy is released during solidification (solidification is exothermic), although the temperatures are different. But overall, pure H2O alone is not a good analog for thinking about the melting and crystallization of rocks because natural magmas have a range of chemical compositions and they form more than one type of mineral when they crystallize.

You can see an example of the mineralogical differences betweensolidified H2O and solidified basalt in the following thin section views (Figure 01 and Figure 02):
Ice Thin Section PPL

Figure 01. Glacial ice core thin section in plane polarized light (PPL). The core is approximately 9 cm across. Individual ice crystals are visible only in the crossed polarized light (XPL) view, which you can see by clicking on the image.

Basalt Thin Section  PPL

Figure 02. Basalt thin section in plane polarized light (PPL). Notice that it consists of several minerals: plagioclase laths (clear), clinopyroxene (beige), olivine (clear but equant), opaque minerals (magnetite and ilmenite), and "mesostasis" (partially devitrified glass). Click on the image to see a larger version with the option of a cross-polarized light (XPL) view.

The solified H2O on a frozen lake or in a glacier, such as the sample from Antarctica in Figure 01, is a rock made of only one mineral (ice) with one chemical composition (H2O). The solidified magma (basalt) in Figure 02 is a rock made of several minerals (plagioclase, augite, olivine, magnetite), each with its own unique chemical composition. The fact that magmas solidify to several minerals, each with a chemical composition different than the magma's composition, makes the solidification of magmas qualitatively different than the freezing of pure water.

To see some of the effects of compositional variation, let's examine a familiar liquid only slightly more complicated than pure H2O by adding salt (NaCl). What would happen if you added the mineral halite (NaCl) to a glass filled with water and stirred the mixture? Please make a prediction and press "Enter":

When added to water, the halite crystals will: