Microstructures of Igneous Rocks

Introduction This chapter is concerned with the shapes of crystals and aggregates formed when melted rocks and glass crystallize, solid rocks melt, and melts boil. These processes are broadly grouped together as ‘magmatic (igneous) processes’. As discussed in introductory geology courses, partial melting of solid rock to form magm a occurs in Earth's mantle, producing mainly mafic (basaltic, silicapoor) magma, and also in the deeper parts of Earth's crust, producing mainly felsic (granitic, silica-rich) magma. Magma may reach the surface as volcanic rocks or solidify in the crust as intrusive rocks. Magma bodies may encounter other magma bodies and mix or mingle with them. Microstructures may preserve evidence of these and other processes, such as magmatic flow and conditions of crystallization, and in this chapter I present this evidence and discuss its reliability. When a liquid becomes supersaturated with a dissolved chemical component (i.e. the dissolved component reaches a concentration atwhich the liquid can hold no more of the component in solution), a new phase theoretically is produced. A phase is a chemically and physically homogeneous part of a system (e.g. a body of magma) that is bound by an interface with other phases (e.g. the melt phase and other crystal phases in a magma).