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Guide to Reading
In any study of rocks, metamorphic rocks always come last. This is logical; some other rock (igneous, sedimentary, or even a different metamorphic rock) must exist first to get changed into a metamorphic rock. There are limits to the changes that may occur and still yield metamorphic rock. An important one is that no matter how extreme the temperatures or pressure involved, the rock undergoing change must remain essentially a solid. If it were to be broken down into sediment or changed to a melt, you’d be back in the realms of sedimentary or igneous rocks.
Once a basic definition of metamorphic rock is established, the chapter examines the causes of metamorphism and the features associated with different types of metamorphism. The causes, known as agents of metamorphism, are heat, pressure, differential stress, and hydrothermal fluids. The processes of metamorphism are recrystallization, phase change, neocrystallization, pressure solution, and plastic deformation. The specific changes that occur depend on many factors, including the chemical/mineralogical composition of the protolith (original rock), the metamorphic agent or agents at work, the metamorphic processes they have set in motion, and the environment where it’s all taking place. You’ll look at phase diagrams and stability fields and read about compositional banding (gneissic layering), metasomatism, veins, normal stress (compression and tension), shear stress, equant and inequant grains, preferred mineral orientation, and supercritical fluid. It does get a bit complex. Keep in mind the basics: existing rock gets subjected to extreme conditions in its environment and gets altered in various ways and to varying degrees, but it never stops being a solid rock.
Classification of metamorphic rock comes next. It’s not very complex; there are only two fundamental divisions: foliated and nonfoliated. Common foliated rocks (which exhibit a layered look) that the author describes are slate, phyllite, schist, flattened-clast conglomerate (meta-conglomerate), and the “hybrid rock” (part igneous, part metamorphic) migmatite. Common nonfoliated rocks described (which don’t have the layered look because they have neither preferred mineral orientation nor compositional banding) are hornfels, amphibolite, quartzite, and marble. Nature often manages to defy rigid classification. As an example of this, the author points out the existence of two rocks that exhibit contradicting characteristics: foliated quartzite and foliated marble.
The significance of the existence of one kind of metamorphic rock instead of another kind is addressed in the chapter section that describes the intensity of metamorphism. Numerous factors are relevant to the issue, including what metamorphic grades are achieved (low, intermediate, or high), and what metamorphic zones, index minerals, mineral assemblages, and metamorphic facies are produced.
Just when you start to feel you have a firm grip on what metamorphism is all about, you learn that a seemingly backward version of everything you’ve just been taught can occur. Retrograde metamorphism can happen to rocks under conditions of decreasing temperatures and pressures, in direct contrast to the usual prograde metamorphism associated with increasing temperatures and pressures.
The chapter concludes with a discussion of locations where you can find metamorphic rocks (environments of metamorphism). These include areas
- adjacent to the intrusion of plutons (contact or thermal metamorphism and contact aureoles)
- in subsiding sedimentary basins (burial metamorphism)
- in fault zones (dynamic metamorphism and mylonites)
- beneath mountains adjacent to subducting plates or between colliding plates (dynamothermal or regional metamorphism)
- at mid-ocean ridges (hydrothermal metamorphism)
- in subduction zones (blueschist facies and glaucophane)
- at impact sites (shock metamorphism)
- on continental shields
- in the mantle
If many of the above environments remind you of the plate tectonics chapters, that’s good! As you were told early in the text, in any geologic discussion today it’s difficult to avoid plate tectonics, because it so often provides the basic answer to the question, “Why does that happen?”
The first eight chapters presented the infrastructure of Earth, what it is, and how it got established. If you don’t have a firm grasp of the concepts presented so far, you may want to review these fundamental chapters, as future chapters build on this material. In the remaining chapters, you will learn about activities that occur within this infrastructure, beginning with some very dramatic action: volcanic activity. |
