Learning Objectives

1. Metamorphism changes the texture or mineralogy (or both) of its parent rock usually in response to high temperature and pressure within the earth's interior and under conditions that produce ductile (plastic) strain.

2. The chemical composition of the parent rock controls that of the metamorphic rock, although mineralogy may change. Minerals are stable within a particular temperature range, but that range varies with pressure and presence of other substances. The upper limits of metamorphism may overlap partial melting.

3. New metamorphic minerals crystallize under high confining pressure and tend to be denser than their low-pressure counterparts. Differential stress (compression and/or shearing) produces foliated textures described as slaty, schistose, or gneissic. Water triggers metamorphism and promotes new mineral formation. Metamorphism is a slow process taking place over millions of years.

4. The classification of metamorphic rocks is based on texture (foliated versus nonfoliated) and mineralogy (chemical composition is controlled by parent rock).

5. Metamorphism is either contact (high temperatures but low confining pressure) or regional (high-temperatures and high confining pressure). Metamorphic rocks produced in a contact aureole ("baked zone" of Chapter 3) include hornfels (shale parent rock), marble (limestone parent rock), and quartzite (quartz sandstone parent rock). All of these rocks have nonfoliated textures.

6. Regional metamorphism usually produces foliated rocks such as greenschist (basalt parent rock) and amphibole schist (basalt parent rock), although marble and quartzite also form, if their appropriate parent is present. Progressive metamorphism of shale can produce slate, phyllite, schist or gneiss as temperature and pressure increase. Partial melting produces migmatites. Retrogressive metamorphism reflect the effects of water movement that allows recrystallization under conditions below the peak of metamorphism.

7. Plate tectonics, particularly subduction, explain differential stress and temperature variations, which increase toward the continents because of rising magma from melting at depth.

8. Water plays an important role in metamorphism. Metasomatism involves hot water transporting ions from outside the rock, and form significant ore deposits. Hydrothermal rocks are formed by crystallization from hot water, most commonly quartz veins and disseminated ore deposits.

9. The water involved in metamorphism originates as either ground water or water trapped in descending oceanic crust in a subduction zone.