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Foliation develops when minerals flatten out or line up in bands. At low levels of metamorphism, the bands are extremely thin, as in slate. With higher pressure and temperature, the mineral mica can grow and make the rock look shiny, as is common in phyllite and schist. At even higher levels of metamorphism, the minerals in the rock tend to separate into light and dark bands, like those in gneiss. How do rocks change as foliation develops? Nonfoliated Rocks Metamorphic rocks that do not show foliation are called nonfoliated rocks. One reason a metamorphic rock may not display foliation is that it is made up mainly of one type of mineral, so that different minerals cannot separate and line up in layers. One common nonfoliated metamorphic rock is marble, which develops from limestone. Marble is used as a decorative stone. It is good for carving and sculpting. Because marble is nonfoliated, it does not split into layers as an artist is working with it. Another example of a nonfoliated rock is quartzite. It forms from sandstone that is made up almost entirely of pieces of quartz. Another reason that a metamorphic rock may lack foliation is that it has not been subjected to high pressure. Hornfels is a metamorphic rock that can form when a rock is subjected to high temperatures. Hornfels, which often forms when magma or lava touches other rock, is nonfoliated. What are two reasons a metamorphic rock might not show foliation? KEY CONCEPTS CRITICAL THINKING 1. What conditions can cause a sedimentary or igneous rock to change into a metamorphic rock? 4. Draw Conclusions Would gneiss be more likely to form at shallow depths or at great depths where mountains are being pushed up? Why? 2. How do new minerals grow within existing rocks? 3. Why do bands of minerals develop in most metamorphic rocks? 5. Infer Would you expect to find foliated or nonfoliated metamorphic rocks next to a lava flow? Why? CHALLENGE 6. Synthesize What features of sedimentary rocks are unlikely to be found in metamorphic rocks? What features of metamorphic rocks do not occur in sedimentary rocks? Chapter 3: Rocks 101