Name:______________________________________Date:_______Pd:_____
METAMORPHIC ROCKS
The third classification of rock—metamorphic—includes igneous or sedimentary rocks that have been transformed
deep within Earth's crust. Preexisting metamorphic rock can also be altered to create new forms of metamorphic
rock. Whatever the original material, some traces of its structure are usually preserved in the resulting metamorphic
rock.
Formation. One of the most important factors in metamorphism—the formation of metamorphic rock—is pressure.
It may be applied by the weight of overlying sedimentary beds; it may be caused by magma making its way into
surrounding rock layers; or it may be due to the mountain-building forces that deform Earth's crust. These colossal
pressures generate tremendous heat that quickens and heightens various chemical reactions within the rock. The
presence of water is another factor in metamorphism. The water may be so scanty that it forms a mere film around
the particles. Yet it provides a medium in which rock substances can pass into solution and from which they can
condense on the surface of new and growing crystals.
Under extreme heat and pressure, the original rock particles are forced into new arrangements. In some cases, the
rock constituents recombine with those in the immediate vicinity and form new minerals, many of which grow with
nearly perfect crystal form. Garnet is such a mineral.
Layering. Metamorphic rocks may exhibit layers resembling those of sedimentary rocks. Metamorphic layering,
or foliation, is due to mechanical and chemical changes in the original rock. Grains, crystals, and fossils are shifted
or broken up and strung out in a linear series. Parallel rows of plate like minerals, not at all related to the original
bedding, may form. Shale is changed by pressure into slate—a dark rock with an extremely fine grain that splits
readily into thin, smooth slabs. Slate may later become a rock called phyllite and, with continued pressure, a
crystalline foliated rock known as schist.
Phyllite represents a stage intermediate between slate and schist. Its grain is very fine, consisting primarily of mica,
but here and there a few larger crystals appear. The foliation is rougher and wavier than that of slate. Schist is visibly
crystalline, with wavy foliation. It consists of such flake like or tabular minerals as mica, chlorite, hornblende, or
talc, as well as distinct crystals of quartz and garnet.
Other kinds of rock, such as sandy shale and granite, for example, are transformed into a more coarsely foliated
crystalline rock called gneiss. Gneiss looks irregular and streaky because it has alternating layers of different
minerals. Gneisses originate from several different kinds of rock. Each type of gneiss has much the same
composition as the mother rock. Granite gneiss, for example, is derived from granite.
Some types of metamorphic rock show no foliation at all. Pure sandstones are changed into a more compact mass
called quartzite, where spaces pore have been compressed, making a very hard and durable rock. Limestone is
converted by heat and pressure into marble, in which the carbonate grains become visibly crystalline. Pure marble is
white. Most lime stones, however, contain impurities that often react under metamorphism to produce new
minerals. These often impart striking colors or mottling to the resulting marble, making it more attractive as a
building stone.
Connections with the Text
a. While Reading- Place a box around unfamiliar terms/vocabulary
b. After Reading- Answer the questions below:
1. Describe the processes that form metamorphic rocks?
2. Where do metamorphic rocks form?
3. Compare and Contrast the formation of Metamorphic Rocks to Igneous and Sedimentary.
Metamorphic
Sedimentary
Igneous
Similarities:
4. Consider the metamorphous a caterpillar goes through to become a butterfly. How is this transformation
similar to igneous rocks and sedimentary rocks becoming metamorphic rocks?