The farther down we go into the Earth, the hotter it gets. In fact, geologists know that part of the Earth's inner core is so hot that it's still molten liquid (even after 4500 million years of time to cool since it first formed!). Closer to Earth's outer surface, the heat can become great enough so that some rocks will change their form ("metamorphose") without melting while other rocks will completely melt and become igneous plutons. Because these rocks often reside in the middle and lower crust (where they are buried by a veneer of sedimentary rocks), we tend to be less familiar with them (especially here in Ohio where the state is completely blanketed by sedimentary rocks (last week's lab). However, it's important to keep in mind that both plutonic and metamorphic rocks make up the bulk of Earth's continents and oceanic crust.
By definition, metamorphic rocks are those which have been altered by physical and chemical processes mostly in response to movement of the earth's outer plates (regional metamorphism) or in response to getting heated by a pluton or other igneous intrusion (contact metamorphism). It's important to realize that these changes occur solely in the solid-state, without melting. If melting does occur, then the resulting rock would be an igneous rock.
All metamorphic rocks started as something else, either a sedimentary rock, an igneous rock, or even an older metamorphic rock (if it was metamorphosed again). There are then 3 questions to ask about any metamorphic rock:
Contact metamorphism occurs locally in response to temperature changes produced by intrusion of magma into cooler country rocks. For instance, a hot dike cools by losing heat to the country rock. The country rock gets baked near the contact with the dike. Because the agent of contact metamorphism is only heat, the resulting metamorphic rock tends to not have any layering in it.
Regional metamorphism occurs on a much grander scale (the size of mountain belts) and is induced by large-scale plate motions (e.g. continent-continent collisions). We know this because the cores of ancient mountain belts (exposed for us to look at today) all contain regional metamorphic rocks. These rocks are also always deformed.
Classification of metamorphic rocks (see display cabinet 8 across the hall just outside the lab door) Like sedimentary and igneous rocks, metamorphic rocks are classified on the basis of texture (including grain size) and composition (mineral content).
Grain size: A general rule of thumb is that grain size increases with increasing degree (or grade) of metamorphism.
Foliation: alignment of flakey minerals into parallel layers as a result of pressure.
High pressures usually result from strong deformation (i.e., from two continents colliding) and deep burial (i.e., from one continent being thrust over another), and therefore the presence of a foliation is characteristic of regional metamorphism. Because grain size increases with metamorphic grade, the foliation tends to become more prominent with increasing metamorphic grade as well (it's simply easier to see with the naked eye).
The names of the foliation for the 4 rocks above are:
slatey cleavage ----> phyllitic cleavage ----> schistosity ----> gneissic banding
Nonfoliated (or simply massive): a metamorphic rock with no alignment of minerals (no fabric). Usually characteristic of contact metamorphism.
Metamorphic grade is also characterized by the growth of new minerals as the rock gets hotter and more deeply buried in the crust. In this regard, micas tend to form at low metamorphic grade whereas the gem mineral garnet starts to grow at medium metamorphic grade. Many of the metamorphic minerals are spectacular to look and therefore some geologists define metamorphism as "a process that turns ugly rocks into beautiful ones."
