Metamorphism: formation of new rocks from old rocks by Temperature and Pressure*
        *metamorphism is NOT lithification: T and P are much higher, deeper in Earth
        *metamorphism does NOT involve melting; minerals transform while solid
    two parts to metamorphism:
        (a) change minerals: new minerals grow from old under influence of T and P
            1.  one old mineral into one new mineral (polymorphs)
                    ex/  graphite to diamond; calcite1 to calcite2, quartz to coesite
            2.  old minerals to new minerals via chemical reactions between solids
                    ex/  Na-feldspar + Ca-pyroxene   =      garnet + Na-pyroxene
                                            basalt                       =               eclogite
        (b) change form: change texture (shape and arrangement) of minerals/objects
            ex. of change form:  stretched out pebbles in a metamorphosed conglomerate
            ex. of change form:  mudstone--slate--phyllite--schist--gneiss
        alignment of minerals (slate-phyllite-schist) versus new metamorphic banding (gneiss)

Metamorphic Variables: what factors control the metamorphic rock you get?  "bake a cake"
    Starting Rock or Protolith; what starting minerals, what composition (ingredients)
        Isochemical metamorphism (constant composition) versus Changing composition
            chemical reactions can produce gases and fluids that burp off into Earth
    Pressure (how much weight) and Temperature (how hot):  depends on depth in Earth
        P and T increase in Earth but at different rates depending upon specific place

Metamorphic Grade or Intensity: how high was P and T, how deep in Earth?
    low grade/intensity (<20 km ±), medium grade (20-40 km ±), high grade (>40 km ±)
    ultra-metamorphism: too hot, too much water will melt rocks = migmatites, magma
        example:  metamorphism of basalt (dark volcanic rock) to make three meta-basalts:
           low grade metabasalt = greenstone:  white Na-feldspar, green amphibole, chlorite
           medium grade metabasalt = amphibolite: black Ca-feldspar, black amphibole, garnet
           high grade metabasalt = eclogite:  red garnet, green Na-pyroxene

Metamorphic Minerals: many common minerals are produced by metamorphism
        also:  many gems are metamorphic minerals; emerald, ruby, tourmaline, garnet, jade...
    Index minerals; special minerals that indicate grade/intensity of metamorphism
        examples:    chlorite, green amph. = low.   garnet, biotite = medium.   pyroxene = high

Metamorphism and Plate Tectonics: metamorphic style depends upon tectonic setting
    (a) contact metamorphism: aka "low pressure " (variable temperature)
        baked rocks around/in contact with magma bodies (plutons) in shallow crust
        highest T metamorphic rock nearest pluton, lowest T rock farthest away
        magma bodies:  tectonic settings such as rifts, in arcs above subduction zones, etc.
    (b) subduction zone metamorphism: aka "low temperature " (variable pressure)
        along the subduction zone itself, cold oceanic crust pushed into mantle
        lowest P at top of subduction zone, highest P deepest along subduction zone
    (c) regional/collisional metamorphism:  T and P both increasing rapidly into crust
        mountain building metamorphism produced by collision of two continents
        lowest T-P in shallow part of mountain belt, highest T-P in deep mountain root
 

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