IGNEOUS ROCKS: THE HEAT

1. Characteristics of igneous rocks
   1. Some terms
      1. plutonic: from Pluto, god of the underworld; = intrusive
      2. volcanic; = extrusive
   2. Texture: what does the rock look like?
      1. names of textures
         1. phaneritic (phanero: greek "visible")
         2. aphanitic (not visible)
         3. porphyritic (visible crystals in a fine matrix or groundmass)
      2. what this tells you about cooling
         1. large crystals, whole rock is crystals = slow cooling
         2. small crystals, few and far in between = fast cooling
         3. where do fast and slow cooling occur?
   3. mineral and chemical composition
      1. some more terms
         1. mafic minerals (MAgnesium, FE (iron))
         2. felsic minerals (FELdspar: Al, Na, K)
         3. mafic rocks (45-50% SiO2), intermediate rocks (50-65% SiO2), felsic rocks (>65% silica) (don't memorize those silica contents)

2. Names of igneous rocks
   1. Ultramafic
      1. highest temperature
      2. contain olivine and pyroxene, no felsic minerals
      3. name: peridotite: the mantle is made of peridotite
      4. only occurs as intrusive rock
   2. mafic
      1. slightly lower temperature
      2. extrusive mafic rock = basalt
      3. intrusive rock = gabbro
   3. intermediate
      1. andesite [from Andes]
   4. felsic
      1. intrusive = granite; extrusive = rhyolite
   5. go here for a chart that relates temperature, texure, and rock name

3. How to make a magma: melt the asthenosphere
   1. at a subduction zone
      1. down-going slab has water in ocean-floor sediment and in crystals in the basalt
      2. as slab begins to heat, water is driven off
      3. water causes peridotite to melt in very small pockets
         1. partial melt
         2. composition = basalt
      4. melted material has lower density than solid, so it rises
      5. small pockets coalesce, become magma at ~ 15-20 km depth
      6. magma begins to cool, crystallize
   2. at a mid-ocean ridge
      1. rising asthenosphere (convection) decompresses, melts
      2. becomes magma at ~5 km depth
      3. begins to cool, crystalize, or erupts due to shallow depth
   3. the moral
      1. adding water lowers the melting temperature
      2. reducing the pressure lowers the melting temperature
   4. straw into gold: turning basalt into granite
      1. crystal fractionation
         1. remove high-T minerals from a melt; go here for a picture of crystal fractionation
         2. separate minerals from melt
      2. assimilation: works best in continental crust; go here for a picture of assimilation (this happens in the crust, not in the mantle)
      3. magma mixing: two magma bodies of different composition are joined (as if Geology were suddenly joined with the Music department); go here for a picture of magma mixing

4. Some names of igneous bodies
   1. dike
   2. sill
   3. pluton
   4. batholith