II. Igneous Rocks

A. Rock cycle

B. Types of Igneous rocks

1. location of formation
(fig. 3.3)
1. intrusive (plutonic)- underground - cool slowly - large crystals form
2. extrusive (volcanic) - cool quickly - small/no crystals form
2. composition (can be defined by chemical composition or mineral components)
(fig. 3.6)
1. felsic - >65% SiO₂, much Na, K
2. intermediate
3. mafic - < 52% SiO₂, much Fe, Mg

C. Composition

1. mineralogy - Bowen's reaction series  (fig 3.16)
   sequence in which minerals form as magma cools; requires slow cooling rate and presence of extra Si to keep progressing down the sequence
1.  discontinuous series
• olivine >>>>pyroxene>>>>amphibole>>>>biotite
• similar chemical compositions (all are Fe-Mg minerals)
• different crystal structures (increasing in complexity)
• replacement of pre-existing minerals
2. continuous series
• Ca-plagioclase feldspar >>>>>Na plagioclase feldspar
• different chemical composition (Ca to Na)
• same crystal structure (3-D framework)
• continuous growth of crystal (zoning)
3. other minerals
• muscovite mica
• orthoclase feldspar
• quartz
2. rock names  (fig 3.7)
1. granite/ rhyolite
2. diorite/andesite
3. gabbro/basalt
4. peridotite
3. felsic and intermediate magma formation
1. differentiation from mafic magma
• crystal settling (fig. 3.17)
• assimilation and xenoliths (fig. 3.18)
2. partial melting (Rocky Road ice cream)
3. melt lower crust

D. Geometry of intrusive bodies (plutons)

1. tabular
   1. dikes
   2. sills
2. irregular
   1. batholiths (>100km²)
   2. stocks (<100km²)