Chapter 2 (Part 2)

II. Igneous Rocks (Picture of different sized crystals)

   A. Form as magma cools and crystallizes 

      1.Rocks formed inside Earth are called plutonic or intrusive rocks

      2.Rocks formed on the surface 

         a.Formed from lava

         b. Called volcanic or extrusive rocks

   B. Crystallization of magma 

      1.Rate of cooling

         a.Slow rate forms large crystals

         b.Fast rates forms microscopic crystals 

         c.Very fast rates forms glass

   C.Classification- based on texture and mineral composition

      1.Texture

         a.Size and arrangement of crystals

         b.Types

            1.Fine Grained- fast rate of cooling

            2.Coarse Grained- slow rate of cooling

            3. Porphyritic- two rates of cooling- different crystal sizes

            4.Glassy- very fast rate of cooling

      2.Mineral composition 

         a.Explained by Bowen's reaction series which shows the order of mineral crystallization 

         b. Influenced by crystal settling in the magma 

   D.Naming Igneous rocks

      1.Basaltic rocks

         a.Derived from the first minerals to crystallize

         b.Rich in iron and magnesium 

         c. Low in silica 

         d.Common rock is basalt 

      2. Granitic Rocks

         a.From the last minerals to crystallize

         b.Mainly feldspar and quartz 

         c.High silica content

         d.Common rock is granite