Chapter 8 (P2)

Chapter 8 

V.Intrusive igneous activity 

   A.Magma emplaced at depth

   B.Underground igneous body is called a pluton 

   C.Plutons are classified according to 

      1.Shape

         a.Tabular (sheetline)

         b.Massive

      2.Orientation with respect to the host (surrounding) rock

         a.Discordant- cuts across sedimentary beds

         b.Concordant- parallel to sedimentary beds

   D.Types of igneous intrusive features

      1.Dike- a tabular, discordant pluton

      2.Sill a tabular, concordant pluton

      3.Laccolith

         a.Forms in same way as sill

         b.Lens shaped mass

         c.Arches overlying strata upward

      4.Batholith

         a.Largest intrusive body

         b.Surface exposure 100+ square

         c.Frequently form the cores of mountains

VI.Igneous activity and plate tectonics 

   A. Origin of magma 

      1.Temperature and magma generation

         a.Magma must originate from solid rock

         b.Temperature melts solid rock

         c.Temperature increases with depth

      2.Role of pressure

         a.Increase in pressure causes an increase in melting temperature

         b. Drop in confining pressure 

            1.Lowers the melting temperature

             2.Occurs when rock ascends 

            3.Partial melting 

               a.Igneous rocks are mixtures of minerals 

               b.Melting over a range of temperature

               c.Forms a melt with a higher silica content

   B.Distribution of igneous activity 

      1.Igneous activity along plate margins

         a.Oceanic ridge spreading center 

            1.Lithosphere pulls apart

            2.Less pressure on underlying rocks

            3.Partial melting occurs

            4.Large quantities of basaltic magma are produced

         b.Convergent plate margin

            1.Subduction zone

            2.Descending plate partially melts

            3.Magma slowly rises upward

            4.Rising magma can from

               a.Island arc in an ocean 

               b.Andesitic-granitic volcanoes on a continent

            5.Associated with the Pacific basin

               a.Called Ring of Fire

               b. Explosive- high gas volcanoes 

      2. Intraplate volcanism 

         a.Activity within a rigid plate

         b.Basaltic magma source 

            1.Partial melting of mantle rock

            2.Plumes of hot mantle material 

               a. From hot spots on the surface 

               b.A plume may be located below Hawaii 

         c.Granitic magma source when the continental crust is remelted over a mantle plume