Physical Geology   Chapter 3   Igneous Rocks

 

The Rock Cycle

 

    Figure 3.1

The beginning of the rock cycle – magma from the mantle produces igneous rocks near the Earth surface

The The

 

 

Igneous rocks – formed  by cooling and solidifying magma

 

  Magma – molten rock

  Lava – magma on the Earth surface

 

Magma and igneous rocks can be:

  Extrusive – on the Earth surface

  Intrusive – magma solidifies underground

 

Figure 3.2  The rock cycle – a plate-tectonic example

 

 

 

            Andes-type convergent margin

            All of the major processes in the rock cycle

 

Textures of igneous rocks

 

Coarse grained – mineral grains greater

    than 1 mm, can see with the eye

 

    magma cools slowly, enough time to

        form large crystals

 

    Intrusive

         rocks are very good insulators

            allows slow cooling

 

Fine grained – mineral grains smaller

    than 1 mm

 

    magma cools quickly

 

    glass forms when quenched quickly,

        such as lava flowing into water

 

    Extrusive

        

Igneous intrusion into country rock

Figure 3.4     Contact metamorphism

Country rock – the pre-existing rock that magma intrudes into

Xenolith – a broken-off piece of country rock that is floating in the magma

 

Composition and texture

 of igneous rocks

Magma type

Coarse grained

Fine grained

Mafic

Gabbro

Basalt

Intermediate

Diorite

Andesite

Silicic

Granite

Rhyolite

 

 

 

 

 

 

Example of an intrusive granite from a batholith:

Figure 3.5

Hand specimen – note coarse texture, different mineral grains

Thin section – note interlocking mineral grains

 

Classification Chart for Igneous Rocks  Figure 3.6

Main trends in chemical composition and mineral composition

  from ultramafic (mantle) to silicic (continental crust) igneous rocks:

increasing silica and aluminum

increasing Na, then K in the feldspars

decreasing Ca, Fe, and Mg

 

  Granite and rhyolite – silicic composition  Figure 3.7

            mostly quartz and K-feldspar

            highest concentrations of Si and Al

 

  Diorite and andesite – intermediate composition 

 

 

 

            little or no quartz

            mostly Na-feldspar

            abundant amphibole and pyroxene

            little or no olivine

 

  Olivine and gabbro (and peridotite) – mafic (and ultramafic) composition

            almost entirely olivine and Ca-feldspar, with some pyroxene

            highest concentrations of Ca, Fe, Mg

 

*** Know intrusive and extrusive equivalents ***

            (coarse-grained and fine-grained rocks in chart above)

           

Porphyry (or porphyritic) – extrusive igneous rock that has large crystals embedded in a fine-grained matrix;  formed by slow cooling of an initial magma (to form large crystals) followed by a second injection of magma that extruded onto the Earth surface (to form the fine-grained matrix)

 

Intrusive bodies – shallow  

    Figure 3.9

    Most volcanism occurs in tectonically active areas (lots of stress, and fractured rock)

 

Dike – a thin, sheet-like or tabular intrusion that cuts across existing rock layers 

discordant     Figure 3.7

 

Ship Rock, New Mexico  Figure 3.8

 

Sill – a thin, sheet-like or tabular intrusion that was injected between existing rock layers

    concordant

 

Deep intrusive bodies

 

Magma diapirs and plutons  Figure 3.12

 

Stocks and batholiths

            A stock is less than 100 square kilometers, a batholith is larger

 

Figure 3.13   Forming a batholith

 

  Sierra Nevada batholith

    Figure 3.14

 

How magma forms

The geothermal gradient

 Is about 3o C in 100 m     Figure 3.15

            higher thermal gradient in volcanic regions

 

 

Factors that control melting temperatures    Figure 3.16

 

 

    Pressure    

Water

 

 

 

Mixed minerals    Figure 3.17

 

            analogous to putting salt on the ice on your sidewalk, lowers the melting point

 

Bowen’s reaction series – Sequence of crystallization

 

Discontinuous branch

            discrete minerals

              olivine

                pyroxene

                  amphibole

                    biotite mica

 

 

 

 

 

 

Continuous branch

            solid-solution series in the feldspars

            (can have varying percentages of Ca, Na, and K) 

  Ca-feldspar

                Na-feldspar

                  K-feldspar

 

Both branches converge to

            K-feldspar, quartz, and muscovite at the silicic end

 

***  Three ways to change the composition of a magma ***

            Differentiation  Assimilation     Mixing

 

Differentiation of magma

High-temp minerals freeze out first    Figure 3.19

    accumulate at the bottom of the magma chamber

Assimilation

 

 

 

    Melted country rock is incorporated into the magma    Figure 3.20

 

 

    The magma now has a different composition  (most rocks near the Earth surface have much more silica and alumina than magma from the mantle)

 

Mixing of magmas

Silicic magmas are thick and viscous    Figure 3.21

 

   Mafic magmas are thin and flow easily

 

Igneous activity & plate tectonics

Divergent boundaries – seafloor spreading    Figure 3.22

 

 

Mantle plume under continental crust    Figure 3.22

 

 

Convergent margins – injection of water into mantle from subducting plate lowers the melting point of mantle rocks, producing magma    Figure 3.22

 

Two modes of forming magma

 (at a subduction zone)

  Observation:  many batholiths are mostly granite, not diorite

 

      which means that continental crust is melting to produce the magma for the batholith

 

Figure 3.26

    Intermediate magma produces diorite

 

    Heating of crust produces granite

 

Granite batholith, Chile

    Figure 3.4

 

Washington-Oregon coast    Juan de Fuca Ridge offshore    Figure 3.25

    very shallow subduction angle because the oceanic crust is still hot, and low density