Igneous Rocks

Fig. 7.16

Complete the following table by identifying which of the characteristics in the left-hand column are present in volcanic and/or plutonic igneous rocks by stating yes or no for the appropriate number. One characteristic has been completed as an example.

Characteristic May form from basaltic magma Form at Earth’s surface Have texture Made of small grains Granite is an example Form as a result of melting Present at Earth’s surface only after erosion Contains minerals Classified based on color Dark-colored examples have low silica content Contain visible grains Volcanic igneous rocks Plutonic igneous rocks 1.

3.

2.

4.

  5.

7.

9.

11.

13.

15.

17 19.

6.

8.

10.

12.

14.

16 18.

20.

Overview of Igneous Rocks

 Form when minerals crystallize from magma Intrusive  Plutonic Extrusive  Volcanic  Magmas derived from below the Earth’s surface in the mantle  Magma is hot and buoyant Koryakskaya Sopka Volcano, Eastern Russia

It’s all about heat and density

 Heat source??

 Geothermal Gradient  Composition  Hot stuff

Igneous Rock Classification

Texture Composition

Phaneritic Porphyritic Phaneritic Aphanitic Porphyritic Aphanitic Felsic Intermediate Mafic Ultramafic

Granite Rhyolite Diorite Gabbro Porphyritic Granite Porphyritic Diorite Porphyritic Gabbro Andesite Basalt Peridotite Porphyritic Rhyolite Porphyritic Andesite Porphyritic Basalt

Intrusive Vs. Extrusive

 Plutonic  Formed within the Earth  Magma  Reach Surface by uplift and erosion of the Earth’s Crust  Volcanic  Formed at the Surface  Lava

Texture

 Related to the cooling history of the rock Really Fast = no grains  Fast = Fine-grained  glassy aphanitic Slow = Coarse-grained  Complex = Mixture  phaneritic porphyritic Why??

 Other textures: Vesicular : trapped gases in lava Pyroclastic : ash and rock fragments formed explosively

Texture



Aphanitic

  

Fine –grained Cooled quickly Crystallized at the Earth’s surface

Porphyritic Aphanitic Phaneritic Porphyritic Phaneritic

Peanut Butter or Sugar Cookie

Aphanitic--peanut butter cookie

Texture

 Aphanitic 

Porphyritic Aphanitic



Two stages of cooling

– 1

st cooled slowly within the Earth (larger cyrstals Phenocrysts)

– 2

nd cooled rapidly on the Earth’s surface (fine-grained matrix)

Phaneritic  Porphyritic Phaneritic

Chocolate Chip Cookie

Porphyritic Aphanitic—Chocolate Chip Cookie

Texture

  Aphanitic Porphyritic Aphanitic 

Phaneritic Coarse –grained Cooled slowly Crystallized within the Earth Ooopps!! I must have eaten the Oatmeal Cookie

 Porphyritic Phaneritic

Phaneritic—Oatmeal Cookie

Texture

   Aphanitic Porphyritic Aphanitic Phaneritic 

Porphyritic Phaneritic Oatmeal Raisin Two stages of cooling Cookie

– 1

st cooled slowly within the Earth (larger crystals Phenocrysts)

– 2

nd cooled faster but still slow enough that crystals fully develop – within the Earth (coarse-grained matrix)

Porphyritic Phaneritic—Oatmeal Raisin Cookie

Texture

    Aphanitic Porphyritic Aphanitic Phaneritic Porphyritic Phaneritic 

Vesicular –voids left by trapped gas

 Glassy  Pyroclastic

Texture

     Aphanitic Porphyritic Aphanitic Phaneritic Porphyritic Phaneritic Vesicular 

Glassy



Very rapid cooling Ions do not have time to from crystalline structures

Pyroclastic

Texture

      Aphanitic Porphyritic Aphanitic Phaneritic Porphyritic Phaneritic Vesicular Glassy 

Pyroclastic – welded shards of rock & ash ejected from a vent during an eruption

Composition of Igneous Rocks

 Silica (Si0 2 ) is primary ingredient of all magmas

Composition of Earth's Crust by Mass

 Viscosity: Resistance to flow Silica content temperature

O-46.6% All Others-9% Si-27.7

Ca-3.6% Al-8.1% Fe-5.0%

Composition—Silica Content

 Felsic:

Feldspar & Silica

>65% silica  High Viscosity  Intermediate: 53-65% silica  45-52% silica  Intermediate Viscosity  Mafic:

Magnesium and Iron (Fe)

Low Viscosity  Ultramafic: <45% silica  Very Low Viscosity

Composition

 Felsic-rhyolitic: <900 E C; Na, K, Al-rich Light colored

Composition

 Mafic-basaltic: >1100 E C; Ca, Fe, Mg-rich Dark Colored

Diorite

Composition

 Intermediate-andesitic: 900-1100 E C; Na, Al, Ca, Fe, Mg, K Salt & Pepper appearance Andesite Porphyry

Bowen’s Reaction Series

Hot

 Different minerals crystallize from magmas at different temperatures

Cold

Magmatic Differentiation



Formation of more than one magma from a single parent magma

Magmatic Differentiation



Crystal Settling

gravity : crystallized minerals have a density greater than the magma and settle to the bottom due to  Because Fe and Mg are first removed, melt becomes rich in SiO 2 , Na, and K Marbles analogy

Magmatic Differentiation



Assimilation

: magma reacts with the “country rock” which is adjacent to the magma chamber Magma composition is altered according to the composition of the assimilated country rock Inclusions are rocks Incompletely melted chunks of country rock

Magmatic Differentiation



Magma Mixing

: Magmas of different compositions are mixed together Resulting magma is of a composition intermediate between the parents

Magma Mixing

Magma Mixing

Magma Mixing

Fig. 7.21

Composition Quiz

 Which type of lava would flow most easily?



Mafic/Ultramafic

 Which type of volcano would erupt most violently?



Felsic