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 Volcanic igneous rocks Plutonic igneous rocks May form from basaltic magma1.2. Form at Earth’s surface3.4. Have texture Made of small grains5.6. Granite is an example7.8. Form as a result of melting9.10. Present at Earth’s surface only after erosion Contains minerals Classified based on color15.16 Dark-colored examples have low silica content1718. Contain visible grains19.20.

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 Composition Texture Felsic Intermediate MaficUltramafic Phaneritic GraniteDioriteGabbroPeridotite Porphyritic Phaneritic Porphyritic Granite Porphyritic Diorite Porphyritic Gabbro Aphanitic RhyoliteAndesiteBasalt Porphyritic Aphanitic 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  glassy Fast = Fine-grained  aphanitic Slow = Coarse-grained  phaneritic Complex = Mixture  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  Porphyritic Phaneritic Ooopps!! I must have eaten the Oatmeal Cookie

Phaneritic—Oatmeal Cookie

Texture  Aphanitic  Porphyritic Aphanitic  Phaneritic  Porphyritic Phaneritic Two stages of cooling – 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) Oatmeal Raisin Cookie

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  Viscosity: Resistance to flow Silica content temperature

Composition—Silica Content  Felsic: Feldspar & Silica >65% silica  High Viscosity  Intermediate: 53-65% silica  Intermediate Viscosity  Mafic: Magnesium and Iron (Fe) 45-52% silica  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

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

Bowen’s Reaction Series  Different minerals crystallize from magmas at different temperatures Melting CrystallizationHotCold

Magmatic Differentiation  Formation of more than one magma from a single parent magma

Magmatic Differentiation  Crystal Settling : crystallized minerals have a density greater than the magma and settle to the bottom due to gravity  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

Fig. 7.21

Composition Quiz  Which type of lava would flow most easily?  Mafic/Ultramafic  Which type of volcano would erupt most violently?  Felsic