Igneous Rocks

Fig. 7.16

Volcanic igneous rocks Plutonic igneous rocks 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 magma 1. 2. Form at Earth’s surface 3. 4. Have texture  Made of small grains 5. 6. Granite is an example 7. 8. Form as a result of melting 9. 10. Present at Earth’s surface only after erosion 11. 12. Contains minerals 13. 14. Classified based on color 15. 16 Dark-colored examples have low silica content 17 18. Contain visible grains 19. 20.

Overview of Igneous Rocks Koryakskaya Sopka Volcano, Eastern Russia 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

It’s all about heat and density Heat source?? Geothermal Gradient Composition Hot stuff Heat source = radioactive decay Geothermal gradient = 25 deg C/km Composition = heavier elements make heavier minerals/rocks Hot Stuff = density of a material is dependent on temperature

Igneous Rock Classification Composition Texture Felsic Intermediate Mafic Ultramafic Phaneritic Granite Diorite Gabbro Peridotite Porphyritic Phaneritic Porphyritic Granite Porphyritic Diorite Porphyritic Gabbro Aphanitic Rhyolite Andesite Basalt 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 Other textures: 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 Peanut Butter or Sugar Cookie Aphanitic Fine –grained Cooled quickly Crystallized at the Earth’s surface Porphyritic Aphanitic Phaneritic Porphyritic Phaneritic

Aphanitic--peanut butter cookie

Texture Porphyritic Aphanitic Aphanitic Two stages of cooling Chocolate Chip Cookie Aphanitic Porphyritic Aphanitic Two stages of cooling 1st cooled slowly within the Earth (larger cyrstals - Phenocrysts) 2nd cooled rapidly on the Earth’s surface (fine-grained matrix) Phaneritic Porphyritic Phaneritic

Porphyritic Aphanitic—Chocolate Chip Cookie

eaten the Oatmeal 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 Porphyritic Phaneritic Aphanitic Porphyritic Aphanitic Oatmeal Raisin Cookie Texture Aphanitic Porphyritic Aphanitic Phaneritic Porphyritic Phaneritic Two stages of cooling 1st cooled slowly within the Earth (larger crystals - Phenocrysts) 2nd cooled faster but still slow enough that crystals fully develop – within the Earth (coarse-grained matrix)

Porphyritic Phaneritic—Oatmeal Raisin Cookie

Texture Vesicular –voids left by trapped gas Aphanitic Porphyritic Aphanitic Phaneritic Porphyritic Phaneritic Vesicular –voids left by trapped gas Glassy Pyroclastic

Texture Glassy 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 (Si02) is primary ingredient of all magmas Viscosity: Resistance to flow Silica content temperature Rocks are made of minerals

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: <900EC; Na, K, Al-rich Light colored

Composition Mafic-basaltic: >1100EC; Ca, Fe, Mg-rich Dark Colored

Composition Intermediate-andesitic: 900-1100EC; Na, Al, Ca, Fe, Mg, K Salt & Pepper appearance Andesite Porphyry Diorite

Bowen’s Reaction Series Melting Crystallization Hot Cold Different minerals crystallize from magmas at different temperatures

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 SiO2, 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