Igneous Rocks

Warm-up Compare and contrast rocks and minerals. Rocks are made of one or more mineral A rock is not always a mineral (ex: coal, obsidian)

Overview of Igneous Rocks Koryakskaya Sopka Volcano, Eastern Russia Igneous means from fire Form when minerals crystallize from magma Intrusive = cools inside = magma = Plutonic Extrusive = cools outside= lava = 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

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

Igneous Rock Classification Composition Texture Felsic Intermediate Mafic Ultramafic Course-Grained Granite Diorite Gabbro Peridotite Porphyritic Course-Grained Porphyritic Granite Porphyritic Diorite Porphyritic Gabbro Fine-Grained Rhyolite Andesite Basalt Porphyritic Fine-Grained Porphyritic Rhyolite Porphyritic Andesite Porphyritic Basalt

Texture Related to the cooling history of the rock Other textures: Really Fast = no mineral grains glassy Fast = Small/ microscopic mineral grains Fine-Grained Slow = large mineral grains Course-Grained Complex = Mixture  porphyritic Why?? Other textures: Vesicular: trapped gases in lava Pyroclastic: ash and rock fragments formed explosively

Texture Fine-Grained Cooled quickly Peanut Butter or Sugar Cookie Fine-Grained Cooled quickly Crystallized at the Earth’s surface

Fine-Grained--peanut butter cookie

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

Porphyritic Fine-Grained —Chocolate Chip Cookie

Texture Course-Grained Cooled slowly Crystallized within the Earth Oatmeal Cookie

Course-Grained—Oatmeal Cookie

Texture Porphyritic Course-Grained Oatmeal Raisin Cookie 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 Course-Grained —Oatmeal Raisin Cookie

Texture Vesicular –voids left by trapped gas

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

Texture 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 Quartz is basically just silicate Viscosity: Resistance to flow Affected by 2 main things: 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 (Ferris) 45-52% silica  Low Viscosity Ultramafic: <45% silica  Very Low Viscosity

Why does it matter? Higher viscosity makes a more explosive volcano!

Composition Felsic-rhyolitic: Light colored Quartz, feldspar, mica

Composition Mafic-basaltic: Dark Colored  pyroxene and olivine

Composition Intermediate-andesitic: Salt & Pepper appearance feldspar, mica, hornblende, and/or pyroxene Andesite Porphyry Diorite

Volcanic igneous rocks Plutonic igneous rocks Warm-up: 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 mafic 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.

Bowen’s Reaction Series Melting Crystallization Hot Cold Different minerals crystallize from magmas at different temperatures Each mineral has it’s own hardening temp

Magmatic Differentiation Formation of more than one type of magma from a single parent magma 3 ways it can happen Crystal setting Assimilation Magma mixing

Magmatic Differentiation Crystal Settling: crystallized minerals are more dense than the magma and settle to the bottom Marbles analogy Because Fe and Mg crystalize first, melt becomes rich in SiO2, Na, and K Different minerals crystalize based on elements available

Magmatic Differentiation Assimilation: magma reacts with the “country rock” which is adjacent to the magma chamber “country rock” is the rock surrounding 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 parent magmas

Magma Mixing

Magma Mixing

Magma Mixing