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Igneous Rocks Summary 1. The Rock Cycle 2. Formation of Igneous Rocks 3. Classification of Igneous Rocks The Rock Cycle Geological Materials Transformation Processes Igneous Rock Solidification Magma Partial Melting Mantle Rock Fig 3.1 Formation of Magma How are rocks melted? 1. Heating 2. Depressurization 3. Partial Melting Where do rocks melt? Subduction zones (Silicic and Intermediate) Mantle Plumes or Hot Spots Partial Melting Different rocks melt at different temperatures. Depending on the temperature, some rocks in the mixture may be liquidified and others might not. Types of Igneous Rock Extrusive Fine grained rock that cooled quickly on the Earth’s surface. Intrusive Course grained rock that cooled slowly under the surface. This is the most common type of rock. Also describe as Plutonic See Kehew, Fig 3-6 Classification of Igneous Rocks These classes of rock are based on the type of magma that cooled to create the rock. These are describe in terms of temperature, composition, viscosity, and pH level. Silicic Rocks Silicic (a.k.a, felsic) Magmas Cool (<700oC) Viscous (sticky, doesn’t flow easily) Gaseous (steam of H2O and C02) Silicic Rocks Usually intrusive, course-grained, Silicic (Granite) to Intermediate (Diorite). If extrusive, fine-grained rocks formed by explosive volcanoes Rhyolite or Andesite Volcanoes Intrusive Silicic Igneous Rock E.g., Granite (Silicic, Phaneritic): Quartz Biotite Na Plagioclase Crystallized (Solidified) Silicic Magma Poor in: Fe, Mg, Ca, (<20%) Rich in: Silica (>70%) Mafic Rocks Mafic Magmas Hot (>1000oC) Non-Viscous (runny, flows easily) “Dry” (no H2O or C02) Mafic Rocks Usually Extrusive, Fine-grained, Mafic (Basalt) rock forms oceanic crust, Shield Volcanoes and Basalt Floods If Intrusive, course-grained mafic rocks are formed Gabbro. If intrusive, Dikes and Sills more common See Kehew, Fig 3-40 Mafic Volcanism, Hawaii Mantle Hot Spot Volcano See Kehew, 3-8 Hawaii A long chain of inactive volcanoes Island ages, millions of years 59.6 55.2 43.4 42.4 56.2 48.1 Midway 27.2 19.9 20.6 12.0 10.3 Hawaii 0-5 Shield Volcanoes Mafic Magma Low-viscosity Non-explosive eruptions Gentle slopes Covering large areas Types of Eruptions (Mafic Volc.) Types of Eruptions Lava floods Lava fountains Fissure eruptions Rock Textures (Table 3-1) Aphanitic Porphyritic Vesicular Glassy (Obsidian) Surface Textures (Extrusive, Mafic Rocks) Ahah (Rubbley) Mostly solid when flowing Pahoehoe (Ropey) mostly liquid when flowing) Mafic Sill: Intruded between layers Mafic magma is less viscous and hotter so Does not form plutons but Cuts along layers (Sills) or even across layers (Dikes) Also Baked Zones of adjacent country rock and Chill Zones within the intrusion Igneous Rock Classification Mineral Percentage Intrusive (Plutonic) Extrusive (Volcanic) Continental Crust Oceanic Mantle Crust Igneous Rock Classification Intermediate Mafic Granite Rhyolite Diorite Andesite Gabbro Basalt Extrusive Intrusive Silicic (Porphyritic) Bowen’s Reaction Series Two series of minerals formed during crystallization of magma Low Silica Magma 1200oC 1000o Intrus. Extrus. Gabbro Basalt Diorite Andesite 750o Framework Sheet Double Chain Single Chain Isolated Temperature of Crystallization Granite Rhyolite High Silica Magma Bowen’s Reaction Series Illustrates the relationship between the cooling magma and the crystallization of the minerals contained in the rock. Rocks on the right side of this chart are rich in calcium and sodium Rocks on the left side represents iron-rich minerals. They cool and create quartz. Terminology Felsic – silicate minerals, magmas and rocks enriched in lighter elements for oxygen, aluminum, sodium, and silicon. Aphanitic – dark-coloured rock where the grains are so fine they cannot be seen by the naked eye. Terminology Mafic – iron and magnesium enriched minerals are found in these rocks. They are dark in colour. Phaneritic – Grains in the rock can be seen by the naked eye. Types of Igneous Rocks Granite Acidic Very pale Coarse-grained Intrusive, felsic Can be pink to dark gray. Rhyolite Extrusive form equivalent to Granite Fine grained. Make up Mt Fuji. Andersite Intermediate rock Less quartz that granite, thus has a darker colour. Both sizes of grains. Not as acidic as Granite. Found in the Andes Extrusive Diorite Coarse grained Grey to dark gray in colour Intrusive. Basalt Black to gray in colour (dark) Fine-grained Mafic Found in Ireland (Giants Causeway) and Hawaii. Basic Extrusive Gabbro Dark Basic Intrusive Coarse-grained Part of the oceanic crust Peridotite Ultrabasic Rare and part of the mantle Course grained Green in colour Obsidian Glassy (volcanic glass) Can be green or black Extrusive Pumice Glassy (Frothy) Light to dark in colour depending on its impurities. Solidified foam Extrusive Homework P. 106 #1-3 P. 113 #1-4 Rocks in the Collection 1. 2. 3. 4. 5. 6. 7. Grantie Rhyolite Andesite Obsidian Pumice Basalt Gabbro Anorthosite 9. Diorite 10. Scoria 11. Syenite 12. Peridotite 8.