Slide 1Names of Igneous RocksVisual Estimation of Modal AbundanceClassification based on Modal MineralogySlide 5Classification of Phaneritic Igneous RocksClassification of Igneous RocksAphanitic rocksClassification of aphanitic Igneous RocksClassification of Igneous RocksClassification of Igneous RocksSlide 12ObsidianNames of Igneous Rocks•Texture + Composition = name•Set up diagrams (many ternary ones again, you remember how these work?) to represent composition changes for rocks of a certain texture•Composition can be related to specific minerals, or even physical characteristics of mineral grains•Modal Composition - % of minerals comprising a rockVisual Estimation of Modal AbundanceClassification based on Modal Mineralogy•Felsic rocks: mnemonic based on feldspar and silica. Also applies to rocks containing abundant feldspathoids, such as nepheline. GRANITE•Mafic rocks: mnemonic based on magnesium and ferrous/ferric. Synonymous with ferromagnesian, which refers to biotite, amphibole, pyroxene, olivine, and Fe-Ti oxides. BASALT•Ultramafic rocks: very rich in Mg and Fe. Generally have little feldspar. PERIDOTITE•Silicic rocks: dominated by quartz and alkali fsp. Sometimes refered to as sialic (Si + Al).granitegranodioriteClassification of Phaneritic Igneous RocksFigure 2-2. A classification of the phaneritic igneous rocks. a. Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS.The rock must contain a total ofat least 10% of the minerals below.Renormalize to 100% Quartz-richGranitoid909060602020Alkali Fs.Quartz SyeniteQuartzSyeniteQuartzMonzoniteQuartzMonzodioriteSyeniteMonzoniteMonzodiorite(Foid)-bearingSyenite5103565(Foid)-bearingMonzonite(Foid)-bearingMonzodiorite90Alkali Fs.Syenite(Foid)-bearingAlkali Fs. Syenite10(Foid)Monzosyenite(Foid) Syenite(Foid)Monzodiorite(Foid) GabbroQtz. Diorite/Qtz. Gabbro510Diorite/Gabbro/Anorthosite(Foid)-bearingDiorite/Gabbro60(Foid)olitesQuartzoliteGraniteGrano-dioriteTonaliteAlkali Feldspar GraniteQAPF60Q=quartzA=Alkali fledspars (An0-An5)P=Plagioclase feldspars (An5-An100)F=FeldspathoidClassification of Igneous RocksFigure 2-4. A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al. (1986) J. Petrol., 27, 745-750. Oxford University Press.Aphanitic rocksbasaltrhyoliteClassification of aphanitic Igneous RocksFigure 2-3. A classification and nomenclature of volcanic rocks. After IUGS. (foid)-bearing Trachyte(foid)-bearing Latite(foid)-bearing Andesite/Basalt(Foid)ites10606035651020206060FAPQRhyolite DaciteTrachyte Latite Andesite/BasaltPhonoliteTephriteClassification of Igneous RocksFigure 2-5. Classification of the pyroclastic rocks. a. Based on type of material. After Pettijohn (1975) Sedimentary Rocks, Harper & Row, and Schmid (1981) Geology, 9, 40-43. b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298.Classification of Igneous RocksFigure 2-2. A classification of the phaneritic igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS.PlagioclaseOlivinePyroxeneOlivine gabbroPlagioclase-bearing ultramafic rocks90(b)AnorthositeOlivineClinopyroxeneOrthopyroxeneLherzoliteHarzburgiteWehrliteWebsteriteOrthopyroxeniteClinopyroxeniteOlivine WebsteritePeridotitesPyroxenites90401010DunitePyroxeneOlivinePlagioclase FeldsparreticulitePele’s tearsPele’s Hair (the really thin strands among the rocks…)ObsidianVolcanic glass formed from more felsic magmas (more silica), typically rhyolitic and usually black from small amount of nanocrystalline magnetite - Glass can get molecularly thin pretty easily, making it very sharp and using through antiquity for knives/spears, and still used today for scalpels (permits a much finer blade than does steel)Obsidian Cliffs, Yellowstone
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