Presentation on theme: "Names of Igneous Rocks Texture + Composition = name Set up diagrams (many ternary ones again, you remember how these work?) to represent composition changes."— Presentation transcript:
Names 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 rock
Visual Estimation of Modal Abundance
Classification 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).
Classification of Phaneritic Igneous Rocks Figure 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 of at least 10% of the minerals below. Renormalize to 100% Quartz-rich Granitoid Alkali Fs. Quartz Syenite Quartz Syenite Quartz Monzonite Quartz Monzodiorite Syenite Monzonite Monzodiorite (Foid)-bearing Syenite (Foid)-bearing Monzonite (Foid)-bearing Monzodiorite 90 Alkali Fs. Syenite (Foid)-bearing Alkali Fs.Syenite 10 (Foid) Monzosyenite (Foid) Syenite (Foid) Monzodiorite (Foid) Gabbro Qtz. Diorite/ Qtz. Gabbro 5 10 Diorite/Gabbro/ Anorthosite (Foid)-bearing Diorite/Gabbro 60 (Foid)olites Quartzolite Granite Grano- diorite Tonalite Alkali Feldspar Granite Q A P F 60 Q=quartz A=Alkali fledspars (An 0 -An 5 ) P=Plagioclase feldspars (An 5 -An 100 ) F=Feldspathoid
Classification of Igneous Rocks Figure 2-4. A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al. (1986) J. Petrol., 27, Oxford University Press.
Aphanitic rocks basalt rhyolite
Classification of aphanitic Igneous Rocks Figure 2-3. A classification and nomenclature of volcanic rocks. After IUGS.
Classification of Igneous Rocks Figure 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, b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1,
Classification of Igneous Rocks Figure 2-2. A classification of the phaneritic igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS. Olivine Clinopyroxene Orthopyroxene Lherzolite Harzburgite Wehrlite Websterite Orthopyroxenite Clinopyroxenite Olivine Websterite Peridotites Pyroxenites Dunite Pyroxene Olivine Plagioclase Feldspar
reticulite Pele’s tears Pele’s Hair (the really thin strands among the rocks…)
Obsidian Volcanic 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 N.P.