The Rock CycleFundamental QuestionsSlide 4Intrusive Rocks in Northern Victoria Land, AntarticaPlate Tectonics and the Rock CycleIgneous Rocks: TerminologyClassification of Igneous and Volcanic RocksClassification Schemes IClassification based on Field Relations and TexturesRhyolite Hand SpecimenGranite Hand SpecimenMore on Fabric ClassificationMagmatic IntrusionDeeply Eroded IntrusionsCoarse Grained Intrusive Rock TexturePhotomicrograph - Phaneritic TextureAndesite Hand SpecimenDiorite Hand SpecimenClassification based on Mineralogy & ChemistryColor IndexGabbro Hand SpecimenBasalt Hand SpecimenSummary Classification for Igneous RocksIgneous Rock Identification: SummaryIntrusive BodiesSan Cristobal Volcano, NicaraguaVolcanic Neck and DikeAerial Photo of Shiprock, New MexicoDikes vs. Sills2 m Dike in Dominica, West IndiesDominica Dike along Strike ViewDikes in northern Victoria Land, AntarticaEdinburgh SillMagmatic DiapirsCoalescing Diapirs and PlutonsPegmatite OutcropSierra Nevada Batholith Igneous Rocks, Intrusive Activity, and the Origin of Igneous RocksChapter 3Photo credit: G. MattioliThe Rock Cycle•A Plate Tectonic ExampleIgneous Rocks •Igneous Rock Textures•Identification of Igneous Rocks•Varieties of Granite•Chemistry of Igneous RocksFundamental Questions•How are rocks sampled in the field and analyzed in the lab to determine their chemical, modal, and mineralogical composition?•What do these analyses tell us about the composition of magmatic rocks?•How can the data be presented to elucidate compositional patterns and contrasts?•How do we classify magmatic rocks to convey meaningful petrogenetic information on the origin and evolution of the magma from which they solidified?The Rock CycleIntrusive Rocks in Northern Victoria Land, AntarticaPlate Tectonics and the Rock CycleIgneous Rocks: Terminology•Igneous rocks are formed as a result of cooling and crystallization from a magma.•Magma is molten rock (fluid), rich in silica (SiO2), which contains dissolved volatiles (e.g. CO2 and H2O).•Lava is magma extruded on or very near the Earth’s surface. Most lavas have been significantly degassed en route to the surface.Classification of Igneous and Volcanic Rocks•Based on hand specimen fabric•Based on field relationships and textures•Based on mineralogy and chemistry–Color IndexClassification Schemes I•Based on Fabric–Phaneritic: rocks with mineral grains that are large enough to be identified by eye. Texture is typical of slowly cooled intrusive rocks.–Aphanitic: rocks with grain too small to be identified by eye. Texture is most common in rapidly solidified extruded magma and marginal facies of shallow intrusions.Classification based on Field Relations and Textures•Extrusive or volcanic rocks: typically aphanitic or glassy. This means that they are generally fine grained in texture. Grains are typically 0.5 to 1 mm. Common example is basalt.–Many varieties are porphyritic. This means that the grain size is bimodal, with a fine grained matrix surrounding larger grains that are called phenocrysts. Common example is andesite.•Intrusive or plutonic rocks: typically phaneritic. This means that they are generally coarse grained and this texture is often quite uniform. Grains can range in size but are often clearly visible to the naked eye (>2-3 mm). Common example is granite.–Amphiboles and biotites are commonly altered to chlorite. Muscovite found in some granites, but rarely in volcanic rocks. Perthitic feldspar, reflecting slow cooling and exsolution, is widespread.Rhyolite Hand SpecimenGranite Hand SpecimenMore on Fabric Classification•Porphyritic texture: magmatic rocks with bimodal grain size distributions. –Larger grains are called phenocrysts–Smaller grains constitute the groundmass or matrix–Porphyritic aphanitic rocks are more common than porphyritic phaneritic rocks•Glassy or vitric texture: rocks that contain variable proportions of glass.–Holocrystalline rocks: wholly composed of crystals–Vitrophyric rocks: porphyritic rock with phenocrysts in a glassy matrixMagmatic IntrusionDeeply Eroded IntrusionsGRANITE INTRUSIONSHALE COUNTRY ROCKTorres del Paine, ChileCoarse Grained Intrusive Rock TexturePotassium Feldspar (stained yellow)Photomicrograph - Phaneritic TexturePhoto credit: C.C. PlummerInterlocking grainsAndesite Hand SpecimenPlagioclase FeldsparphenocrystsMatrix or groundmassDiorite Hand SpecimenInterlocking grainswith uniform sizeClassification based on Mineralogy & Chemistry•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 referred to as sialic (Si + Al).Color Index•Defined as the modal proportion of dark-colored minerals in a rock. Should really be based on the proportion of ferromagnesian minerals as feldspars may range in color.–Leucocratic: 0-30% mafics–Melanocratic: 60-100% maficsGabbro Hand SpecimenBasalt Hand SpecimenFined grained mafic volcanic rockSummary Classification for Igneous RocksIgneous Rock Identification: SummaryIntrusive Bodies•Shallow Intrusive Structures–Sills–Dikes–Volcanic necks•Intrusive Rocks That Crystallize at Depth•Abundance and Distribution of Plutonic Rocks•Sierra Nevada Batholith•PegmatitesSan Cristobal Volcano, NicaraguaPhoto Credit: G. MattioliVolcanic Neck and DikeAerial Photo of Shiprock, New MexicoFrom: http://www.ngdc.noaa.gov/seg/hazardDikes vs. SillsSills: Concordant structuresParallel to pre-existing layersDikes: Discordant structuresCross-cut pre-existing layers2 m Dike in Dominica, West IndiesPhoto Credit: G. MattioliDominica Dike along Strike ViewPhoto Credit: G. MattioliDikes in northern Victoria Land, AntarticaEdinburgh SillFrom: http://www.ngdc.noaa.gov/seg/hazardMagmatic DiapirsA diapir is a dome that is cored by plasticmaterial, in this case,partially molten rock.Concept first appliedto salt domes.Coalescing Diapirs andPlutonsRise because ofbuoyancy-magmais lower densitythan rocks. But siliceous magmas have high viscosity or resistanceto flow, whichmakes eruptiondifficult withouthigh gas