1Earth Materials: Intro to rocks & Igneous rocks• Three main categories of rocks: • Igneous•Sedimentary1•Sedimentary• Metamorphic• The most common minerals in rocks (a.k.a. the “rock-forming” minerals)• Igneous Rocks – description and classificationThe three major categories of rocks Fig 3.1 Understanding Earth2The three major categories of rocks Fig 3.1 Understanding Earth3Composed of crystals and/or glassMay have layers if volcanicClassified using composition and textureMost are composed of fragments (e.g. sand); Some may be crystalline.Always layered/stratified at some scaleClassified using composition and textureAlways crystallineSome have aligned crystals (foliation), some do notClassified using composition and textureRelative abundance of rock types in the earth’s crust4Fig 3.4 Understanding EarthThe “Rock-forming” mineralsThe 8 most abundant mineral in the earth’s crust and the most common minerals in rocks are often called the “rock-forming” minerals. All 8 can be found in igneous rocks, and all 8 are silicates.Quartz (1)Feldspars: Alkali feldspar (2)Plil fld5Plagioclase feldspar(3)Micas: Muscovite mica (4)Biotite mica (5)Pyroxene (6)Amphibole (7)Olivine (8)Understanding Earth62Igneous rocks• General categories based on where they form• Intrusive (plutonic) • Extrusive (volcanic)• Types of Intrusions• Description and classification•Texture7Texture• crystal size(s)• bubbles•fragments• Composition (minerals and color)•Felsic• Intermediate•Mafic• Origin of magmaFormation of igneous rocks: intrusive and extrusive rocksMagma vs. Lava8gMagma is molten rock underground. Intrusive or PlutonicLava is molten rock which has reached the surface. Extrusive or VolcanicFig 3.2 Understanding EarthExtrusive rocks:Volcanoes and Lava flowsFig 4.8 Understanding Earth9October 1915 eruption of Mt. Lassen (photo by Chester Mullen)Fig 4.8 Understanding EarthSome major types of igneous intrusions (plutons)Vein - very thin, tabular intrusion- the smallest type of intrusionSill - intrudes between rock layers, tabular shapeDike - cuts across pre-existing rock layers, tabular shape10pgy, p- the main way in which magmas reach the surfaceStock - cuts across rock layers, irregular shapeBatholith - like a stock, but much larger (area of at least 100 km2)- the largest type of igneous body- usually comprised of numerous individual intrusionsVeins of granite cutting through metamorphosed sedimentary rock11Fig 4.2 Understanding EarthSill12Intrusive rocks:Sill and dikeFig 4.15 & 4.16 Understanding EarthDike3Part of the Sierra-Nevada batholith in Yosemite N. P.A huge mass of intrusive rocks composed of many individual intrusions (plutons)13Classification of igneous rocks by texture and compositionIgneous textures: crystal sizeall large crystalsd d b l ti l l li d d d14-produced by relatively slow cooling deep underground-found in intrusive igneous rocks(also known as plutonic igneous rocks)phaneritictexture - large crystalspegmatitictexture - extremely large crystalsClassification of igneous rocks by texture and compositionIgneous textures: crystal sizeat least partly small crystals and/or glass- produced by relatively fast cooling, usually at or near the earth’s surface15- typical of extrusive (volcanic) rocks - sometimes in intrusive rocks (e.g. shallow dikes)glassytexture - no crystalsaphanitictexture - very small crystalsporphyritictexture - mixed sizes of crystalsGlassy texture: Obsidian, pumice, and volcanic ashFig 4.3 Understanding Earth16Igneous texturesGeology 2ndedChernicofffine = aphanitic mixed = porphyritic17Geology 2ed. -Chernicoffcoarse = phaneritic extremely coarse = pegmatiticClassification of igneous rocks by texture and compositionIgneous textures: other texturesbubbles:vesicular textureGas bubbles trapped in an igneous rock are called vesicles.Examples: vesicular basalt and pumice18Examples: vesicular basalt and pumicebroken fragments: pyroclastic textureIf there is enough gas, expanding bubbles may cause lava to explosively break into fragments.Examples: volcanic ash and pumicePyroclastic deposits may have some of the same features as sedimentary rocks.4Vesicular textures:basalt with vesicles (left) and pumice (below)Understanding Earth19Pyroclastic (fragmental) texturesVolcanic ash20Tuff (this one contains both ash and pumice)PumiceClassification of igneous rocks by texture and compositionCompositionobserved as variations in:• mineral abundance (quartz, feldspar, etc.), • elemental abundance (silicon, oxygen, iron, etc.), 21• color (dark, medium, light)Ultramafic i.e. peridotiteMafic i.e. basalt and gabbroIntermediate i.e. andesite and dioriteFelsic i.e. rhyolite and graniteClassification of igneous rocks: texture and compositionMafic rocks: basalt (fine) and gabbro (coarse)22Fig 4.5 Understanding EarthExtrusive/Volcanic(glassy, aphanitic, or porphyritic)Intrusive (coarse or porphyritic)Felsic rocks: rhyolite (fine) and granite (coarse)Ultramafic rocksUnderstanding EarthClassification of igneous rocks: texture and composition23Example: Peridotite Composition Felsic Intermediate Mafic Ultramafic Extrusive (glassy, aphanitic, porphyritic, Rhyolite Dacite Andesite Basalt Komatiite (rare)re Igneous rock classification24fragmental, vesicular, etc.) (rare)TexturIntrusive (phaneritic, pegmatitic) Granite Granodiorite Diorite Gabbro Peridotite more silica; less iron & magnesiumless silica; more iron & magnesiumlighter darker5Igneous rockcompositionsMore silica (SiO2), sodium, potassiumMore iron, magnesium, calciumLighter color Darker colorg 4.6 Understanding Earth25modified from FigRhyolite or GraniteDacite or GranodioriteAndesite or DioriteBasalt or GabbroKomatiite or PeridotiteHow are magmas formed?by partial melting of crust or mantle rocks Three ways to melt rocks in the Earth’s mantle and crust:26• increase temperature• decrease pressure on already hot rock (decompression)• add fluids (water) to already hot rockMagma formation at plate boundaries and hot spots27Fig 4.8 Understanding EarthMagma formation by addition of fluids at subduction zones28Modified from Fig 4.19 Understanding EarthWhere and how are magmas formed?Mid-ocean ridges – decompressionmafic (basaltic) magmaContinental rifts and other areas of extension – decompressionmostly mafic (basaltic) magmaSbd tilti d t dditi f fl id29Subduction zones –melting due to addition of fluidsmafic, intermediate, and felsic