How 2 nd half labs will work Continue to use your mineral identification skills Learn to describe, classify, interpret rock hand samples: Igneous sedimentary metamorphic volcanic plutonic (1 week) (1 wk) Assignments: 1. You get a lab handout with each set of rocks Spend 2 lab periods working on the questions Turn in at beginning of next lab 2. Formal rock descriptions: one per lab handout (= 4 total) One-page detailed description of one rock sample
Igneous Petrology: study of melts (magmas) and the rocks that crystallize from them Igneous rocks Volcanic vs. Plutonic Erupts onto the surface (=cools fast) Doesn t make it to the surface (= cools slowly underground) Extrusive Intrusive Mt. Etna fire fountain Lake Ann stock, Mt Baker Our goal: identify, classify, interpret these rocks
How we classify rocks: (1) mineralogical composition (2) texture Importance of Textures Texture is: Size, shape and arrangement of crystals (+- glass) Helps us understand processes that formed a rock: its origin and history Textures in igneous rocks controlled by: 1 Magma Composition 2 Cooling Rate
Cooling Rates Control: 1 Nucleation Rate How fast are new, tiny mineral seeds born 2 Growth Rate How fast do the seeds grow bigger
How do minerals know when to start nucleating & growing? Expected to begin at liquidus.but doesn t Always starts below the liquidus because: 1 2 3 Nucleation Growth Diffusion of elements These factors control crystallization, and cannot keep up with temperature drop = Undercooling The degree to which the temperature of a melt falls below the true crystallization temperature before minerals begin to form
Situation #1: slow cooling (= plutonic rocks) Slow cooling results in minor undercooling (at T a ) and crystals form here molten Result: Rapid growth rate Slow nucleation rate or rate liquidus Few coarse-grained (large, >5mm) crystals Where crystals should ideally begin to form
Situation #2: faster cooling (= volcanic rocks) Rapid cooling permits more undercooling (at T b ) and crystals form here molten Result: Slower growth rate Rapid nucleation rate or rate liquidus Many fine-grained (small, <1 mm) crystals Where crystals should ideally begin to form
Extremely rapid cooling causes the most undercooling (at T c ) but crystals don t grow Situation #3: Super fast cooling (= volcanic rocks like obsidian or the rinds on pillow basalt) molten Result: Little, if any, nucleation or growth or rate liquidus No crystals: The magma quenches to a glass Where crystals should ideally begin to form
Textures in igneous rocks controlled by: 1 Magma Composition 2 Cooling Rate Why?
How Magma Composition Affects Texture: Through Viscosity Basalt Andesite Dacite Rhyolite Log Viscosity Low temp Hi temp 50 60 70 Weight % SiO 2 in magma Viscosity causes undercooling
Igneous rocks Volcanic vs. Plutonic Erupts onto the surface (=cools fast) Doesn t make it to the surface (= cools slowly underground) Extrusive Many fine-grained crystals to No crystals (totally glass) Glass forms easiest in SiO 2 -rich magmas (like rhyolites), which are viscous. This inhibits diffusion of elements and prevents crystals from forming & growing. Intrusive Few coarse-grained crystals Picture of obsidian lava flow
Big Obsidian Flow Newberry Volcano, OR
Igneous Textures 3 major ones 1 2 3 Aphanitic - Crystals (if any) too small to see with naked eye Phaneritic - Crystals visible with naked eye Fragmental - Accumulated chunks of igneous rocks, crystal fragments, glass, pumice - Forms in violent eruptions - Collectively known as Pyroclastic
Igneous Textures 3 major ones 1 2 Aphanitic - Crystals (if any) too small to see with naked eye Phaneritic - Crystals visible with naked eye Either may also be described as Porphyritic = contains phenocrysts 3 Fragmental - Accumulated chunks of igneous rocks, crystal fragments, glass, pumice - Forms in violent eruptions - Collectively known as Pyroclastic Phenocrysts Crystals that are conspicuously larger than others Groundmass or Matrix Fine-grained part of rock that surrounds the phenocrysts Many other common textures: See handout
Porphyritic? Yes 3 types of phenocrysts: Chocolate chips Walnuts Raisins Aphanitic groundmass (= the dough) Groundmass: glassy or fine-grained? Hopefully not glassy! This rock, cookiite, is porphyritic with aphanitic groundmass
Crystal Habits (aka Shape) Equant Tabular Bladed Columnar Acicular Prismatic Platy Look in your lab handout
Quartz, Olivine Equant habit (equidimensional) Look for conchoidal fracture (no cleavage) Quartz translucent white to gray Olivine translucent yellow-green Plagioclase, Sanidine, Orthoclase (Microcline usu. metamorphic only) Usually Tabular or Bladed habit Look for flat faces (due to cleavages) Plagioclase Albite + Anorthite white polysynthetic (=albite) twinning (fine striations on faces: use hand lens) Amphibole: (usually black hornblende) Prismatic or Acicular (pencil-like) habit Cleavages at 60/120 Micas: Biotite black Phlogopite Muscovite gold tan clear Platy habit Super-shiny flat cleavage surfaces K-spars: Orthoclase Sanidine plutonic rocks only pink (usually) or white volcanic rocks only clear (rarer) Pyroxene: Prismatic or Columnar habit Orthopyroxene brown Clinopyroxene green Cleavages at 90 deg
Igneous rocks Volcanic vs. Plutonic Erupts onto the surface (=cools fast) Doesn t make it to the surface (= cools slowly underground) Extrusive Spectrum ranging from: Many fine-grained crystals to No crystals (totally glass) Aphanitic or porphyritic-aphanitic or fragmental Hypocrystalline, Holohyaline, or Holocrystalline Intrusive Few coarse-grained crystals Phaneritic or porphyritic-phaneritic Holocrystalline
Volcanic Rocks 1 Determine the mode: volume percent of each phenocryst mineral in the rock = MODE Example: Mode of some average granite might be 25% quartz 30% plagioclase 5% biotite 30% kspar 10% hornblende 2 Color of rock: felsic vs. mafic Felsic light-colored minerals feldspar silica Mafic dark-colored minerals magnesium ferric iron (except for anorthite!) Apply terms to rocks: Felsic rocks: light color; has mostly felsic minerals Mafic rocks: dark color; mostly mafic minerals Ultramafic rocks >90% mafic minerals
Finally, put it all together: Color Grain Size Phenocrysts + Groundmass Basalt: Dark-colored Fine grained (± porphyritic) ~ equal amounts plag + cpx Maybe olivine No olivine Andesite: Grayish Commonly porphyritic Plag, opx, cpx, hornblende G mass: aphanitic or glassy No quartz Has quartz *subdivided into tholeiitic, alkalic, calc-alkaline* Dacite: Light-colored Commonly porphyritic Plag, qtz, sanidine, hbl, bio G mass: fine-grained or glassy Rhyolite: Light-colored Can be porphyritic or be Plag, qtz, sanidine totally glassy (obsidian) G mass: glassy or very fine-grained *often occurs as pyroclastic rocks (tuffs, ignimbrites, pumice)*