Phenocrysts in Volcanic RocksGroundmass in Volcanic RocksTextures of Volcanic RocksGrain SizeMineralogyMuscovite - silvery white, Biotite - jet blackBreeds of Plutonic RocksIGNEOUS ROCKS -- LAB PREPDefinitions:Felsic - mnemonic derived from feldspar + silica describing igneous rocks having abundant lightcolored minerals (mostly quartz, feldspars). HighSi, low Fe, Mg.Mafic - mnemonic derived from magnesium + ferric describing igneous rocks containing chieflydark ferromagnesian minerals. Higher Fe, Mg,lower Si.Intermediate - intermediate composition between mafic and felsicUltramafic - said of a rock that is composed mostly of mafic (ferromagnesian) minerals, such asperidotite.Felsic lavas are the most viscous (and hence mostexplosive), due to the stronger interactionsbetween silica tetrahedra, while mafic lavas are theleast viscous and flow readily (hence Plateau orFlood Basalts)Volcanic (extrusive) basalt, andesite, rhyoliteThe rocks are characterized by a fine-grained groundmass (often aphanitic) which resultsfrom rapid cooling after eruption. The much coarser phenocrysts are minerals that crystallizedbefore the lava erupted at the surface. All of the minerals that did not crystallize before eruptionmake up the groundmass.Definitions:Groundmass - the material between the phenocrysts in a porphyritic igneous rockPhenocryst - any of the large crystals in porphyritic igneous rocksPorphyritic - an igneous rock in which large crystals are enclosed in a matrix (or groundmass) ofmuch finer grained minerals (or obsidian)Aphanitic - describing an fine-grained igneous rock with a groundmass too small to be distinguishedwith the unaided eye.Phenocrysts in Volcanic Rocks Quartz - gray to black grains with greasy to glassy luster. Irregular cleavage. Only found in rhyolite.Feldspar - pink to white to dark gray grains with shiny luster with good cleavage in 2 directions(only 1 obvious in phenocrysts).Biotite - black grains with one excellent cleavageOlivine - greenish-yellow crystals with glassy luster and irregular cleavage.Groundmass in Volcanic Rocks The porphyritic rocks can usually be recognized by the phenocrysts and the color of thegroundmass -- black for basalt, light colored for rhyolites and gray to grayish-green for andesites.The non-porphyritic rocks are trickier as they contain no phenocrysts (the most definitivetest). Obsidians are volcanic glass with the composition of rhyolites, Felsite which is light coloredmicrocrystalline material corresponds to rhyolites or andesites, and Trap which is dark-coloredmicrocrystalline material and corresponds to basalt.Textures of Volcanic Rocks The textures of volcanic rocks gives some indication of the way in which they erupted:Volcanic rocks may be Flow Rocks which erupted as lava flowing onto the surface of theEarth, or they may be Pyroclastic which are expelled into the air from the volcanic vent.Pyroclasts range from dust, ash (up to 2mm), or bombs (greater than 6cm). Flow rocks havea range of textural features, notably the shape of the flow surface, with Pillow Lavas, Pahoehoe (flowlavas that have a surface ropey texture), Aa (slower moving lava with rock broken jagged surface).Pyroclastic (Tephra) deposits (Tuf) often form layered structures due to a sequence of eruptions.Sedimentary ash deposits are particularly common.When there is a lot of gas dissolved in a magma as it reaches the Earth's surface (gasesinclude H2O, SO2, H2S, CH4, CO2, HCl etc), these gases may be trapped in the lava forming gasbubbles. Depending on how fast the lava cools, you will get vesicular basalt (slower cooling) orpumice (very fast cooling often associated with pyroclasts).Plutonic (intrusive) gabbro, diorite, graniteThese rocks are characterized by a relatively larger grain size, due to the much longer time forcrystallization to occur from the magma in an intrusive (insulating) environment. The compositionsof the plutonic rocks parallel those of the volcanic.Questions: How are Plutonic rocks diferent from Volcanic rocks?Volcanic rocks are extruded onto the surface of the EarthPlutonic rocks are formed at depth in the Earth (greater than several km down).Volcanic rocks have mostly fine grain sizes, as they cooled rapidlyPlutonic rocks have coarse grain sizes, because they cooled slowlyVolcanic rocks are often porphyritic (contain phenocrysts) in a fine grained groundmassPlutonic rocks are rarely porphyritic due to the more uniform and slower rate of coolingVolcanic rocks often contain pores where gas came out of the magma and formed bubblesPlutonic rocks rarely contain gas bubbles or poresPlutonic rocks are generally equigranular aggregates of phaneritic mineral grains (grains thatare large enough to be visible as individual grains in hand specimen without the aid of amicroscope). Although this is largely true, the mafic grains (micas etc) are often smaller and havedifferent shapes from the feldspar and quartz grains.Grain Size The grain size of a plutonic rock mostly reflects the depth at which the rock was emplaced andcooled. The deeper the emplacement, the slower the cooling and the more time for the grains togrow. Although this observation is generally true, it is not the whole truth, as we have rocks such aspegmatites which have extremely large grain sizes, which rather reflect anomalous water-richconditions in the growth region.Mineralogy The properties of quartz, feldspars and olivine have been discussed in volcanic rocks whenwe talked about phenocrysts.Quartz - white to dark gray grains with greasy to glassy luster. Irregular cleavage, sometimesconchoidal fracture. Only found in granite (of the plutonic rocks).Feldspar - pink to white to dark gray grains with shiny luster with good cleavage in 2 directionsMuscovite (mica) and Biotite - dark grains with one excellent cleavage, shiny flakes.Muscovite - silvery white, Biotite - jet blackMuscovite is not found in volcanic rocks as it is the last mineral to crystallize from a magmaand there is not enough time in a lava to reach this far, so that the mica is tied up in the aphaniticgroundmassPyriboles (pyroxenes and amphiboles) may also occur as black grains with 2 cleavage directionsOlivine - greenish-yellow crystals with glassy luster and irregular cleavage. Olivine does not appearthat often in plutonic rocks as it usually reacts with the magma during cooling to produce