common rock types colby geology. igneous rocks classification based on composition and texture....
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Igneous Rocks
Classification based on composition and texture.
COMPOSITIONPrincipally the rock-forming minerals
OlivinePyroxenesAmphibolesMicasFeldsparsQuartz
Constitute 95% of minerals in ALL igneous rocks.
COMPOSITION
You can determine basic igneous rock compositionjust by the COLOR of the rock.
DARK COLORED igneous rocks = MAFIC
LIGHT COLORED igneous rocks = FELSIC
GRAY COLORED igneous rocks = INTERMEDIATE
Color allows an approximation of igneous rock composition
Related to Bowen’s Reaction Series(red vs. green)
TEXTURE
TEXTURE reflects cooling rate of magma or lava.
SLOW COOLING = LARGE Crystals
RAPID COOLING = SMALL Crystals
SLOW COOLING = PLUTONIC
RAPID COOLING = VOLCANIC
TEXTURETypes of Textures
PhaneriticLarge crystals > 2 mmSlow cooling rate = Plutonic
Granite, Diorite, GabbroLight Gray DarkFelsic Intermediate Mafic
TEXTURETypes of Textures
AphaniticSmall crystals < 2 mmFast cooling rate = Volcanic
Rhyolite, Andesite, Massive BasaltFelsite ScoriaLight Gray DarkFelsic Intermediate Mafic
TEXTURETypes of Textures
GlassyNo crystals, lava cools too fastExtremely fast cooling rate = Volcanic
Obsidian, PumiceLightFelsic
Obsidian
Pumice
TEXTURETypes of Textures
PorphyriticTwo distinct sizes of crystalsStarts cooling slowly, remainder cools quickly
Phenocrysts
Groundmass
TEXTURETypes of Textures
TEXTURE is related to origin of magma or lava.
Phaneritic = Coarse-grained = IntrusivePlutonic
Aphanitic = Fine-grained = ExtrusiveGlassy or smaller Volcanic
Identification/Classification of Igneous RocksKey on page 75-77 in lab manualChart on page 91 in lab manual
IGNEOUS ROCK CLASSIFICATION
COMPOSITION
Ultramafic (Very Dark)
Dunite Peridotite
Mafic (Dark) GabbroBasalt Scoria
Intermediate (Gray) Diorite AndesiteFelsic (Light) Granite
Rhyolite Felsite
Obsidian Pumice
PHANERITIC APHANITIC GLASSY
Sedimentary RocksClastic Sedimentary Rocks
COMPOSITIONDerived from pre-existing rocks.Five main constituents:
QuartzFeldspar
Rock fragmentsClay (and clay minerals)
Calcite
TEXTURE
CLASTIC – fragments of rock debris
Classified by analyzing grain-size
Sediment Size(mm)
Particle Name Rock Name
<0.004
64-256
4-64
2-4
0.063-2
0.004-0.063
>256 Boulder
Cobble
Pebble
Granule
Sand
Silt
Clay
Gravel
Mud
ConglomerateBreccia
Sandstone
Shale
SiltstoneMudstone
Particles>2mm = coarse-grained CONGLOMERATE
(rounded clasts) BRECCIA (angular clasts)
1/16 - 2mm = medium-grained SANDSTONE<2mm = fine-grained SHALE
Sandstones further classified on the basis of theirmineralogical composition:
Mostly quartz = QUARTZ SS.>25% feldspar = ARKOSE1/3 silt, clay and rock fragments = GREYWACKE
Sedimentary RocksChemical Sedimentary Rocks
CHEMICAL SEDIMENTARY ROCKSprecipitated from sea water (evaporites)initially dissolved in watervery similar in appearance to igneous rocks
except MONOMINERALIC.texture is crystalline if visible
microcrystalline if not
OOLITIC LIMESTONECHERTDOLOMITE
CHEMICAL SEDIMENTARY ROCKSROCK SALT or HALITE
composed of halite (NaCl)microcrystalline to crystallineprecipitates from sea waterforms economic deposits
Sedimentary RocksOrganic Sedimentary Rocks
ORGANIC or BIOCLASTIC SEDIMENTARY ROCKScan be crystalline or clastic (bioclastic)all particles are derived from organismstypically composed of calcite or aragonite (CaCO3)
Lithographic Limestone or Micrite
Very fine-grained limestoneFormed from calcareous
algaeFizzes in HCl
NONCARBONATE ORGANIC SED. ROCKSCHERT
microcrystallinecomposed of skeletons, microscopic in sizemade of SiO2
hardness = 7may exhibit conchoidal fracture
NONCARBONATE ORGANIC SED. ROCKSPEAT
composed of compressed plant debrisbasic building block of coalused as a fuel source in some areas
Identification/Classification of Sedimentary RocksKey on page 79-80 in lab manualChart on page 93 in lab manual
Metamorphic RocksRocks are fundamentally changed by HEAT and/or
PRESSUREMay be derived from ANY pre-existing rock type
CLASSIFICATIONbased on the presence or absence of FOLIATION
FOLIATION is the parallel alignment of the tabularminerals (micas and clay minerals) and by varying degrees of banding.
Metamorphic RocksTypes of Foliation
Slatey cleavagevery fine-grained rocks
Usually found in slate and phyllite
Metamorphic RocksTypes of Foliation
Gneissic bandingminerals segregated into bands
Usually found in gneiss
Metamorphic RocksFoliated Rocks
SLATE Have slatey cleavagePHYLLITE Rock splits along parallel planes
SCHIST Has schistosity
GNEISS Has gneissic banding
Metamorphic RocksNonfoliated Rocks
NO structure is apparent in the rockRock is typically MASSIVE (monomineralic)
Metamorphic RocksNonfoliated Rocks
MARBLEIs composed of calcite (CaCO3), soft H = 3Will react to dilute hydrochloric acid
ANTHRACITE COALFormed from organic material (plant debris)Contains no minerals
Metamorphic RocksNonfoliated Rocks
Identification/Classification of Metamorphic RocksKey on page 82 in lab manualChart on page 95 in lab manual
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