Common Rock Types

Colby Geology

Igneous RocksClassification 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)

Granite-Rhyolite

Diorite-Andesite

Gabbro-Basalt

1200°C

600°C

900°C

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

Rhyolite Felsite

Andesite

Massive Basalt Scoria

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

Coarse-grained Clastic Sedimentary Rocks

Conglomerate Breccia

Medium-grained Clastic Sedimentary RocksSandstones

Arkose

Greywacke Qtz. Sandstone

Fine-grained Clastic Sedimentary RocksShale

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

Oolitic Limestone

Chert

Dolostone

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

Fossiliferous Limestone

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

Schistosityplaty minerals visible

Usually found in schist

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

QUARTZITEMetamorphosed quartz sandstoneHardness = 7

Metamorphic RocksNonfoliated Rocks

ANTHRACITE COALFormed from organic material (plant debris)Contains no minerals

Metamorphic RocksNonfoliated Rocks

GREENSTONEComposed of metamorphosed mafic igneous rocks

Metamorphic RocksNonfoliated Rocks

Identification/Classification of Metamorphic RocksKey on page 82 in lab manualChart on page 95 in lab manual