CEE 437 Lecture 11

Rock Classification

Thomas Doe

Translation of Mineral Properties to Rock Properties

Comparison of mineral properties to rock properties

Rocks have lower strength, especially tensile strengthAnisotropy of minerals and heterogeneity of minerals

ElasticityThermal expansionDiversity of mineral orientation

Creation of microcracks on mineral boundaries

Minerals – Summing Up

Most earth materials are minerals, that is, they are crystallineMineral structures can lead to anisotropic propertiesSilicates are the dominant rock-forming mineralsSheet silicates are important for engineering – micas and claysMineral heterogeneity and anisotropy leads to microcrackformation which greatly influences rock properties

Sedimentary Rocks

Clastics, Siliciclastics, and EvaporitesClastic rocks, depositional medium, and energyDiagenesis — chemical changes after deposition

Sedimentary Rocks and Rock Properties

Properties for a given geologic description vary wildly based on cementation, porosity and other diagenetic factors.Properties can be strongly anisotropic and heterogeneous based on bedding

Clastic Sedimentary Rocks

Clastic — broken like iconoclast)Often referred to as Siliciclastics as having Si based rock forming mineralsBased on grain size and to a lesser extent compositionGrain size related to energy of depositional environment

Relationship of medium velocity to maximum grain size)

Clastic Sedimentary Rocks

Clay, muds → shales, mudstones, claystones (difference based on fissility)Silts → siltstonesSands → sandstonesGravels → Conglomerates (Breccia if angular, breccia may also be a term for tectonically fragmented rock)

Weathering Cycle

Clastic Sediments

Lithification

Cementationdeposition of a material different from clasts

Crystallizationcrystal growth on clasts to fill pore space

CompactionDiagenesis

Early post-depositional chemical transformation of sediments, e.g. calcite to dolomite

Carbonates

Generally like siliciclastics — carbonate muds, sands, etc.Often deposited in reefsMajor portion of world oil depositsProperties depend strongly on post-depositional pore chemistry

CementationDissolution

Karst topography, cave formation

Carbonate Environments

Evaporites

Rock salt (NaCl), Gypsum-Anhydrite (CaSO4), Sylvite (KCl)Deposition in regions where evaporation exceeds recharge

desert lakesrestricted seas (Mediterranean)lagoons, back-reef areas

Subject to flow and diapirism

Other Sedimentary Rocks

Chert: finely crystalline silicaas replacement/diagenetic nodulesas bedded material from silica-shelled biota

CoalDerived from vegetation

Banded Iron FormationLikely bacteria derived, mainly Pre-Cambrian

Igneous Origins

IntrusiveBatholithic or plutonic: phaneriticDikes or sills that chill rapidly: aphanitic

Extrusivedeposition as melt (lava)pyroclastic

tufftephrapyroclastic flows

Geologic Settings for Igneous Rocks

OceanicHi Fe, Mg, Ca, low Sibasalt, gabbro

Continental Hi Si, Na, Kgranite, rhyolite, andesite

Differentiation of Crustal Composition

Weathering differentiating towards higher Silica

Preferential melting of high-silica materials

Concentration of C, Ca, Na, K in sea and air

Original basaltic composition of crust

Carbonate concentrated by organic processes

Identifying Igneous Rocks

ChemistryAcidic: Basic (more Si, less Si)

TextureAphanitic: crystals not visiblePhaneritic: made of visible crystal componentsPorphyritic: Larger crustals in aphanitic or phaneritic ground mass

Bowen’s Reaction Series

Igneous Rock Classification

SERPENTINITE

Acidic, Felsic Basic, Mafic Ultramafic

Extrusives

Viscosity varies with Si and water contentBasalt — low viscosityRhyolite — high viscosity

Rhyolite flows relatively unusual as rhyolite does not flow well

ExplosiveTuffs, pyroclastics

Volcano Types

Basaltic: low viscosity — Hawaii, Columbia Plateau

Andesitic/Rhyolitic

Structures of Basalt Flows

Lava TubesFlow Stratigraphy

collonadeentablatureflow top breccia/scoria

Hawaii Basalt Flows

Basalt Flow Structures

Eruptions of Acid-Rock Volcanoes

Rhyolite Dome

Mt. St. Helen’s Blast Zone

Mt. Mazama Ash Distribution

Basic Metamorphic Types

Quartz Sandstone → QuartziteLimestone, Dolomite → MarbleShale →

Slate — cleavage, no visible xl’sPhyllite — foliation, mica sheen but xl’s not visibleSchist — clear foliation, visible micaGneiss — like granite but with foliation/gneissosity

Basalt → greenschist, amphibolite

Non-foliated Metamorphic Rocks

Sandstone —> QuartziteLimestone —> MarbleDolomite —> Dolomitic Marble

Foliated Metamorphic Rocks

Shale/MudstoneSlatePhyllite (Greek for leaves e.g. phyllo dough)SchistGneiss

Origin of Foliation (gneissosity, schistosity)

Engineering Properties

Anisotropy of strength and elastic propertiesPreferred failure on foliation

Slate

Phyllite

Schist

Gneiss

Banded Gneiss

Metamorphic Grade

Subduction-Zone Metamorphism

Metamorphism at Continental Collisions

Contact Metamorphism

CEE 437 �Lecture 11� Rock ClassificationTranslation of Mineral Properties to Rock PropertiesMinerals – Summing UpSlide Number 4Slide Number 5Sedimentary RocksSedimentary Rocks and Rock PropertiesClastic Sedimentary RocksClastic Sedimentary RocksWeathering CycleClastic SedimentsLithificationCarbonatesCarbonate EnvironmentsEvaporitesOther Sedimentary RocksIgneous OriginsGeologic Settings for Igneous RocksDifferentiation of Crustal CompositionSlide Number 20Identifying Igneous RocksBowen’s Reaction SeriesIgneous Rock ClassificationExtrusivesVolcano TypesStructures of Basalt FlowsHawaii Basalt FlowsBasalt Flow StructuresEruptions of Acid-Rock VolcanoesRhyolite DomeMt. St. Helen’s Blast ZoneMt. Mazama Ash DistributionBasic Metamorphic TypesNon-foliated Metamorphic RocksFoliated Metamorphic RocksOrigin of Foliation (gneissosity, schistosity)Engineering PropertiesSlatePhylliteSchistGneissBanded GneissMetamorphic GradeSubduction-Zone MetamorphismMetamorphism at Continental CollisionsContact Metamorphism