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 strength Anisotropy of minerals and heterogeneity of minerals Elasticity Thermal expansion Diversity of mineral orientation Creation of microcracks on mineral boundaries
Minerals Summing Up Most earth materials are minerals, that is, they are crystalline Mineral structures can lead to anisotropic properties Silicates are the dominant rock-forming minerals Sheet silicates are important for engineering micas and clays Mineral heterogeneity and anisotropy leads to microcrack formation which greatly influences rock properties
Sedimentary Rocks Clastics, Siliciclastics, and Evaporites Clastic rocks, depositional medium, and energy Diagenesis 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 minerals Based on grain size and to a lesser extent composition Grain 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 siltstones Sands sandstones Gravels Conglomerates (Breccia if angular, breccia may also be a term for tectonically fragmented rock)
Weathering Cycle
Clastic Sediments
Lithification Cementation deposition of a material different from clasts Crystallization crystal growth on clasts to fill pore space Compaction Diagenesis Early post-depositional chemical transformation of sediments, e.g. calcite to dolomite
Carbonates Generally like siliciclastics carbonate muds, sands, etc. Often deposited in reefs Major portion of world oil deposits Properties depend strongly on post-depositional pore chemistry Cementation Dissolution Karst topography, cave formation
Carbonate Environments
Evaporites Rock salt (NaCl), Gypsum-Anhydrite (CaSO 4 ), Sylvite (KCl) Deposition in regions where evaporation exceeds recharge desert lakes restricted seas (Mediterranean) lagoons, back-reef areas Subject to flow and diapirism
Other Sedimentary Rocks Chert: finely crystalline silica as replacement/diagenetic nodules as bedded material from silica-shelled biota Coal Derived from vegetation Banded Iron Formation Likely bacteria derived, mainly Pre-Cambrian
Igneous Origins Intrusive Batholithic or plutonic: phaneritic Dikes or sills that chill rapidly: aphanitic Extrusive deposition as melt (lava) pyroclastic tuff tephra pyroclastic flows
Geologic Settings for Igneous Rocks Oceanic Hi Fe, Mg, Ca, low Si basalt, gabbro Continental Hi Si, Na, K granite, rhyolite, andesite
Differentiation of Crustal Composition Weathering differentiating towards higher Silica Carbonate concentrated by organic processes Preferential melting of high-silica materials Concentration of C, Ca, Na, K in sea and air Original basaltic composition of crust
Identifying Igneous Rocks Chemistry Acidic: Basic (more Si, less Si) Texture Aphanitic: crystals not visible Phaneritic: made of visible crystal components Porphyritic: Larger crustals in aphanitic or phaneritic ground mass
Bowen s Reaction Series
Igneous Rock Classification Acidic, Felsic Basic, Mafic Ultramafic SERPENTINITE
Extrusives Viscosity varies with Si and water content Basalt low viscosity Rhyolite high viscosity Rhyolite flows relatively unusual as rhyolite does not flow well Explosive Tuffs, pyroclastics
Volcano Types Basaltic: low viscosity Hawaii, Columbia Plateau Andesitic/Rhyolitic
Structures of Basalt Flows Lava Tubes Flow Stratigraphy collonade entablature flow 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 Quartzite Limestone, Dolomite Marble Shale Slate cleavage, no visible xl s Phyllite foliation, mica sheen but xl s not visible Schist clear foliation, visible mica Gneiss like granite but with foliation/gneissosity Basalt greenschist, amphibolite
Non-foliated Metamorphic Rocks Sandstone > Quartzite Limestone > Marble Dolomite > Dolomitic Marble
Foliated Metamorphic Rocks Shale/Mudstone Slate Phyllite (Greek for leaves e.g. phyllo dough) Schist Gneiss
Origin of Foliation (gneissosity, schistosity)
Engineering Properties Anisotropy of strength and elastic properties Preferred failure on foliation
Slate
Phyllite
Schist
Gneiss
Banded Gneiss
Metamorphic Grade
Subduction-Zone Metamorphism
Metamorphism at Continental Collisions
Contact Metamorphism