CEE 437 Lecture 10 Rock Classification Thomas Doe
Igneous Origins Intrusive Batholithic or plutonic: phaneritic Dikes or sills that chill rapidly: aphanitic Extrusive deposition as melt (lava) pyroclastic tuff tephra pyroclastic flows
Tectonic Plates
Off-Shore and Under Washington Ocean Ridge Mafic/Basic rocks (basalt) forming from partial melting of ultramafic material Subduction Zone Intermediate and Felsic/Acidic rocks forming from partial melting of oceanic crust and sediments
Geologic Settings for Igneous Rocks Oceanic Hi Fe, Mg, Ca, low Si basalt, gabbro Continental Hi Si, Na, K granite, rhyolite, andesite
Bowen s Reaction Series
Igneous Rock Classification Acidic, Felsic Basic, Mafic Ultramafic SERPENTINITE
What s Happening at the Ridge? A simplified structure of an ophiolite suite: 1. axial magma chamber 2. sediments 3. pillow basalts 4. sheeted basaltic dykes 5. layered gabbro 6. dunite/peridotite cumulates
Snoqualmie Batholith Miocene (~12 my)
Northwest Igneous and Metamorphic Rocks Plate Moving Over Mantle Hot Spot? Cascade Batholiths (Felsic, Cret- Miocene) Columbia River Basalts (16-8 my, Miocene)) North American Plate Motion Snake River Basalts (8-6 my Pliocene) Yellowstone Region Acidic Volcanics (Pleistocene to recent)
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
Volcano Types Basaltic: low viscosity Hawaii, Columbia Plateau Andesitic/Rhyolitic
Basalt Flow Structures
Rhyolite Dome
Mt. Mazama Ash Distribution
Weathering Fates Feldspars to clays (clays, shales) Quartz endures (siltstones, sandstones) Calcium recirculated into carbonate minerals by organic processes (limestones) Consequence: Over time, evolution of less dense more silicic continental crust
Weathering Cycle
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)
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
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, Peat Derived from vegetation Banded Iron Formation Likely bacteria derived, mainly Pre-Cambrian
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
Subduction-Zone Metamorphism
Metamorphic Grade
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
Contact Metamorphism