cee 437 lecture 10 rock...
TRANSCRIPT
CEE 437 Lecture 10
Rock Classification
Thomas Doe
Igneous Origins
IntrusiveBatholithic or plutonic: phaneriticDikes or sills that chill rapidly: aphanitic
Extrusivedeposition as melt (lava)pyroclastic
tufftephrapyroclastic 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
OceanicHi Fe, Mg, Ca, low Sibasalt, gabbro
Continental Hi Si, Na, Kgranite, rhyolite, andesite
Bowen’s Reaction Series
Igneous Rock Classification
SERPENTINITE
Acidic, Felsic Basic, Mafic Ultramafic
A simplified structure of an ophiolite suite:1. axial magma chamber2. sediments3. pillow basalts4. sheeted basaltic dykes5. layered gabbro6. dunite/peridotite cumulates
What’s Happening at the Ridge?
Snoqualmie Batholith – Miocene (~12 my)
Northwest Igneous and Metamorphic Rocks
Columbia River Basalts (16-8 my,
Miocene))
Snake River Basalts (8-6 my Pliocene)
Yellowstone Region Acidic
Volcanics (Pleistocene to
recent)
Cascade Batholiths (Felsic, Cret-Miocene)
Plate Moving Over Mantle Hot Spot?
North American Plate Motion
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
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 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)
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
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
Coal, PeatDerived from vegetation
Banded Iron FormationLikely bacteria derived, mainly Pre-Cambrian
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
Subduction-Zone Metamorphism
Metamorphic Grade
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
Contact Metamorphism