CEE 437 Lecture 10 Rock...
Transcript of CEE 437 Lecture 10 Rock...
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CEE 437 Lecture 10
Rock Classification
Thomas Doe
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Igneous Origins
IntrusiveBatholithic or plutonic: phaneriticDikes or sills that chill rapidly: aphanitic
Extrusivedeposition as melt (lava)pyroclastic
tufftephrapyroclastic flows
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Tectonic Plates
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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
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Geologic Settings for Igneous Rocks
OceanicHi Fe, Mg, Ca, low Sibasalt, gabbro
Continental Hi Si, Na, Kgranite, rhyolite, andesite
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Bowen’s Reaction Series
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Igneous Rock Classification
SERPENTINITE
Acidic, Felsic Basic, Mafic Ultramafic
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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?
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Snoqualmie Batholith – Miocene (~12 my)
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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
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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
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Volcano Types
Basaltic: low viscosity — Hawaii, Columbia Plateau
Andesitic/Rhyolitic
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Basalt Flow Structures
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Rhyolite Dome
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Mt. Mazama Ash Distribution
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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
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Weathering Cycle
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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
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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)
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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)
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Clastic Sediments
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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
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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
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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
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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
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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
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Subduction-Zone Metamorphism
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Metamorphic Grade
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Non-foliated Metamorphic Rocks
Sandstone —> QuartziteLimestone —> MarbleDolomite —> Dolomitic Marble
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Foliated Metamorphic Rocks
Shale/MudstoneSlatePhyllite (Greek for leaves e.g. phyllo dough)SchistGneiss
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Origin of Foliation (gneissosity, schistosity)
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Engineering Properties
Anisotropy of strength and elastic propertiesPreferred failure on foliation
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Slate
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Phyllite
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Schist
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Gneiss
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Contact Metamorphism