Northwest igneous and metamorphic rocks

Rock CycleMetamorphic Rocks Burial, metamorphism, recrystallization Melting Crystallization at depth or extrusion at surface Burial, metamorphism, recrystallization... Identifying Igneo

CEE 437 Rocks!

Thomas Doe

Rock Cycle

Metamorphic Rocks

Burial, metamorphism, recrystallization

Melting

Crystallization at depth or

extrusion at surface

Burial, metamorphism, recrystallization

Northwest Igneous and Metamorphic Rocks

Columbia River Basalts (miocene)

Snake River Basalts (pliocene)

Yellowstone Region Acidic Volcanics (Pleistocene to recent)

Geologic Settings for Igneous

Igneous Origins

 Intrusive

 Batholithic or plutonic: phaneritic

 Dikes or sills that chill rapidly: aphanitic

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

Igneous Rock Classification

SERPENTINITE

Acidic, Felsic Basic, Mafic Ultramafic

Magma Generation on Continental

Margins

Magma Generation in Convergent

Continental Plate Margins

 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

Hawaii Basalt Flows

Basalt Flow Structures

Eruptions of Acid-Rock Volcanoes

Rhyolite Dome

Caldera

Mt St Helen’s Blast Zone

Mt Mazama Ash Distribution

Sedimentary Rocks

 Clastics, Siliciclastics, and Evaporites

 Clastic rocks, depositional medium, and energy

 Diagenesis — chemical changes after deposition

Rock Cycle

Metamorphic Rocks

Burial, metamorphism, recrystallization

Melting

Crystallization at depth or

extrusion at surface

Burial, metamorphism, recrystallization

Sediment Sources

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

Clastic Sedimentary Rocks

 Clay, muds → shales, mudstones, claystones

(difference based on fissility)

Weathering Cycle

Clastic Sediments

Clay Minerals

 kaolinite: single gibbsite layer

 montmorillonite:weak water bonding between layers, moderated by Ca, Na, or K (near-shore

environments)

 illite: K bonds between layers (off-shore environments)

 bentonite: highly expansive, volcanic-derived, Na-rich montmorillonite

Clay Structure

Clay Structure Cont’d.

Kaolinite

Illite

Montmorillonite

 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

Carbonate Environments

 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

Sedimentary Rocks and Rock

Properties

 Properties for a given geologic description vary wildly based on cementation, porosity and other diagenetic factors

 Properties can be strong anisotropic and

heterogeneous based on bedding

Depositional Environments

 A rock unit is not everywhere the same age: Bright Angel Shale

 (example channels and banks in fluvial systems)

activity

Sediment Sorting

Sedimentary Structure — Cross

Bedding

Fluvial and Lacustrine Environments

 Fluvial

 Channelization

 Complex and close interrelationship of fine and course sediments

 Challenge for characterization due to high variability

 Special examples: glacial environments

 Lacustrine

 Deltaic deposits at margins, finer materials in lake beds

Deltaic Environments

 Variability based on proximity to source

 Stratigraphy effected by progradation

Deltaic Development and

Sedimentary Facies

Continental Slope Environments

 Turbidites and turbidity currents

 Graded bedding

 poor sortingpoor sorting

 vertical zonation with fining upwards

Turbidites and Turbidity

Currents

Metamorphic Classification

 Original Material

 sandstone, limestone, shale, basalt)

 Metamorphic Grade (Temperature, Pressure)

 Source of Metamorphism (Regional, Contact)

Basic Metamorphic Types

 Slate — cleavage, no visible xl’s

 Phyllite — foliation, mica sheen but xl’s not visiblePhyllite — foliation, mica sheen but xl’s not visible

 Schist — clear foliation, visible mica

 Gneiss — like granite but with foliation/gneissosity

Non-foliated Metamorphic Rocks

 Sandstone —> Quartzite

 Limestone —> Marble

 Dolomite —> Dolomitic Marble

Foliated Metamorphic Rocks

Origin of Foliation (gneissosity,

schistosity)

Engineering Properties

 Anisotropy of strength and elastic properties

 Preferred failure on foliation

Slate

Phyllite

Schist

Chlorite Schist

Gneiss

Banded Gneiss

Metamorphic Grade

Subduction-Zone Metamorphism

Metamorphism at Continental

Collisions

Contact Metamorphism