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