UW CE437 Lecture #3: Rocks!

Aaron Fox and Thomas Doe

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1. A brief introduction to the major rock types

2. Rock Provinces and Environments

�Where do we expect to see certain types of rocks?� How does the environment control rock formation?

3. The Rock Cycle

4. How to Identify Rocks

� In hand sample?

� Chemically?

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LEFT: Image courtesy the NASA Remote Sensing Tutorial,

http://rst.gsfc.nasa.gov/Sect2/rock_cycle_800x609.jpg

RIGHT: Image courtesy the Lynchburgh,VA Rock Club: http://www.lynchburgrockclub.org/images/rock%20cycle.gif

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Igneous: A rock that solidified from molten or partly molten material (magma); derived from the Latin ignis (‘fire’).

Sedimentary: A rock formed from either

• Solid fragmental material transported and deposited by wind, water, or ice

• Chemically precipitated from solution

• Secreted by (does not apply to snot) or formed from the dead skeletons of....

Metamorphic: Rocks formed by the mineralogical, chemical, and structural adjustment of solid rocks to physical and chemical conditions different from those under which the original rocks (sedimentary / igneous) were formed.

All definitions after “The Dictionary of Geological Terms, 3rd Edition” by R. Bates and J. Jackson, published by the American Geologic Institute

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• Intrusive�Batholiths and plutons: visible minerals

(phaneritic) and relatively slow cooling�Dikes and sills: microscopic minerals

(aphanitic) and relatively fast cooling

• Extrusive�Lavas�Pyroclastic deposits

�tuff�tephra�pyroclastic flows (ignimbrites)

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� Extrusive: Quick cooling

• Pillow basalts: Normal basalt on the inside, a glassy rind on the outside formed by quenching of lava erupted into water

� Intrusive: Slow cooling; rock is a natural insulator

� Viscosity ~ Silica and H20 content�Basalt = Low viscosity, lower silica content, less volatile

�Rhyolite = High viscosity, high silica content, more volatile

� Viscosity controls volcano morphology

• High Viscosity = composite cones / stratovolcanoes

• Low Viscosity = flood basalts, ocean plates, shield volcanoes

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Both images courtesy (and copyrighted by) the USGS Volcanic Hazards Program, http://volcanoes.usgs.gov/

Basalt: ‘A’a (pronounced “ah-ah”)

Basalt: Pahoehoe

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Dacite, Lassen Peak, CA Andesite, Brokeoff Volcano, CA

Rhyolite, Mono-Inyo Craters, CA. The black bands are obsidian (volcanic glass)

All images courtesy (and copyrighted by) the USGS Volcanic Hazards Program, http://volcanoes.usgs.gov/

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Tephra: ash (two left piles) and lapilli(right two piles)

Pyroclastic Flow: MayonVolcano, The Philippines

Upper left and right images courtesy (and copyrighted by) the USGS Volcanic Hazards Program, http://volcanoes.usgs.gov/. Lower left image copyright Richard Robinson (Santa Monica College, CA) http://homepage.smc.edu/robinson_richard/rocktest/igneous_web/pages/tuff%20breccia%202.html

Tuff: A general term for all rocks formed from consolidated pyroclasticmaterials

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Batholith: Half Dome, Yosemite National Park, CA:http://www.stanford.edu/~wgupta/images/yosemite%20-%20half%20dome.jpg

Pluton: Chita Pluton, NW ArgentinaImage courtesy Dr. Aaron Yoshinobu, Texas Tech University: http://www.gesc.ttu.edu/Fac_pages/Yoshinobu/ARCS/Chita.html

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Dike (and volcanic plug): Shiprock, NM

Image courtesy Martha Schoene, Seton Hall University: http://pirate.shu.edu/~schoenma/

Sill (basalt, note columnar jointing)

Image courtesy Louis J. Maher Jr., University of Wisconsin Geology: “Geology by Light Plane”

http://www.geology.wisc.edu/~maher/air/air03.htm

�������� !������� ����Granite (left) : K-feldspar (pink), quartz (clear), biotite mica (black)Images courtesy of Geologic Image Archive, University of Pittsburg, PA: http://www.pitt.edu/~cejones/GeoImages/index.html

Diorite: Plagioclase feldspar (white), Quartz, and Biotite micaImage courtesy Lynn Fichter, James Madison University: http://csmres.jmu.edu/geollab/Fichter/IgnRx/IgHome.html

Peridotite (below) : Olivine (green), orthopyroxene (black)

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• Chemistry� Acidic: Basic (more Si, less Si)� Trace elements

• Texture� Aphanitic: crystals not visible� Phaneritic: made of visible crystal components� Porphyritic: Large crystals in aphanitic or phaneritic ground mass

• Mineralogy� % of quartz, feldspars, mafic minerals

• Geologic setting� ocean volcanoes, flood basalts, mountain ranges

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Acidic (Felsic) Basic (Mafic) Ultramafic

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Images courtesy About.com (Homework Help > Geology > Rocks&Sediments > More on Igneous Rocks > Phaneritic Igneous Rock Classification): http://geology.about.com/library/bl/bligneouseasyQAP.htm

To be useful, you must either:

1) Be able to see the minerals (phaneritic)

2) Have access to a microscope (aphanitic)

3) Have access to mass spec / XRD / SEM (glasses)

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Oceanic• High Mg, Fe, Ca, low Si

• Basalt, Gabbro

• Hawaii

Continental• High Si, Na, K

• Quite often more volatiles (H20, dissolved gases)

• Granite, Rhyolite, Andesite

• WA/OR Cascades

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Columbia River Basalts

(Miocene)

Snake River Basalts

(Pliocene)

Yellowstone Region Acidic Volcanics(Pleistocene to

recent)

Cascade Volcanoes (recent)

Recent Basaltic Volcanism (Newberry Crater)

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The physical appearance of the volcano is controlled by:

• Magma Viscosity / SiO2 content

• Eruptive ‘style’ ~ % volatiles

• Where might you expect each type of volcano to form?

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Flood basalt flows (Deccan Traps, India; the Columbia River Basalts, WA) are decidedly NOT homogenous

•Interbedded: Sediments between basalt flows (sandstones, diatomite)

•Internal structures of lava flows

� Related largely to speed of cooling

� Pillow lavas > palagonite clay

� colonnade

� entablature

� flow top breccias

� pumice / scoria from smaller cinder cones and fissures

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Basaltic cinder cone

Nuée Ardente (a type of pyroclastic flow)

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Metamorphic rocks are not unlike graduate students; they undergo many strange changes in response to an increase in pressure.....

Metamorphic rocks result from subjecting igneous or sedimentary rocks to increased temperatures and pressures by:

• Adding heat (i.e. basalt flowing through existing rock at a volcano)

• Burial (sedimentary basins, subduction zones)

• Tectonic compression (mountain building, faulting, continental collision)

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• Structure: Foliated or Non-Foliated (we’ll talk about this)

• What the original rock was (protolith)

• The metamorphic grade (P, T) and the metamorphic path

• The source of the metamorphism (regional, contact)

• Mineral assemblages

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1) Get your minds out of the gutter.... �

2) Low metamorphic grade, caused by growth of fine grained chlorite and quartz crystals

3) Now you have TWO planes of weakness in the rock!

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• As the pressure and temperature goes up, so does the size of the growing crystals

• Eventually, the rock develops a new planar foliation, defined by the growth of sheet silicates (micas: biotite, muscovite)

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• At still higher pressures and temperatures, we start to get mineral segregations (felsic and mafic minerals split into distinct layers).

• The onset of partial melting of the original rock

• Mica becomes chemically unstable; higher-T (further down Bowen’s Reaction series) minerals such as hornblende (amphibole) and pyroxenes now grow.

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� Rock strength and elasticity now becomes HIGHLY anisotropic

� Foliations lead to new preferred failure planes not necessarily parallel to original bedding (wedges)

� Increased P-T leads to rock dehydration

� Low-grade metamorphism leads to development of significant clay minerals, which affects rock strength. Generally, the feldspar minerals will decay to clays

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Grade: A measure of the degree of the metamorphism; does NOT describe a unique path through P-T space

Facies: An assemblage of minerals that reached equilibrium under a specific set of P, T conditions. Named for the most recognizable mineral in that P-T regime

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Quartz Sandstone > Quartzite (non-foliated)

Limestone, Dolomite > Marble (not usually foliated, but can exhibit structure)

Shale >• Slate — cleavage, no visible crystals

• Phyllite — foliation, mica sheen but crystals not visible

• Schist — clear foliation, visible mica

• Gneiss — like granite but with foliation / gneissosity

Basalt >• Greenschist – low grade metamorphism, foliated

• Amphibolite – higher grade metamorphism, foliated

• Blueschist – high P, relatively low T (ocean trench)

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Unknown Slate, Unknown Location (Ask Tom!)

Sulphide-enriched Precambrian (really old) Slate, Sandford Lake, Western Ontario, Canada

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Mt. Stevens Formation, early Cambrian – Precambrian in age, Canadian Rockies, B.C.

Locality unknown

Note the shiny appearance (due to mica growth)

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Locality unknown. Reddish to black objects are garnet porphyroblasts

Corea Creek Schist, sillimanitezone metamorphism, GlenelgRiver Metamorphic Complex, Victoria, Australia

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Gneiss (location unknown)

Gneiss (right) in contact with younger schist (left), Södermalm district, Stockholm, Sweden

2 ��7 �Shelburne marble, Middlebury, VTImage courtesy Vermont Geological Survey: http://www.anr.state.vt.us/dec/geo/photogalleryp2.htm

Pink marble, somewhere in CA

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Moretown Fm. quartzite, Middlesex, VTImage courtesy Vermont Geological Survey: http://www.anr.state.vt.us/dec/geo/photogalleryp4.htm

Quartzite (close up), North Carolina, approximately 700 million years old

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1) Building materials (rip-rap, marble countertops, aggregate)

2) Foundations

3) Rock slope, excavation, and tunnel stability

4) Economics: minerals, oil and gas, coal

5) Hazards: Radon, subsidence (karst), volcanoes

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1) Fine-grained igneous rocks cannot be used as aggregate in Portland cement due to excessive volume expansion caused by alkali – silica reaction

2) Coarse-grained igneous rocks (granites) are not good construction aggregate material due to low abrasion resistance

3) Foundations (dams, bridge piers) need to avoid weathered igneous rocks due to high clay content, low strength / shear resistance

4) Volcanoes are not good places to build houses. Geothermal plants, however, are a different story.

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1) Same alkali – silica reaction problem as igneous rocks; in particular, argillite/phyllite, granite gniess, and impure quartzite

2) Coarse-grained gneiss can be severely abraded when used as aggregate

3) Rock mass stability is affected by the degree and direction of foliation

4) Marble can cause the same dissolution problems (sinkholes, reservoir leakage, dissolution cavities) as limestone (Tom will discuss on Tuesday)

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Thanks!Any questions, email me:

afox@golder.comor talk to Drs. Doe or Dershowitz