GO 324 Rocks and Minerals
Emporia State University
Igneous rock has a variety of origins, including volcanic or extrusive, hypabyssal, and plutonic or intrusive. If the magma cools and solidifies at depth, then the igneous rock is termed plutonic or intrusive. An example of intrusive igneous rock is granite, which is the primary constituent of continent crust. If the magma cools after traveling near to or onto the surface, then it is termed volcanic or extrusive. An example of extrusive rock is basalt, which is the primary constituent of oceanic plates. Hypabyssal igneous rock crystallizes at medium depths and these rock types occur at boundaries of the continents and oceanic crustal rocks. An example of hypabyssal rock is andesite. These intermediate rocks may form in geologic structures such as sills, dikes, and batholiths.
Ah, an optional opportunity for you! If you are enrolled in this course, visit Wikipedia on Large Igneous Province, en.wikipedia.org/wiki/Large_igneous_province. Write a summary paragraph of what is meant by large igneous province. In addition, choose one of the following links and write an additional paragraph summary of the topic and its igneous geology significance. Choose from: Columbia River Basalt Group (http://en.wikipedia.org/wiki/Columbia_River_Basalt_Group), Geology of Venus (http://en.wikipedia.org/wiki/Geology_of_Venus), OR Ethiopian Higlands (http://en.wikipedia.org/wiki/Ethiopian_Highlands). Send this paper by March 27, to email@example.com for one extra credit point on the second exam. Place Igneous Rock Extra Points for Exam 2 in the subject line of the email!
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Glassy - a smooth, glassy-looking texture in which no mineral phenocrysts occur; the resulting rock is actually amorphous or without a crystalline form or structure. See an example at http://volcanoes.usgs.gov/images/pglossary/obsidian.php
Vesicular - vesicles or holes are obvious creating a sponge-like appearance to the volcanic rock. This term is also applied to igneous rock without a crystalline mineral make-up, or amorphous material such as pumice or scoria. See http://www.snopes.com/photos/natural/maiken.asp for the birth of an island in the South Pacific as seen from a nearby boat. It is fascinating to see the rafts of pumice. Also a wannabe island is nearing "completion" on the surface in the Solomon Islands... http://www.huffingtonpost.com/2011/01/24/birth-of-an-island_n_811134.html. And finally, one additional note on an older "birth of an island" at http://www.platetectonics.com/book/page_10.asp Pumice rafting from New Island and Tongas can be seen in a NASA MODIS image http://earthobservatory.nasa.gov/IOTD/view.php?id=7124. Uses of pumice can be seen at http://en.wikipedia.org/wiki/Pumice. Check out some fresh basalt from Hawaiian Islands with vesicular texture at http://meteorites.wustl.edu/id/lavarocks.htm. Also an example of scoria can be seen at http://www.geospectra.net/fieldgeology/scoria_0468.jpg.
Aphanitic - a dense texture where mineral grains are not visible to the unaided eye. See an example at http://www.geospectra.net/fieldgeology/rhyolite2_0469.jpg.
Phaneritic - an even-granular texture with mineral phenocrysts visible to the unaided eye. See an example at http://www.geospectra.net/fieldgeology/granite_0478.jpg.
Aphanitic Porphyry - visible phenocrysts embedded in the rock with an invisible or aphanitic groundmass or background. See an example at http://www.geospectra.net/fieldgeology/basalt2_0463.jpg and http://www.geospectra.net/fieldgeology/basalt3_0463.jpg.
Phaneritic Porphyry - an uneven-granular texture where larger visible phenocrysts are embedded in a rock with smaller, but visible, phenocrysts making up the groundmass or background. See an example at http://www.geospectra.net/fieldgeology/monzonite_0476.jpg.
Pegmatitic - very coarse, uneven-granular texture with phenocrysts larger than 1 cm in size. See amazonite that came from a pegmatite deposit in Brazil at the 2004 Tucson Gem and Mineral show - weighing 105 kg from Irene Ching of Bolva Group Inc., http://www.minsocam.org/msa/special/Pig/PIG_best_shots/TGMS04_amznt01.jpg.
Fragmental or Tuff - pyroclastic fragments, solidified ash particles, crystal and glass debris, cemented together. For an example see http://en.wikipedia.org/wiki/Tuff.
See other text and images for igneous rock at http://geology.csupomona.edu/drjessey/class/gsc101/igneous.html
|Structural features in igneous rock may aid in identification as they may reveal rock mode of formation. Vesicular is a structural term that refers to a rock with numerous vesicles or cavities. This structure is formed when the rock quench cools and the former gas filled cavities simply solidify. Igneous rock displaying this structure include scoria and pumice. This structure is nicely explained and illustrated at http://meteorites.wustl.edu/id/vesicles.htm. Spherulitic and amygdaloidal are two structures related to vesicular. When the vesicles are rounded or spherical, they may be filled sometime after the rock's initial formation with needlelike crystals (usually quartz or feldspars) and this is termed spherulitic. If the vesicles are almond-shaped, elliptical with pointed ends, an amygdaloidal structure results; these cavities are filled with secondary mineralization, usually quartz or zeolite minerals.|
|See amygdaloidal basalt as an example, http://www.geospectra.net/fieldgeology/basalt2_0463.jpg. When the mineral grains are roughly spherical the structure is termed orbicular (usually in phaneritic, felsic rocks). Flow banding is an igneous rock structure produced when continued flowing lava solidifies, and alternating layers result (Chesterman, 1978, p. 599). Xenoliths are stranger rock that is when older surrounding rock is broken and included in a magma forming an unrelated inclusion in the rock (Chesterman, 1978, p. 599-600). A wonderful example is found at http://en.wikipedia.org/wiki/File:Peridotitic_mantle_xenolith.JPG of peridot in a peridotite.||
Olivine in basalt bomb!Image taken from http://commons.wikimedia.org/wiki/File:Peridotitic_mantle_xenolith.JPG
To investigate more on the topic of xenolith and our upcoming field trip, visit Limestone Xenoliths in Hills Pond Lamproite, Woodson County, Kansas at JStor, http://www.jstor.org/stable/3628095?seq=1
The color index is divided into three categories: leucocratic, mesocratic, and melanocratic. The lightest color term, leucocratic, contains two-thirds light minerals, one-third dark. Reds or pinks are considered a light mineral color; for example, orthoclase in rhyolite. Mesocratic include rocks of an intermediate status, one-half dark, one-half light; for example, albite and hornblende in diorite. Melanocratic is the darkest color term with two-thirds dark, one-third light colored minerals; for example olivine in basalt and peridotite. For the most part, greens are considered a dark mineral color.
Color is dependent upon the mineral make-up. See the minerals of igneous rock at http://facweb.bhc.edu/academics/science/harwoodr/geol101/labs/igneous/CIndex.htm.
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Felsic - a composition high in potassium and sodium rich silicates with a silica content of about 70%; these igneous rocks are mainly in continental crust.
Intermediate - variable between felsic and mafic.
Mafic - a composition high in calcium, magnesium, and iron rich silicates with a silica content of about 50%; these igneous rocks are mainly in oceanic crust.
Ultramafic - a composition very high in magnesium and iron with a silica content of less than 50%; these igneous rocks are found in the mantle.
Some of the common minerals found in igneous rock include feldspars, quartz, micas, hornblende, augite, and olivine. A more detailed mineral assemblage is given below.
Minerals associated with slow cooling molten magma include:
silicates: olivine, pyroxenes (augite), amphiboles (hornblende), micas, feldspars, quartz
sulfides: pyrite, pyrrhotite
oxides: magnetite, chromite
native elements: diamond, platinum
Minerals associated with molten magma and vapors forming pegmatites include:
silicates: beryl, topaz, tourmalines, micas, spodumene (kunzite/hiddenite)
oxides: corundum, cassiterite, columbite, uraninite
sulfides: molybdenite, arsenopyrite
Minerals associated with hydrothermal veins, forming in cracks and fissures as a result of precipitation from solutions include:
silicates: feldspars, quartz, epidote
carbonates: calcite, rhodochrosite, ankerite
oxides: hematite, ilmenite, rutile
sulfides: galena, sphalerite, chalcopyrite, pyrite
Minerals associated with igneous rock formed from vapors or sublimation of volcanic fumes include:
silicates: topaz, zeolites
sulfides: pyrite, cinnabar, realgar, stibnite
native elements: sulfur, arsenic, mercury.
Go to the class presentation introducing igneous rocks, http://academic.emporia.edu/abersusa/go324/Rocks-Igneous.pdf. Also an excellent reference online to bring this all together is at http://www.geologyclass.org/Igneous Concepts.htm, which is an igneous rock and processes supplemental lecture for students enrolled in Paradise Valley Community College in Black Mountain, Maricopa County, Arizona, near Phoenix, and surrounding county high schools (http://www.geologyclass.org/). Another easy to read website is at http://facweb.bhc.edu/academics/science/harwoodr/geol101/labs/igneous/, Black Hawk College, Moline or Galva, Illinois.
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This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. The curriculum is © by the author, 2001-2013. Creation and last update January 23, 2013. For more information contact the course instructor, S. W. Aber, e-mail: firstname.lastname@example.org.
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