Field Geology 2009:
Identifying Rocks from the Flint Hills of Kansas,
Raton Basin and Southern Rocky Mountains of Colorado
www.geospectra.net/fieldgeology/rocks.htm

Emporia State University
Susan W. Aber

 rocks
Photo by S.W Aber; Photo date June 5, 2009

Rocks are a natural aggregate of minerals and are classified into sedimentary, igneous, and metamorphic. Identifying texture and mineral components are key to naming rocks. Texture is the size, shape, and arrangement of mineral grains, while characteristic mineral components vary with environments and conditions of formation.

Sedimentary Rock Igneous Rock Metamorphic Rock
Identification Clues Rock Pictures Website Links

Sedimentary rocks are formed at or near the surface in layers of mineral grains that compact and cement into solid rock. They are grouped according to depositional environments or source area of sediments. Sedimentary groups are clastic (detrital), biogenic, and chemical. Clastic or detrital rocks are composed of mineral grains produced by the weathering and erosion of preexisting igneous, metamorphic, or sedimentary rocks, all derived from land. Clastic texture is described by size such as clay, silt, sand, gravel, etc., according to the Wentworth Scale, and includes rock types such as shale, sandstone, and conglomerate. Biogenic rocks form directly or indirectly from organisms and are subdivided into bioclastic and biogenic. Bioclastic rocks are created from the hard skeletal parts of corals, mollusks, brachiopods, echinoderms, sponges, radiolaria, and other marine invertebrates. These organisms are abundant in shallow, warm, clear seas, and are quickly cemented and may undergo mineralogical and textural diagenesis to create rocks such as limestone and chert. Organic biogenic rocks are derived from decomposition of plant and animal tissues. The accumulation of carbon or hydrocarbon-rich sediments takes place in stagnant, oxygen-poor environments. This produces sediment with lithification accomplished by compaction and examples include coal and oil shales. Chemical rocks form from precipitation of low temperature/pressure minerals in saline or freshwater solutions onto a depositional surface or within sediment pores. The chemical sedimentary rock types include opal, chert, iron-oxides, oolitic limestone, rock salt, bauxite, etc. For detailed information visit:

Igneous rock solidifies from molten material or emanating volatile constituents, hot aqueous or gaseous solutions and are broadly grouped as plutonic/intrusive, hypabyssal/intermediate, and volcanic/extrusive rocks. If magma cools and solidifies at great depth, phaneritic textures or large crystal grains result in the intrusive rock such as granite. If the magma cools and solidifies after moving up to the surface, aphanitic textures or fine crystal grains result in the extrusive rock such as basalt. Magma that cools and solidifies at medium depths, porphyry textures or a mix of fine and coarse grains result in the hypabyssal rock such as andesite. These intermediate rocks may form in geologic structures such as sills, dikes, and stocks. Igneous rock colors and compositions are grouped as leucocratic/felsic, mesocratic/intermediate, melanocratic/mafic, and malanocratic/ultramafic. Felsic rock compositions are potassium and sodium-rich silicates such as granite, syenite, rhyolite, and trachyte. Intermediate rock compositions are variable and between felsic and mafic; they include granodiorite, dacite, monzonite, latite, diorite, and andesite. Mafic rock compositions are calcium, magnesium, and iron rich silicates such as gabbro and basalt. Ultramafic rock compositions are high in magnesium and iron with a silica content of less than 50%; these igneous rocks are found in the mantle and include kimberlite and lamproite. For detailed information visit:

Metamorphic rock is formed when preexisting rocks are transformed or altered in a solid state under relatively high pressure, temperature, and/or hot circulating fluids. During metamorphism, the parent rock, igneous, sedimentary, or metamorphic, recrystallizes without melting and with rearranging textures and structures. New minerals can form with hydrothermal metamorphism and changes in shape and size of mineral grains, with other methods of metamorphism. These changes are dependent on the properties of the parent rock, type of metamorphic environment present, and the duration of the process until the new rock is finally exposed at the earth's surface. For detailed information on metamophic rock visit:

Rock Identification Clues

Successful identification of rocks can begin with observations of composition and texture. These observations can lead to inference on formation and taken together, naming the rock (charts modified from ES 110 handout by Aber & Thompson). Some rock types will be listed below, followed by photographs of some of the rocks you are likely to encounter to Kansas and Colorado field excursions.

Sedimentary Rock

Observations Inference Rock Type
one mineral dominant biogenic-skeletal fragments or
chemical precipitate 		limestone, chert, gypsum, rock salt
one mineral dominant; fine-grained
mud cemented 		sediment transported-deposited by
calm water in lake, lagoon, swamp, or ocean 		shale, limestone
multiple minerals; coarse-grained
gravel cemented 		sediment transported-deposited by
glacier, swift water, rockslide 		conglomerate
multiple minerals; medium-grained
sand cemented 		sediment transported-deposited by
wind, water in stream or on beach 		sandstone
rounded mineral grains transport great distance rounded conglomerate, sandstone
angular mineral grains transport short distance brecciated conglomerate or sandstone
small rounded spheres constant motion, warm shallow seawater;
chemical precipitate 		oolitic limestone
well sorted, mineral grains same size sediment transport-wind or water shale, siltstone
poor sorted, mineral grains varying in size sedment transport-ice, flood water or mudflow conglomerate

Igneous Rock

Observations Inference Rock Type
light minerals dominant low temperature crystallization granite, rhyolite
mixed color minerals medium temperature crystallization diorite, andesite
dark minerals dominate high temperature crystallization gabbro, basalt
minerals easily visible to unaided eye slow cooling; intrusive granite, gabbro
minerals not visible to unaided eye rapid cooling; extrusive rhyolite, basalt
minerals easily visible to unaided eye
mixed within minerals not visible to unaided eye 		minerals forming within cooling magma; magma
rises to the surface forcing the remaining melt
to freeze or crystallize more quickly 		granite porphyry,
basalt porphyry
no minerals visible, glassy look quench cooling obsidian
no minerals visible, glassy or
dull look with numerous holes 		pressure release as rise to surface, gas escapes
with openings preserved as cavities 		pumice, scoria

Metamorphic Rock

Observations Inference Rock Type
one mineral dominant; crystalline and sugary regional stresses marble, quartzite
one mineral dominant; fine-grained
parallel planes, smooth and shine 		directed stresses; low grade metamorphism
where plane surfaces are perpendicular to
applied forces 		pipestone, slate
multiple minerals; medium-grained
parallel planes, wrinkles and sheen 		directed stresses; medium grade metamorphism phyllite, schist
alternating minerals; coarse-grained,
alternating colored layers 		directed stresses; higher grade metamorphism gneiss, ecologite

Igneous Rock Images

Click to enlarge.
Color/Composition Texture Image Name
Leucocratic/Felsic Phaneritic Biotite Granite
Leucocratic/Felsic Phaneritic MicroGranite,
Aplite
Leucocratic/Felsic Aphanitic Rhyolite
Leucocratic/Felsic Aphanitic Porphyry Rhyolite Porphyry/
close-up view
Leucocratic/Felsic Phaneritic Syenite
Leucocratic/Felsic Phaneritic Porphyry Hornblende Syenite/close-up view
Leucocratic/Felsic Aphanitic Porphyry Trachyte
Mesocratic/Intermediate Phaneritic Porphyry Granodiorite
Mesocratic/Intermediate Aphanitic Porphyry Dacite
Mesocratic/Intermediate Phaneritic Porphyry Quartz Monzonite Porphyry
Mesocratic/Intermediate Aphanitic Porphyry Hornblende Andesite
Melanocratic/Mafic Aphanitic Porphyry Amygdaloidal Basalt
Melanocratic/Mafic Aphanitic Basalt
Melanocratic/Mafic Phaneritic Lamprophyry - Dark Dike rock
Melanocratic/Mafic Phaneritic Hornblende Gabbro
Melanocratic/Mafic Aphanitic-Vesicular Scoria

Sedimentary Rock Images

Click to enlarge.
Sedimentary Group Texture Image Name
Clastic Coarse Sand Arkose Sandstone
Clastic Medium Sand Sandstone
Clastic Fine Sand Sandstone
Clastic Silt Siltstone
Clastic Clay Shale
Carbonate -
Bioclastic Biogenic 		Clay and Fossil remains Fossiliferous Limestone

Metamorphic Rock Images

Click to enlarge.
Metamorphic Facies-
Method of Metamorphism 		Texture Image Name
Amphibolite-Granulite
Regional or Dynamothermal 		Crystalloblastic -
Foliated 		Granitic Gneiss
Hornfels
Contact 		Granoblastic -
Non-foliated 		Hornfel
Amphibolite - Ecologite
Regional or Dynamothermal 		Granoblastic -
Non-foliated 		Quartzite

Website Links

Course Informational Links

Webpage created May 26, 2009; latest update June 24, 2009. Copyright S.W. Aber. E-mail: saber@emporia.edu, Susie Ward Aber.