Environmental Engineering Reference
In-Depth Information
TABLE 5.7
Characteristics of some Igneous Rocks
Rock
Characteristics
Plate
Coarse to Medium Grained — Very Slow to Slow Cooling
Pegmatite
Abundant as dikes in granite masses and other large bodies. Chiefly quartz and feldspar
5.1
appearing separately as large grains ranging from a centimeter to as large as a meter
in diameter
Granite
The most common and widely occurring igneous rock. Fabric roughly equigranular
5.2
normally. Light colors contain chiefly quartz and feldspar; gray shades contain biotite
mica or hornblende
5.3
Syenite
Light-colored rock differing from granite in that it contains no quartz, consisting almost
5.4
entirely of feldspar but often containing some hornblende, biotite, and pyroxine
Diorite
Gray to dark gray or greenish, composed of plagioclase feldspar and one or more of the
5.5
ferromagnesian minerals. Equigranular fabric
Gobbro
Dark-colored rock composed chiefly of ferromagnesian minerals and plagioclase feldspar
5.6
Peridotite
Dark-colored rocks composed almost solely of ferromagnesian minerals. Olivine
5.7
predominant: negligible feldspar. Hornblende or pyroxenes associated. Readily altered
Pyroxenite As above but pyroxene alone or predominant
Hornblendite As above but hornblende alone or predominant
Dunite
Major constituent is olivine, which alters readily to serpentine
Dolerite
(or
Dark-colored rock intermediate in grain size between gabbro and basalt. Abundant
diabase)
as thick lava flows that have cooled slowly.
Fine-Grained — Rapid Cooling
Andesite
Generally dark gray, green, or red. Pure andesite is relatively rare, and it is usually
found with phenocrysts. Porphyritic andesite and basalt compose about 95%
of all volcanic materials
Basalt
Most abundant extrusive rock: found in all parts of the world and beneath the oceans.
5.8
Colors range from grayish to greenish black to black. Fine-grained with a dense
compact structure. Often contains numerous voids (vesicular basalt)
5.9
Rhyolite
The microcrystalline equivalent of granite formed at or near the surface.
5.10
Characteristically white, gray, or pink, and nearly always contains a
few phenocrysts of quartz or feldspar
Felsite
Occurs as dikes, sills, and lava flows. The term felsite is used to define the finely
crystalline varieties of quartz porphyries or other light-colored porphyries that
have few or no phenocrysts and give but slight indications to the unaided eye
of their actual mineral composition.
Glassy Rocks — Very Rapid Chilling
Obsidian
Solid natural glass devoid of all crystalline grains, generally black with a brilliant
luster and a remarkable conchoidal fracture
Pitchstone
A variety of obsidian with a resinous luster
Pumice
Extremely vesicular glass: a glass froth
Scoriae
Formations that have as much void space as solids
Note
: The more common types are italicized.
Chemical Precipitates (nondetrital)
Materials are carried in solution in flowing water to the sea or other large water bodies
where they precipitate from solution. Chemical precipitates include the immense thick-
ness of marine carbonates (limestones and dolomites) and the less abundant evaporites
(gypsum, anhydrite, and halite).
In addition to being formed from physical-chemical processes, many nondetrital rocks are
formed from the dissolved matter precipitated into the sea by the physiological activities of
living organisms.
