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ROCK
“PICTIONARY”
ROCK TYPES
IGNEOUS ROCKS
Rocks formed when magma (molten material beneath the
earth’s surface) or lava (molten material on the earth’s
surface) cools and becomes solid.
SEDIMENTARY ROCKS
Rocks composed of pieces of other rocks, remains of animals
or plants (i.e. fossils), or minerals formed by chemical
precipitation or evaporation.
METAMORPHIC ROCKS
Rocks formed when existing rocks (parent rock) are changed
or altered due to pressure, heat, or chemically active fluids.
IGNEOUS ROCK TEXTURES
TEXTURE DEFINITION
APHANITIC
Crystalline texture with individual crystals too small to
be visible to the unaided eye. Caused by rapid
extrusive cooling of lava.
PHANERITIC
Crystalline texture in which the individual components
are distinguishable with the unaided eye. Caused by
slow intrusive cooling of magma.
PORPHRYTIC
Crystalline texture in which larger crystals
(phenocrysts) are set in an aphanitic groundmass.
Caused by initial slow intrusive cooling of magma then
subsequently fast extrusive cooling of lava.
PHOTOGRAPH
PEGMATITIC
Crystalline texture with exceptionally large crystals.
Caused by extremely slow intrusive cooling of magma.
VESICULAR
Texture of a rock, esp. a lava, characterized by
abundant vesicles (holes) formed as a result of the
expansion of gases during the fluid stage of the lava.
GLASSY
Having the appearance of glass. Caused by extremely
fast extrusive cooling of lava. So quick the atoms are all
in a chaotic distribution and not crystallized.
FELSIC
Is a light colored igneous rock rich in potassium and
sodium silicates.
INTERMEDIATE
Is a dark and light igneous rock with a composition in
between felsic and mafic.
MAFIC
Is a dark colored igneous rock rich in iron and
magnesium silicates.
ULTRA-MAFIC
Is made almost entirely of dark-colored iron and
magnesium silicates.
SEDIMENTARY ROCK TEXTURES
ROUNDED CLASTS
Sharp edges of the clasts have been rounded off due to
transportation within a stream.
ANGULAR CLASTS
Clasts are angular and have sharp edges since the
material has not been transported in a river.
WELL SORTED
Composed of grains or clasts of approximately the
same size.
POORLY SORTED
Composed of grains or clasts of a variety of different
sizes.
FOSSILIFEROUS
Contains fossils (evidence of ancient life) embedded in
a fine grained matrix.
EFFERVESENCE (i.e. the acid test)
Reaction to acid of some sedimentary rocks (limestone,
dolostone) which contain calcite or dolomite. Bubbles
are generated when dilute HCl acid is applied.
Video:
http://www.youtube.com/watch?v=Lw3bWR0Ws7M
CROSS-BEDDING
Is a primary sedimentary structure in which one set of
inclined sedimentary layers is beveled off by an
erosional process and a new set of sedimentary layers
is deposited on top of the truncated original layers; this
type of primary sedimentary structure is common in
stream sediments and sand dunes.
METAMORPHIC ROCK TEXTURES
FOLIATED: SLATY CLEAVAGE
Fine-grained texture caused by the parallel orientation
of microscopic grains, causing the rock to separate
along parallel planes. This feature is a property known
as slaty cleavage. (Slaty cleavage or rock cleavage is
not to be confused with cleavage in a mineral, which is
related to the internal atomic structure of the mineral.)
FOLIATED: PHYLLITIC TEXTURE
Fine-grained texture formed by the parallel
arrangement of platy minerals, usually micas, that are
barely macroscopic (visible to the naked eye). The
parallelism is often silky, wavy or crenulated. The
predominance of micaceous minerals imparts a sheen
to the hand specimens.
FOLIATED: SCHISTOSE TEXTURE
Coarse-grained texture resulting from the suhparallel to
parallel orientation of large platy minerals such as
chlorite or micas, which cause the rock to sparkle.
Other common minerals present are quartz and
amphiholes. The average grain size of the minerals is
generally smaller than in a gneissic texture.
FOLIATED: GNEISSIC TEXTURE
Coarse-grained texture in which the minerals have
been segregated into bands, each of which is
dominated by one or two minerals. These bands range
in thickness from 1 mm to several centimeters. The
individual mineral grains are macroscopic and impart a
striped appearance to a hand specimen. Light-colored
bands commonly contain quartz and feldspar. And the
dark hands are commonly composed of hornblende and
ochise. Accessory minerals are common and are
useful in applying specific names to these rocks.
NON-FOLIATED
Contains no linear structure, is massive in appearance.
Named on the basis of its composition (e.g. quartz,
calcite).
EFFERVESENCE (i.e. the acid test)
Reaction to acid of some metamorphic rocks (marble)
which contain calcite or dolomite. Bubbles are
generated when dilute HCl acid is applied.
Video:
http://www.youtube.com/watch?v=Lw3bWR0Ws7M
PHOTOGRAPH CREDIT
angular clasts
aphanitic
cross-bedding
effervescence
“acid test” video
felsic fine
felsic large
fossiliferous
glassy
gneissic texture
intermediate fine
Intermediate large
mafic fine
mafic large
non-foliated
pegmatitic
phanaritic
phyllitic texture
poorly sorted
porphrytic
rounded clasts
schistose texture
slaty cleavage
ultra-mafic
vesicular
well sorted
http://skywalker.cochise.edu/wellerr/rocks/sdrx/breccia1.htm
http://skywalker.cochise.edu/wellerr/rocks/igrx/rhyolite2.htm
http://skywalker.cochise.edu/wellerr/rocks/sdrx/sedstruct7.htm
http://www.geol.umd.edu/~jmerck/geol100/lectures/09.html
http://www.youtube.com/watch?v=Lw3bWR0Ws7M
http://skywalker.cochise.edu/wellerr/rocks/igrx/rhyolite007.htm
http://skywalker.cochise.edu/wellerr/rocks/igrx/granite-red2.htm
http://skywalker.cochise.edu/wellerr/rocks/sdrx/limestone3.htm
http://skywalker.cochise.edu/wellerr/rocks/igrx/obsidian2.htm
http://skywalker.cochise.edu/wellerr/rocks/mtrx/gneiss10.htm
http://skywalker.cochise.edu/wellerr/rocks/igrx/andesite-misc7.htm
http://skywalker.cochise.edu/wellerr/rocks/igrx/diorite-misc5.htm
http://skywalker.cochise.edu/wellerr/rocks/igrx/basalt-fresh6.htm
http://skywalker.cochise.edu/wellerr/rocks/igrx/gabbro-misc1.htm
http://skywalker.cochise.edu/wellerr/rocks/mtrx/quartzite1.htm
http://skywalker.cochise.edu/wellerr/rocks/igrx/pegmatite4.htm
http://skywalker.cochise.edu/wellerr/rocks/igrx/granite-red3.htm
http://skywalker.cochise.edu/wellerr/rocks/mtrx/phyllite5.htm
http://skywalker.cochise.edu/wellerr/rocks/sdrx/conglomerate2.htm
http://skywalker.cochise.edu/wellerr/rocks/igrx/andesite2657.htm
http://skywalker.cochise.edu/wellerr/rocks/sdrx/conglomerate6.htm
http://skywalker.cochise.edu/wellerr/rocks/mtrx/schist1.htm
http://skywalker.cochise.edu/wellerr/rocks/mtrx/slate1.htm
http://skywalker.cochise.edu/wellerr/rocks/igrx/dunite1.htm
http://skywalker.cochise.edu/wellerr/rocks/igrx/basalt-vesic3.htm
http://skywalker.cochise.edu/wellerr/rocks/sdrx/sandstone-orthoquartzite2.htm