· GRANITE ROCKS
Granite is a common and widely-occurring group of intrusive
felsic igneous rocks that forms at great depths and pressures
under continents. Granite consists of orthoclase, plagioclase
quartz, hornblende, biotite, muscovite and minor accessories
such as magnetite, garnet, zircon and apatite minerals. Rarely a
pyroxene is present. Ordinary granite always carries a small
amount of plagioclase, but when this is absent the rock is
referred to as alkali granite. An increasing proportion of
plagioclase feldspar causes granite to pass into granodiorite. A
rock consisting of equal proportions of orthoclase and
plagioclase plus quartz may be considered a quartz monozonite. A
granite containing both muscovite and biotite micas is called a
The word granite comes from the Latin granum, a grain, in
reference to the grained structure of such a crystalline rock.
Granite occurs as stock-like masses and as batholiths often
associated with mountain ranges and frequently of great extent.
Granite has been intruded into the crust of the Earth during all
geologic periods, except perhaps the most recent; much of it is
of Precambrian age. Granite is widely distributed throughout the
Because of its hardness and comparative cheapness in relation
to marble, granite is often used to make kitchen countertops. A
granite countertop can be cut in any shape, and it is virtually
unscratchable. Very hot pots must not be placed onto it though,
because the temperature differential could possibly crack the
Average density: 2750 kg/m3 (range 1741 to 2800)
· IGNEOUS ROCKS GENERAL
Igneous rocks are crystalline or glassy rocks formed by the
cooling and solidification of molten magma. Igneous rocks
comprise one of the three principal classes of rocks, the others
being metamorphic and sedimentary.
Igneous rocks are formed from the solidification of magma,
which is a hot (600 deg.C - 1300 deg.C, or 1100 deg. - 2400 deg.
F) molten or partially molten rock material. The Earth is
composed predominantly of a large mass of igneous rock with a
very thin covering of sedimentary rock. Whereas sedimentary
rocks are produced by processes operating mainly at the Earth's
surface such as weathering and erosion, igneous--and
metamorphic--rocks are formed by internal processes that cannot
be directly observed.
Magma is thought to be generated within the asthenosphere
(the layer of partially molten rock underlying the Earth's
crust) at a depth below about 60-100 kilometers (40-60 miles).
Because magma is less dense than the surrounding solid rocks, it
rises toward the surface. It may settle within the crust or
erupt at the surface from a volcano as a lava flow. Rocks formed
from the cooling and solidification of magma deep within the
crust are distinct from those erupted at the surface mainly
owing to the differences in conditions in the two environments.
Within the Earth crust the temperatures and pressures are much
higher than at its surface; consequently, the hot magma cools
slowly and crystallizes completely. The slow cooling promotes
the growth of minerals large enough to be identified visually
without the aid of a microscope (called phaneritic, from the
Greek phaneros, meaning "visible"). On the other hand, magma
erupted at the surface is chilled so quickly that the individual
minerals have little or no chance to grow. As a result, the rock
is either composed of minerals that can be seen only with the
aid of a microscope (called aphanitic, from the Greek aphanes,
meaning "invisible") or contains no minerals at all (in the
latter case, the rock is composed of glass, which is really a
viscous, non-crystalline liquid). This results in two groups of
igneous rocks: (1) plutonic or intrusive igneous rocks that
solidified deep within the earth and (2) volcanic, or extrusive,
igneous rocks formed at the Earth's surface.
The deep-seated plutonic rocks can be exposed at the surface
for study only after a long period of weathering or by some
tectonic forces that push the crust upward or by a combination
of the two. The exposed intrusive rocks are found in a variety
of sizes, from small dikes to massive dome-shaped batholiths,
which cover hundreds of square miles and make up the cores of
many mountain ranges.
Extrusive rocks occur in two forms: (1) as lava flows that
flood the land surface much like a river and (2) as fragmented
pieces of magma of various sizes (pyroclastic materials), which
often are blown through the atmosphere and blanket the Earth's
surface upon settling. The coarser pyroclastic materials
accumulate around the erupting volcano, but the finest
pyroclasts can be found as thin layers located hundreds of miles
from the opening. Most lava flows do not travel far from the
volcano, but some low-viscosity flows that erupted from long
fissures have accumulated in thick sequences. Both intrusive and
extrusive magmas have played a vital role in the spreading of
the ocean basin, in the formation of the oceanic crust, and in
the formation of the continental margins. Igneous processes have
been active since the formation of the Earth some 4.6 billion
· IGNEOUS ROCKS MORE DETAILS
Igneous rocks are formed when molten rock (magma derived from
the mantle, or, pre-existing rocks molten by extreme
temperature) cools and solidifies, with or without
crystallization. Over 700 types of igneous rocks have been
described, most of them intrusive.
Igneous rock are geologically important because:
their minerals and global chemistry gives information about the
composition of the mantle where they were extracted from, and
the temperature and pressure conditions that allowed this
extraction, or (below) their minerals and global chemistry gives
information about the composition of the country pre-existing
rock that melted
their absolute ages can be obtained from various forms of
radiometric dating and thus can be compared to adjacent strata,
allowing a time sequence of events
their features are usually characteristic of a specific tectonic
environment, allowing tectonic reconstitutions
in some special circumstances they host important mineral deposits,
of, for example, tungsten, tin or uranium, commonly associated
with granites they can be explored as ornamental stone
Igneous rocks are classified according to mode of occurrence,
texture, chemical composition, and the geometry of the igneous
Modes Of Occurrence
In terms of modes of occurrence, igneous rocks can be either
intrusive (plutonic) or extrusive (effusive).
Intrusive rocks crystallize within the crust interior.
Extrusive rocks are the result of volcanic eruptions and,
therefore, solidify in atmospheric conditions.
The most important distinction in igneous rocks textures is
related to grain size.
Phaneritic rocks contain minerals with grains (crystals) visible to
the unaided eye and are commonly intrusive (as the slower
cooling rates allow the formation of large crystals). In the
extreme, such rocks may contain extremely large crystals, in
which case they are termed pegmatitic.
In extrusive rocks, where cooling is much more rapid, the
individual mineral crystals are usually not visible and these
rocks are termed aphanitic.
Porphyritic textures are an intermediate situation between the
previous two: the groundmass of the rock has an aphanitic
texture, but crystals (termed in this particular occurrence as
phenocrystals) are visible to unaided eye.
If a molten magma cools at extremely high rates, allowing no
crystallization, the result is a vulcanic glass called obsidian.
Crystal shape is also an important factor in the texture of
an igneous rock. Crystals may be euedral, subeuedral or anedral:
Euedral, if the crystallographic shape is preserved
Subeuedral, if only part is preserved
Anedral, if the crystal presents no recognizable crystallographic
· Chemical Composition
Igneous rocks can be subdivided according to two main
contents of silica:
acid igneous rocks present a high silica content (ex: granite)
basic igneous rocks have low silica content (ex: basalt)
intermediate igneous rocks
contents of quartz, alkali feldspar and feldspatoids: the
felsic rock, with predominance of felsic minerals; these rocks (ex:
granite) are usually light coloured
mafic rock, with predominance of mafic minerals (all other
minerals, excluding the felsic); these rocks (ex: basalt) are
usually dark coloured
ultramaphic rock, with more that 90% of mafic minerals (ex: dunite)
Note that light coloured rocks, such as limestone or
sandstone cannot be classified as felsic, because their origin
is not igneous, is sedimentary.
The following table is a simple subdivision of igneous rocks
according both to their composition and mode of occurrence.
Mode of occurrence
· Geometry Of The Igneous Body
Igneous rocks can also classified according to the shape and
size of the intrusive body and its relation to the other
formations into which it intrudes. Typical intrusive formations
are batholiths, laccoliths, sills and dikes. The extrusive types
usually are called lavas.
· Example Of Classification
Granite is an igneous, intrusive rock (crystallised at
depth), with felsic acid composition (rich in silica and with
more than 10% of felsic minerals) and phaneritic, subeuedral
texture (minerals are visible for the unaided eye and some of
them retain original crystallographic shapes).
· CLASSIFICATION OF ROCKS