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 binary granite.
word granite comes from the Latin granum, a grain, in reference
to the grained structure of such a crystalline rock.
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 Earth.
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 granite.
density: 2750 kg/m3 (range 1741 to 2800)
IGNEOUS ROCKS GENERAL
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
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.
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.
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.
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 years ago.
IGNEOUS ROCKS MORE DETAILS
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:
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
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
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
rocks are classified according to mode of occurrence, texture,
chemical composition, and the geometry of the igneous body.
terms of modes of occurrence, igneous rocks can be either intrusive
(plutonic) or extrusive (effusive).
rocks crystallize within the crust interior.
rocks are the result of volcanic eruptions and, therefore, solidify
in atmospheric conditions.
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.
extrusive rocks, where cooling is much more rapid, the individual
mineral crystals are usually not visible and these rocks are
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.
shape is also an important factor in the texture of an igneous
rock. Crystals may be euedral, subeuedral or anedral:
if the crystallographic shape is preserved
if only part is preserved
if the crystal presents no recognizable crystallographic direction
rocks can be subdivided according to two main chemical parameters:
igneous rocks present a high silica content (ex: granite)
igneous rocks have low silica content (ex: basalt)
of quartz, alkali feldspar and feldspatoids: the felsic minerals
rock, with predominance of felsic minerals; these rocks (ex:
granite) are usually light coloured
rock, with predominance of mafic minerals (all other minerals,
excluding the felsic); these rocks (ex: basalt) are usually
rock, with more that 90% of mafic minerals (ex: dunite)
that light coloured rocks, such as limestone or sandstone cannot
be classified as felsic, because their origin is not igneous,
following table is a simple subdivision of igneous rocks according
both to their composition and mode of occurrence.
Geometry Of The Igneous Body
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
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
CLASSIFICATION OF ROCKS