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Q1: Igneous Rocks {march 2001 2002 20004 2005 2007 2008 2009 2010-2016} OU Telangana
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FORMS
STRUCTURES
TEXTURES
CLASSIFICATIONS
Ans: Igneous rock: (derived from the Latin word ignis meaning fire), or magmatic rock, is one
of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is
formed through the cooling and solidification of magma or lava. The magma can be derived
from partial melts of existing rocks in either a planet's mantle or crust. Typically, the melting is
caused by one or more of three processes: an increase in temperature, a decrease in pressure,
or a change in composition. Solidification into rock occurs either below the surface
as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form
with crystallization to form granular, crystalline rocks, or without crystallization to form natural
glasses.
FORMS :
In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic and hypabyssal)
or extrusive(volcanic).
Intrusive
Close-up of granite (an intrusive igneous rock) exposed in Chennai, India.
Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a
planet, surrounded by pre-existing rock (called country rock); the magma cools slowly and, as a
result, these rocks are coarse-grained. The mineral grains in such rocks can generally be
identified with the naked eye. Intrusive rocks can also be classified according to the shape and
size of the intrusive body and its relation to the other formationsinto which it intrudes. Typical
intrusive formations are batholiths, stocks, laccoliths, sills and dikes. When the magma solidifies
within the earth's crust, it cools slowly forming coarse textured rocks, such as granite, gabbro, or
diorite.
The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite.
When exposed by erosion, these cores (called batholiths) may occupy huge areas of the Earth's
surface.
Intrusive igneous rocks that form at depth within the crust are termed plutonic (or abyssal) rocks
and are usually coarse-grained. Intrusive igneous rocks that form near the surface are
termed subvolcanic or hypabyssal rocks and they are usually medium-grained. Hypabyssal rocks
are less common than plutonic or volcanic rocks and often form dikes, sills, laccoliths,lopoliths,
or phacoliths.
Extrusive
Extrusive igneous rock is made from lava released by volcanoes
Sample of basalt (an extrusive igneous rock), found in Massachusetts
Extrusive igneous rocks, also known as volcanic rocks, are formed at the crust's surface as a
result of the partial melting of rocks within the mantle and crust. Extrusive igneous rocks cool and
solidify quicker than intrusive igneous rocks. They are formed by the cooling of molten magma on
the earth's surface. The magma, which is brought to the surface through fissures or volcanic
eruptions, solidifies at a faster rate. Hence such rocks are smooth, crystalline and finegrained. Basalt is a common extrusive igneous rock and forms lava flows, lava sheets and lava
plateaus. Some kinds of basalt solidify to form long polygonal columns. The Giant's Causeway in
Antrim, Northern Ireland is an example.
The molten rock, with or without suspended crystals and gas bubbles, is called magma. It rises
because it is less dense than the rock from which it was created. When magma reaches the
surface from beneath water or air, it is called lava. Eruptions ofvolcanoes into air are
termed subaerial, whereas those occurring underneath the ocean are termed submarine. Black
smokers and mid-ocean ridge basalt are examples of submarine volcanic activity.
The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting.
Extrusive rock is produced in the following proportions:[3]
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divergent boundary: 73%
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convergent boundary (subduction zone): 15%
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hotspot: 12%.
Magma that erupts from a volcano behaves according to its viscosity, determined by
temperature, composition, crystal content and the amount of silica. High-temperature magma,
most of which is basaltic in composition, behaves in a manner similar to thick oil and, as it
cools, treacle. Long, thin basalt flows with pahoehoe surfaces are common. Intermediate
composition magma, such as andesite, tends to form cinder cones of intermingled ash, tuff and
lava, and may have a viscosity similar to thick, cold molasses or even rubber when
erupted. Felsic magma, such as rhyolite, is usually erupted at low temperature and is up to
10,000 times as viscous as basalt. Volcanoes with rhyolitic magma commonly erupt explosively,
and rhyolitic lava flows are typically of limited extent and have steep margins, because the
magma is so viscous.
Felsic and intermediate magmas that erupt often do so violently, with explosions driven by the
release of dissolved gases—typically water vapour, but also carbon dioxide. Explosively
erupted pyroclastic material is called tephra and includes tuff,agglomerate and ignimbrite. Fine
volcanic ash is also erupted and forms ash tuff deposits, which can often cover vast areas.
Because lava usually cools and crystallizes rapidly, it is usually fine-grained. If the cooling has
been so rapid as to prevent the formation of even small crystals after extrusion, the resulting rock
may be mostly glass (such as the rock obsidian). If the cooling of the lava happened more slowly,
the rock would be coarse-grained.
Because the minerals are mostly fine-grained, it is much more difficult to distinguish between the
different types of extrusive igneous rocks than between different types of intrusive igneous rocks.
Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be
determined by examination of thin sections of the rock under a microscope, so only an
approximate classification can usually be made in the field.
Classification
Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical
composition, and the geometry of the igneous body.
The classification of the many types of different igneous rocks can provide us with important
information about the conditions under which they formed. Two important variables used for the
classification of igneous rocks are particle size, which largely depends on the cooling history, and
the mineral composition of the
rock.Feldspars, quartz or feldspathoids, olivines, pyroxenes, amphiboles, and micas are all
important minerals in the formation of almost all igneous rocks, and they are basic to the
classification of these rocks. All other minerals present are regarded as nonessential in almost all
igneous rocks and are called accessory minerals. Types of igneous rocks with other essential
minerals are very rare, and these rare rocks include those with essential carbonates.
In a simplified classification, igneous rock types are separated on the basis of the type of feldspar
present, the presence or absence of quartz, and in rocks with no feldspar or quartz, the type of
iron or magnesium minerals present. Rocks containing quartz (silica in composition) are silicaoversaturated. Rocks with feldspathoidsare silica-undersaturated, because feldspathoids cannot
coexist in a stable association with quartz.
Igneous rocks that have crystals large enough to be seen by the naked eye are called phaneritic;
those with crystals too small to be seen are called aphanitic. Generally speaking, phaneritic
implies an intrusive origin; aphanitic an extrusive one.
An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is
termed porphyry. Porphyritic texture develops when some of the crystals grow to considerable
size before the main mass of the magma crystallizes as finer-grained, uniform material.
Igneous rocks are classified on the basis of texture and composition. Texture refers to the size,
shape, and arrangement of the mineral grains or crystals of which the rock is composed.
Texture
Gabbro specimen showingphaneritic texture; Rock Creek Canyon, eastern Sierra Nevada, California; scale
bar is 2.0 cm.
Main article: Rock microstructure
Texture is an important criterion for the naming of volcanic rocks. The texture of volcanic rocks,
including the size, shape, orientation, and distribution of mineral grains and the intergrain
relationships, will determine whether the rock is termed a tuff, a pyroclastic lava or a simple lava.
However, the texture is only a subordinate part of classifying volcanic rocks, as most often there
needs to be chemical information gleaned from rocks with extremely fine-grained groundmass or
from airfall tuffs, which may be formed from volcanic ash.
Textural criteria are less critical in classifying intrusive rocks where the majority of minerals will
be visible to the naked eye or at least using a hand lens, magnifying glass or microscope.
Plutonic rocks also tend to be less texturally varied and less prone to gaining structural fabrics.
Textural terms can be used to differentiate different intrusive phases of large plutons, for
instance porphyritic margins to large intrusive bodies, porphyry stocks
and subvolcanic dikes (apophyses). Mineralogical classification is most often used to classify
plutonic rocks. Chemical classifications are preferred to classify volcanic rocks, with phenocryst
species used as a prefix, e.g. "olivine-bearing picrite" or "orthoclase-phyric rhyolite".
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see also List of rock textures and Igneous textures
Basic classification scheme for igneous rocks on their mineralogy. If the approximate volume fractions of
minerals in the rock are known, the rock name and silica content can be read off the diagram. This is not an
exact method, because the classification of igneous rocks also depends on other components than silica,
yet in most cases it is a good first guess.
Chemical classification and Petrology
Total alkali versus silica classification scheme (TAS) as proposed in Le Maitre's 2002 Igneous Rocks - A
classification and glossary of terms[4]:237
Igneous rocks can be classified according to chemical or mineralogical parameters.
Chemical: total alkali-silica content (TAS diagram) for volcanic rock classification used when
modal or mineralogic data is unavailable:
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felsic igneous rocks containing a high silica content, greater than 63%
SiO2 (examples granite and rhyolite)
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intermediate igneous rocks containing between 52 – 63% SiO2 (example andesite
and dacite)
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mafic igneous rocks have low silica 45 – 52% and typically high iron – magnesium content
(example gabbro and basalt)
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ultramafic rock igneous rocks with less than 45% silica.
(examples picrite, komatiite and peridotite)
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alkalic igneous rocks with 5 – 15% alkali (K2O + Na2O) content or with a molar ratio of alkali
to silica greater than 1:6. (examples phonolite and trachyte)
Chemical classification also extends to differentiating rocks that are chemically similar according
to the TAS diagram, for instance;
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Ultrapotassic; rocks containing molar K2O/Na2O >3
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Peralkaline; rocks containing molar (K2O + Na2O)/ Al2O3 >1
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Peraluminous; rocks containing molar (K2O + Na2O)/ Al2O3 <1
An idealized mineralogy (the normative mineralogy) can be calculated from the chemical
composition, and the calculation is useful for rocks too fine-grained or too altered for identification
of minerals that crystallized from the melt. For instance, normative quartz classifies a rock as
silica-oversaturated; an example is rhyolite. In an older terminology, silica oversaturated rocks
were called silicic or acidic where the SiO2 was greater than 66% and the family
term quartzolite was applied to the most silicic. A normative feldspathoid classifies a rock as
silica-undersaturated; an example is nephelinite.
Mineralogical classification
For volcanic rocks, mineralogy is important in classifying and naming lavas. The most important
criterion is the phenocryst species, followed by the groundmass mineralogy. Often, where the
groundmass is aphanitic, chemical classification must be used to properly identify a volcanic
rock.
Mineralogic contents – felsic versus mafic
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felsic rock, highest content of silicon, with predominance of quartz, alkali feldspar and/or
feldspathoids: the felsic minerals; these rocks (e.g., granite, rhyolite) are usually light
coloured, and have low density.
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mafic rock, lesser content of silicon relative to felsic rocks, with predominance of mafic
minerals pyroxenes, olivines and calcic plagioclase; these rocks (example, basalt, gabbro)
are usually dark coloured, and have a higher density than felsic rocks.
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ultramafic rock, lowest content of silicon, with more than 90% of mafic minerals (e.g., dunite).
For intrusive, plutonic and usually phaneritic igneous rocks (where all minerals are visible at least
via microscope), the mineralogy is used to classify the rock. This usually occurs on ternary
diagrams, where the relative proportions of three minerals are used to classify the rock.
The following table is a simple subdivision of igneous rocks according to both their composition
and mode of occurrence.
Composition
Mode of occurrence Felsic
Intermediate Mafic
Ultramafic
Intrusive
Granite Diorite
Gabbro Peridotite
Extrusive
Rhyolite Andesite
Basalt Komatiite
Essential rock forming silicates
Felsic
Intermediate Mafic
Ultramafic
Coarse-grained Granite Diorite
Gabbro Peridotite
Medium-grained
Diabase
Fine-grained
Rhyolite Andesite
Basalt
Komatiite
For a more detailed classification see QAPF diagram.
Example of classification
Granite is an igneous intrusive rock (crystallized at depth), with felsic composition (rich in silica
and predominately quartz plus potassium-rich feldspar plus sodium-rich plagioclase) and
phaneritic, subeuhedral texture (minerals are visible to the unaided eye and commonly some of
them retain original crystallographic shapes).
NOTE:
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