IGNEOUS ROCKS 9
Rockis a solid aggregate comprising of one or more minerals. There arethree types of rocks classified as igneous, sedimentary andmetamorphic rocks. Metamorphic rocks are formed as a result ofchanges that occur due to extreme heat and pressure below the earthsurface. Rocks resulting from such condition appear to have sparklingcrystals and ribbon-like layers. Examples are gneiss and marbletemperature (Gill, 2010). Gneiss is mainly used in construction andother structural purposes while marble is used as an ingredient inthe manufacture of refracting materials. Sedimentary rocks are formedfrom lithification and compaction of sediments over an extendedperiod. The deposits are comprised of particles of sand, pebbles orshells. These rocks are soft, break easily and contain fossils.Examples are conglomerates and limestone. Igneous rocks havedifferent textures, features, classifications, reaction series andapplications as discussed in the paper.
Igneousrocks are formed when magma cools and solidifies on or below theearth’s surface. There are two types of igneous rocks bothextrusive and intrusive rocks. Extrusive rocks are also known asvolcanic rocks. They are formed when magma cools and solidifies afterreaching the earth’s surface (Gill, 2010). Extrusive igneous rocksinclude basalt, andesite, pumice, rhyolite, scoria, obsidian andtuff. Intrusive rocks are also referred as plutonic rocks and areformed when magma cools and solidifies before reaching the surface ofthe earth. Examples are gabbro, diorite, and granite.
Volcanicrocks crystallize faster since they are exposed to air and thereforeform fine-grained crystals. The minerals can only be viewed under amicroscope .The fine-grained nature of the crystals makes itdifficult in distinguishing various types. In some cases, the coolingmay be so rapid that there is no formation of crystals such asobsidian (Gill, 2010).Plutonic rocks cool slowly beneath the earth’ssurface forming coarse grained crystals. The minerals are largeenough to be seen using naked eyes. In addition to these types ofigneous rocks, there are also hypabyssal rocks that are formed atmedium to shallow depths of the earth’s crust mainly in fissuressuch as sills and dikes. They have a medium grain size and texturethat is between that of extrusive and intrusive rocks. Examples ofhypabyssal rocks are micro-granite, diabase, and diorite.
They can be either light colored or dark colored.
Made up of two or more minerals.
Volcanic rocks do not contain fossils.
They have no layering.
They are made of minerals crystals that are of different sizes.
They may be coarse-grained, fine-grained or glassy.
Some igneous rocks may have openings
Thetexture is the measure of the size of crystals in a rock (Jerram,2011).  Rock texture is important to igneous petrologistsbecause it reveals how the rock was formed. There are different typesof igneous textures and are described by some terms (Jerram, 2011).These include
Phaneritic texture or coarse-grained texture – the minerals can be seen using naked eyes. The grains are in several millimeters or larger than a centimeter.
Aphanitic or fine-grained texture – the particles cannot be identified with our naked eyes. Use of a microscope or hand lens is required to see the minerals since the minerals grains are smaller than one millimeter.
Porphyritic texture – it contains both large and small crystals.
Glassy texture – has no visible crystals.
Pyroclastic texture – results from compaction of volcanic ash and tuff deposits. The texture has a gritty feel.
Vesicular texture – has open spaces and bubbles caused when volatile compounds or substances which erupt from the liquid due to pressure decrease.
Frothy texture – has many small bubbles than the rock.
Thereare various criteria that are used to classify igneous rocks. Theyinclude:
i.   Mineral present in the rocks
Itis also known as the mode of a rock. The pattern and its relativeproportions in a rock depend on the chemical composition of themagma. The type of classification performs well with slow coolingplutonic rocks when all the minerals from the magma have beencrystallized. In volcanic rocks, there is rapid cooling, andtherefore the grains are not identified or may be transparent.
ii.   Texture of the rock
Therock`s texture depends on the cooling history of a rock. Rocks withthe same minerals and same chemical compositions may have differenttextures (Gill, 2010). The texture subdivides igneous rocks intovolcanic (extrusive) which are usually fine-grained or glassy andplutonic (intrusive) rocks which are coarse-grained.
Themineral content and the size of grain determine the color of rocks.Rocks with large quantities of quartz and feldspars are light coloredwhile those which comprises olivine, pyroxenes and amphiboles aredark colored. Sometimes the use of color in classification may bemisleading especially with rocks having the same composition butdifferent size of a particle. For example, granite is light coloredsince it has more of feldspars and quartz and a rock that is rapidlycooled and has the same composition as granite can be black coloredor transparent such as obsidian. Rocks can also be classified as thefelsic rocks because of having large amounts of feldspars and quartzand mafic rocks containing more ferromagnesium minerals.
iv.   Chemical composition
Chemicalcomposition is the most distinctive feature of igneous rocks since itreveals the magma content that determines the origin of a rock. Itverifies the type of minerals that would crystallize and theirpercentages (Gill, 2010). It is not easy to identify the chemicalcomposition in the field making it impractical. Therefore, thiscriterion of classifying igneous rocks has its limitations andtherefore an approach based on the information obtained fromdifferent stages is used. A simple field-based classification is doneon the field involving the texture and the mineralogical content. Forthe intrusive igneous rocks, the IUGS classification is used. Thenthe samples are brought to the laboratory where thin sections arecut. These helps in determining the mineralogical content andtextural classification is done.
Volcanicrocks are classified by total alkali content versus the silica in theabsence of model data (Gill, 2010).Classification based on silicacontent divides the rocks into three categories silica oversaturated,and silica saturated and silica under saturated. If magma isunder-saturated with silica content, then a mineral such as quartz isnot present in the rock and should not precipitate. In saturatedsilica rocks, quartz does not appear due to enough silica content.These rocks contain olivine or hypersthene plus olivine when leucite,nepheline or quartz is absent (Gill, 2010). Silica saturated rocksare also classified into
Felsic rocks which contain more than 63% of silica content. Examples are rhyolite and granite.
Intermediate rocks which contain between 52-63% silica content. Examples are dacite and andesite.
Mafic igneous rocks which contain silica content of 45-52% and high iron-magnesium content. Examples are basalts and gabbros.
Ultramafic rocks containing less than 45% silica content. An example is a peridotite.
Aluminais the second oxide most present in igneous rocks after silica.Alumina concept is based on whether there is more of Al that makes upthe feldspars since it is the most abundant mineral in igneous rocks.Three conditions exist regarding alumina saturation.
Peraluminous rocks formed when a rock is preferred to have more alumina than the required over feldspars. Muscovite, corundum, topaz, sillimanite and kyanite are good examples of such rocks.
Met aluminous rocks are the most common types of igneous rocks. They do not contain aluminum-rich mineral, sodic pyroxenes, and amphiboles in the mode.
Peralkaline rocks have high alkalis percentages and are therefore oversaturated on aluminum. They are distinguished by their high Na- precious minerals such as aegerine.
Theother classification distinguishes rocks that are alkaline from thosethat are sub-alkaline by use of silica versus alkalis diagram. Silicaunder-saturated rocks usually have high levels of alkaline.
TheBowen Reaction Series
Bowencreated a model known as Bowen reaction series that is essential inexplaining the origin of various types of igneous rocks. The seriessuggests that the type of rocks that form from solidification ofmagma depends on the chemical composition depending on where themagma is originating and the crystallization temperature (Gill,2010). According to Bowen the mineral that constitutes igneous rocksstarts from two different sequences at higher temperatures but latermerge into a single series at low temperature. One sequence is thediscontinuous series which involves the formation of chemicallydifferent minerals at distinct temperature intervals starting frommagnesium and iron rich mafic mineral. Continued decrease in thetemperature changes the chemical composition of mineral frompyroxenes to amphiboles and finally to biotite.
Theother sequence is the continuous series where the temperaturedecrease leads to gradual changes in the chemistry of the mineralsthat form sodium and calcium rich felsic magma. These sequences formlight-colored rocks that are composed of plagioclase feldsparminerals (Jerram, 2011). At high temperatures, the rocks are mostlydominated by calcium which upon cooling they are replaced by sodium.The decrease in the temperature of magma results in the convergenceof the two series. The minerals that are found in the merged seriesare rich in potassium and silica. There is also the formation of theMuscovite while quartz is the last mineral to be formed in the Bowenreaction series (Gill, 2010).
Magmafrom the mantle occurs as a result of increased temperature, changein composition or decrease in pressure. These are the conditions thatbring about partial melting in the mantle. Igneous rocks are composedof many primary minerals that are mostly black, white or gray such asfeldspars, quartz, muscovite, biotite, olivine, pyroxenes and manyothers. These minerals harden the rock because they get compactedtogether during cooling, and this gives igneous rocks their valuableuses. Igneous rocks form at different places such as at differentboundaries especially the mid-oceanic ridge where magma fills gapsafter the drift, subduction zones, convergent boundaries and hotspots. Igneous rocks are geologically important because they give usthe information about the mantle due to their chemistry, the mineralscontained, and pressure and temperature conditions in the formation.Igneous rocks also contain valuable mineral ores such as uranium andtin.
Gill,R. (2010).Igneousrocks and processes: A practical guide.Chichester, West Sussex, UK: Wiley-Blackwell.
Jerram,D. A., &Petford, N. (2011). Thefield description of igneous rocks.Chichester: Wiley-Blackwell.