GEOGRAPHY NOTES FOR UPSC CIVIL SERVICES PRELIMINARY EXAM BY 'BEST CURRENT AFFAIRS'

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ROCKS For IAS Prelims 2022

The earth’s crust is composed of rocks. A rock is an aggregate of one or more minerals. Rock may be hard or soft and in varied colours. For example, granite is hard, sandstone is soft. Gabbro is black and quartzite can be milky white. Rocks do not have definite composition of mineral constituents. Feldspar and quartz are the most common minerals found in rocks. The crustal rocks are classified on the basis of mode of formation, physical and chemical properties, location etc. On the basis of mode of formation the rocks are divided into three categories (i) igneous rocks (ii) sedimentary rocks (iii) metamorphic rocks.

IGNEOUS ROCKS

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They are classified on several grounds as mentioned below:

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(b) On the basis of chemistry and mineralogical composition: (i) Felsic igneous rock (feldspar is dominant) (ii) Mafic igneous rock (magnesium and ferrous are dominant) (iii) Ultra mafic igneous rock (Peridotite and dunite are dominant).

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(a) On the basis of silica content: (i) Acidic igneous rocks have more silica e.g. Granites (ii) Basic igneous rocks have less silica e.g. Gabbro.

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As igneous rocks form out of magma and lava from the interior of the earth, they are known as primary rocks. The igneous rocks are formed when magma cools and solidifies. When magma in its upward movement cools and turns into solid form it is called igneous rock.

The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth.

Igneous rocks are characterized on the basis of texture. Texture depends upon size and arrangement of grains or other physical conditions of the materials. If molten material is cooled slowly at great depths, mineral grains may be very large.

Sudden cooling (at the surface) results in small and smooth grains. Intermediate conditions of cooling would result in intermediate sizes of grains making up igneous rocks. Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of the examples of igneous rocks.

Igneous rocks are roughly hard rocks and water percolates with great difficulty. They do not have strata and are less affected by chemical weathering. They don’t contain fossils. The number of joints increases upwards. They are mostly associated with volcanic activity.

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(c) On the mode of occurrence: (i) Intrusive igneous rocks— they are cooled and solidified below the surface of the earth. They are further divided into plutonic and hypabyssal igneous rocks. Plutonic rocks cool deep beneath the earth e.g. Granite. Hypabyssal rocks cool just beneath the earth surface e.g. Batholith, laccolith, phacolith, lapolith, sills, dykes, etc. (ii) Extrusive igneous rocks are formed due to cooling and solidification of hot and molten lava at the earth’s surface e.g. Basalt, Gabbro, obsidian.

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Types of Igneous rocks:  Adakite – Class of intermediate to felsic volcanic rocks containing low amounts of yttrium and ytterbium  Andesite – Intermediate volcanic rock  Alkali feldspar granite – Granitoid in which at least 90% of the total feldspar is alkali feldspar  Anorthosite – Mafic intrusive igneous rock composed predominantly of plagioclase  Aplite – Fine-grained intrusive igneous rock type similar to granite  Basalt – Magnesium- and iron-rich extrusive igneous rock  A'a – Basaltic lava with a crumpled appearance  Pahoehoe – Basaltic lava with a flowing, often ropy appearance  Basaltic trachyandesite  Mugearite – Oligoclase-bearing basalt, comprising olivine, apatite, and opaque oxides  Shoshonite – Potassium-rich variety of basaltic trachyandesite  Basanite – A silica-undersaturated basalt  Blairmorite – Rare porphyritic volcanic rock  Boninite – Ultramafic extrusive rock high in both magnesium and silica  Carbonatite – Igneous rock with more than 50% carbonate minerals  Charnockite – Type of granite containing orthopyroxene  Enderbite – Igneous rock of the charnockite series  Dacite – Volcanic rock intermediate in composition between andesite and rhyolite  Diabase, also known as dolerite – An intrusive mafic rock forming dykes or sills  Diorite – Intermediate intrusive igneous rock composed principally of plagioclase feldspar  Napoleonite, also known as corsite – Variety of diorite with orbicular structure  Dunite – Ultramafic and ultrabasic rock from Earth's mantle which is made of the mineral olivine  Essexite – Dark gray or black holocrystalline plutonic rock  Foidolite – Rare coarse-grained intrusive igneous rock in which more than 60% of light-coloured minerals are feldspathoids  Gabbro – Coarse-grained mafic intrusive rock  Granite – common type of intrusive, felsic, igneous rock with granular structure  Granodiorite – A phaneritic-textured intrusive igneous rock similar to granite  Granophyre – Subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths  Harzburgite – Ultramafic and ultrabasic mantle rock. Found in ophiolites.

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Hornblendite – Plutonic rock consisting mainly of the amphibole hornblende Hyaloclastite – Volcaniclastic accumulation or breccia Icelandite – Iron rich, aluminium poor andesite Ignimbrite – Variety of hardened tuff Ijolite – Igneous rock consisting essentially of nepheline and augite Kimberlite – Igneous rock which sometimes contains diamonds Komatiite – Ultramafic mantle-derived volcanic rock Lamproite – Ultrapotassic mantle-derived volcanic or subvolcanic rock Lamprophyre – An ultramafic, ultrapotassic intrusive rock dominated by mafic phenocrysts in a feldspar groundmass Latite – A silica-undersaturated form of andesite Lherzolite – An ultramafic rock, essentially a peridotite Monzogranite – A silica-undersaturated granite with <5% normative quartz Monzonite – Igneous intrusive rock with low quartz and equal plagioclase and alkali feldspar – a plutonic rock with <5% normative quartz Nepheline syenite – A silica-undersaturated plutonic rock of nepheline and alkali feldspar Nephelinite – A silica-undersaturated plutonic rock with >90% nepheline Norite – A hypersthene-bearing gabbro Obsidian – Naturally occurring volcanic glass Pegmatite – Igneous rock with very large interlocked crystals Peridotite – A coarse-grained ultramafic igneous rock Phonolite – A silica-undersaturated volcanic rock; essentially similar to nepheline syenite Phonotephrite – A volcanic rock with a composition between phonolite and tephrite Picrite – An olivine-bearing basalt Porphyry – Textural form of igneous rock with large grained crystals in a fine matrix Pumice – Light coloured highly vesicular volcanic rock Pyroxenite – Igneous rock - a coarse grained plutonic rock composed of >90% pyroxene Quartz diorite – A diorite with >5% modal quartz Quartz monzonite – Type of igneous rock – An intermediate plutonic rock, essentially a monzonite with 5–10% modal quartz Quartzolite – An intrusive rock composed mostly of quartz Rhyodacite – Volcanic rock rich in silica and low in alkali metal oxides – A felsic volcanic rock which is intermediate between a rhyolite and a dacite Rhyolite – Igneous, volcanic rock, of felsic (silica-rich) composition  Comendite – Hard, peralkaline igneous rock, a type of light blue grey rhyolite  Pantellerite – Peralkaline rhyolite type of volcanic rock Scoria – Dark vesicular volcanic rock Shonkinite – Intrusive igneous rock – a plutonic rock Sovite – A coarse-grained carbonatite rock Syenite – Intrusive igneous rock – A plutonic rock dominated by orthoclase feldspar; a type of granitoid Tachylyte – Essentially a basaltic glass Tephriphonolite – A volcanic rock with a composition between phonotephrite and phonolite Tephrite – A silica-undersaturated volcanic rock Tonalite – A plagioclase-dominant granitoid Trachyandesite – An alkaline intermediate volcanic rock  Benmoreite – Silica-undersaturated volcanic rock of intermediate composition - sodic trachyandesite Trachybasalt – A volcanic rock with a composition between basalt and trachyte  Hawaiite – Volcanic rock – a sodic type of trachybasalt, typically formed by ocean island (hot spot) volcanism Trachyte – Extrusive igneous rock – A silica-undersaturated volcanic rock; essentially a feldspathoid-bearing rhyolite Troctolite – Igneous rock – A plutonic ultramafic rock containing olivine, pyroxene and plagioclase Trondhjemite – Light-colored intrusive igneous rock – A form of tonalite where plagioclase-group feldspar is oligoclase Tuff – Rock consolidated from volcanic ash Websterite – A type of pyroxenite, composed of clinoproxene and orthopyroxene Wehrlite – Ultramafic rock - An ultramafic plutonic or cumulate rock, a type of peridotite, composed of olivine and clinopyroxene

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SEDIMENTARY ROCKS

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The word ‘sedimentary’ is derived from the Latin word sedimentum, which means settling. Rocks (igneous, sedimentary and metamorphic) of the earth’s surface are exposed to denudational agents, and are broken up into various sizes of fragments.  Such fragments are transported by different exogenous agencies and deposited. These deposits through compaction turn into rocks. This process is called lithification. (Course Book by BestCurrentAffairs.com)  In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification.  Hence, we see a number of layers of varying thickness in sedimentary rocks like sandstone, shale etc.

Depending upon the mode of formation, sedimentary rocks are classified into three major groups: (i) Mechanically formed - e.g. sandstone, conglomerate, shale, loess etc. (ii) Organically formed - e.g. chalk, limestone, coal etc. (iii) Chemically formed – e.g. chert, halite, potash etc.  These rocks are formed due to aggregation and compaction of sediments. These rocks contain fossils of plants and animals.

They cover 75 percent of surface area of the globe. However they form only 5 percent of the volume of earth’s crust. They contain several layers or strata but these are seldom crystalline rocks.  They are seldom found in original and horizontal manner. They may be well consolidated, poorly consolidated and even unconsolidated.  They are characterized by different sizes of joints. Most sedimentary rocks are porous and permeable.

The formation of sedimentary rocks takes place in three stages: 1. Transportation: after weathering and erosion the fragments of parental rocks are transported by the agents of erosion like stream, wind, air, etc. 

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Deposition: transported materials are deposited in sea, lakes, etc. The particles are deposited in parallel layers and their process of layer formation is called “stratification”.

Consolidation: when the number of layer is large, the weight of upper layer begins to affect the lower layers and the further compression solidifies the sediments into rocks.

They are classified under different schemes:

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1. On the basis of nature of sediments: (a) Mechanically formed or clastic rocks e.g. Sandstones. Conglomerates, clay rock, shale, loess. (b) Chemically formed sedimentary rocks e.g. gypsum, salt rock. (c) Organically formed sedimentary rocks e.g. Limestone, dolomites, coal, peats, etc.

2. On the basis of transporting agents: (i) Argillaceous or aqueous rocks: (a) Marine rocks, (b) Lacustrine rocks, (c) Riverine rocks (ii) Aeolian rocks e.g. Loess. (iii) Glacial sedimentary rocks e.g. Till, moraine.

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Types of Sedimentary rocks:  Argillite – Sedimentary rock, mostly of indurated clay particles  Arkose – Type of sandstone containing at least 25% feldspar  Banded iron formation – Distinctive layered units of iron-rich sedimentary rock that are almost always of Precambrian age  Breccia – Rock composed of broken fragments cemented by a matrix  Calcarenite – Type of limestone that is composed predominantly of sand-size grains  Chalk – Soft, white, porous sedimentary rock made of calcium carbonate  Chert – Hard, fine-grained sedimentary rock composed of cryptocrystalline silica  Claystone – Clastic sedimentary rock composed primarily of clay-sized particles  Coal – Combustible sedimentary rock composed primarily of carbon  Conglomerate – Coarse-grained clastic sedimentary rock with mainly rounded to subangular clasts  Coquina – Sedimentary rock that is composed mostly of fragments of shells  Diamictite – Lithified sedimentary rock of non- to poorly sorted terrigenous sediment in a matrix of mudstone or sandstone  Diatomite – Soft, siliceous sedimentary rock that is easily crumbled  Dolomite (rock), also known as Dolostone – Sedimentary carbonate rock that contains a high percentage of the mineral dolomite  Evaporite – Water-soluble mineral deposit formed by evaporation from an aqueous solution  Flint – Cryptocrystalline form of the mineral quartz  Geyserite – Form of opaline silica that is often found around hot springs and geysers  Greywacke – A hard, dark sandstone with poorly sorted angular grains in a compact, clay-fine matrix  Gritstone – A hard, coarse-grained, siliceous sandstone  Itacolumite – A porous, yellow sandstone that is flexible when cut into thin strips  Jaspillite – Banded mixture of hematite and quartz  Laterite – Product of rock weathering in wet tropical climate rich in iron and aluminium  Lignite – Soft, brown, combustible, sedimentary rock  Limestone – Sedimentary rocks made of calcium carbonate  Marl – Lime-rich mud or mudstone which contains variable amounts of clays and silt  Mudstone – Fine grained sedimentary rock whose original constituents were clays or muds  Oil shale – Organic-rich fine-grained sedimentary rock containing kerogen  Oolite – Sedimentary rock formed from ooids  Phosphorite – A non-detrital sedimentary rock that contains high amounts of phosphate minerals  Sandstone – Type of sedimentary rock  Shale – Fine-grained, clastic sedimentary rock  Siltstone – Sedimentary rock which has a grain size in the silt range  Sylvinite – A sedimentary rock made of a mechanical mixture of sylvite and halite  Tillite – Till which has been indurated or lithified by burial  Travertine – Form of limestone deposited by mineral springs  Tufa – Porous limestone rock formed when carbonate minerals precipitate out of ambient temperature water  Turbidite – Geologic deposit of a turbidity current  Wackestone – Mud-supported carbonate rock that contains greater than 10% grains

METAMORPHIC ROCKS

The word metamorphic means ‘change of form’. These rocks form under the action of pressure, volume and temperature (PVT) changes. Metamorphism occurs when rocks are forced down to earth’s interior by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks or the underlying rocks are subjected to great amounts of pressure by overlying rocks.

Metamorphism is a process by which already consolidated rocks undergo recrystallization and reorganization of materials within original rocks.

Mechanical disruption and reorganization of the original minerals within rocks due to breaking and crushing without any appreciable chemical changes is called dynamic metamorphism.

The materials of rocks chemically alter and recrystallize due to thermal metamorphism. There are two types of thermal metamorphism - contact metamorphism and regional metamorphism.

In contact metamorphism the rocks come in contact with hot intruding magma and lava and the rock materials recrystallize under high temperatures. Quite often new materials form out of magma or lava are added to the rocks.

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In regional metamorphism, rocks undergo recrystallization due to deformation caused by tectonic shearing together with high temperature or pressure or both. In the process of metamorphism in some rocks grains or minerals get arranged in layers or lines. Such an arrangement of minerals or grains in metamorphic rocks is called foliation or lineation.

Sometimes minerals or materials of different groups are arranged into alternating thin to thick layers appearing in light and dark shades. Such a structure in metamorphic rocks is called banding and rocks displaying banding are called banded rocks. Types of metamorphic rocks depend upon original rocks that were subjected to metamorphism.

Metamorphic rocks undergo complete alteration in the appearance of pre-existing rocks due to change in mineral composition and texture through temperature and pressure changes. Gneiss, granite, slate, schist, marble, quartzite etc. are some examples of metamorphic rocks. They are classified as mentioned below: 1. Contact or thermal metamorphism: here metamorphism occurs when the mineral composition of the surrounding rocks is changed due to intense heat e.g. Limestone is changed to marble.

2. Regional or dynamic metamorphism: here pressure plays an important role so that rocks are altered in their forms in an extensive area.

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Types of Metamorphic rocks:  Anthracite – Hard, compact variety of coal that has a submetallic luster  Amphibolite – A metamorphic rock containing mainly amphibole and plagioclase  Blueschist – Metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition  Cataclasite – A rock formed by faulting  Eclogite – A dense metamorphic rock formed under high pressure  Gneiss – Common high-grade metamorphic rock  Granulite – Class of high-grade medium to coarse grained metamorphic rocks  Greenschist – A mafic metamorphic rock dominated by green amphiboles  Hornfels  Calcflinta – A type of hornfels found in the Scottish Highlands  Litchfieldite – Nepheline syenite gneiss  Marble – Non-foliated, metamorphic rock, commonly used for sculpture and as a building material – a metamorphosed limestone  Migmatite – Mixture of metamorphic rock and igneous rock  Mylonite – Metamorphic rock – A metamorphic rock formed by shearing  Metapelite – A metamorphic rock with a protolith of clay-rich (siltstone) sedimentary rock  Metapsammite – A metamorphic rock with a protolith of quartz-rich (sandstone) sedimentary rock  Phyllite – A low grade metamorphic rock composed mostly of micaceous minerals  Pseudotachylite – A glass formed by melting within a fault via friction  Quartzite – Hard, non-foliated metamorphic rock which was originally pure quartz sandstone – A metamorphosed sandstone typically composed of >95% quartz  Schist – A medium-grained metamorphic rock with lamellar texture  Serpentinite – Rock formed by hydration and metamorphic transformation of olivine  Skarn – Hard, coarse-grained, hydrothermally altered metamorphic rocks  Slate – Metamorphic rock - A low grade metamorphic rock formed from shale or silts  Suevite – Rock consisting partly of melted material formed during an impact event – A rock formed by partial melting during a meteorite impact  Talc carbonate – A metamorphosed ultramafic rock with talc as an essential constituent; similar to a serpentinite  Soapstone – Talc-bearing metamorphic rock – Essentially a talc schist  Tectonite – A rock whose fabric reflects the history of its deformation  Whiteschist – A high pressure metamorphic rock containing talc and kyanite

ROCK CYCLE

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Rocks do not remain in their original form for long but may undergo transformation. Rock cycle is a continuous process through which old rocks are transformed into new ones. Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and change as they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.  Transition to igneous rock: When rocks are pushed deep under the Earth's surface, they may melt into magma. If the conditions no longer exist for the magma to stay in its liquid state, it cools and solidifies into an igneous rock. A rock that cools within the Earth is called intrusive or plutonic and cools very slowly, producing a coarse-grained texture such as the rock granite. As a result of volcanic activity, magma (which is called lava when it reaches Earth's surface) may cool very rapidly on the Earth's surface exposed to the atmosphere and are called extrusive or volcanic rocks. These rocks are fine-grained and sometimes cool so rapidly that no crystals can form and result in a natural glass, such as obsidian, however the most common fine-grained rock would be known as basalt. Any of the three main types of rocks (igneous, sedimentary, and metamorphic rocks) can melt into magma and cool into igneous rocks.  Secondary changes: Epigenetic change (secondary processes occurring at low temperatures and low pressures) may be arranged under a number of headings, each of which is typical of a group of rocks or rock-forming minerals, though usually more than one of these alterations is in progress in the same rock. Silicification, the replacement of the minerals by crystalline or crypto-crystalline silica, is most common in felsic rocks, such as rhyolite, but is also found in serpentine, etc. Kaolinization is the decomposition of the feldspars, which are the most common minerals in igneous rocks, into kaolin (along with quartz and other clay minerals); it is best shown by granites and syenites. Serpentinization is the alteration of olivine to serpentine (with magnetite); it is typical of peridotites, but occurs in most of the mafic rocks. In uralitization, secondary hornblende replaces augite; chloritization is the alteration of augite (biotite or hornblende) to chlorite, and is seen in many diabases, diorites and greenstones. Epidotization occurs also in rocks of this group, and consists in the development of epidote from biotite, hornblende, augite or plagioclase feldspar.

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Transition to metamorphic rock: Rocks exposed to high temperatures and pressures can be changed physically or chemically to form a different rock, called metamorphic. Regional metamorphism refers to the effects on large masses of rocks over a wide area, typically associated with mountain building events within orogenic belts. These rocks commonly exhibit distinct bands of differing mineralogy and colors, called foliation. Another main type of metamorphism is caused when a body of rock comes into contact with an igneous intrusion that heats up this surrounding country rock. This contact metamorphism results in a rock that is altered and re-crystallized by the extreme heat of the magma and/or by the addition of fluids from the magma that add chemicals to the surrounding rock (metasomatism). Any pre-existing type of rock can be modified by the processes of metamorphism.

Transition to sedimentary rock: Rocks exposed to the atmosphere are variably unstable and subject to the processes of weathering and erosion. Weathering and erosion break the original rock down into smaller fragments and carry away dissolved material. This fragmented material accumulates and is buried by additional material. While an individual grain of sand is still a member of the class of rock it was formed from, a rock made up of such grains fused together is sedimentary. Sedimentary rocks can be formed from the lithification of these buried smaller fragments (clastic sedimentary rock), the accumulation and lithification of material generated by living organisms (biogenic sedimentary rock - fossils), or lithification of chemically precipitated material from a mineral bearing solution due to evaporation (precipitate sedimentary rock). Clastic rocks can be formed from fragments broken apart from larger rocks of any type, due to processes such as erosion or from organic material, like plant remains. Biogenic and precipitate rocks form from the deposition of minerals from chemicals dissolved from all other rock types.

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Forces that drive the rock cycle:  Plate tectonics  Spreading ridges  Subduction zones  Continental collision  Accelerated erosion  An evolving process  Water

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The following are terms for rocks that are not petrographically or genetically distinct but are defined according to various other criteria; most are specific classes of other rocks, or altered versions of existing rocks. Some archaic and vernacular terms for rocks are also included.  Adamellite – A variety of quartz monzonite  Appinite – A group of varieties of lamprophyre, mostly rich in hornblende  Aphanite – Igneous rock composed of very small crystals invisible to the naked eye  Borolanite – Variety of nepheline syenite from Loch Borralan, Scotland – A variety of nepheline syenite from Loch Borralan, Scotland  Blue Granite – Variety of monzonite, an igneous rock  Epidosite – Hydrothermally altered epidote- and quartz-bearing rock  Felsite – Very fine grained felsic volcanic rock that may or may not contain larger crystals  Flint – Cryptocrystalline form of the mineral quartz  Ganister – Hard, fine-grained quartzose sandstone, or orthoquartzite  Gossan – Intensely oxidized, weathered or decomposed rock  Hyaloclastite – Volcaniclastic accumulation or breccia  Ijolite – Igneous rock consisting essentially of nepheline and augite  Jadeitite – Metamorphic rock found in blueschist-grade metamorphic terranes  Jasperoid – A hematite-silica metasomatite analogous to a skarn  Kenyte – Variety of porphyritic phonolite or trachyte with rhomb shaped phenocrysts of anorthoclase with variable olivine and augite in a glassy matrix - A variety of phonolite, first found on Mount Kenya  Lapis lazuli – Contact metamorphic rock containing lazurite, pyrite and calcite - A rock composed of lazurite and other minerals  Larvikite – Variety of monzonite, an igneous rock  Litchfieldite – A metamorphosed nepheline syenite occurrence near Litchfield, Maine  Llanite – A hypabyssal rhyolite with microcline and blue quartz phenocrysts from the Llano Uplift in Texas  Luxullianite – Rare type of granite  Mangerite – Plutonic intrusive igneous rock, that is essentially a hypersthene-bearing monzonite  Minette – A variety of lamprophyre  Novaculite – A type of chert found in Oklahoma, Arkansas, and Texas  Pietersite – Breccia rock of hawk's eyes and tiger's eyes  Pyrolite – A chemical analogue considered to theoretically represent the earth's upper mantle  Rapakivi granite – Hornblende-biotite granite containing large round crystals of orthoclase each with a rim of oligoclase  Rhomb porphyry – A type of latite with euhedral rhombic phenocrysts of feldspar  Rodingite – A mafic rock metasomatized by serpentinization fluids  Shonkinite – Intrusive igneous rock – melitilic and kalsititic rocks  Taconite – Iron-bearing sedimentary rock, in which the iron minerals are interlayered with quartz, chert, or carbonate  Tachylite – Form of basaltic volcanic glass  Teschenite – A silica undersaturated, analcime bearing gabbro  Theralite – A nepheline gabbro  Unakite – An altered granite  Variolite – Igneous rocks which contain varioles  Vogesite – A variety of lamprophyre  Wad – A rock rich in manganese oxide or manganese hydroxide

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SPECIFIC VARIETIES OF ROCKS