Your Answer : (Not Answered)

Correct Answer : C

The categories are used in metamorphic rock classification.

Metamorphic rocks are divided by texture. Foliated rocks, such as slate, schist and gneiss, have minerals aligned in layers by directed pressure. Non-foliated rocks, such as marble and quartzite, formed without strong directed stress and show no layering.

Sedimentary rocks are classified as clastic, chemical or organic, and igneous rocks as intrusive or extrusive and by silica content. Schist is a single foliated metamorphic rock, not a group being classified.

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Correct Answer : C

Snowball garnets are the product of Dynamothermal metamorphism.

Snowball (rotated) garnets contain spiral trails of inclusions, recording quartz or other grains that were overgrown while the garnet grew and rotated, or while the matrix foliation rotated around it, during deformation. They therefore need growth under elevated temperature during active deformation, which is syntectonic regional or dynamothermal metamorphism.

Dynamic and cataclastic metamorphism involve deformation with little heating, so new garnet does not grow. Plutonic metamorphism is high-temperature recrystallisation at depth without the shearing needed to make snowball structure.

Your Answer : (Not Answered)

Correct Answer : B

An orthopyroxene-bearing granitoid is called Charnockite.

Charnockite is the name for granitic rocks containing orthopyroxene (hypersthene), formed under the high-temperature, low-water-activity conditions of the granulite facies. It is named after Job Charnock, whose tombstone in Kolkata is cut from it, and it is widespread in South India.

Enderbite is the orthopyroxene-bearing tonalite member of the charnockite series, a narrower term than the one asked for. Plagiogranite and tonalite are defined by their feldspar and quartz content and do not imply orthopyroxene.

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Correct Answer : A

Amba Dongar is known for its Carbonatite complex.

Located in Chhota Udaipur district of Gujarat, Amba Dongar is a ring complex of carbonatite and alkaline rocks emplaced at about 65 Ma, linked to Deccan volcanism. It is India's main fluorite deposit and has been studied for its rare earth element potential.

India's anorthosite complexes lie in the Eastern Ghats and Oddanchatram, its chromite deposits mainly in Sukinda (Odisha), and its kimberlites in the Wajrakarur field of Andhra Pradesh and at Majhgawan near Panna.

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Correct Answer : C

The environment is lacustrine.

Every feature in the description points to a lake. Fault grabens and basins with internal drainage are typical lake basins. Finely laminated, kerogen-rich mudstones (oil shales) form in quiet, stratified lake water. Marl, freshwater limestone and evaporites with mudcracks record lake chemistry and drying of its margins. Nonmarine ostracods, diatoms, molluscs, fish and insects are lake fossils.

Glacial deposits are poorly sorted tills without such fine laminae. Eolian deposits are well-sorted, large-scale cross-bedded sands with few fossils. Alluvial fans are coarse, poorly sorted gravels at mountain fronts, not laminated organic mudstones.

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Correct Answer : A

Both (i) and (ii) are true.

(i) Most dune sand is made of quartz grains, often well rounded and frosted by wind abrasion. (ii) Quartz has a hardness of 7 on the Mohs scale and is chemically very stable, so it survives weathering and transport long after feldspars and ferromagnesian minerals have broken down to clay.

In fact (ii) is the reason for (i): because quartz resists both mechanical and chemical breakdown, it is concentrated in mature sediments such as dune and beach sands. Options calling either statement false are therefore wrong. (Gypsum dunes, as at White Sands, and carbonate dunes are exceptions, not the rule.)

Your Answer : (Not Answered)

Correct Answer : C

Partial melts differ from their source because of the difference in melting temperatures of minerals.

A rock is a mixture of minerals that do not all melt at the same temperature. On heating, the first liquid forms from the lowest-melting combination (near the eutectic or minimum), so it is rich in the components of those minerals, such as feldspar and quartz in a crustal rock or clinopyroxene in peridotite, while refractory minerals like olivine stay behind.

If minerals melted at the same temperature (option D) the melt would match the rock. Buoyancy moves melt but does not set its composition, and fractional crystallisation acts on a melt after it has formed, not during melting.

Your Answer : (Not Answered)

Correct Answer : D

The statement not true of komatiites is They are characterized by ophitic texture.

Komatiites are characterised by spinifex texture: long, bladed or skeletal olivine and pyroxene crystals that grew rapidly in a quenched ultramafic lava. Ophitic texture, with pyroxene enclosing plagioclase laths, is typical of dolerite.

The other statements are true. Komatiites are ultramafic lavas with over 18% MgO, erupted at very high temperatures. They are mostly Archaean, because the mantle was hotter then, and they occur in greenstone belts such as Barberton in South Africa, where they were first described.

Your Answer : (Not Answered)

Correct Answer : B

The Dharwar craton grades from greenschist through amphibolite to granulite facies from north to south.

Across Karnataka and Tamil Nadu the craton shows a progressive increase in metamorphic grade southwards: greenschist-facies schist belts in the north, amphibolite-facies gneisses in the middle, and charnockites and other granulite-facies rocks in the south, across the transition zone near Kabbaldurga and Krishnagiri. This tilted section exposes progressively deeper crust southwards.

Blueschist and eclogite are high-pressure rocks of subduction zones and do not form this regional sequence, and the Barrovian zones are a sequence of mineral zones in pelitic rocks rather than this craton-scale facies change.

Your Answer : (Not Answered)

Correct Answer : D

The correct matching is (a)-(i), (b)-(ii), (c)-(iii), (d)-(iv).

1. Monzonite has roughly equal potash feldspar and plagioclase.
2. Norite is a gabbroic rock of calcic plagioclase (labradorite) with orthopyroxene such as hypersthene.
3. Picrite is an olivine-rich basaltic rock with augite and calcic plagioclase.
4. Rhyolite is the volcanic equivalent of granite: quartz with alkali feldspar exceeding plagioclase.

Option A gives monzonite the norite assemblage, option B gives rhyolite the monzonite assemblage, and option C gives monzonite the rhyolite assemblage.

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Correct Answer : D

The rock is ijolite.

Ijolite is a plutonic alkaline rock made essentially of nepheline and aegirine-augite (a sodic pyroxene), with little or no feldspar. It typically occurs in alkaline ring complexes associated with carbonatites, such as those of the East African rift.

Wehrlite (olivine and clinopyroxene) and harzburgite (olivine and orthopyroxene) are ultramafic peridotites without nepheline. Limburgite is a glassy volcanic rock rich in olivine and augite, not an intrusive nepheline rock.

Your Answer : (Not Answered)

Correct Answer : C

A discordant igneous intrusion is a dyke.

An intrusion is discordant when it cuts across the bedding or layering of the rocks it intrudes. A dyke is a sheet-like body of magma that fills a near-vertical fracture and cuts across the layers, such as the dolerite dykes of the Deccan.

Sills, laccoliths and lopoliths are concordant: they are injected parallel to the bedding. A sill is a flat sheet between layers, a laccolith is a lens that domes up the overlying strata, and a lopolith is a large saucer-shaped intrusion that sags downward, all parallel to the layers.

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Correct Answer : B

The feature is a crevasse-splay.

When a flood breaches a natural levee, water pours through the gap (the crevasse) onto the floodplain and spreads out in a fan of small distributary channels that divide and rejoin. The sand and silt it drops form a lobe-shaped crevasse-splay deposit on the floodplain side of the levee.

A debris cone is a steep cone of coarse debris at the foot of a gully. A fan delta is an alluvial fan built directly into standing water. Meander scrolls are the curved ridges left on the inside of a migrating meander bend by successive point bars, not a fan of channels off a levee.

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Correct Answer : B

Of these, pyrite melts incongruently.

A mineral melts incongruently when, on heating, it breaks down into a liquid and a different solid instead of a liquid of its own composition. Pyrite, FeS₂, does not melt as such: at about 743°C it breaks down to pyrrhotite and a sulphur-rich liquid.

Forsterite, anorthite and quartz melt congruently at atmospheric pressure, each to a liquid of its own composition (at about 1890°C, 1553°C and 1713°C respectively). The textbook example of a silicate melting incongruently, enstatite (to forsterite plus liquid), is not among the options.

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Correct Answer : D

These are surge deposits.

A pyroclastic (base) surge is a turbulent, low-density cloud of ash and gas that moves out horizontally from the vent at very high speed after a volcanic explosion, often in phreatomagmatic eruptions. Its deposits are thin, cross-bedded and dune-like, and they mantle the topography.

Air-fall deposits settle vertically out of an eruption column. Solfataras are vents emitting sulphurous gases, not deposits from an ash cloud. 'Blister' is not a type of pyroclastic deposit.

Your Answer : (Not Answered)

Correct Answer : D

The rocks are komatiites.

Komatiites are ultramafic volcanic rocks with very high MgO (more than 18 per cent), mostly of Archaean age, often showing spinifex texture formed by rapid cooling of olivine. Their eruption required melting temperatures of about 1600 °C, which is taken as evidence that the Archaean mantle was up to several hundred degrees hotter than today's.

Eclogite is a high-pressure metamorphic rock, pegmatite is a coarse felsic igneous rock, and anorthosite is a plagioclase-rich rock; none is an ultramafic lava recording a hotter mantle.

Your Answer : (Not Answered)

Correct Answer : B

The youngest of these provinces is the Columbia River Province.

The Columbia River flood basalts of the north-western United States erupted mainly between about 17 and 15 million years ago, in the Miocene, which makes them the youngest major continental flood basalt province.

The others are much older: the Deccan Traps about 66 million years (at the end of the Cretaceous), the Parana-Etendeka province about 133 million years (Early Cretaceous), and the Siberian Traps about 252 million years (end of the Permian).

Your Answer : (Not Answered)

Correct Answer : B

The typical marine biogenic sediments are Calcareous Ooze & Siliceous Oozes.

A biogenic sediment is made mostly of the skeletal remains of organisms; an ooze contains at least 30% such material. Calcareous ooze is built of the calcium carbonate shells of foraminifera and coccolithophores, and siliceous ooze of the silica tests of diatoms and radiolarians.

Pelagic muds (red clays) and turbidites are lithogenous, derived from land, and turbidites are deposited by density currents. Calcareous sandstone and sandy limestone are lithified rocks, not the typical deep-sea biogenic sediments, so 'none of the above' is also wrong.

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Correct Answer : D

The incorrect statement is that a reaction forming Q from P at constant pressure will require an input of heat.

The P-Q boundary slopes up towards higher pressure, so at any fixed pressure Q lies on the low-temperature side and P on the high-temperature side. Forming Q from P at constant pressure therefore means crossing the boundary on cooling: P is the high-entropy phase, and the reaction releases heat rather than absorbing it.

The other statements fit the diagram. The P-Q boundary is sloping, not horizontal, so two coexistence points at different temperatures must be at different pressures. P, Q and liquid meet at a single invariant point. The Q-liquid curve rises with pressure, so Q melts at a higher temperature at higher pressure.

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Correct Answer : A

The combination characteristic of volcanic arcs is basalt-andesite-dacite.

Arc magmas form when water released from the subducting slab lowers the melting point of the overlying mantle wedge. They belong mainly to the calc-alkaline series, whose typical volcanic suite runs from basalt through andesite to dacite, with andesite the hallmark rock of arcs.

Basalt-trachyte-rhyolite is an alkaline or bimodal suite typical of continental rifts and ocean islands. Andesite-syenite-dacite mixes a plutonic rock, syenite, into a volcanic series and is not a natural suite. Trachybasalt-basaltic andesite-andesite is a potassic, mildly alkaline trend, more typical of intraplate or back-arc settings than of the main arc.