Classification of Igneous Rocks Questions and Answers

Q: Which one of the following rock combinations is characteristic of volcanic arcs?

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.

Q: Aluminium saturation index determines whether a rock is metaaluminous or peraluminous, the peraluminous rocks contain aluminous minerals including muscovite, garnet sillimanite and ...

The missing mineral is Cordierite.A peraluminous rock has more alumina than is needed to make feldspars, so the excess forms aluminous minerals: muscovite, almandine garnet, the aluminosilicates sillimanite and andalusite, and cordierite, a magnesium-iron aluminosilicate typical of S-type granites and pelitic rocks.Coesite is a high-pressure form of silica with no aluminium. Cummingtonite (spelt 'Comingtonite' in the paper) is a magnesium-iron amphibole, not an aluminous mineral, and 'Covanzite' is not one of the minerals used to define peraluminous rocks.

Q: Which one of the following is the largest felsic volcanic province of India ?

The largest felsic volcanic province of India is the Malani Suite.The Malani Igneous Suite of western Rajasthan, about 750 million years old (Neoproterozoic), covers some 50,000 square kilometres and is dominated by rhyolites, felsic tuffs and associated granites. It is described as the largest felsic igneous province of India and one of the largest in the world.The Deccan, Rajmahal and Sylhet Traps are flood basalt provinces, dominated by mafic lava, so they are not felsic volcanic provinces.

Q: Undersaturated alkaline lavas with abundant felspathoids sanidine or anorthoclase, aegine or aegirine augite is termed as.....................

Such a lava is a phonolite.Phonolite is the fine-grained, extrusive rock of silica-undersaturated alkaline magma. It consists of alkali feldspar (sanidine or anorthoclase) with abundant feldspathoids such as nepheline or leucite and sodic pyroxene such as aegirine or aegirine-augite.Syenite has the feldspar mineralogy but is a coarse-grained plutonic rock, not a lava. Keratophyre is an altered sodic lava without abundant feldspathoids, and limburgite is a glassy ultramafic lava of olivine and augite with no feldspar.

Q: Metaluminous granites that are typically magnetite bearing and inferred to be produced by differentiation of andesite or partial melting of an igneous source are designed as :

They are I-type granites.In Chappell and White's classification, I-type granites derive from igneous (infracrustal) sources, by partial melting of older igneous rock or by differentiation of mafic to intermediate magma such as andesite. They are metaluminous to weakly peraluminous, carry hornblende, and are typically oxidised and magnetite bearing (Ishihara's magnetite series).S-type granites come from melting sedimentary rocks, are strongly peraluminous with muscovite, cordierite or garnet, and are ilmenite series. A-type granites are anhydrous, alkaline and anorogenic, and M-type granites are small mantle-derived plagiogranites of island arcs.

Q: A discordant igneous intrusion is known as :

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.

Q: The variety of picrific basalt having abundant olivine (20-50 volume %) and less frequent augite phenocrysts with the groundmass of augite, olivine and plagioclase is called :

This variety is oceanite.Oceanite is a picritic basalt crowded with olivine phenocrysts (roughly 20 to 50 per cent of the rock) and fewer augite phenocrysts, in a groundmass of augite, olivine and plagioclase. It is typical of oceanic island volcanoes such as Reunion and Hawaii, where olivine accumulates in the magma.Ankaramite is the opposite picritic variety, with abundant augite phenocrysts and less olivine. Diabase (dolerite) is an ordinary medium-grained basaltic rock, and kimberlite is a volatile-rich ultramafic rock known for diamonds.

Q: The mafic rocks high in both magnesium and silica concentration and are thought to be usually formed in fore-arc environments, typically during the early stages of subduction are called :

These rocks are boninites.Boninites, named after the Bonin Islands south of Japan, are mafic volcanic rocks unusually high in both MgO and SiO2 but low in titanium. They form by melting of very depleted mantle wedge peridotite with added water, in fore-arc settings at the start of subduction, and are common in the lower parts of many ophiolites.Komatiites are ultramafic lavas high in magnesium but low in silica, mostly Archean. Kimberlites and lamproites are volatile- and potassium-rich rocks from deep in continental mantle, not fore-arc magmas.

Q: Intrusive igneous rock that is composed essentially of nepheline and an alkali pyroxene, usually aegirine-augite is called :

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.

Which one of the following rock combinations is characteristic of volcanic arcs?

A.Basalt-andesite-dacite
B.Basalt-trachyte-rhyolite
C.Andesite-syenite-dacite
D.Trachybasalt-basaltic andesite-andesite

Exam Year(s)

IIT JAM - 2026,

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.

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Aluminium saturation index determines whether a rock is metaaluminous or peraluminous, the peraluminous rocks contain aluminous minerals including muscovite, garnet sillimanite and ...

A.Cordierite
B.Coesite
C.Covanzite
D.Comingtonite

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: A

The missing mineral is Cordierite.

A peraluminous rock has more alumina than is needed to make feldspars, so the excess forms aluminous minerals: muscovite, almandine garnet, the aluminosilicates sillimanite and andalusite, and cordierite, a magnesium-iron aluminosilicate typical of S-type granites and pelitic rocks.

Coesite is a high-pressure form of silica with no aluminium. Cummingtonite (spelt 'Comingtonite' in the paper) is a magnesium-iron amphibole, not an aluminous mineral, and 'Covanzite' is not one of the minerals used to define peraluminous rocks.

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Which one of the following is the largest felsic volcanic province of India ?

A.Malani Suite
B.Deccan traps
C.Rajmahal Traps
D.Sylhet Traps

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: A

The largest felsic volcanic province of India is the Malani Suite.

The Malani Igneous Suite of western Rajasthan, about 750 million years old (Neoproterozoic), covers some 50,000 square kilometres and is dominated by rhyolites, felsic tuffs and associated granites. It is described as the largest felsic igneous province of India and one of the largest in the world.

The Deccan, Rajmahal and Sylhet Traps are flood basalt provinces, dominated by mafic lava, so they are not felsic volcanic provinces.

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Undersaturated alkaline lavas with abundant felspathoids sanidine or anorthoclase, aegine or aegirine augite is termed as.....................

A.Syenite
B.Phonolite
C.Keratophyne
D.Limburgite

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: B

Such a lava is a phonolite.

Phonolite is the fine-grained, extrusive rock of silica-undersaturated alkaline magma. It consists of alkali feldspar (sanidine or anorthoclase) with abundant feldspathoids such as nepheline or leucite and sodic pyroxene such as aegirine or aegirine-augite.

Syenite has the feldspar mineralogy but is a coarse-grained plutonic rock, not a lava. Keratophyre is an altered sodic lava without abundant feldspathoids, and limburgite is a glassy ultramafic lava of olivine and augite with no feldspar.

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Metaluminous granites that are typically magnetite bearing and inferred to be produced by differentiation of andesite or partial melting of an igneous source are designed as :

A.M-type granites
B.A-type granites
C.I-type granites
D.S-type granites

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: C

They are I-type granites.

In Chappell and White's classification, I-type granites derive from igneous (infracrustal) sources, by partial melting of older igneous rock or by differentiation of mafic to intermediate magma such as andesite. They are metaluminous to weakly peraluminous, carry hornblende, and are typically oxidised and magnetite bearing (Ishihara's magnetite series).

S-type granites come from melting sedimentary rocks, are strongly peraluminous with muscovite, cordierite or garnet, and are ilmenite series. A-type granites are anhydrous, alkaline and anorogenic, and M-type granites are small mantle-derived plagiogranites of island arcs.

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Which of the following igneous complex is not associated with Deccan Volcanic Province ?

A.Girnar
B.Phenaimata
C.Mundevara
D.Sittampundi

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: D

The complex not associated with the Deccan Volcanic Province is Sittampundi.

Sittampundi in Namakkal district, Tamil Nadu, is an Archaean layered anorthosite complex with chromitite layers in the granulite terrain of southern India, far older than the Deccan Traps and unrelated to them.

The other three are alkaline or mafic complexes emplaced with the Deccan volcanism at the end of the Cretaceous: Girnar in Saurashtra (Gujarat), Phenaimata in Gujarat, and Mundwara (printed 'Mundevara') in Rajasthan.

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A discordant igneous intrusion is known as :

A.Lopolith
B.Laccolith
C.Dyke
D.Sill

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

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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The variety of picrific basalt having abundant olivine (20-50 volume %) and less frequent augite phenocrysts with the groundmass of augite, olivine and plagioclase is called :

A.Oceanite
B.Diabase
C.Kimberlite
D.Ankaramite

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: A

This variety is oceanite.

Oceanite is a picritic basalt crowded with olivine phenocrysts (roughly 20 to 50 per cent of the rock) and fewer augite phenocrysts, in a groundmass of augite, olivine and plagioclase. It is typical of oceanic island volcanoes such as Reunion and Hawaii, where olivine accumulates in the magma.

Ankaramite is the opposite picritic variety, with abundant augite phenocrysts and less olivine. Diabase (dolerite) is an ordinary medium-grained basaltic rock, and kimberlite is a volatile-rich ultramafic rock known for diamonds.

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The mafic rocks high in both magnesium and silica concentration and are thought to be usually formed in fore-arc environments, typically during the early stages of subduction are called :

A.Komatiites
B.Kimberlites
C.Boninites
D.Lamproites

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: C

These rocks are boninites.

Boninites, named after the Bonin Islands south of Japan, are mafic volcanic rocks unusually high in both MgO and SiO₂ but low in titanium. They form by melting of very depleted mantle wedge peridotite with added water, in fore-arc settings at the start of subduction, and are common in the lower parts of many ophiolites.

Komatiites are ultramafic lavas high in magnesium but low in silica, mostly Archean. Kimberlites and lamproites are volatile- and potassium-rich rocks from deep in continental mantle, not fore-arc magmas.

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10.

Intrusive igneous rock that is composed essentially of nepheline and an alkali pyroxene, usually aegirine-augite is called :

A.Wehrlite
B.Harzburgite
C.Limburgite
D.Ijolite

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

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.

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