Match the magma series in Column 1 to the geological settings in Column 2.Column 1Column 2P. Alkaline Seriesi. Divergent oceanic marginQ. Shoshonite Seriesii. Oceanic intraplateR. Calc-alkaline Seriesiii. Convergent plate marginS. Tholeiitic Seriesiv. Convergent plate margin close to back arc

The correct matching is P-ii; Q-iv; R-iii; S-i.Alkaline series magmas, low-degree melts of enriched mantle, typify oceanic intraplate islands such as Hawaii's late stages and the Canaries (ii).The shoshonite series is the high-potassium suite found on the far side of mature arcs, close to the back arc (iv).The calc-alkaline series is the typical suite of convergent margins, as in the Andes and Japan (iii).The tholeiitic series dominates divergent oceanic margins as mid-ocean ridge basalt (i).Option A sends the tholeiites to the intraplate setting and the alkaline rocks to the ridge, the reverse of the usual pattern, and the remaining options misplace the shoshonites.

Magma and Igneous Processes Questions and Answers

Q: Which one of the following is the youngest "Continental Flood Basalt Province" ?

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).

Q: In AFM diagram, the suffix 'A' stands for :

In the AFM diagram of igneous petrology, A stands for \(Na_{2}O + K_{2}O\).The AFM triangle plots A = alkalis (Na2O + K2O), F = total iron as FeO (FeO + Fe2O3) and M = MgO. It separates tholeiitic from calc-alkaline magma series: tholeiites show iron enrichment towards the F corner, while calc-alkaline rocks trend straight towards the A corner.FeO + Fe2O3 is the F apex and MgO the M apex. Al2O3 is the A of a different diagram, Thompson's AFM projection for metamorphosed pelites, not of the igneous AFM diagram the key refers to.

Q: The typical record of the early crust of ultramafic high Fe-Mg rocks depicting upto 500°C hotter mantle are :

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.

Q: Magma undering extensive differentiation in \(F_{e}^{tot}/(F_{e}^{tot} + MgO)\) ratio coincide with a miminal change in silica content resulting in crystallization of olivine and pyroxene without Fe-Ti oxides leads to formation of .................. .

Such differentiation leads to ferrobasalt.In tholeiitic magmas at mid-ocean ridges, fractional crystallisation of olivine, plagioclase and clinopyroxene without Fe-Ti oxides removes magnesium faster than iron. The FeO(total)/(FeO(total) + MgO) ratio of the residual liquid therefore rises sharply while silica barely changes (the 'Fenner' or tholeiitic trend). Basalts that have gone far along this trend, with FeO(total) above about 12 per cent, are called ferrobasalts, as found on the Galapagos spreading centre.N-MORB and E-MORB are distinguished by their trace-element enrichment, not by iron enrichment. 'Fast-spreading EPR' names a tectonic setting, not a rock type.

Q: Melting of pure enstatite leads to :

Melting pure enstatite produces forsterite + liquid.In the system forsterite-silica at atmospheric pressure, enstatite (MgSiO\(_{3}\)) melts incongruently at about 1557 °C: it breaks down into crystals of forsterite (Mg\(_{2}\)SiO\(_{4}\)) and a liquid richer in silica than enstatite.A liquid of enstatite composition would form only if melting were congruent, which it is not at low pressure. Fayalite is the iron end member of olivine and cannot form from pure magnesium enstatite.

Q: ........................... deposits are formed from cloud of ash that move out horizontally from a vent at hurricane velocities following a volcanic explosion.

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.

Q: Which one of the following minerals melts incongruently ?

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.

Q: A ...................... is a pipe-like body of breccia composed of fragments that are mainly derived from the neighboring country rock and also has rocks from stratigraphically higher and lower level mantle origin.

The body described is a diatreme.A diatreme is a carrot-shaped volcanic pipe drilled through the crust by a gas-charged explosive eruption and filled with breccia. Most of its fragments are country rock from the walls, but it also contains blocks that have moved up or down from higher and lower stratigraphic levels and material of mantle origin, as in kimberlite pipes.A maar is the broad, low crater that a diatreme may produce at the surface, not the pipe itself. Tuffisite is fine fragmental material injected as veins and dykes, and a stock is a small intrusion of crystallised magma, not a breccia.

Q: Which of the following is not true for Komatiites ?

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.

Match the magma series in Column 1 to the geological settings in Column 2.

Column 1	Column 2
P. Alkaline Series	i. Divergent oceanic margin
Q. Shoshonite Series	ii. Oceanic intraplate
R. Calc-alkaline Series	iii. Convergent plate margin
S. Tholeiitic Series	iv. Convergent plate margin close to back arc

A.P-i; Q-iv; R-iii; S-ii
B.P-ii; Q-iv; R-iii; S-i
C.P-ii; Q-i; R-iii; S-iv
D.P-iv; R-ii; Q-i; S-iii

Exam Year(s)

IIT JAM - 2026,

Answer: B

The correct matching is P-ii; Q-iv; R-iii; S-i.

Alkaline series magmas, low-degree melts of enriched mantle, typify oceanic intraplate islands such as Hawaii's late stages and the Canaries (ii).
The shoshonite series is the high-potassium suite found on the far side of mature arcs, close to the back arc (iv).
The calc-alkaline series is the typical suite of convergent margins, as in the Andes and Japan (iii).
The tholeiitic series dominates divergent oceanic margins as mid-ocean ridge basalt (i).

Option A sends the tholeiites to the intraplate setting and the alkaline rocks to the ridge, the reverse of the usual pattern, and the remaining options misplace the shoshonites.

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Which one of the following is the youngest "Continental Flood Basalt Province" ?

A.Deccan Traps
B.Columbia River Province
C.Siberian Traps
D.Parana Province

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

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).

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In AFM diagram, the suffix 'A' stands for :

A.\(Al_{2}O_{3}\)
B.\(FeO + Fe_{2}O_{3}\)
C.MgO
D.\(Na_{2}O + K_{2}O\)

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: D

In the AFM diagram of igneous petrology, A stands for \(Na_{2}O + K_{2}O\).

The AFM triangle plots A = alkalis (Na₂O + K₂O), F = total iron as FeO (FeO + Fe₂O₃) and M = MgO. It separates tholeiitic from calc-alkaline magma series: tholeiites show iron enrichment towards the F corner, while calc-alkaline rocks trend straight towards the A corner.

FeO + Fe₂O₃ is the F apex and MgO the M apex. Al₂O₃ is the A of a different diagram, Thompson's AFM projection for metamorphosed pelites, not of the igneous AFM diagram the key refers to.

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The typical record of the early crust of ultramafic high Fe-Mg rocks depicting upto 500°C hotter mantle are :

A.Eclogite
B.Pegmatite
C.Anorthosite
D.Komatiites

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

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.

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Magma undering extensive differentiation in \(F_{e}^{tot}/(F_{e}^{tot} + MgO)\) ratio coincide with a miminal change in silica content resulting in crystallization of olivine and pyroxene without Fe-Ti oxides leads to formation of .................. .

A.N-MORB
B.E-MORB
C.Ferrobasalt
D.Fast-spreading EPR

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: C

Such differentiation leads to ferrobasalt.

In tholeiitic magmas at mid-ocean ridges, fractional crystallisation of olivine, plagioclase and clinopyroxene without Fe-Ti oxides removes magnesium faster than iron. The FeO(total)/(FeO(total) + MgO) ratio of the residual liquid therefore rises sharply while silica barely changes (the 'Fenner' or tholeiitic trend). Basalts that have gone far along this trend, with FeO(total) above about 12 per cent, are called ferrobasalts, as found on the Galapagos spreading centre.

N-MORB and E-MORB are distinguished by their trace-element enrichment, not by iron enrichment. 'Fast-spreading EPR' names a tectonic setting, not a rock type.

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Melting of pure enstatite leads to :

A.Liquid of enstatitic composition
B.Forsterite + liquid
C.Fayalite + liquid
D.Forsterite + fayalite + liquid

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: B

Melting pure enstatite produces forsterite + liquid.

In the system forsterite-silica at atmospheric pressure, enstatite (MgSiO\(_{3}\)) melts incongruently at about 1557 °C: it breaks down into crystals of forsterite (Mg\(_{2}\)SiO\(_{4}\)) and a liquid richer in silica than enstatite.

A liquid of enstatite composition would form only if melting were congruent, which it is not at low pressure. Fayalite is the iron end member of olivine and cannot form from pure magnesium enstatite.

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........................... deposits are formed from cloud of ash that move out horizontally from a vent at hurricane velocities following a volcanic explosion.

A.Air-fall
B.Solfataras
C.Blister
D.Surge

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

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.

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Which one of the following minerals melts incongruently ?

A.Forsterite
B.Pyrite
C.Anorthite
D.Quartz

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

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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A ...................... is a pipe-like body of breccia composed of fragments that are mainly derived from the neighboring country rock and also has rocks from stratigraphically higher and lower level mantle origin.

A.Maars
B.Tuffisite
C.Diatreme
D.Stock

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: C

The body described is a diatreme.

A diatreme is a carrot-shaped volcanic pipe drilled through the crust by a gas-charged explosive eruption and filled with breccia. Most of its fragments are country rock from the walls, but it also contains blocks that have moved up or down from higher and lower stratigraphic levels and material of mantle origin, as in kimberlite pipes.

A maar is the broad, low crater that a diatreme may produce at the surface, not the pipe itself. Tuffisite is fine fragmental material injected as veins and dykes, and a stock is a small intrusion of crystallised magma, not a breccia.

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10.

Which of the following is not true for Komatiites ?

A.They are associated with greenstone belts
B.They are of Archaean age
C.They are ultramafic lavas
D.They are characterized by ophitic texture

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

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.

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