Metamorphism and Its Types Questions and Answers

Q: Metaperidotite composed of olivine and pyroxene wholly or partially converted to ........................ at subduction zones with interaction with sea water.

The olivine and pyroxene are converted to serpentine.When peridotite reacts with water at low to moderate temperatures, at mid-ocean ridges or in the mantle wedge above a subduction zone where fluids are released from the sinking slab, olivine and pyroxene are hydrated to serpentine minerals (antigorite, lizardite, chrysotile), with magnetite and brucite. The resulting rock is serpentinite.Rodingite is a calcium-rich metasomatic rock formed in mafic dykes next to serpentinite, not from the peridotite itself. Sericite is fine-grained white mica from altered feldspar, and kaolinite is a clay formed by weathering of feldspar.

Q: A texture consisting of large crystal of quartz with mosaic substructure surrounded by a fine-grained aggregate of quartz, are termed as .................. texture.

This is a mortar texture.In mortar texture large relict grains (porphyroclasts), commonly quartz or feldspar showing internal subgrains, are surrounded by a fine-grained aggregate of recrystallised grains of the same mineral, like stones set in mortar. It forms when deformation recrystallises the edges of grains in shear zones, as in mylonites.Diaphoresis (diaphthoresis) is retrograde metamorphism, not a texture, and phyllonite is a rock (a mylonitised rock with a phyllitic sheen). Reaction rims are coronas of new minerals formed by reaction around a grain, not a fine aggregate of the same mineral.

Q: The snowball garnet are the product of :

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.

Q: Choose the correct sequence of deformation and metamorphism :

The correct sequence is Shale → Slate → Phyllite → Schist → Gneiss.With increasing deformation and metamorphic grade, a mudrock first becomes slate with a fine slaty cleavage, then phyllite with a silky sheen as micas grow, then schist with visible mica giving schistosity, and finally gneiss, in which minerals segregate into light and dark bands.Option B puts schist after gneiss, option C puts slate after schist, and option D runs the series backwards, so each breaks the order of increasing grade.

Q: If the dark portion is on a vanishing stage and is recognised by dark schlieren streaks within the granitic portion (Leucosome), this migmatite is known as :

This migmatite is a nebulite.In a nebulitic migmatite, melting and mixing have gone so far that the dark, older portion (the melanosome or palaeosome) has almost disappeared. It survives only as faint, cloudy dark schlieren and streaks within the light granitic leucosome, giving the rock a nebulous appearance close to a granite.An agmatite is a breccia-like migmatite in which angular dark blocks remain distinct within a granitic network. The melanosome is the dark, mafic part of a migmatite, not a type of migmatite. The neosome is the newly formed part, leucosome plus melanosome, again a component rather than a type.

Q: The reaction Opx + Plagioclase = Garnet + Cpx + Quartz is :

It is a solid-solid net-transfer reaction.In a net-transfer reaction, phases are produced and consumed, so the modal amounts of minerals change and material is transferred from one set of phases to another. Here orthopyroxene and plagioclase are used up to make garnet, clinopyroxene and quartz, all solids, with no fluid involved. Its strong pressure dependence makes it a widely used geobarometer in granulites.An exchange reaction only swaps elements, such as Fe and Mg, between existing minerals without changing their amounts; such reactions are used as geothermometers. Exsolution is the unmixing of one mineral from another during cooling, as in perthite. 'Ion exchange' is a surface process in clays and resins.

Metaperidotite composed of olivine and pyroxene wholly or partially converted to ........................ at subduction zones with interaction with sea water.

A.Rodingite
B.Sericite
C.Serpentine
D.Kaolinite

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: C

The olivine and pyroxene are converted to serpentine.

When peridotite reacts with water at low to moderate temperatures, at mid-ocean ridges or in the mantle wedge above a subduction zone where fluids are released from the sinking slab, olivine and pyroxene are hydrated to serpentine minerals (antigorite, lizardite, chrysotile), with magnetite and brucite. The resulting rock is serpentinite.

Rodingite is a calcium-rich metasomatic rock formed in mafic dykes next to serpentinite, not from the peridotite itself. Sericite is fine-grained white mica from altered feldspar, and kaolinite is a clay formed by weathering of feldspar.

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A texture consisting of large crystal of quartz with mosaic substructure surrounded by a fine-grained aggregate of quartz, are termed as .................. texture.

A.Mortar
B.Diaphoresis
C.Phyllonite
D.Reaction rims

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: A

This is a mortar texture.

In mortar texture large relict grains (porphyroclasts), commonly quartz or feldspar showing internal subgrains, are surrounded by a fine-grained aggregate of recrystallised grains of the same mineral, like stones set in mortar. It forms when deformation recrystallises the edges of grains in shear zones, as in mylonites.

Diaphoresis (diaphthoresis) is retrograde metamorphism, not a texture, and phyllonite is a rock (a mylonitised rock with a phyllitic sheen). Reaction rims are coronas of new minerals formed by reaction around a grain, not a fine aggregate of the same mineral.

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The snowball garnet are the product of :

A.Dynamic metamorphism
B.Cataclastic metamorphism
C.Dynamothermal metamorphism
D.Plutonic metamorphism

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

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.

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New metamorphic mineral due to chemical reaction changes its :

A.Phase
B.Texture
C.Form
D.Size

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: A

A new metamorphic mineral formed by a chemical reaction marks a change of Phase.

In petrology each mineral is a phase, a physically and chemically distinct part of the rock. When a metamorphic reaction consumes some minerals and produces others, for example andalusite giving way to sillimanite or garnet growing from chlorite, the rock's phase assemblage changes.

Texture, form and grain size can all change by recrystallisation alone, without any new mineral appearing, so they do not describe what a chemical reaction producing a new mineral changes.

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Choose the correct sequence of deformation and metamorphism :

A.Shale → Slate → Phyllite → Schist → Gneiss
B.Shale → Slate → Phyllite → Gneiss → Schist
C.Shale → Phyllite → Schist → Slate → Gneiss
D.Gneiss → Phyllite → Slate → Schist → Shale

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: A

The correct sequence is Shale → Slate → Phyllite → Schist → Gneiss.

With increasing deformation and metamorphic grade, a mudrock first becomes slate with a fine slaty cleavage, then phyllite with a silky sheen as micas grow, then schist with visible mica giving schistosity, and finally gneiss, in which minerals segregate into light and dark bands.

Option B puts schist after gneiss, option C puts slate after schist, and option D runs the series backwards, so each breaks the order of increasing grade.

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If the dark portion is on a vanishing stage and is recognised by dark schlieren streaks within the granitic portion (Leucosome), this migmatite is known as :

A.Agmatite
B.Melanosome
C.Nebulite
D.Neosome

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: C

This migmatite is a nebulite.

In a nebulitic migmatite, melting and mixing have gone so far that the dark, older portion (the melanosome or palaeosome) has almost disappeared. It survives only as faint, cloudy dark schlieren and streaks within the light granitic leucosome, giving the rock a nebulous appearance close to a granite.

An agmatite is a breccia-like migmatite in which angular dark blocks remain distinct within a granitic network. The melanosome is the dark, mafic part of a migmatite, not a type of migmatite. The neosome is the newly formed part, leucosome plus melanosome, again a component rather than a type.

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The reaction Opx + Plagioclase = Garnet + Cpx + Quartz is :

A.Exsolution Reaction
B.Solid-solid Net-Transfer Reaction
C.Exchange Reaction
D.Ion Exchange Reaction

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: B

It is a solid-solid net-transfer reaction.

In a net-transfer reaction, phases are produced and consumed, so the modal amounts of minerals change and material is transferred from one set of phases to another. Here orthopyroxene and plagioclase are used up to make garnet, clinopyroxene and quartz, all solids, with no fluid involved. Its strong pressure dependence makes it a widely used geobarometer in granulites.

An exchange reaction only swaps elements, such as Fe and Mg, between existing minerals without changing their amounts; such reactions are used as geothermometers. Exsolution is the unmixing of one mineral from another during cooling, as in perthite. 'Ion exchange' is a surface process in clays and resins.

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