Magma and Igneous Processes Questions and Answers

Q: The process by which a solid melts to a liquid and another solid, both of different composition than the original is known as :

This is Incongruent melting.In incongruent melting a solid does not melt to a liquid of its own composition; it breaks down into a liquid and a different solid. The classic example is orthoenstatite, which melts to forsterite plus a more silica-rich liquid, and potassium feldspar, which melts to leucite plus liquid.Congruent melting gives a liquid of the same composition as the solid. Eutectic melting is the melting of two or more solids together at the lowest melting temperature of the system, and equilibrium melting is melting in which solid and liquid stay in equilibrium throughout; neither produces a new solid phase.

Q: Composition of the resultant partial melts is different from the composition of original rock primarily because of :

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.

Q: Crystallization of new phlogopite mineral in the existing mantle peridotite is an example of which of the following process ?

The growth of new phlogopite is patent metasomatism.Mantle metasomatism is divided by whether new minerals appear. In patent (modal) metasomatism, fluids or melts introduce new phases, such as phlogopite, amphibole, apatite or ilmenite, into the peridotite. In cryptic metasomatism the chemistry of the existing minerals changes (for example their trace element contents) but no new mineral grows.Since phlogopite is a new mineral, the process is not cryptic. Sulphide metasomatism concerns sulphide phases, and mantle depletion is the extraction of melt, the opposite of adding phlogopite-forming components.

Q: Which of the following processes is responsible for the incorporation of sulphides in the komatiite magmas ?

Sulphides enter komatiite magmas mainly by Assimilation of crustal material.Komatiites form by very high degrees of mantle melting and leave the mantle undersaturated in sulphur. To form the nickel sulphide ores of Kambalda in Western Australia or the Thompson belt in Canada, the hot, fluid lava had to melt and assimilate sulphur-rich sediments in its floor (thermal erosion), which saturated the magma in sulphide.Higher partial melting is what makes komatiites sulphur-undersaturated in the first place, very low melting would not produce komatiite at all, and extreme fractionation is uncharacteristic of these rapidly erupted lavas.

11.

The process by which a solid melts to a liquid and another solid, both of different composition than the original is known as :

A.Congruent melting
B.Incongruent melting
C.Eutectic melting
D.Equilibrium melting

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: B

This is Incongruent melting.

In incongruent melting a solid does not melt to a liquid of its own composition; it breaks down into a liquid and a different solid. The classic example is orthoenstatite, which melts to forsterite plus a more silica-rich liquid, and potassium feldspar, which melts to leucite plus liquid.

Congruent melting gives a liquid of the same composition as the solid. Eutectic melting is the melting of two or more solids together at the lowest melting temperature of the system, and equilibrium melting is melting in which solid and liquid stay in equilibrium throughout; neither produces a new solid phase.

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12.

Composition of the resultant partial melts is different from the composition of original rock primarily because of :

A.Melt induced buoyancy
B.Varying amounts of fractional crystallization
C.The difference in melting temperatures of minerals
D.Similarity in melting temperatures of minerals

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

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.

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13.

Crystallization of new phlogopite mineral in the existing mantle peridotite is an example of which of the following process ?

A.Cryptic metasomatism of Dawson (1984)
B.Patent Metasomatism of Harte (1983)
C.Sulphide metasomatism
D.Mantle depletion

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: B

The growth of new phlogopite is patent metasomatism.

Mantle metasomatism is divided by whether new minerals appear. In patent (modal) metasomatism, fluids or melts introduce new phases, such as phlogopite, amphibole, apatite or ilmenite, into the peridotite. In cryptic metasomatism the chemistry of the existing minerals changes (for example their trace element contents) but no new mineral grows.

Since phlogopite is a new mineral, the process is not cryptic. Sulphide metasomatism concerns sulphide phases, and mantle depletion is the extraction of melt, the opposite of adding phlogopite-forming components.

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14.

Which of the following processes is responsible for the incorporation of sulphides in the komatiite magmas ?

A.Very low amounts of partial melting
B.Assimilation of crustal material
C.Extreme fractionation of magma
D.Higher amounts of partial melting

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: B

Sulphides enter komatiite magmas mainly by Assimilation of crustal material.

Komatiites form by very high degrees of mantle melting and leave the mantle undersaturated in sulphur. To form the nickel sulphide ores of Kambalda in Western Australia or the Thompson belt in Canada, the hot, fluid lava had to melt and assimilate sulphur-rich sediments in its floor (thermal erosion), which saturated the magma in sulphide.

Higher partial melting is what makes komatiites sulphur-undersaturated in the first place, very low melting would not produce komatiite at all, and extreme fractionation is uncharacteristic of these rapidly erupted lavas.

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