Match the following textures with their characterstic crystal size :TextureCharacteristic Crystal size(a) Glassy(i) Fine grained rocks(b) Aphanitic(ii) Coarse grained rocks(c) Phaneritic(iii) Early crystalizing mineral from a matrix of interlocking euhedral grains(d) Cumulate(iv) Rapidly cooling with no crystal

The correct matching is (a)-(iv), (b)-(i), (c)-(ii), (d)-(iii).A glassy texture forms when lava cools so fast that no crystals grow.Aphanitic rocks are fine grained, with crystals too small to see with the naked eye.Phaneritic rocks are coarse grained, with visible crystals.A cumulate texture is made of early-crystallising minerals that settled or accumulated, set in interstitial material.Option A gives glassy the coarse grain size, option B gives aphanitic the no-crystal description, and option C uses (ii) and (iii) twice each.

Igneous Rock Textures and Structures Questions and Answers

Q: .............................. is a microstructure of glassy basaltic rocks characterised by radiating aggregates of fibrous crystal.

This microstructure is variolitic.A variolitic texture consists of varioles, small rounded spherulites of radiating fibrous crystals (usually plagioclase or pyroxene), set in glassy or fine-grained basalt. It forms by rapid crystallisation of quickly chilled lava, as in the rims of pillow lavas.A vugh is a small cavity in a rock, and a vesicle is a gas bubble cavity in lava; neither is made of crystals. Tektites are natural glass bodies produced by meteorite impacts.

Q: If the groundmass of a porphyritic texture in a rock is hemicrystalline, the texture is known as :

The key gives Mesostasis. I could not find a source that names a texture this way, so this rests on elimination.A hemicrystalline groundmass is one that is partly crystalline and partly glass. Mesostasis is the last-formed interstitial material between the larger crystals of a rock, which may be glass or very fine crystals, so it is the option tied to a mixed glassy and crystalline groundmass.The other options do not describe that. Vitrophyric means phenocrysts set in a glassy groundmass, which is wholly glass and not partly crystalline. Felsophyric means a fine felsitic groundmass of quartz and feldspar intergrown, which is crystalline. Glomerophyritic (properly glomeroporphyritic) describes phenocrysts gathered in clusters and says nothing about the groundmass.

.............................. is a microstructure of glassy basaltic rocks characterised by radiating aggregates of fibrous crystal.

A.Vugh
B.Vesicle
C.Variolitic
D.Tiktites

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: C

This microstructure is variolitic.

A variolitic texture consists of varioles, small rounded spherulites of radiating fibrous crystals (usually plagioclase or pyroxene), set in glassy or fine-grained basalt. It forms by rapid crystallisation of quickly chilled lava, as in the rims of pillow lavas.

A vugh is a small cavity in a rock, and a vesicle is a gas bubble cavity in lava; neither is made of crystals. Tektites are natural glass bodies produced by meteorite impacts.

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Match the following textures with their characterstic crystal size :

Texture	Characteristic Crystal size
(a) Glassy	(i) Fine grained rocks
(b) Aphanitic	(ii) Coarse grained rocks
(c) Phaneritic	(iii) Early crystalizing mineral from a matrix of interlocking euhedral grains
(d) Cumulate	(iv) Rapidly cooling with no crystal

A.(a)-(ii), (b)-(i), (c)-(iv), (d)-(iii)
B.(a)-(i), (b)-(iv), (c)-(ii), (d)-(iii)
C.(a)-(iii), (b)-(iii), (c)-(ii), (d)-(ii)
D.(a)-(iv), (b)-(i), (c)-(ii), (d)-(iii)

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: D

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

A glassy texture forms when lava cools so fast that no crystals grow.
Aphanitic rocks are fine grained, with crystals too small to see with the naked eye.
Phaneritic rocks are coarse grained, with visible crystals.
A cumulate texture is made of early-crystallising minerals that settled or accumulated, set in interstitial material.

Option A gives glassy the coarse grain size, option B gives aphanitic the no-crystal description, and option C uses (ii) and (iii) twice each.

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If the groundmass of a porphyritic texture in a rock is hemicrystalline, the texture is known as :

A.Vitrophyric
B.Felsophyric
C.Glomerophyritic
D.Mesostasis

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: D

The key gives Mesostasis. I could not find a source that names a texture this way, so this rests on elimination.

A hemicrystalline groundmass is one that is partly crystalline and partly glass. Mesostasis is the last-formed interstitial material between the larger crystals of a rock, which may be glass or very fine crystals, so it is the option tied to a mixed glassy and crystalline groundmass.

The other options do not describe that. Vitrophyric means phenocrysts set in a glassy groundmass, which is wholly glass and not partly crystalline. Felsophyric means a fine felsitic groundmass of quartz and feldspar intergrown, which is crystalline. Glomerophyritic (properly glomeroporphyritic) describes phenocrysts gathered in clusters and says nothing about the groundmass.

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