Some important MCQs with explanations related to Petrology for the Jharkhand Academic Council (JAC) 12th Exam -2

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11. Which of the following is an intrusive igneous rock?

a) Basalt
b) Granite
c) Obsidian
d) Pumice

Answer: b) Granite
Explanation:
Intrusive igneous rocks form when magma cools slowly beneath the Earth's surface, resulting in coarse-grained textures. Granite is a classic example. Basalt, Obsidian, and Pumice are extrusive igneous rocks.

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12. What is the main characteristic of a sedimentary rock?

a) Foliation
b) Layering or Bedding
c) Vesicular texture
d) Glassy texture

Answer: b) Layering or Bedding
Explanation:
Sedimentary rocks often exhibit layering (bedding) due to the deposition of sediments over time. Foliation is a feature of metamorphic rocks, while vesicular and glassy textures are seen in igneous rocks.


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13. Which of the following is NOT a metamorphic rock?

a) Slate
b) Quartzite
c) Schist
d) Sandstone

Answer: d) Sandstone
Explanation:
Sandstone is a sedimentary rock. Slate, Quartzite, and Schist are metamorphic rocks formed under heat and pressure.


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14. Which igneous rock is glassy in appearance and forms from rapid cooling?

a) Basalt
b) Obsidian
c) Granite
d) Gabbro

Answer: b) Obsidian
Explanation:
Obsidian is a volcanic glass that forms when lava cools so rapidly that crystals do not have time to form, resulting in a glassy texture.

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15. Which rock is formed by the metamorphism of Shale?

a) Gneiss
b) Marble
c) Slate
d) Quartzite

Answer: c) Slate
Explanation:
Slate is formed when shale is subjected to low-grade metamorphism, resulting in fine-grained, foliated rock.


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16. Which mineral is most abundant in granite?

a) Feldspar
b) Quartz
c) Mica
d) Olivine

Answer: a) Feldspar
Explanation:
Granite is composed primarily of feldspar and quartz, with feldspar being the most abundant mineral, followed by quartz and minor amounts of mica.

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17. What is the difference between magma and lava?

a) Magma is solid; lava is liquid.
b) Magma is found underground; lava is on the surface.
c) Magma is extrusive; lava is intrusive.
d) Magma forms rocks; lava does not.

Answer: b) Magma is found underground; lava is on the surface.
Explanation:
Magma is molten rock located beneath the Earth's surface, while lava is magma that has erupted onto the surface through a volcano.


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18. Which of the following is a non-clastic sedimentary rock?

a) Shale
b) Conglomerate
c) Limestone
d) Sandstone

Answer: c) Limestone
Explanation:
Non-clastic sedimentary rocks, such as limestone, form from chemical precipitation or biological processes, unlike clastic rocks, which form from fragments of pre-existing rocks.


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19. What type of metamorphism occurs at convergent plate boundaries?

a) Contact metamorphism
b) Regional metamorphism
c) Hydrothermal metamorphism
d) Dynamic metamorphism

Answer: b) Regional metamorphism
Explanation:
Regional metamorphism occurs over large areas under intense pressure and temperature, typically at convergent plate boundaries where mountain-building processes take place.


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20. Which of the following rocks contains fossils?

a) Granite
b) Shale
c) Quartzite
d) Basalt

Answer: b) Shale
Explanation:
Shale is a sedimentary rock that forms in low-energy environments, making it an excellent medium for preserving fossils.


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21. What is the difference between foliated and non-foliated metamorphic rocks?

a) Foliated rocks are clastic; non-foliated rocks are non-clastic.
b) Foliated rocks have layered structures; non-foliated rocks do not.
c) Foliated rocks are igneous; non-foliated rocks are sedimentary.
d) Foliated rocks are soft; non-foliated rocks are hard.

Answer: b) Foliated rocks have layered structures; non-foliated rocks do not.
Explanation:
Foliated rocks, like schist, exhibit a banded or layered appearance due to the alignment of minerals under pressure. Non-foliated rocks, like marble, do not have this feature.




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22. What causes the vesicular texture in igneous rocks?

a) High temperature during cooling
b) Trapped gas bubbles
c) Rapid cooling
d) Large crystal growth

Answer: b) Trapped gas bubbles
Explanation:
Vesicular texture occurs when gas bubbles are trapped in cooling lava, creating holes or vesicles in the rock (e.g., pumice, scoria).

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23. Which rock is commonly associated with the formation of coal?

a) Sandstone
b) Shale
c) Limestone
d) Peat

Answer: d) Peat
Explanation:
Peat is an organic material that undergoes compaction and chemical changes over time to form coal. Shale may be associated with coal seams, but coal originates from peat.

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