Some important MCQs with explanations related to Petrology for the Jharkhand Academic Council (JAC) 12th Exam -3

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24. Which of the following is an example of an ultramafic rock?

a) Basalt
b) Peridotite
c) Granite
d) Rhyolite

Answer: b) Peridotite
Explanation:
Ultramafic rocks are rich in magnesium and iron and contain very low silica. Peridotite is a classic ultramafic rock, while Basalt, Granite, and Rhyolite have higher silica content.
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25. What is the primary difference between conglomerate and breccia?

a) Conglomerate is clastic; breccia is non-clastic.
b) Conglomerate contains rounded grains; breccia contains angular grains.
c) Conglomerate forms underwater; breccia forms on land.
d) Conglomerate is older than breccia.

Answer: b) Conglomerate contains rounded grains; breccia contains angular grains.
Explanation:
Conglomerates have rounded clasts, indicating transport over long distances, while breccias have angular clasts, suggesting little transport.
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26. Which rock type is formed from the cooling of silica-rich lava?

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

Answer: b) Rhyolite
Explanation:
Rhyolite is an extrusive igneous rock formed from silica-rich (felsic) lava. Granite is its intrusive counterpart. Basalt and Gabbro are mafic rocks with lower silica content.
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27. What is the main process involved in the formation of coal?

a) Crystallization
b) Compression and Carbonization
c) Lithification
d) Metamorphism

Answer: b) Compression and Carbonization
Explanation:
Coal forms from the compression of organic material, mainly plant debris, over millions of years. This process, called carbonization, increases the carbon content of the material.

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28. Which of the following textures is typical of intrusive igneous rocks?

a) Aphanitic
b) Phaneritic
c) Vesicular
d) Glassy

Answer: b) Phaneritic
Explanation:
Intrusive igneous rocks cool slowly underground, allowing large crystals to form, which gives them a phaneritic (coarse-grained) texture. Aphanitic, vesicular, and glassy textures are associated with extrusive rocks.

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29. Which mineral is most resistant to weathering and is commonly found in sedimentary rocks?

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

Answer: b) Quartz
Explanation:
Quartz is highly resistant to chemical and physical weathering, making it one of the most common minerals found in sedimentary rocks like sandstone.

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30. What is the dominant mineral in limestone?

a) Quartz
b) Calcite
c) Feldspar
d) Gypsum

Answer: b) Calcite
Explanation:
Limestone is primarily composed of calcite (CaCO₃), which forms through chemical precipitation or biological activity in marine environments.

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31. Which process leads to the formation of metamorphic rocks?

a) Weathering and erosion
b) Heat and pressure
c) Cooling and crystallization
d) Deposition and cementation

Answer: b) Heat and pressure
Explanation:
Metamorphic rocks are formed when existing rocks are subjected to heat, pressure, or chemically active fluids, leading to physical and mineralogical changes.

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32. Which rock has a foliated texture?

a) Marble
b) Schist
c) Quartzite
d) Limestone

Answer: b) Schist
Explanation:
Schist is a foliated metamorphic rock, characterized by parallel alignment of minerals. Marble and Quartzite are non-foliated, and Limestone is sedimentary.

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33. What is the parent rock of Quartzite?

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

Answer: c) Sandstone
Explanation:
Quartzite is formed by the metamorphism of sandstone. Heat and pressure recrystallize the quartz grains, giving the rock a harder, non-foliated structure.
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34. What type of rock is formed at a mid-ocean ridge?

a) Granite
b) Basalt
c) Sandstone
d) Schist

Answer: b) Basalt
Explanation:
At mid-ocean ridges, magma rises and cools rapidly to form basalt, a mafic extrusive igneous rock that makes up the oceanic crust.
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35. Which rock is formed when lava cools so quickly that no crystals form?

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

Answer: c) Obsidian
Explanation:
Obsidian is a natural volcanic glass that forms when lava cools so rapidly that atoms do not have time to arrange into a crystalline structure.
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36. What is the geological significance of sedimentary rocks?

a) They form the oceanic crust.
b) They preserve records of Earth's history.
c) They are sources of diamonds.
d) They are resistant to weathering.

Answer: b) They preserve records of Earth's history.
Explanation:
Sedimentary rocks often contain fossils and layers that provide insights into past environments, climate, and life forms.

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37. What causes foliation in metamorphic rocks?
a) Cooling and crystallization
b) Alignment of minerals under pressure
c) Deposition of sediments
d) Rapid cooling

Answer: b) Alignment of minerals under pressure
Explanation:
Foliation occurs when pressure causes platy minerals like mica to align perpendicular to the stress direction, forming a layered appearance.
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38. Which rock type is associated with volcanic eruptions?

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

Answer: b) Rhyolite
Explanation:
Rhyolite is an extrusive igneous rock formed from silica-rich lava during volcanic eruptions. Marble, Gneiss, and Quartzite are metamorphic rocks.
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39. Which of the following sedimentary rocks is organic in origin?

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

Answer: d) Coal
Explanation:
Coal forms from the accumulation of plant material in swampy environments. It is classified as an organic sedimentary rock.

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40. Which of the following is an example of a plutonic rock?

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

Answer: b) Granite
Explanation:
Plutonic rocks, like granite, form deep underground as magma cools slowly. Basalt, Obsidian, and Pumice are extrusive rocks.

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