Rocks

1. Which one of the following is a permeable rock that allows water to pa

Which one of the following is a permeable rock that allows water to pass through it ?

Granite
Limestone
Quartzite
Sill
This question was previously asked in
UPSC CAPF – 2023
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The correct answer is B. Permeability is the measure of the ability of a material to transmit fluids. Among the given options, limestone is generally considered permeable, especially when fractured or dissolved to form karst features.
– Permeable rocks contain interconnected pores or fractures that allow water (or other fluids) to flow through them.
– Limestone is a sedimentary rock primarily composed of calcium carbonate. While the rock matrix itself can have some porosity, its permeability is often significantly enhanced by dissolution of the carbonate material by acidic groundwater, creating channels, caves, and fractures. This forms karst topography and makes limestone important aquifers.
– Granite, Quartzite, and Sill (intrusive igneous) are typically dense, crystalline rocks with low primary porosity and permeability unless they are significantly fractured.
Permeability is distinct from porosity, which is the percentage of void space in a material. A material can be porous but not permeable if the pores are not connected. For example, clay can be highly porous but has very low permeability. Limestone can exhibit both porosity and, importantly, high permeability, particularly secondary permeability developed through dissolution and fracturing.

2. Which one of the following is not an igneous rock?

Which one of the following is not an igneous rock?

Granite
Gneiss
Pumice
Basalt
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UPSC CAPF – 2018
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Igneous rocks are formed through the cooling and solidification of molten rock (magma or lava). Granite, pumice, and basalt are all types of igneous rocks: Granite is an intrusive igneous rock formed from slow cooling magma; Basalt is an extrusive igneous rock formed from rapidly cooling lava; Pumice is a highly porous extrusive igneous rock (volcanic glass). Gneiss, however, is a metamorphic rock formed from pre-existing rocks (like granite, shale, or volcanic rocks) that have been subjected to high temperature and pressure, resulting in recrystallization and often a banded appearance (foliation).
Igneous rocks form from molten rock. Metamorphic rocks form from pre-existing rocks altered by heat and pressure.
The three main rock types are igneous, sedimentary, and metamorphic. Igneous rocks are classified based on their composition and texture (which depends on cooling rate). Gneiss is a common high-grade metamorphic rock.

3. ‘Evaporites’ is :

‘Evaporites’ is :

a rock
a meteorite
a brackish water pan
an evaporimeter
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UPSC CAPF – 2015
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‘Evaporites’ is a term for a type of sedimentary rock. Evaporite rocks are formed by the precipitation of minerals out of water that is evaporating. Common examples include rock salt (halite), gypsum, and anhydrite.
Evaporites are chemical sedimentary rocks whose formation is directly linked to the evaporation of water containing dissolved salts, leading to the concentration and crystallization of these salts.
Evaporite deposits are found in environments where evaporation rates exceed precipitation rates, such as restricted marine basins, playa lakes, and salt pans. They can be economically important sources of salt and gypsum.

4. Which one among the following is a mechanically formed sedimentary

Which one among the following is a mechanically formed sedimentary rock:

Salt rock
Limestone
Sandstone
Gypsum
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UPSC CAPF – 2012
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Mechanically formed sedimentary rocks, also known as clastic sedimentary rocks, are composed of fragments (clasts) of pre-existing rocks that have been eroded, transported, deposited, and then cemented together. Sandstone is a classic example of a clastic sedimentary rock, formed from cemented sand grains. Salt rock (halite) and Gypsum are typically chemical sedimentary rocks formed by evaporation and precipitation of minerals from water. Limestone can be chemical or organic (biogenic), but sandstone is definitively mechanically formed.
Sandstone is a clastic (mechanically formed) sedimentary rock composed of rock fragments (sand grains).
Clastic sedimentary rocks are classified based on the size of the clasts (fragments). Examples include conglomerate (rounded pebbles/cobbles), breccia (angular pebbles/cobbles), sandstone (sand-sized grains), siltstone (silt-sized grains), and shale (clay-sized grains).

5. Which one of the following is an example of both an igneous as well as

Which one of the following is an example of both an igneous as well as a metamorphic rock ?

Gabbro
Geyserite
Gneissoid
Granite
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UPSC NDA-2 – 2024
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Gneissoid texture is characteristic of Gneiss, which is a metamorphic rock often formed from igneous rocks (like granite) or sedimentary rocks. While Gneiss is the metamorphic rock, the term ‘Gneissoid’ refers to the banded texture found in such rocks, highlighting its metamorphic nature and potential igneous (or sedimentary) parentage. Among the options, Gneissoid is the best fit to represent a rock type that bridges both categories.
– Igneous rocks are formed from the cooling and solidification of magma or lava (e.g., Granite, Gabbro).
– Metamorphic rocks are formed from pre-existing rocks (igneous, sedimentary, or other metamorphic rocks) that are subjected to high heat, pressure, or chemically active fluids, causing changes in their mineralogy, texture, or chemical composition (e.g., Gneiss, Marble, Slate).
– Gneiss is a high-grade metamorphic rock characterized by distinct banding (gneissoid texture) formed by the segregation of different minerals. It can be derived from various parent rocks, including igneous rocks like granite (forming granitic gneiss) or sedimentary rocks like shale.
– Gabbro is an igneous rock. Geyserite is a form of opal or chalcedony deposited by hot springs or geysers (often considered a chemical sedimentary deposit or mineraloid). Granite is an igneous rock.
– Gneissoid specifically describes the texture of Gneiss, a metamorphic rock that frequently originates from igneous precursors like granite, thus representing a link between the two rock types.
The transformation of granite into gneiss is a common metamorphic pathway. Regional metamorphism, associated with mountain building, is a primary process by which rocks like granite are transformed into gneiss under conditions of high temperature and pressure.

6. The formation of ‘tors’ on small rocky hills is associated with which

The formation of ‘tors’ on small rocky hills is associated with which among the following?

Granite
Limestone
Alluvial
Dolomite
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UPSC NDA-2 – 2022
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Tors are distinctive rock landforms that consist of outcrops of bedrock weathered into piles of rounded boulders. They are typically formed in areas underlain by igneous rocks, most commonly granite. The formation process involves the intersection of joint patterns in the granite bedrock, followed by subsurface weathering (like hydrolysis) along these joints and subsequent erosion of the weathered material, leaving the unweathered core stones exposed as tors.
Granite, being susceptible to weathering along its natural joint systems and through chemical processes like hydrolysis, provides the ideal conditions for the development of tors.
While tors are most characteristic of granite, similar landforms can occasionally develop on other jointed rock types like dolerite, but they are overwhelmingly associated with granitic landscapes. Limestone and dolomite typically form karst topography through dissolution, and alluvial deposits are unconsolidated sediments, neither of which forms tors.

7. Which one of the following sedimentary rocks is organically formed ?

Which one of the following sedimentary rocks is organically formed ?

Shale
Chert
Halite
Chalk
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UPSC NDA-2 – 2019
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Sedimentary rocks are formed from the accumulation or deposition of mineral or organic particles. Organically formed sedimentary rocks are composed primarily of the remains of organisms (plants or animals). Chalk is a type of limestone composed mainly of the microscopic calcium carbonate shells (coccoliths) of marine algae called coccolithophores. Shale is a clastic sedimentary rock made of clay and silt. Chert is a hard, fine-grained rock composed of microcrystalline quartz, typically formed chemically or biochemically (though often grouped with chemical rocks). Halite (rock salt) is a chemical sedimentary rock formed by the evaporation of saline water. Therefore, Chalk is the only rock among the options that is definitively and primarily formed from organic remains.
– Organically formed sedimentary rocks are composed of organic remains.
– Chalk is made of fossilized microscopic marine organisms.
– Shale is clastic, Chert is chemical/biochemical, Halite is chemical.
Other examples of organically formed sedimentary rocks include coal (formed from accumulated plant remains) and some types of limestone composed of larger shell fragments or coral reefs. The classification of some rocks like Chert can be debated depending on the origin of the silica (chemical vs. biogenic). However, chalk is a classic example of an organically formed rock.

8. Which one of the following is not an igneous rock ?

Which one of the following is not an igneous rock ?

Gabbro
Granite
Dolomite
Basalt
This question was previously asked in
UPSC NDA-2 – 2017
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Dolomite is not an igneous rock.
Igneous rocks are formed through the cooling and solidification of magma or lava.
– Gabbro is a coarse-grained, mafic intrusive igneous rock.
– Granite is a coarse-grained, felsic intrusive igneous rock.
– Basalt is a fine-grained, mafic extrusive igneous rock.
– Dolomite is a sedimentary rock composed primarily of the mineral dolomite (calcium magnesium carbonate). It forms through the post-depositional alteration of calcium carbonate sediments or rocks.
Rocks are classified into three main types based on their origin: igneous, sedimentary, and metamorphic. Gabbro, Granite, and Basalt are all fundamental examples of igneous rocks. Dolomite belongs to the class of sedimentary rocks.

9. Which one of the following is the lowermost/innermost intrusive igneou

Which one of the following is the lowermost/innermost intrusive igneous rock ?

Laccolith
Batholith
Lopolith
Phacolith
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UPSC NDA-1 – 2023
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The correct answer is B) Batholith.
Intrusive igneous rocks form when magma cools and solidifies beneath the Earth’s surface. Among the given options, a batholith is the largest and typically the deepest type of intrusive igneous rock body. Batholiths are massive, irregular plutonic bodies that form deep within the crust and are often considered the root of mountain ranges. Laccoliths, lopoliths, and phacoliths are generally smaller and form at shallower depths compared to batholiths, often causing deformation of overlying strata.
– Laccoliths are dome-shaped intrusions that push overlying layers upwards.
– Lopoliths are saucer-shaped intrusions.
– Phacoliths are lens-shaped intrusions conforming to the crests or troughs of folded rock layers.
– Batholiths are typically vast, extending over 100 square kilometers, and are uncovered by erosion of the overlying rock.

10. Which one of the following is not an igneous rock ?

Which one of the following is not an igneous rock ?

Granite
Slate
Basalt
Gabbro
This question was previously asked in
UPSC NDA-1 – 2023
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The correct option is B.
Igneous rocks are formed from the cooling and solidification of molten rock (magma or lava). Metamorphic rocks are formed from existing rocks (igneous, sedimentary, or other metamorphic rocks) that have been changed by heat, pressure, or chemical reactions. Slate is a metamorphic rock.
Granite is a coarse-grained intrusive igneous rock. Basalt is a fine-grained extrusive igneous rock. Gabbro is a coarse-grained intrusive igneous rock, compositionally similar to basalt. Slate is a fine-grained, foliated metamorphic rock formed by the low-grade regional metamorphism of shale or mudstone.
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