Important Questions: Minerals & Rocks | Geography Class 11 - Humanities/Arts PDF Download

Very Short Answer Type Questions

Q1: Why are coal and petroleum called fossil fuels?
Ans: Because they are associated with fossils (remnants of organisms).

Q2: Define a rock.
Ans: A rock is any natural mass of mineral matter that makes up the earth’s crust.

Q3: Which are two main types of igneous rocks?
Ans: On the basis of chemical differentiation of magma, there are two types of igneous rocks, viz., mafic and felsic.

Q4: Name the scale that grades between the size of mineral grains.
Ans: The system of grading of mineral grains according to their size is called the Wentworth scale.

Q5: What is ‘diagenesis’?
Ans: It is the process of physical and chemical changes affecting sediments during their conversion into solid rocks.

Q6: What does the term ‘lithosphere’ mean?
Ans: The term lithosphere means a sphere of rocks.

Q7: What kind of rock is granite?
Ans: Granite is a coarse-grained igneous rock that is formed by the slow cooling of magma.

Q8: What is metamorphism?
Ans: Metamorphism is the change of form. It refers to the change in the form of rock when it is subjected to heat or pressure.

Short Answer Type Questions

Q9: How would you classify the igneous rocks based on the size and nature of crystals?
Ans: There are five types of igneous rocks based on the size and nature of crystals viz.

• rocks with phaneritic textured crystals
• rocks with orphan textured crystals
• equigranular textured rocks, and
• porphyritic textured rocks.

The crystals large enough to be seen with naked eyes or with the help of hand lens are called phaneritic textured crystals, whereas those too small to be distinguished without the aid of a microscope are – called orphan textured crystals of the igneous rocks. Where crystals in the rocks are all within the same size range, the texture is described as equigranular, whereas few large crystals are embedded in a matrix of smaller crystals, the texture is porphyritic.

Q10: What are schists?
Ans: As metamorphism continues a large percentage of the minerals assume the plate-like shape and are assembled in parallel orientation in the rock, a structure is known as foliation. Strongly developed foliation is known as schist.

Q11: What are minerals and how are they formed?
Ans: The mineral is a naturally occurring homogeneous solid that is an inorganic substance having an orderly atomic structure and definite chemical composition. The basic source of all minerals is hot magma in the interior of the earth. When magma cools, crystals of mineral appear. These first crystals may sink in the magma so that the composition of the magma changes with depth. Thus, a sequence of minerals is formed in the rocks as the magma cools. Besides this, certain minerals such as coal, petroleum and natural gas are organic substances found in solid, liquid and gaseous forms respectively.

Q12: What is mineral ‘lustre’?
Ans: The appearance of a mineral surface under reflected light is referred to as its mineral lustre. It is described by several descriptive adjectives, such as metallic (metal like), adamantine (diamond), vitreous glass), resinous (oil), pearly or silky.

Q13: What is the Deccan Trap?
Ans: The Deccan trap is an extensive area in the north-west part of India. It covers about 5 lakh sq. km, area. It has been formed by lava flows. Lava has solidified to form basalt. It is useful for cotton cultivation.

Long Answer Type Questions

Q14: Write short notes on:
(i) Sedimentary rocks
Ans: Exogenous agencies such as rain, wind, ice, running water, plants and animals are constantly hurried in rock disintegration producing loose and broken rock fragments in all sizes. These minerals are carried by wind, ice and running water in depressions such as lakes and seas on the earth’s surface. Such dropped materials are called sediments. Accumulation of these sediments in course of time gives rise to sedimentary rocks. The word ‘sedimentary’ is derived from the Latin word ‘sedimented’ means setting down. The sediments are usually deposited in distinct layers or strata. Therefore, these rocks are also known as stratified rocks.

(ii) Types of metamorphism
Ans: Metamorphism is the process of transformation of pre-existing rock into a new rock. Thus, metamorphic rocks are found when a rock is subjected to heat and/or pressure. Their characteristics are altered by forming new mineral forms. There are three types of metamorphism, viz. foliation, lineation and banding. When, under continued metamorphism, its minerals assume the plate-like shape and are assembled in parallel orientation in the rock, it is called foliation, e.g., Schist is formed in this manner.
When the mineral grains are drawn out into long, thin, pencil-like objects, all in parallel alignment, it is referred to as lineation. In another type of metamorphism, i.e., banding, the minerals of different varieties or groups are segregated into alternate layers. These layers are usually of light or dark shades, making the banding conspicuous.

Q15: Describe the categorization of metamorphic rocks.
Ans: The metamorphic rocks can be broadly grouped into two major classes – cataclastic rocks and recrystallised rocks. Cataclastic rocks are formed by mechanical disruption (breaking and crushing) of the original materials. The process is described as dynamic metamorphism. The recrystallised rocks are further divided into two sub-classes – contact and regional metamorphic rocks. The contact metamorphic rocks are formed by recrystallisation under high temperature caused by intruding magma. The rocks are not subjected to bending or breaking and new minerals emanating from magma are added to metamorphosed rocks. The regional metamorphic rocks undergo recrystallisation during the process of being deformed by sharing often under the condition of high pressure or high temperature or bolts.

Q16: Describe the formation of igneous rocks, giving suitable examples of various types.
Ans: The igneous rocks are divided into extrusive rocks and intrusive rocks. When the magma solidifies on the surface of the earth, we call it an extrusive rock. A typical example of this kind of rock is basalt. It is a very fine-grained rock, usually Sf black in colour.
On the other hand, when the magma solidifies in the crust, below the surface, it is called an intrusive rock. The few most common examples of intrusive rocks are granite and dolerite. More frequent, however, is the solidification of magma below the earth’s surface leading to the formation of intrusive rocks. Igneous rocks are classified on the basis of chemical composition and texture. Chemical differentiation of magma gives rise to mafic and felsic types of igneous rocks.
Texture relates to the sizes and pattern of the mineral crystals present in the rock. The size of mineral crystals in an igneous rock depends largely upon the rate of cooling of magma. As a general rule, rapid cooling results in small crystals and slow cooling results in large crystals. Extremely sudden cooling results in the formation of natural glass which is non-crystalline. Large bodies of magma trapped beneath the surface cool very slowly because the surrounding rocks conduct the heat slowly. Rapid cooling occurs in lava that loses heat rapidly to the atmosphere or to the overlying ocean water.

Q17: What is the economic importance of minerals?
Ans: Mineral resources can be divided into four main groups – essential resources, energy resources, metal resources and industrial resources. The most basic group, essential resources, comprises soil and water. Energy resources can be divided into fossil fuels (crude oil, natural gas, coal, oil shale and tar sand) and nuclear fuels (uranium, thorium and geothermal power). Metal resources range from structural metals such as iron, aluminium and titanium to ornamental and industrial metals such as gold, platinum and gallium.
Mineral deposits have two geological characteristics that make them a real challenge to modem civilization. First, all of them are non-renewable resources. The geological processes that form them are much slower than the rate at which we exploit them. There is no likelihood of our ability to grow mineral deposits at a rate equal to our consumption. Second, mineral deposits have a place value. We cannot decide from where to extract them; nature made that decision for us when the deposits were formed.

Q18: Discuss various types of rocks in detail.
Ans: There are three main groups of rocks: igneous, sedimentary and metamorphic rocks.
Igneous rocks: These are formed from lava hurled out of a volcano or from the cooling of hot magma below the crust. Granite is a coarse-grained rock that was formed by the slow cooling of magma. Basalt is a fine-grained igneous rock, almost black, that was formed by quick cooling of lava. Chemical differentiation of magma gives rise to mafic and felsic types of igneous rocks.
The size of mineral crystals in an igneous rock largely depends upon the rate of cooling of magma. As a general rule, rapid cooling results in small crystals and slow cooling in large crystals. Extremely sudden cooling results in the formation of a natural glass which is non-crystalline. Large bodies of magma trapped beneath the surface cool very slowly because the surrounding rocks conduct the heat slowly. Rapid cooling occurs in lava that loses heat rapidly to the atmosphere or to the overlying ocean water.

Q19: Why fossils are preserved in sedimentary and not in igneous rocks?
Ans: Fossils are primarily found in sedimentary rocks rather than igneous rocks due to the different formation processes of these rock types.

Formation Process:

• Sedimentary Rocks: These rocks are formed from the accumulation and cementation of mineral and organic particles, such as sand, silt, clay, and the remains of plants and animals. As these particles settle and accumulate in layers, they can entrap and preserve fossils.
• Igneous Rocks: These rocks are formed from the cooling and solidification of molten magma or lava. The extreme heat and pressure involved in the formation of igneous rocks usually destroy any organic material, including fossils, that might be present in the original rock.

Porous Nature of Sedimentary Rocks:

• Sedimentary Rocks: Sedimentary rocks often have a porous structure, allowing water and minerals to seep through the rock. This percolating water can dissolve the original organic material in the rock and leave behind a cavity that can later be filled with minerals, preserving the shape of the organism as a fossil.
• Igneous Rocks: Igneous rocks generally lack the porous structure of sedimentary rocks, making it difficult for organic material to be preserved. The high temperatures and pressure involved in the formation of igneous rocks also contribute to the destruction of organic matter.

Surface Exposure:

• Sedimentary Rocks: Sedimentary rocks are often found at the Earth's surface or in shallow layers, where fossils can be exposed through erosion and weathering processes, making them accessible to paleontologists.
• Igneous Rocks: Igneous rocks are often found deep beneath the Earth's surface and are usually only exposed after significant geological processes, such as uplift and erosion. By this time, any fossils that might have been present would likely have been destroyed.

These factors collectively explain why fossils are predominantly found in sedimentary rocks. However, in some rare cases, fossils can be found in certain types of igneous rocks, such as volcanic ash deposits, where rapid burial can preserve organic material before it is completely destroyed.

Q20: How is coal formed?
Ans: Coal is a combustible black or brownish-black sedimentary rock primarily composed of carbon, along with various other elements, including hydrogen, sulfur, oxygen, and nitrogen. It is formed from the remains of plants that lived and died in swampy conditions millions of years ago. The process of coal formation, known as coalification, occurs in several stages:

Plant Material Accumulation:

• Coal begins its formation when plant material, mainly trees, ferns, and other vegetation, accumulates in a swampy environment. These plants die and fall into stagnant water, where they are preserved without fully decomposing due to the lack of oxygen.

Peat Formation:

• Over time, layers of dead plants accumulate and form a spongy, brown material called peat. Peat is the first precursor to coal. It consists of organic material that is not fully decayed, preserving much of the original plant structure.

Lithification:

• As more layers of plant material accumulate on top of the peat, the weight of the overlying sediment compacts the peat. The water content is gradually squeezed out, and the peat becomes more compacted and solid. This process, known as lithification, results in the formation of lignite, which is a low-grade form of coal with a high moisture content.

Coalification:

• With increasing burial depth and geological time, the lignite undergoes further heat and pressure, transforming into higher-grade coals such as bituminous coal and anthracite. This process, called coalification or metamorphism, results in the loss of moisture, volatile components, and oxygen content, increasing the carbon content of the coal.
• Bituminous coal is intermediate in rank and is widely used for electricity generation and industrial processes.
• Anthracite coal is the highest rank and has a high carbon content and low moisture and volatile matter. It is often used for heating due to its high energy content and relatively clean-burning properties.

The specific type of coal that forms depends on factors such as the types of plants present, the conditions of the swamp, and the depth of burial. The entire process of coal formation can take millions of years, and different stages of coal can be found in different geological layers, reflecting the varying conditions under which they formed.