Q. 1. Why are fossils preserved in Sedimentary and not in Igneous rocks ?
Ans. Fossils are remains of vegetation and animals buried under the sediments. The sedimentary rocks are stratified rocks and are found in layers. These fossils are preserved in between these layers. But in Igneous rocks, the fossils are destroyed due to high temperature of lava.
Q. 2. What relationship is explained by rock cycle between the three types of rocks ?
Ans. Rocks are mainly of three types. The change of one rock into another type is known as one rock cycle. Sedimentary rocks are formed from the material obtained by the erosion of Igneous rocks. These rocks are changed into Metamorphic rocks by the agency of pressure and heat. Metamorphic rocks are again changed into Igneous rocks due to melting.
Q. 3. How are Igneous rocks formed ?
Ans. Igneous rocks are formed by the solidification of Lava. Lava cools rapidly and solidifies on the surface of earth. Crystals are formed due to the cooling of Lava.
These rocks are intrusive as well as extrusive.
Q. 4. What do you mean by ‘Deccan trap’ ?
Ans. Deccan trap is an extensive area in N. W. part of India covering about 5 lakh sq. km. It has been formed by lava flows. Lava has solidified to form Basalt. It is useful for cotton cultivation.
Q. 5. Describe the economic importance of minerals.
Ans. Economic Importance of Minerals.
Mineral resources can be divided into four main groups :
(i) essential resources,
(ii) energy resources,
(iii) metal resources, and
(iv) industrial resources.
The most basic group, essential resources, comprises soil and water. Energy resources can be divided into the fossil fuels (crude oil, natural gas, coal, oil shale and tar sand) and the nuclear fuels (including 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. Industrial minerals include more than 30 commodities, such as salt, asbestos and sand.
Mineral deposits have two geological characteristics that make them a real challenge to modern civilisation. First, almost all of them are nonrenewable 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 where to extract them ; nature made that decision for us when the deposits were formed.
Q. 6. Describe the main physical characteristics of minerals.
Ans. Some main physical characteristics of minerals
1. External crystal form—determined by internal arrangement of the molecules-cubes, octahedrons, hexagonal prisms, etc.
2. Cleavage—tendency to break in given directions producing relatively plane surfaces; result of internal arrangement of the molecules; may cleave in one or more directions and at any angle to each other.
3. Fracture—internal molecular arrangement is so complex that there are no planes of molecules; the crystal will break in an irregular manner, not along planes of cleavage.
4. Lustre—appearance of a material without regard to colour; each mineral has a distinctive lustre like metallic, silky, glossy etc.
5. Colour—some minerals have characteristic colour determined by their molecular structure— malachite, azurite, chalcopyrite, etc. and some minerals are coloured by impurities. For example, because of impurities, quartz may be white, green, red, yellow, etc.
6. Streak—colour of the ground powder of any mineral. It may be of the same colour as the mineral or may differ—malachite is green and gives green streak; fluorite is purple or green but gives a white streak.
7. Transparency—transparent light rays pass through so that objects can be seen plainly; translucent—light rays pass through but will get diffused so that objects cannot be seen; opaque—light rays will not pass at all.
8. Structure—particular arrangement of the individual crystals; fine, medium or coarse grained; fibrous—separable, divergent, radiating.
9. Hardness—relative resistance being scratched; ten minerals are selected to measure the degree of hardness from 1 to 10. They are : (1) talc, (2) gypsum, (3) calcite, (4) fluorite, (5) apatite, (6) feldspar, (7) quartz (8) topaz, (9) corundum (10) diamond.
Compared to this, for example, fingernail is 2.5 and glass or knife blade is 5.5.
10. Specific gravity—the ratio between the weight of a given object and the weight of an equal volume of water; object weighed in air and then weighed in water and divide weight in air by the difference of the two weights.
Q. 7. What do you mean by foliation and Banded rocks ?
Ans. Foliation : In metamorphic rocks, grains or minerals get arranged in layers or lines. This is called foliation.
Banded rocks : Sometimes minerals or materials of different groups are arranged into alternating thin to thick layers, appearing in light and dark shades.
Such a structure is called banded rocks.