This set of Engineering Geology Multiple Choice Questions & Answers (MCQs) focuses on “Important Igneous Rocks”.
1. Granite belongs to which mode of occurrence of igneous rock?
a) Volcanic rocks
d) Volcanic and hypabyssal
Explanation: Granites may be defined as plutonic light coloured igneous rocks. These are among the most common igneous rocks.
2. The two most common mineral found in granites are
a) Diamond and mica
b) Mica and orthoclase
c) Quartz and felspar
d) Felspar and corundum
Explanation: Two most common and essential mineral constituents of granite are: Quartz and Felspar.
3. Which is the most important accessory mineral in granite?
Explanation: Among the accessory minerals in granites, micas deserve the first mention. Both varieties are present in small proportions in most granites.
4. The texture of granite is
Explanation: Granites are generally coarse to medium grained, holocrystalline and equigranular rocks.
5. Syenites belong to which mode of occurrence?
a) Volcanic rocks
Explanation: Syenites are defined as igneous, plutonic, even-grained rocks in which alkali-felspars are the chief constituent minerals.
6. State true or false. Syenites show similar textures of granites.
Explanation: Syenites show textures broadly similar to those of granites, that is, they are coarse to medium-grained, holocrystalline in nature and exhibiting graphic, inter-growth or porphyiritic relationship among its constituents.
7. What is the usual colour of gabbro?
Explanation: Gabbros are usually found in black colour. Sometimes they are also found in other shades of dark grey and dark brown.
8. What is the mode of formation of gabbro?
Explanation: Gabbros are coarse-grained plutonic rocks of basic character. Plagioclase felspars of lime-soda composition are the chief constituents of gabbros.
9. Pick the igneous rock whose mode of occurrence is volcanic.
Explanation: Basalts are volcanic igneous rocks formed by rapid cooling from lava flows from volcanoes either over the surface or under water on oceanic floors.
10. What can be said about the grain size of pegmatite?
a) Fine grained
b) Medium grained
c) Fine to medium grained
d) Coarse grained
Explanation: Pegmatites are exceptionally coarse-grained igneous rocks formed from hydrothermal solutions emanating from magmas that get cooled and crystallized in cavities and cracks around magmatic intrusions.
11. Pick the rock which is not volcanic in terms of mode of occurrence.
Explanation: Granite is of plutonic occurrence whereas the rest, viz., Basalt, Obsidian. Pumice are volcanic in occurrence.
12. The igneous rock with glassy texture is
Explanation: Obsidian is of glassy texture and it is distinct property of the igneous rock which makes it easier for its identification.
13. The igneous rock with very low specific gravity is
Explanation: Pumice has a low specific gravity whereas all the other igneous rocks have medium specific gravity. This property makes the igneous rock “pumice” very light. The main reason behind this is, pumice has many pores.
14. The igneous rock with flow texture is
Explanation: Rhyolite is an igneous rock which exhibits the flow type of texture which is not found in other igneous rocks.
15. Pegmatites are formed due to displacement reactions. State true or false.
Explanation: Pegmatites have formed due to replacement reactions between the hydrothermal solutions and the country rock through which these liquids happen to pass.
Sanfoundry Global Education & Learning Series – Engineering Geology.
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