This set of Engineering Geology Questions and Answers for Campus interviews focuses on “Forms of Igneous Rocks – 02”.
1. The intrusive bodies in which injection of lava occurs without any influence of dip and strike is
a) Concordant bodies
b) Discordant bodies
c) Non-cordant bodies
Explanation: All those intrusive bodies that have been injected into the strata without being influenced by their structural disposition (dip and strike) and thus traverse across or oblique to the bedding planes etc. are grouped as discordant bodies.
2. The example not belonging to discordant body is
b) Volcanic necks
Explanation: Bysmalith is a type of laccolith which is again a type of concordant body and not a discordant body. The dykes, volcanic necks and batholiths are usually the widely studied types of discordant bodies.
3. Which discordant body is columnar or column shaped?
b) Volcanic necks
Explanation: Dykes may be defined as columnar bodies of igneous rocks that cut across the bedding plane or uncoformities or cleavage planes and similar structures.
4. Texture shown by dykes is
d) All types of textures
Explanation: In composition, dykes are generally made up of hypabyssal rocks like dolerites, porphyrites and lamprophyres, showing all textures between glassy and phaneritic types.
5. Dykes tend to occur individually. State true or false.
Explanation: Dykes generally tend to occur in groups or sets. Thus, the term dyke-set is used for a couple of parallel and closely spaced dykes.
6. When the vents of quiet volcanoes become sealed with intrusions it leads to
d) Volcanic necks
Explanation: In some cases vents of quiet volcanoes have become sealed with the intrusions. Such congealed intrusions are termed volcanic necks or volcanic plugs.
7. The bodies which show both concordant and discordant relations are
Explanation: Batholiths are huge bodies of igneous masses that show both concordant and discordant relations with the country rocks.
8. What should be the area and depth respectively, for an igneous body to be called a batholith?
a) 100 square km and depth not traceable
b) 10 square km and depth up to 100km
c) Not traceable and depth 10 km
d) 50 km and depth 10 km
Explanation: The dimensions of batholiths vary considerably but is generally agreed that to qualify as a batholith the igneous mass should be greater than 100 square kilometres in area and its depth should not be traceable.
9. What is the term used for a batholith when surface area is less than 100 km?
Explanation: When the surface area of batholith-like igneous mass is less than 100 km, it is commonly termed as stock. When such a stock has roughly circular outline, it is further distinguished as a boss.
10. Majority of batholiths show which composition?
Explanation: In composition, batholiths may be made of any type of igneous rock. They also exhibit many types of textures and structures. But as a matter of observation, majority of batholiths shows predominantly granitic composition, texture and structure.
11. Pick the wrong statement about granitization.
a) It is a set of processes rather than a single step
b) It involves already existing sedimentary and other rocks
c) It involves magmatic stage
d) It doesn’t actually require magmatic stage
Explanation: Granitization may be broadly described as a set of processes by which already existing sedimentary and other rocks are changed into granit-like without actually passing through a magmatic stage.
12. The volcanic sheets may vertical. State true or false.
Explanation: The volcanic sheets may be horizontal or slightly inclined depending upon the original topography and subsequent geological history of the area.
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