This set of Engineering Geology Multiple Choice Questions & Answers (MCQs) focuses on “Forms of Igneous Rocks – 01”.
1. Form of igneous rock where magma is injected and cooled along planes of host rocks is
a) Discordant bodies
b) Concordant bodies
c) Parallel bodies
d) Synchronous bodies
Explanation: All those intrusions in which the magma has been injected and cooled along or parallel to the structural planes of the host rocks are called concordant bodies.
2. Pick the type of concordant body among the following.
Explanation: The various types of concordant bodies are sills, phacoliths, laccoliths and lopoliths.
3. The type of concordant body whose thickness is much smaller than its length and width is
Explanation: It is typical of sills that their thickness is much small than their width and length. Moreover, this body commonly thins out or tapers along its outer margins.
4. The upper and lower margins of sills are relatively
a) Coarser grain size
b) Medium grain size
c) Finer grain size
d) Can be of any size
Explanation: The upper and lower margins of sills commonly show a comparatively finer grain size than their interior portions.
5. Sills in length are restricted to hundreds of metres. State true or false.
Explanation: In length, sills may vary from a few centimetres to hundreds of meters. Minor and local projections from big sills may rise above into the overlying strata.
6. Most common rocks composing the sills are
Explanation: The most common rocks composing the sills are intermediate and basic igneous rocks like syenites and gabbros. They may show aphinitic and porphyritic textures.
7. The small sized intrusives that occupy bends in the folds are called
Explanation: Phacoliths are concordant, small sized intrusions that occupy positions in the troughs and crests of bends called folds.
8. The concordant bodies associated with structural basins is
Explanation: Those igneous intrusions, which are associated with structural basins, that are sedimentary beds inclined towards a common centre, are termed as lopoliths. They may form huge bodies of consolidated magma.
9. The shape arch or dome is shown by which type of concordant body?
Explanation: Laccoliths are concordant intrusions due to which the invaded strata have been arched up or deformed into a dome. The igneous mass itself has a flat or concave base and a dome-shaped top.
10. What is the type of lava which leads to formation of laccolith?
c) Partially crystallized
d) Gaseous lava
Explanation: Laccoliths are formed when the magma being injected is considerably viscous so that it is unable to flow and spread for greater distances. Instead, it gets collected in the form of a heap about the orifice of eruption.
11. What is the type of laccolith in which fracture is formed?
d) Volcanic neck
Explanation: Extreme types of laccoliths are called bysmaliths and in these the overlying strata get ultimately fractured at the top of the dome because of continuous injections from below.
12. A sill is not considered to be a sill when two or more injections of different types of magma are involved. State true or false.
Explanation: Composite sills are the sills which result from two or more injections of different types of magma. Hence it cannot be told that it is not a sill, instead, it is a type of sill.
Sanfoundry Global Education & Learning Series – Engineering Geology.
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