Question

Rock salt and gypsum are _ _ _ _ _ _ formed sedimentary rocks.
A. Chemically
B. Mechanically
C. Physically
D. All of these

Answer 1

Hint:- Rock salt is formed due to evaporation of the water containing salt in it. When it evaporates it leaves behind the dry lake beds on which this salt is deposited.
- Sedimentary rocks formed by such a process undergo changes in their composition.

Complete answer:
Process of formation: Deposition in layers, consolidation and compaction of sediments shapes sedimentary rocks. Therefore being of various thicknesses, they are layered or stratified. Sediments are a result of the weathering and erosion of other types of rocks. 75% of the earth's surface is covered by such rocks, but as they are available only in the upper part of the crust, only 5% of the total rock percentage of Earth is volumetrically occupied. Sedimentary rocks deposited with ice are called till or tillite and Loess is the term used for wind-deposited sediments.

Sedimentary rocks are categorised according to the mode of formation into-

A. Those formed chemically: Mineral-containing water evaporates at the mouths of springs or salt lakes, forming stalactites and stalagmites which are the deposits of lime left over by the lime-mixed water as it evaporates in the underground caves. Examples of such rocks are gypsum, limestone, rock salt, potash etc.

B. Those formed mechanically: Mechanical agents such as flowing water, wind, ocean waves, ice, etc. shape them. These examples are shale, Loess, conglomerate etc.

C. Those formed organically: The remains of plants and animals are buried beneath the soil, and their composition varies due to heat and pressure from overlying layers. Well-known examples are coal, limestone, shale, chalk, geyserite etc.

Thus the correct answer is A.

Note:- The name "gypsum" derives from the old Greek word "gypsos" meaning plaster. In the windows of their temples, the Greeks used a kind of gypsum because of their moonlight effects, which were created on the altar by the sun.