How is unconformity formed?

Formation of an Unconformity:

An Unconformity may develop due to the changes in the process of deposition due to which there is a break in the deposition. This may also take place due to Upheaval of the deposition surface (or basin). Such upheavals consequently lead to erosion and/or deformation of the uplifted rock strata on account of its exposure to the active deforming geological agents such as water, wind, glaciers, etc.

Upon the upper surface of the pre-existing strata which was being uplifted (and which may or may not have undergone any deformation), a second set of mutually Conformable rock beds may be deposited during the geological periods of fresh sedimentation cycles. This may also take place due to Submergence of the Uplifted Strata followed by the deposition of the younger rock beds.
The two formation which then lie in contact with each other are actually Unconformably related to each other. And thus the resulting structure is termed as an Unconformity.

The time period of no deposition or erosion is called as "Hiatus."