ASBOG Practice Exam

What type of unconformity forms when granitic or metamorphic rocks are covered by sedimentary rocks after exposure to erosion?

1. Disconformity

2. Angular unconformity

3. Nonconformity

4. Paraconformity

The correct answer is: Nonconformity

The correct choice highlights a nonconformity, which is characterized by a significant geological feature where older granitic or metamorphic rocks are in contact with younger sedimentary rocks. This type of unconformity occurs specifically when the older rocks undergo a period of erosion, leaving them exposed before sedimentary layers are deposited on top of them. In this process, the previously formed sedimentary rocks may not have been laid down in a sequence directly upon the igneous or metamorphic rocks, leading to a lack of chronological continuity. Nonconformities are indicative of considerable geological time between the formation of the older crystalline rocks and the subsequent sedimentary deposits, marking a distinct gap in the geological record. The other types of unconformities do not apply here. A disconformity involves sedimentary layers being interrupted but remains parallel to the older layers, while an angular unconformity shows a tilting or folding of older strata before being overlain by a younger sedimentary layer at an angle. A paraconformity represents a gap where there is no visible erosion or inclination between strata, making it distinct from the scenario described.