Which term describes a time gap in the rock record caused by erosion or non-deposition?

• A. Law of Superposition
• B. Faulting
• C. Extrusion
• D. Unconformity

Answer: (D)

This question tests understanding of gaps in the rock record. An unconformity is a surface that marks a missing interval in geologic time, created when deposition stops for a long period or erosion wears away already deposited rock. That means the rocks above and below don’t line up in time, even though they may be continuous in space. Over long times, different forms appear: disconformities occur when parallel layers miss a time span, angular unconformities when tilted layers are overlain by newer horizontal ones, and nonconformities when sedimentary rocks sit atop old igneous or metamorphic rocks. The other terms aren’t about missing time in the record: the law of superposition describes the arrangement of ages in layers, faulting involves movement along fractures, and extrusion refers to lava reaching the surface. Unconformity is the best descriptor for a time gap caused by erosion or non-deposition.