Which of the following is an indicator of past tectonic activity in rocks?

• A. Deformed strata with faults, folds, foliations, metamorphic grade changes, igneous intrusions
• B. Uniform mineral content
• C. No evidence of deformation
• D. Presence of only one rock type

Answer: (A)

Evidence of past tectonic activity in rocks is shown by deformation features such as faults, folds, foliations, metamorphic grade changes, and igneous intrusions. When tectonic forces shape the crust, rocks are squeezed, stretched, and heated. Faults record breaks and slip; folds show bending from directional stress; foliations reveal minerals realigned under pressure; metamorphic grade changes reflect minerals recrystallizing under heat and pressure; and igneous intrusions mark magma moving through and crystallizing within already-deformed rocks. Collectively, these features signal a history of tectonic disturbance rather than simple, undeformed formation.

Other possibilities describe rocks that aren’t evidencing such stress: uniform mineral content suggests a lack of differentiation from deformation or metamorphism; no deformation means the rocks haven’t been stressed; and having only one rock type can occur in a mostly calm setting and doesn’t by itself indicate tectonic activity.