Final answer:
The relative ages of geological events can be established using principles such as superposition and cross-cutting relationships, and scientists use methods like seismic reflection and borehole data to understand subsurface structures.
Step-by-step explanation:
To determine the relative ages of geological events in a stratigraphic cross section, one must use geological principles such as superposition and cross-cutting relationships. For example, in Cross Section I, if a rock layer G is folded and the fold is cut by a fault, then the folding must have occurred before the faulting. This is because the fault cuts through the folded layer, indicating that the folding was already present. Similarly, when considering an unconformity, such as C, if it is overlain by sedimentary layers, the unconformity represents a time gap between the older underlying layers and the newer layers above.
Geologists utilize tools like seismic refraction and reflection, borehole data, and gravity and magnetic surveys to determine the depths of rock layers within Earth's interior. These methods allow scientists to image and analyze subsurface structures without directly accessing them, which is essential for understanding the geological history recorded in the rock strata.