Final answer:
To determine the relative ages of rock layers, geologists use the law of superposition, examine igneous intrusions, note the presence of faults, utilize fossil correlation, and complement these methods with radiometric dating for numerical ages.
Step-by-step explanation:
Mia is interested in understanding the methods used to determine the relative ages of rock layers. One of the key techniques is the law of superposition, which states that in an undisturbed sequence of rocks, each layer is older than the one above and younger than the one below. Therefore, by examining the sequence of rock layers, scientists can establish a relative timeline for the rocks.
Another method is looking at igneous intrusions, where magma cuts through different layers of rock. Since the intrusion cuts through the pre-existing rocks, it is assumed to be younger than the rocks it disrupts. Additionally, if there are faults, or cracks in the Earth's crust along which movement has taken place, these can also be used to determine relative ages. The rocks that are disrupted by the fault must have been there before the fault occurred, allowing geologists to infer which rocks are older.