Final answer:
Geologists determine the relative age of a rock by studying fossil content and using relative dating techniques. Radiometric dating, a form of absolute dating, can provide an exact numerical age, but is not used directly to establish relative age. It is often combined with relative dating methods, especially for sedimentary rocks.
Step-by-step explanation:
To determine the relative age of a rock, geologists use several methods, including studying its fossil content and using relative dating techniques. Fossil content can provide a relative age because certain fossils are known to appear in the geological record at specific times. By studying which fossils are present, scientists can infer the rock's placement within the geological timescale.
Relative dating techniques involve determining the sequence of events in Earth's history without necessarily knowing their numerical age. This can be done by observing the relationships between rock layers and the principles of stratigraphy such as superposition, where younger layers are deposited on top of older ones. However, radiometric dating is a form of absolute dating that utilizes radioactive isotopes to calculate the exact numerical age of rocks, minerals, and sometimes fossils. This method relies on the known decay rates of radioactive materials and the measured ratios of parent and daughter isotopes within geologic materials.
It's important to note that while radiometric dating can give an absolute age, it is typically not used to establish relative age directly. Instead, it can provide absolute dates for events that can then be used to refine the relative age determinations made through stratigraphic methods. For minerals found in igneous and metamorphic rocks, radiometric dating can indicate crystallization and metamorphism times, respectively. For sedimentary rocks, combining relative dating with radiometric dating of intersecting igneous or metamorphic events is needed to determine age, as radiometric dating alone cannot often date these rocks directly.