Final answer:
To determine which rock layers are older, geologists use the principles of superposition, original horizontality, and lateral continuity, along with geochronology and the content of rock layers, such as fossils and isotopes.
Step-by-step explanation:
To determine which rock layers are older, geologists use the principle of superposition, which states that in a sequence of undeformed sedimentary rocks, the oldest layers are at the bottom and the youngest ones are at the top. This principle, along with others such as the principle of original horizontality and the principle of lateral continuity, helps geologists to establish relative ages of rock layers.
Additionally, by examining the content of the rock layers, we can find fossils and use other dating methods like geochronology to understand Earth's history. For instance, the Acasta Gneiss in Canada is one of the oldest known Earth rocks at 3.96 billion years old, which is determined through methods like measuring the decay of radioactive isotopes. By looking at the position of the rock layer and the contents of the rocks, scientists can deduce the relative ages of different rock strata within locations like the Grand Canyon or Zion National Park.