Explain how scientists know that rock layers at the bottom of Arizona's Grand Canyon are older than those found on the rim.

The principle of superposition is a fundamental concept in geology. It states that in an undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom, and the youngest layers are at the top.
This idea is simple but powerful. Think of it like stacking books – the book on the bottom was placed first, so it is the oldest. Similarly, rock layers are deposited over time, with each new layer forming on top of the previous one.
This principle helps scientists deduce the relative ages of rock layers. For instance, if you look at the bottom of the Grand Canyon, those rocks are older than the ones at the top because they were deposited first.
Any disturbance, such as an earthquake or volcanic activity, can disrupt this sequence. Scientists look for signs of these disturbances to understand the history of the rock layers better.

Radiometric dating is a technique used to determine the actual age of rocks by measuring the decay of radioactive elements within them. Radioactive elements, like uranium, decay into stable elements, like lead, at a predictable rate.
When scientists examine a rock, they can measure the ratio of the original radioactive element to its decay products. This ratio tells the scientist how long the decay has been happening, which translates into the age of the rock.
For example, if scientists find a particular rock layer at the bottom of the Grand Canyon and use radiometric dating to measure it as 1 billion years old, they know precisely how old that layer is. They can then compare it to rock layers above it to confirm the principle of superposition.
Radiometric dating is highly reliable and has been used to date rocks from the Earth, moon, and even meteorites.

Fossil evidence plays a crucial role in understanding the relative ages of rock layers. Fossils are the remains of ancient organisms preserved in rock. These can be bones, shells, or even imprints of plants.
Scientists know that different types of organisms lived at different times in Earth's history. For instance, simpler organisms like trilobites are found in older layers, while more complex organisms, like mammals, are found in newer layers.
By studying fossils in various rock layers, scientists can piece together the history of life on Earth. Finding a particular fossil in a rock layer helps them identify the approximate age of that layer.
In the Grand Canyon, simpler fossilized life forms are found in the deeper, older layers, while more advanced organisms are found in the upper, newer layers. This fossil evidence supports the principle of superposition and radiometric dating results.