How do you figure out how to order the rock layers? What strategy or clues did you use?
Ordering rock layers can be done using various strategies and clues. Here are some common methods used by geologists to determine the relative age of rock layers:
1. Law of Superposition: This principle states that in undisturbed rock layers, the youngest rocks are at the top while the oldest rocks are at the bottom. By observing the vertical sequence of layers and their relative positions, it is possible to establish their relative ages.
2. Principle of Original Horizontality: This principle states that sedimentary rock layers are originally deposited in horizontal, flat layers. Any significant tilting or folding of the layers occur after their formation. Therefore, if an undisturbed rock layer is found to be tilted or folded, the layers that were originally horizontal can be determined and used to establish their relative ages.
3. Principle of Lateral Continuity: This principle states that sedimentary rock layers initially extend laterally until they thin out, grade into a different type of rock, or end at the edge of a basin or cliff. By identifying the same rock layers in different locations, geologists can correlate and establish the relative ages of the layers.
4. Principle of Cross-Cutting Relationships: This principle suggests that a rock layer must be older than any feature (e.g., faults, igneous intrusions) that cuts across it. For example, if a fault is observed cutting across several rock layers, the fault is younger than the rocks it cuts through.
5. Fossil Correlation: Fossils are useful clues for relative dating because they have a specific range of time in which they existed. By comparing the fossils found in different rock layers, geologists can determine the relative ages of the layers. Fossil assemblages can be used to correlate rocks across different locations.
Combining these strategies and clues enables geologists to create a relative age sequence of rock layers based on their positions, compositions, and relationships with other rock formations. It is important to note that these methods provide relative ages, which indicate the order of events, but not absolute ages in years. For determining absolute ages, radioactive dating methods are used.