Overview
This lecture explains the Law of Superposition, a principle used to determine the relative ages of rock layers and fossils, by stating that the lowest layers are the oldest and the upper layers are the youngest.
Law of Superposition Principle
- The Law of Superposition states that in undisturbed sedimentary rock layers, the oldest layers are at the bottom and the youngest at the top.
- This principle helps determine the relative (approximate) age of rock layers and the fossils within them.
- Relative age does not provide an exact age, but establishes a sequence of events.
Determining Relative Age
- The sequence of rock layers allows scientists to infer which fossils or rocks are older or younger compared to each other.
- Deeper layers represent older geological time periods; digging deeper means looking further back in time.
- This law can be used in real-world examples, such as examining the layers of the Grand Canyon.
Diagram Interpretation
- In diagrams, the topmost layer is always the youngest, and the bottommost layer is the oldest, unless disrupted.
- Geological forces such as erosion, faulting, or intrusion can alter layers, but the law still applies with careful observation.
- Intrusions (magma that cuts through layers) are younger than the layers they penetrate.
- Faults (cracks) that cut through layers occur after the formation of those layers.
Limitations and Exceptions
- The Law of Superposition works best with undisturbed, horizontal layers.
- Even when forces like tectonic shifts alter the layers, scientists use clues to reconstruct the original order.
Key Terms & Definitions
- Law of Superposition — In sedimentary rock, each layer is older than the one above and younger than the one below.
- Relative Age — The age of an object compared to others, not an exact number.
- Intrusion — Magma that cuts through rock layers, younger than the layers it intrudes.
- Fault — A crack in the Earth's crust that can shift rock layers.
- Index Fossil — A fossil used to define and identify geologic periods (mentioned for later study).
Action Items / Next Steps
- Review diagrams demonstrating the Law of Superposition.
- Be able to identify the oldest and youngest layers in any given rock sequence.
- Watch upcoming lesson on index fossils for another way to determine relative ages.