Here's why:
* Sedimentation: Rocks form from layers of sediment (like sand, mud, or volcanic ash) that settle over time.
* Time and Accumulation: The oldest layers of sediment are deposited first, and then newer layers build on top of them.
* Compression and Lithification: Over millions of years, the weight of the layers above compresses and cements the sediment, turning it into rock.
Exceptions:
While superposition is a powerful principle, there are situations where the oldest rocks are not at the bottom:
* Folding and Faulting: Earthquakes and tectonic movements can fold and break rock layers, causing the oldest layers to be on top of the youngest.
* Erosion and Uplift: Weathering and erosion can remove the top layers of rock, exposing older layers. Also, tectonic forces can uplift rock layers, bringing deeper layers to the surface.
In summary: While superposition is a great starting point, it's important to remember that geological processes can make rock layers appear in unexpected ways.
