ASBOG Practice Exam

The law of superposition does not account for the presence of which of the following?

1. Faults

2. Igneous intrusions

3. Weathered layers

4. Unconformities

The correct answer is: Igneous intrusions

The law of superposition states that in any undisturbed sequence of sedimentary rocks, older layers will be found at the bottom, while younger layers are at the top. This principle is critical for understanding the chronological sequence of geological formations. Igneous intrusions are bodies of igneous rock that have formed by the injection of magma into preexisting rock. These intrusions can cut through and disrupt the original layers of sedimentary rock. Since the formation of intrusions occurs after the sedimentary layers have been deposited, they are not accounted for by the law of superposition—the layers are disrupted rather than simply lying above one another in a straightforward chronological order. This distinguishes igneous intrusions from other geological features that can be evaluated using the principle of superposition, where the order of layers is maintained. In contrast, faults, weathered layers, and unconformities can still be interpreted in relation to the law of superposition, though they might imply a more complex history regarding the deposition of layers or periods of erosion. However, they do not necessarily contradict the basic principle itself in terms of the relative ages of the rock layers that can still be organized chronologically.