1. Erosion: Imagine a layer of rock being formed, representing a long period of time. Then, imagine a period of intense erosion where wind, water, or ice remove that layer completely. This leaves a gap in the record, as the time period represented by that layer is now missing. This is common in areas with tectonic uplift, where rocks are exposed and vulnerable to erosion.
2. Non-deposition: Sometimes, instead of erosion, there might be a period where sediments simply don't accumulate. This could be due to a lack of source material, a change in environmental conditions (like a dry period), or a shift in the landscape that prevents sediment deposition. This leads to a gap in the rock record, as there is no rock formed to represent that time period.
Both erosion and non-deposition can result in unconformities, which are breaks in the rock record that show a significant missing interval of time. These unconformities are valuable to geologists because they tell us about the past environment and how it changed over time.
