The active layer in geology refers to the uppermost layer of soil or rock that undergoes seasonal freezing and thawing in permafrost regions. This layer experiences significant physical and chemical changes due to the fluctuating temperatures throughout the year, leading to processes such as frost heaving, soil moisture changes, and nutrient cycling. Understanding the dynamics of the active layer is crucial for predicting the impacts of climate change on permafrost regions, as changes in the active layer thickness can affect land surface stability, water availability, and ecosystem productivity. Researchers study the active layer through field observations, monitoring instruments, and numerical modeling to better understand its role in permafrost degradation and feedbacks to the climate system.