Geological theories explain the processes that shape the Earth’s structure and surface over time. One prominent theory is plate tectonics, which describes how the Earth's outer shell is divided into several large plates that move and interact, causing earthquakes, volcanic activity, and the formation of mountains. Another important theory is uniformitarianism, which suggests that the geological processes we observe today, such as erosion and sedimentation, have been occurring in a similar manner throughout Earth's history. This principle helps scientists understand past geological events by studying current processes and their effects on the landscape.