Why did Charles Lyell believe small geological changes could have large effects over time?

Charles Lyell believed that small geological changes could produce large effects over time because he championed the doctrine of uniformitarianism.12

Contrary to the "catastrophist" school of thought—which attributed Earth's features to sudden, violent events—Lyell argued that geological history is shaped by the same physical, chemical, and biological processes observed today. He maintained that these processes have always operated at the same average levels of intensity and frequency throughout Earth's history.132

By assuming that past geological events were no more frequent or intense than modern ones, Lyell concluded that Earth's surface features are not the result of occasional cataclysms. Instead, he posited that these features are the cumulative result of gradual, continuous changes acting over vast periods of geological time. This "steady-state" view allowed scientists to interpret the history of the Earth by observing ongoing natural processes, providing a foundational methodology for the geological sciences.123