Catastrophism is the theory that most changes to the Earth's crust happened because of major events. No matter how cool it would be to see a mountain form, according to this theory, you wouldn't be around to watch.
Catastrophism was first proposed by the French scientist Georges Cuvier in the early 1800s. His work was done before a lot of important discoveries about geology, evolution, and the fossil record, so his original theories aren't used anymore. But the idea of sudden events that have massive effects on Earth and its species is still used sometimes in different ways, and that's sometimes called neocatastrophism. Catastrophism is contrasted with uniformitarianism or gradualism, the idea that slow incremental changes have shaped Earth's past.