Final answer:
Good effects of volcanic eruptions include fertile soil, new land formation, and the creation of new habitats. Mt. St. Helens' eruption led to a process akin to mountain building where the cleared area was covered with nutrient-rich volcanic debris, which could support new plant growth.
Step-by-step explanation:
Three good effects of volcanic eruptions include: fertile soil, new land formation such as islands, and the creation of new habitats. Fertile soil arises from volcanic material that is rich in nutrients, making it highly productive for agriculture once it has weathered. Volcanic eruptions can also lead to the creation of new islands, such as those in the Hawaiian chain, as molten rock hardens upon reaching the ocean. Additionally, over the long term, eruptions can create new habitats for wildlife as the land recovers and vegetation takes root.
Referring to Mt. St. Helens, the replacement of the forest with volcanic debris that formed new soil would trigger a process known as primary succession. This is a biological process where a previously uninhabited land is colonized by living organisms for the first time. Given the options provided, the answer to the question related to the 1980 volcanic explosion of Mt. St. Helens would be C. mountain building, as it refers to processes that create topographical relief - in this case, the depositing of volcanic material that forms new soil and potentially alters the landscape.