A lot of scientists believe that really big volcanic eruptions can change weather around the world. Ash clouds sometimes float very high in the air for years, so sunlight cannot get to the earth, causing snow and frost in the summer in a lot of countries.
The main good effect that volcanoes have on the environment is to give vitamins to the soil around them. Volcanic ash contains minerals that help plants grow, and if the ash is very soft, it will quickly get mixed into the soil.
Another good thing about volcanoes is that their sides are often steep and hard to reach, making them a safe place for plants and animals to live.
Also, volcanoes are a very important source of life. Their gases are what created our water and atmosphere. This has been happening for 4.5 billion years.
Other important good effects of volcanoes include the creation of natural glass, sulfur, copper, and nickel