Volcanic eruptions can affect Earth's climate. For example, debris thrown high into the air by the eruption of Mount Pinatubo in the Philippines in 1991 reflected a small amount of sunlight back to space in the year or two afterward, resulting in a "global cooling" of a degree or so.

This weekend is the anniversary of the most famous volcanic eruption in the U.S. in the last few decades. On May 18, 1980, Mt. St. Helens in southwestern Washington state blew its top, shooting volcanic ash nearly 15 miles into the air. However, the amount of debris produced by Mt. St. Helens was many times less than that produced by Pinatubo. And most of it stayed in the lower atmosphere, so the cloud of dust and ash settled out relatively quickly, with only short-term regional effects on the weather and climate in the northwestern United States.

Still, the eruption and the avalanche of rock and other debris that accompanied it caused 57 deaths, and the mountain was forever altered: its height was reduced by more than 1300 feet.