Mount St. Helens, Washington 2004-2007: Volcanic dome growth renewed on September 20, 2004 and has been continuous.
The largest eruption in the world this century occurred in 1912 at Novarupta on the Alaska Peninsula.
The May 18, 1980 eruption of Mount St. Helens (Washington) was the most destructive in the history of the United States.
The most recent eruptive activity occurred at Lassen Peak (California) in 1914-1917 A.D.
Climb A Volcano - 'Family Fun - Picnic at the Top'
The USGS Photo Glossary of volcanic terms includes photo examples of each term.  
All these links have volcano information: USGS Volcano Observatories
The United States ranks third, behind Indonesia and Japan, in the number of historically active volcanoes (that is, those for which we have written accounts of eruptions).
Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America.
Mauna Loa (Hawaii) is the world's largest active volcano, projecting 13,677 feet above sea level, its top being over 28,000 feet above the deep ocean floor. From it's base below sea level to it's summit, Mauna Loa is taller than Mount Everest.