4 - 8 years. There are thousands of volcanoes all over the world. Some erupted millions of years ago and are quiet now. Others are newer and still active. Still others are being formed right now. Geologists study volcanoes-and so can you. Learn how the very structure of the earth predicts where they are found, and explains how they form and what makes them suddenly, and violently, erupt. Paperback. 32 pages.