The majestic Cascade Mountains serve as the centerpiece for the Northwest's renowned beauty, with their rugged, snow-capped peaks drawing thousands of visitors to their flanks each summer. Several of the region's national parks are centered on these mountains, including North Cascades National Park, Mount Rainier National Park, and Crater Lake National Park. Underlying their elegance, however, is the disquieting fact that the range's major peaks are potentially hazardous volcanoes. On average, two eruptions occur per century. In the last century, explosive eruptions at Lassen Peak and Mount...