Newberry Volcano, which was last active about 1,300 years ago. It has one of the largest collections of cinder cones, lava domes, lava flows and fissures in the world. Medicine Lake Volcano Medicine Lake Volcano has erupted about eight times in the past 4,000 years and was last active about 1,000 years ago when rhyolite and dacite erupted at Glass Mountain and associated vents near the caldera's eastern rim. Mount Shasta Mount Shasta last erupted around 1250 and has been the most active volcano in California for about 4,000 years. Previous claims of a 1786 eruption have been discredited. Eruptions in the Cascade Range Eleven of the thirteen volcanoes in the Cascade Range have erupted at least once in the past 4,000 years, and seven have done so in just the past 200 years. The Cascade volcanoes have had more than 100 eruptions over the past few thousand years, many of them explosive eruptions. However, certain Cascade volcanoes can be dormant for hundreds or thousands of years between eruptions, and therefore the great risk caused by volcanic activity in the regions is not always readily apparent. When Cascade volcanoes do erupt, pyroclastic flows, lava flows, and landslides can devastate areas more than away; and huge mudflows of volcanic ash and debris, called lahars, can inundate valleys more than downstream. Falling ash from explosive eruptions can disrupt human activities hundreds of miles downwind, and drifting clouds of fine ash can cause severe damage to jet aircraft even thousands of miles away. All of the known historical eruptions have occurred in Washington, Oregon and in Northern California. The three most recent were Lassen Peak in 1914 to 1921, a major eruption of Mount St. Helens in 1980, and a minor eruption of Mount St. Helens from 2004 to 2008. In contrast, volcanoes in southern British Columbia, central and southern Oregon are currently dormant. The regions lacking new eruptions keep in touch to positions of fracture zones that offset the Gorda Ridge, Explorer Ridge and the Juan de Fuca Ridge. The volcanoes with historical eruptions include: Mount Rainier, Glacier Peak, Mount Baker, Mount Hood, Lassen Peak, and Mount Shasta. Renewed volcanic activity in the Cascade Arc, such as the 1980 eruption of Mount St. Helens, has offered a great deal of evidence about the structure of the Cascade Arc. One effect of the 1980 eruption was a greater knowledge of the influence of