Cascade Volcanoes - Eruptions of The Cascades

Eruptions of The Cascades

11 of the 13 volcanos in the U.S. portion of the Cascade Range have erupted at least once in the past 4,000 years, and 7 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 10 or more miles away; and huge mudflows of volcanic ash and debris, called lahars, can inundate valleys more than 50 miles (80 km) 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 two most recent were Lassen Peak in 1914 to 1921 and a major eruption of Mount St. Helens in 1980. Minor eruptions of Mount St. Helens have also occurred, most recently in 2006. 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 landslides and volcanic development in the evolution of volcanic terrain. A vast piece on the north side of Mount St. Helens dropped and formed a jumbled landslide environment several kilometers away from the volcano. Pyroclastic flows and lahars moved across the countryside. Parallel episodes have also happened at Mount Shasta and other Cascade volcanoes in prehistoric times.