Mount Baker erupted in the mid-1800s for the first time in several thousand years. Activity at steam vents (fumaroles) in Sherman Crater, near the volcano’s summit, increased in 1975 and is still vigorous, but there is no evidence that an eruption is imminent.

Glacier Peak has erupted at least six times in the past 4,000 years. About 13,000 years ago, an especially powerful series of eruptions deposited volcanic ash at least as far away as Wyoming.

Mount Rainier has produced at least four eruptions and numerous lahars in the past 4,000 years. It is capped by more glacier ice than the rest of the Cascade volcanoes combined, and parts of Rainier’s steep slopes have been weakened by hot, acidic volcanic gases and water. These factors make this volcano especially prone to landslides and lahars.

Mount St. Helens is the most frequently active volcano in the Cascades. During the past 4,000 years, it has produced many lahars and a wide variety of eruptive activity, from relatively quiet outflows of lava to explosive eruptions much larger than that of May 18, 1980.

Mount Adams has produced few eruptions during the past several thousand years. This volcano’s most recent activity was a series of small eruptions about 1,000 years ago.

Mount Hood last erupted about 200 years ago, producing pyroclastic flows, lahars, and a prominent lava dome (Crater Rock) near the volcano’s summit. Most recently, a series of steam blasts occurred between 1856 and 1865.

Mount Jefferson last erupted more than 20,000 years ago. However, eruptions nearby have produced several lava flows and small volcanic cones in the past 10,000 years.

Three Sisters Volcanic Center in central Oregon includes five large volcanoes—North Sister, Middle Sister, South Sister, Broken Top, and Mount Bachelor. About 2,000 years ago, eruptions occurred on South Sister, as well as from several small volcanoes north of North Sister. Since 1997, a broad area centered 3 miles west of South Sister has domed upward by more than 5 inches. Scientists think that this doming reflects the ongoing accumulation of magma at a depth of 3 to 4 miles. The outcome of this activity is uncertain, but there is no evidence that an eruption is imminent. The USGS and its partners have increased monitoring efforts in the area to detect any changes that might warrant more concern.

Newberry Volcano, a broad shield covering more than 500 square miles, is capped by Newberry Crater, a large volcanic depression (caldera) 5 miles across. Its most recent eruption was about 1,300 years ago.

Crater Lake occupies a 6-mile-wide caldera formed 7,700 years ago when the summit of an ancient volcano (referred to as Mount Mazama) collapsed during a huge explosive eruption. More than 10 cubic miles of magma was erupted, 10 times as much as in any other eruption in the Cascades during the past 10,000 years. Smaller eruptions ending about 5,000 years ago formed Wizard Island and several submerged cones and lava domes on the lake floor.

Medicine Lake Volcano, a broad shield capped by a 4- by 7-mile-wide caldera, has erupted at least eight times in the past 4,000 years, most recently about 900 years ago. With a volume of more than 130 cubic miles, it is the largest volcano in the Cascades.

Mount Shasta has been the most active volcano in California during the past 4,000 years, second in the entire Cascade Range to Mount St. Helens. During that time, Shasta has erupted on average about once every 300 years, producing many pyroclastic flows and lahars.
probably last erupted in 1786.

Lassen Volcanic Field, including Lassen Peak, is the southernmost volcanic center in the Cascades. The most recent volcanic eruptions in California occurred at Lassen Peak from 1914 to 1917. An explosive eruption on May 22, 1915, produced a large pyroclastic flow, lahars, and ash that fell as far away as Elko, Nevada, 300 miles to the east.

After the 1980 eruption of Mount St. Helens, Congress provided increased funding that enabled the USGS to establish a volcano observatory for the Cascade Range. Located in Vancouver, Washington, the David A. Johnston Cascades Volcano Observatory (CVO) was named for a USGS scientist killed at a forward observation post by the May 18, 1980, eruption.

Scientists at CVO quickly recognized that was not economically feasible to fully monitor all potentially active Cascade volcanoes. To address this and similar problems elsewhere in the United States and abroad, the USGS developed a suite of portable volcano-monitoring instruments—essentially, a portable volcano observatory. In the Pacific Northwest, when regional networks of earthquake sensors, operated in cooperation with the University Washington’s Pacific Northwest Seismograph Network, detect unusual seismic activity at volcano, CVO staff will rapidly deploy this portable equipment to evaluate the hazard and, if needed, provide timely warnings to local officials and the public.

CVO also uses remote sensing as an early-detection tool. A technique called interferometric synthetic-aperture radar (InSAR) allows scientists to measure subtle movements of the ground surface, using radar images obtained by Earth-orbiting satellites. The current ground doming at Three Sisters was first detected using this technique.

CVO was able to issue accurate warnings of the small eruptions that occurred until 1986 at Mount St. Helens. Observatory scientists also have conducted many new studies of the past geologic behavior of Cascade volcanoes. These studies allow assessments of long-term hazards (including lahars and landslides, which can occur even when a volcano is not erupting) and provide the data for hazard-zone maps that enable communities to make wise land-use-planning decisions and to prepare emergency-response plans.

The work of CVO scientists is helping residents of the Pacific Northwest live with volcanic risk and plan for volcano emergencies. This work is only part of the USGS Volcano Hazards Program’s ongoing efforts to protect people’s lives and property in all of the volcanic regions of the United States, including the Pacific Northwest, eastern California, Wyoming, Alaska, and Hawaii.