Just when we start to think Mount St. Helens is settling down, the volcano vents or rumbles. Here's what you need to know about past history and recent activity.
- 2005 to present - Mount St. Helens continues to experience low rates of seismicity, low emissions of steam and volcanic gases, minor production of ash, and the growth of a new lava dome inside the crater.
- March 8, 2005 - The Mount St. Helens volcano experienced a small explosive event, with the resulting steam-and-ash plume reaching an altitude of approximately 36,000 feet above sea level.
- January 16, 2005 - Explosive eruption that scattered ash and rocks as large as 1 meter in the crater and ash eastward onto the volcano's east side.
- October 11, 2004, to present - A new and distinctive lava dome became evident; it continues to grow and change.
- October 5, 2004 - The most vigorous steam-and-ash eruption since the start of the unrest. It lasted over one hour. The ash rose to about 3,700 m (12,000 ft) and drifted north-northeastward. A light ash dusting fell in the towns of Morton, Randle, and Packwood, about 50 km (30 mi) away. A light dusting affected the east side of Mount Rainier National Park, 110 km (70 mi) north-northeast.
- October 1, 2004 - A small steam eruption, with minor ash, issued from a vent just south of the 1980-86 lava dome
- September 23-25, 2004 - A swarm of small, shallow earthquakes (smaller than magnitude 1) began on the morning of September 23, peaked in midday on September 24, then declined through the afternoon of September 25.
The Last Major Eruption
It all started on March 15, 1980, when Mount St. Helens began a period of low-level seismic activity. As the activity escalated, the volcano kept us all on the edge of our seats. Here are the highlights from the events leading up to the major May 18 eruption, in reverse chronological order.
- May 17, 1980 - Law enforcement officials escorted about 50 carloads of property owners into the Red Zone to retrieve possessions.
- May 7-13, 1980 - Small explosions of steam and ash are emitted from the volcano. Intermittent earthquakes up to magnitude 4.9.
- April 29, 1980 - State officials asked the governor to close a large area around the volcano. The plan called for a Red Zone (no public access) and a Blue Zone (restricted access). Emergency services officials are frustrated because the public appeared to remain unaware of the danger.
- March 27 to April 18, 1980 - Earthquakes and steam-driven explosions occur off and on during this period.
- March 20, 1980 - A magnitude 4.1 earthquake, unlike any that had been previously detected in the area, occurred just northwest of the summit of Mount St. Helens. Seismologists were uncertain as to whether or not these first earthquakes were related to volcanic activity. They decided to deploy additional seismometers in order to better monitor future activity.
- March 15-19, 1980 - A number of very small earthquakes are recorded, but are not recognized as immediate precursors to possible volcanic activity.
A Brief History
As mountains go, Mount St. Helens is young. The volcano's oldest known deposits erupted about 50-40 thousand years ago, and the cone that partly collapsed in 1980 is only 2200 years old. Some Indians of the Pacific Northwest variously called Mount St. Helens "Louwala-Clough," or "smoking mountain." The modern name, Mount St. Helens, was given to the volcanic peak in 1792 by Captain George Vancouver of the British Royal Navy, a seafarer and explorer. He named it in honor of a fellow countryman, Alleyne Fitzherbert, who held the title Baron St. Helens and who was at the time the British Ambassador to Spain. Vancouver also named three other volcanoes in the Cascades--Mounts Baker, Hood, and Rainier--for British naval officers.
Here are the highlights of Mount St. Helens activity over the last 2000 years:
- Goat Rocks Eruptive Period - (Approximately 1800 A.D.) This eruptive period lasted for 100-150 years. Known events include ash explosions in 1842, which was followed by the extrusion of the Goat Rocks dome. Contemporary accounts indicate activity several times during the 1840s and 1850s but are non-specific and even contradictory. The last significant activity before 1980 was "dense smoke and fire" in 1857, although minor, unconfirmed eruptions were reported in 1898, 1903, and 1921
- Kalama Eruptive Period - (1479 to 1482 A.D.) This eruptive period included two major ejections of ash, as well as lava flows and dome building.
- Sugar Bowl Eruptive Period - (Approximately 800 A.D.) Mount St. Helens was reshaped by a combination of dome building, a lateral blast, and pyroclastic flows during this period of volcanic activity.
- Castle Creek Eruptive Period - (200 B.C. to 300 A.D.) Major activity during this era included ejections of ash, pyroclastic flows, and lava flows.
Data source: USGS/Cascades Volcano Observatory