What are the warning signs of an eruption? The most important sign of an impending volcanic eruption is seismic activity beneath the volcanic area. Seismologists can interpret subtle differences between earthquakes related to the rise of magma and the more familiar quakes caused by tectonic faulting.
Other warning signs of magma rising into the shallow subsurface might include increased release of volcanic gases from small openings called fumaroles, such as those found in the Bumpass Hell area of Lassen Volcanic National Park, and changes in the gas composition.
Deformation of the ground surface in the vicinity of a volcano may also indicate that magma is approaching the surface. Typically, these warning signs appear a few weeks to months before an eruption, but can last for decades or even centuries without leading to an eruption.
What is being done to monitor the Lassen volcanic center? After the eruption of Mount St. Helens in , the U. Monitoring of the Lassen area includes periodic measurements of ground deformation and volcanic gas emissions and continuous transmission of data from a local network of nine seismometers to USGS offices in Menlo Park, California. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers table 1. No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.
Table 1. Number of earthquakes at northern California volcanic centers during hour periods following major earthquakes on 25 April Events with coda durations less than or equal to 10 seconds and greater than 10 seconds are tallied separately. Earthquakes were identified from film records of seismograms from nearby stations.
Courtesy of Stephen Walter. Film was also scanned for the 24 hours following the M 7. Furthermore, both the 25 April main shock and a M 6. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.
Lassen Report. About 10 minutes after the S-wave's arrival and while surface waves were still being recorded, a M 2. Film records showed 9 more earthquakes in the first hour, and 22 events were identified during the first 24 hours.
Although most were M 1 or smaller, at least two and perhaps as many as four were of magnitude greater than or equal to 2. Nine were detected by the RTP system. Click on the index link or scroll down to read the reports. This compilation of synonyms and subsidiary features may not be comprehensive.
Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.
The Lassen volcanic center consists of the andesitic Brokeoff stratovolcano SW of Lassen Peak, a dacitic lava dome field, and peripheral small andesitic shield volcanoes and large lava flows, primarily on the Central Plateau NE of Lassen Peak. A series of eruptions from Lassen Peak from to marks the most recent eruptive activity in the southern Cascade Range.
Activity spanning about , years began with eruptions of the Rockland caldera complex and was followed beginning about , years ago by construction of Brokeoff stratovolcano. Beginning about , years ago activity shifted to the north flank of Brokeoff, where episodic, more silicic eruptions produced the Lassen dome field, a group of 30 dacitic lava domes including Bumpass Mountain, Mount Helen, Ski Heil Peak, and Reading Peak.
At least 12 eruptive episodes took place during the past , years, with Lassen Peak being constructed about 27, years ago. The Chaos Crags dome complex was constructed about years ago north of Lassen Peak. The Cinder Cone complex NE of Lassen Peak was erupted in a single episode several hundred years before present and is considered part of the Lassen volcanic center Clynne et al. The eruptions of Lassen Peak began with phreatic eruptions and included emplacement of a small summit lava dome, subplinian explosions, mudflows, and pyroclastic flows.
The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Clynne M A, Stratigraphic, lithologic, and major element geochemical constraints on magmatic evolution at Lassen volcanic center, California.
Res , 19,, How old is "Cinder Cone"? Volcanism in National Parks: U. Volcano hazards assessment for the Lassen region, northern California. United States of America. Rheologic properties and kinematics of emplacement of the Chaos Jumbles rockfall avalanche, Lassen Volcanic National Park. Res , Green J, Short N M, New York: Springer-Verlag, p. Late Cenozoic volcanism, subduction, and extension in the Lassen region of California, southern Cascade Range. Heiken G, Plinian-type eruptions in the Medicine Lake Highland, California, and the nature of the underlying magma.
Hildreth W E, Quaternary magmatism in the Cascades--geologic perpectives. Time-variation of hydrothermal discharge at selected sites in the western United States: implications for monitoring. Age of Lassen Peak, California, and implications for the ages of late Pleistocene glaciations in the southern Cascade Range.
Geol Soc Amer Bull , Williams H, Univ Calif Pub Geol Sci , Wood C A, Kienle J eds , Volcanoes of North America. Cambridge, England: Cambridge Univ Press, p.
Poland, M. Bawden, M. Lisowski, and D. Dzurisin, Eos Trans. AGU, 85 47 , Fall Meet. The maps shown below have been scanned from the GVP map archives and include the volcano on this page. Clicking on the small images will load the full dpi map. Very small-scale maps such as world maps are not included.
The maps database originated over 30 years ago, but was only recently updated and connected to our main database. We welcome users to tell us if they see incorrect information or other problems with the maps; please use the Contact GVP link at the bottom of the page to send us email.
Catalog number links will open a window with more information. Lassen Volcanic Center. Seismic events in the Lassen area that were apparently triggered by the M 7. Squares mark seismic stations. The Lassen Peak eruption consisted mostly of sporadic steam blasts. In May of , however, partially molten rock oozing from the vent began building a precarious lava dome. The dome collapsed on May 19 sending an avalanche of hot rock down the north flank of the volcano. Three days later, a vertical column of ash exploded from the vent reaching altitudes of 30, feet.
The ash column spawned a high-speed ground flow of hot gas and fragmented lava. Ash from the top of the column drifted downwind miles to the east, as far as Winnemucca, NV. On both days, melting snow fueled mudflows, flooding drainages miles away. The older Chaos Crags eruption was similar in style, but considerably larger in magnitude. Volcanic earthquakes are common at Lassen, although most are too small to be felt. Non-volcanic earthquakes along regional faults also occur—earthquake swarms in , , and included several events above magnitude 4.
Ground surveys show localized subsidence of the volcano, probably due to motion on regional faults. If you've noticed any changes to the earthquake counts released in CalVO's weekly updates, don't worry - some behind-the-scenes improvements to our monitoring system have been implemented which allow us to focus on unrest specifically related to our volcanoes and volcanic regions. Location: California, Shasta County Latitude:
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