By: Robin Koelewijn

Stud. Nr: 0518123

Nov. 1998

The Lassen volcanic centre consists of an andesitic stratovolcano, a dacite dome field and peripheral small andesitic shield volcanoes. Lassen's long and complex eruptive history is a 600,00-year-long record of volcanism associated with the generation, rise, emplacement and evolution of a plutonic magma body in the crust.

The evolution of the Lassen volcanic centre began with the construction of Brokeoff volcano, an andesitic stratovolcano. Glacial erosion, enhanced by hydrothermal alteration of permeable cone rocks, has resulted in deep erosion of Brokeoff volcano. The major erosional remnants, Brokeoff Mountain, Mount Diller, Mount Conrad and Diamond Peak enclose a central depression that marks the position of Brokeoff volcano which was approximately 3,350 meters high, had a basal diameter of approximately 12 kilometers, and a volume of about 80 cubic kilometers. Potassium-argon ages of lavas from Brokeoff volcano range from 0.59-0.39 million years ago. Thus Brokeoff was active for approximately 200,000 years.

Coincident with the extinction of Brokeoff volcano was a major change in the character of volcanism at the Lassen volcanic center. Activity shifted to the north flank of Brokeoff volcano and became more silicic and episodic. Three sequences of silicic lavas and a group of hybrid lavas were erupted in the last 400,000 years

The first expression of the silicic magma system was eruption of small rhyodacite lava flows and a rhyolite dome at 400,000 years ago. They were quickly followed by eruption of more than 50 cubic kilometers of rhyolitic magma as air-fall tephra and ash flows. This eruption is thought to have produced a small caldera on the northern flank of Brokeoff volcano that is now filled by two subsequent sequences of lava comprising the dacitic dome field. The lavas of the early sequence are best preserved on Raker Peak and Mount Conard. The ash flows are preserved only to the west in the Manton area; the air-fall tephra is widely recognised in nearby states.

The second and third sequences of lavas at the Lassen volcanic center comprise the dacite dome field. The second sequence erupted from 250,000-200,000 years ago and produced a cluster of 12 lava domes and associated thick flows with a total volume of approximately 15-25 cubic kilometers. The vents for these lavas were concentrated along the inferred edge of the caldera on the north flank of Brokeoff volcano. Bumpass Mountain, Mount Helen, Ski Heil Peak and reading Peak are the most prominent domes.

The third sequence consists of porphyritic hornblende-biotite rhyodacite erupted lava domes, short thick lava flows and pyroclastic flows erupted in at least 12 episodes during the past 100,00 years. Rocks of this sequence form the northern and western portions of the dacite dome field and their vents are concentrated in linear chains near the inferred western edge of the old caldera. The most prominent features of this sequence are Eagle Peak (55,000 years ago), Sunflower Flat domes (35,000 years ago), Lassen Peak (approximately 25,000 years ago) and Chaos Crags (1,050 years ago).

A group of hybrid andesite lavas are associated with the silicic lavas. They form lava flow complexes with agglutinate cones marking their vents. A volume of approximately 10 cubic kilometers of hybrid andesite erupted around the margins of the dacite dome field in ten episodes over the past 300,000 years. Hat Mountain, Raker Peak and Cinder Cone are the most prominent features. The products of the 1915 eruption of Lassen Peak belong to this sequence.

Lassen Peak is one of the youngest major Cascade volcanoes. It is situated in the north of California and its present height is 3,187 meters. Most of the mountain is a single mass of dacite rock, emplaced in a relatively short time. Lassen Peak is a volcanic dome. This type of volcanoes are mounds that form when very viscous lava is erupted slowly and piles up over the vent, rather than moving away as a lava flow. A dome grows largely by extension from within. In the past 200 years the volcano erupted only once in 1914-1917, the present thermal activity is reduced to small steaming fumaroles in summit craters and some hot grounds on the northern flank.

Long before the recent activity at Mount St. Helens, a series of spectacular eruptions from Lassen Peak between 1914 and 1917 demonstrated the explosive potential of cascade volcanoes. A small phreatic eruption occurred at a new vent near the summit of the peak on May 30, 1914 and was followed during the next 12 months by more than 150 explosions of various sizes. The activity changed character in May 1915 when a lava flow was observed in the summit crater. The lava subsequently flowed about 100 meters over the west and probably over the east crater walls. Disruption of the sticky lava on the upper east side of Lassen Peak on May 19 resulted in an avalanche of hot rocks onto a snowfield. A lahar was triggered that reached more than 18 kilometers down Lost Creek.

On May 22 an explosive eruption occurred which produced a pyroclastic flow that devastated an area as far as 6.5 kilometers northeast of the summit. It was a nearly horizontal (lateral) blast that reached only about one-fifth as far as the recent Mount St. Helens lateral blast. The eruption created lahars that travelled more than 20 kilometers down several valleys radiating from the volcano. A vertical eruption column resulting from the pyroclastic eruption rose to an altitude of more than 9 kilometers above the vent and deposited a lobe of pumiceous tephra that can be traced as far as 30 kilometers to the east-northeast. The fall of fine ash was reported as far away as Elko Nevada, more than 500 kilometers to the east of Lassen Peak. Intermittent eruptions of variable intensity continued until about the middle of 1917. Approximately 11,000 years ago Lassen Peak generated dozens of debris flows. In the Lassen Peak region eruptions of more than 20 center producing lava flows and (or) cinder cones took place during the last 10,000 years. Small volumes of mafic tephra erupted at many centers during the Holocene time. Around 80 square kilometers covered with fine ash from Cinder Cone eruption about 400 years ago (cinder cone is only 10-12 kilometers to the northwest of Lassen Peak). Some four kilometers to the northeast is chaos Crags which erupted some 1,100 years ago producing several pyroclastic flows.

Personnel of the U.S Geological Survey’s Cascades Volcano Observatory established trilateration networks at Mount Baker, Mount Tainier, Mount Hood, Crater Lake, Mount Shasta and Lassen Peak in 1980-1984. These networks are capable of detecting changes in slope distance of several centimeters or more.

Magma rising beneath a volcano forcefully displaces the surrounding rock and the resulting deformation can be measured at the ground surface. Since about 1910, attempts have been made to measure this deformation in order to help understand magmatic processes and predict eruptions at active volcanoes. The vertical component of deformation has generally been determined by levelling techniques and by the less precise measurement of vertical angles. Early attempts to measure the horizontal component of deformation utilised relatively imprecise triangulation methods. But in the mid-60’s the development of electronic distance meters (EDM) initiated studies of horizontal strain at shield volcanoes on Hawaii. Few such precise monitoring efforts had been attempted on active stratovolcanoes or composite volcanoes before 1980, when the reawakening of Mount St. Helens provided an ideal opportunity to test the utility of such measurements. The unprecedented success of horizontal strain monitoring at this location suggested that this technique could be used for surveillance of other cascade volcanoes.

The distance measurements indicate no significant deformation of any of the monitored volcanoes. Between 1982 and 1984 there was a slight aerial contraction at Lassen Peak but nothing was detected after this period.

The record of late Pleistocene and Holocene eruptive activity at the Lassen volcanic centre suggests that the most likely hazardous future events include pyroclastic eruptions that produce pyroclastic flows and tephra. Christiansen (1982) regards the Lassen volcanic centre as one of the principal candidates in the Cascade Range for future silicic, probably explosive, eruptions. Based on the eruptive history, pyroclastic flows could endanger areas within several tens of kilometres of an active vent. Lahars and floods caused by these vents could affect low-lying areas even farther from the vent, particularly if eruptions occur during periods of thick snow cover. Eruptions that produce lava flows are less dangerous, although both lava flows and domes become unstable and produce pyroclastic flows and rockfall avalanches that could affect areas as far as several kilometres away. Mixing of hot debris with snow can generate lahars that could inundate valley bottoms for tens of kilometres as in 1915. The older eruptive history of the volcanic centre suggests that considerably larger and more devastating eruptions are possible. The presence of a vigorous hydrothermal system (Muffler and others, 1982), the early-20^th-century eruption, continuing seismicity and the cluster of young domes suggested the existence of an active silicic magmatic system (Christiansen, 1982). This system lies within a large negative gravity anomaly (LaFehr, 1965), which suggest the presence of a large pluton (Heiken and Eichelberger, 1980). Future eruptions at vents within the Lassen volcanic centre could produce voluminous air-fall tephra and pyroclastic flows that could devastate broad areas. This suggestion is supported by evidence of three caldera-forming events in the Lassen region. Estimated is that about 50 km³ of rhyolite pyroclastic flows and air-fall pumice was erupted during each of these episodes. Although the consequences of such a large eruption would be severe, the annual probability of such a large event is small.

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