Pumice falling onto the northeastern slope of Lassen Peak generated a high-speed avalanche of hot ash, pumice, rock fragments, and gas, called a pyroclastic flow, that swept down the side of the volcano, devastating a 3 square miles 8 km2 area. The pyroclastic flow rapidly incorporated and melted snow in its path. The water from the melted snow transformed the flow into a highly fluid lahar that followed the path of the lahar of May 19—20, and it rushed nearly 10 miles 16 km down "Lost Creek" to the Old Station.
This new lahar released a large volume of water that flooded the lower Hat Creek Valley a second time. The eruption produced smaller mudflows on all flanks of Lassen Peak, deposited a layer of volcanic ash and pumice traceable for 25 miles 40 km to the northeast, and rained fine ash at least as far away as Winnemucca, Nevada, miles km to the east.
Together these events created the Devastated Area which is still sparsely populated by trees due to the low nutrient and high porosity of the soil. Phreatic activity For several years after the large eruption in , spring snowmelt water percolating down into Mount Lassen triggered steam explosions, an indication that magma beneath the surface of the volcano remained quite hot.
Particularly vigorous steam explosions in May blasted out the second of the two craters that are now seen near the northwest corner of the volcano's summit. The two older craters were buried. Latest satellite images. Show more. First visit to our site? Try our free app! Android iOS version. Volcanic ash. The term for all fine-grained volcanic products fragmented during explosive eruptions. Map of currently active volcanoes See which volcanoes are erupting at the moment!
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Stauffer, and James W. Hendley II. Forest Service. See a list of other volcano-related fact sheets published by the U. Download a PDF version of this fact sheet Kb. Download a free copy of Adobe Acrobat Reader. For questions about the content of this report, contact Mike Clynne.
On May 22, , an explosive eruption at Lassen Peak, California, the southernmost active volcano in the Cascade Range, devastated nearby areas and rained volcanic ash as far away as miles to the east.
Helens, Washington. Recent work by scientists with the U. The May 22, , explosive eruption of Lassen Peak, California, blasted rock fragments and pumice high into the air and rained fine volcanic ash as far away as Winnemucca, Nevada, miles to the east. In this photograph taken from near the town of Red Bluff, 40 miles west of the volcano, the huge column of volcanic ash and gas produced by the eruption rises to a height of more than 30, feet.
Photograph taken by R. Stinson; courtesy of the National Park Service. The series of explosive eruptions at Lassen Peak, California, were some of the first volcanic eruptions to be extensively photographed, largely by Benjamin Franklin Loomis, a local businessman and amateur photographer. As shown in the left photograph, the northeastern flank of Lassen Peak was covered by mature conifer forest before The photograph on the right, taken by Loomis in June from nearly the same location shows the devastation caused by the two most powerful eruptions in the series, those of May 19 and 22 of that year.
Today, this rock can be seen on the Devastated Area Interpretive Trail. Photographs courtesy of the National Park Service. The May eruptions of Lassen Peak destroyed a 3-square-mile area, now simply called the Devastated Area, on the flank of the volcano. Although compositions of some pyroxene microphenocrysts in the dark andesite resemble those in the andesitic inclusions, these grains are sparse. They are both significantly smaller than and lack the intergrowths observed in the latter.
The dark andesite solidified from magma that was of approximately the same composition as that which formed the andesitic inclusions. It was undercooled to the point of nucleation of pyroxene, but not plagioclase, and was quenched upon eruption.
The dark andesite also contains unreacted phenocrysts derived from either the black dacite magma or light dacite magma, reacted phenocrysts with overgrowth rims, and microphenocrysts derived from andesitic inclusions.
Hence, although dark andesite lacks large andesitic inclusions, it shows evidence of contamination by both andesitic-inclusion material and crystals derived from the silicic end member. Contamination could occur by: 1 syneruptive mixing between end-member mafic magma and light dacite magma; 2 syneruptive mixing of an already hybrid dark andesite magma with light dacite magma; or 3 preeruptive homogeneous mixing between end-member or slightly hybridized mafic magma and black dacite magma.
The small range of composition of the most mafic dark andesite is difficult to explain by process 1 or 2. Process 3 , in which the dark andesite magma originates by homogeneous mixing between the mafic end-member and black dacite magma, avoids the need for a constant minimum amount of syneruptive mixing with light dacite magma. Production of a homogeneous hybrid magma is a logical consequence of downward propagation of the layer of mafic foam.
The downward propagation of mafic foam, followed by formation and disaggregation of andesitic inclusions, has several important consequences for the overlying silicic magma. Heat released by the crystallizing mafic magma would diffuse into the overlying silicic magma and be carried to higher levels by convection.
The andesitic inclusions could be transported, either by convection or their own buoyancy, to higher levels in the silicic magma, where their disaggregation would add material to the latter.
The result is a decrease in the compositional and thermal difference between the two magmas, which are the two principal factors that control formation of undercooled inclusions Bacon, Continued formation and disaggregation of inclusions into the overlying magma reduces the ability of the overlying magma to cool the mafic magma; eventually, formation of mafic foam ceased. In the magma chamber, some or all of the remaining mafic magma mixed with black dacite magma to form the magma erupted as the dark andesite.
The formation of andesitic inclusions and dark andesite magma are two extremes of the same mixing process. Early in the mixing history, the temperature and composition contrast between the mafic and silicic magmas was large, and mafic foam formed. Instability of this foam leads to its breakup, and the formation of andesitic inclusions.
Eventually, as thermal equilibration greatly reduced the thermal contrast, the addition of a significant portion of mafic magma to the silicic magma by disaggregation of andesitic inclusions reduced the compositional contrast so that the two magmas could mix homogeneously.
When two liquids of different viscosity are simultaneously drawn into a conduit, the resultant flow is unstable and the two liquids mix. As magma continues to ascend in the conduit, the bands become thin and the two magmas mix until homogeneity is achieved. The nature of the banding displayed by erupted magma is primarily a function of viscosity contrast, flow velocity, and the length of the conduit.
Even though all four of the rock types are part of the same mixing event between basaltic andesite and dacite, four distinct types of mixing are required to explain them. Basaltic andesite magma intruded the base of the reservoir of dacite magma as a turbulent fountain Fig.
Mixing in the fountain produced the hybrid andesitic magma, which fell back and accumulated at the base of the chamber. Phenocrysts from the host dacite that were mixed into the mafic magma were reacted. Heat loss to the overlying host dacite caused rapid crystallization and vesiculation of the hybrid magma and produced a layer of mafic foam. Instability and breakup of the foam layer formed the andesitic inclusions Fig. Foam was added to the base of the layer by cooling and crystallization, and removed from the top by flotation of andesitic inclusions, so that a more or less constant thickness was maintained.
The andesitic inclusions were stirred into the host dacite, both by their own buoyancy and by convection in the dacite magma that was induced by addition of heat from the cooling mafic magma and crystallization of the foam layer Fig.
Disaggregation of andesitic inclusions into the host dacite magma hybridized the dacite in the main part of the chamber, and produced the black dacite Fig.
Continued transfer of material and heat from the mafic magma to the silicic magma by foaming and convection caused the temperature of the black dacite magma to rise and its viscosity to decrease. Eventually, the black dacite magma could no longer cool the mafic magma to the point of vesiculation Fig. Mixing then occurred directly across the interface of the two magmas, and a new layer of hybrid magma, the dark andesite magma, was formed.
This may have triggered the eruption. At the end of this step, the magma chamber consisted of three magma types—dark andesite magma, overlain by black dacite magma zoned upward to the light dacite magma. The rise in temperature and volatile pressure, and concomitant decrease in viscosity and density of the black dacite magma caused it to rise through the light dacite magma. This provoked fracturing of the wallrock, and initiated propagation of a conduit to the surface Sparks et al.
Black dacite magma began to empty from the chamber into the conduit and rose toward the surface. Heating of groundwater in the volcano and degassing of the conduit magma caused the phreatic activity from May to May From May 14 to May 19, , the magma in the conduit was squeezed out into the crater, building the lava dome at the summit of Lassen Peak Fig.
On the evening of May 19, new, less degassed black dacite reached the vent, and caused the explosion that destroyed the lava dome and initiated the avalanche. Black dacite then erupted from the vent and formed the two lava flow lobes. Enough of the black dacite magma erupted so that the magma withdrawal front intersected the interface between black dacite magma and dark andesite magma. Simultaneous tapping of the two magmas and mixing in the conduit during ascent produced the banded pumice erupted on May 22 Fig.
Magma dragged into the conduit from near the top of the chamber erupted as light dacite pumice on May 22, All four rock types in the eruption sequence of Lassen Peak show evidence of magma mixing. Homogeneous mixing of magmas produced the andesitic inclusion magma and the dark andesite magma.
Back mixing by disaggregation of the andesitic inclusions into light dacite magma formed the black dacite magma. Heterogeneous or incomplete mixing formed the banded pumice. Disaggregation of andesitic inclusions played an important role in forming the intermediate magmas that erupted in Cycling of phenocrysts through partially crystallized undercooled inclusions explains the coexistence of strongly reacted and unreacted phenocryst populations.
This process explains the generation of disequilibrium phenocryst assemblages and features in many volcanic rocks of intermediate composition. Schematic depiction of the magma chamber and formation of the rocks: a mafic foam and andesitic inclusions; b black dacite magma; c magma chamber during eruption of the black dacite, May 14—19; d magma chamber during eruption of the banded pumice and light dacite, May The size and shape of the magma chamber depicted is for the convenience of the presentation.
The actual configuration may have been much different. This paper is an outgrowth of the Lassen project, which was proposed, begun, and supported at all stages by Patrick Muffler, and I am grateful for the opportunity to have worked with him. My interpretation of the petrology depends heavily on the crystal chemistry, and I thank Lew Calk for teaching me many of the intricacies of electron microscopy and ensuring that the Menlo Park microprobe functioned well.
Joe Taggart, A. Stewart, and D. Vivit performed the major element chemical analyses. Ellen Lougee drafted the magma chamber schematic illustrations. Their comments significantly improved the exposition of the ideas presented herein.
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Sign In or Create an Account. Sign In. Advanced Search. Search Menu. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents Abstract. Geologic Context. The Eruption. Analytical Techniques. Lithologic Descriptions. Mineralogy of the Rocks. Mineralogy and Magma Mixing. Mixing Calculations. Origin of the Rock Types. Clynne Michael A. US Geological Survey. Oxford Academic. Cite Cite Michael A. Select Format Select format.
Permissions Icon Permissions. Abstract The eruption of Lassen Peak in May produced four volcanic rock types within 3 days, and in the following order: 1 hybrid black dacite lava containing 2 undercooled andesitic inclusions, 3 compositionally banded pumice with dark andesite and light dacite bands, and 4 unbanded light dacite.
Open in new tab Download slide. Table 1: Occurrence and relative abundance of mineral populations in the rock types. A, abundant; C, common; S, sparse; R, rare; —, absent. Open in new tab.
Table 2: Representative analyses of amphibole in rock types. Andesitic inclusion. Light dacite. SiO 2 Table 3: Representative analyses of plagioclase in the rock types. Dark andesite. Black dacite. Table 4: Representative analyses of olivine in rock types. Table 5: Representative analyses of chromian spinel included in olivine in the rock types. SiO 2 0. Table 6: Representative analyses of pyroxene in rock types. Table 7: Representative analyses of titanomagnetite in rock types.
MgO 1. Table 8: Major- and trace-element geochemistry of rock types. Variation of a Ba and b Sr vs SiO 2 of the rocks. Table 9: Calculated end-member compositions, and comparison with dacite of Chaos Crags. Calculated compositions. Chaos Crags. SiO 2 48 52 54 70 Correction factors for electron microanalysis of silicates, oxides, carbonates, phosphates, and sulfates. Google Scholar Crossref. Search ADS. Compositional evolution of the zoned calc-alkaline magma chamber of Mount Mazama, Crater Lake, Oregon.
Primitive magmas at five Cascades volcanic fields: melts from hot, heterogeneous sub-arc mantle. Primitive basalts and andesites from the Mt. Shasta region, N. California: products of varying melt fraction and water content. The amphibole effect: a possible mechanism for triggering explosive eruptions. Empirical correction factors for the electron microanalysis of silicates and oxides. The variable role of slab-derived fluids in the generation of a suite of primitive calc-alkaline lavas from the southernmost Cascades, California.
Trace element and isotopic constraints on magmatic evolution at Lassen volcanic center. The climactic eruptions of Lassen Peak, California, in May, abstract. Disaggregation of quenched magmatic inclusions contributes to chemical heterogeneity in silicic lavas of Lassen Peak, California. Stratigraphic, lithologic, and major element geochemical constraints on magmatic evolution at Lassen volcanic center, California.
Geologic studies of the Lassen volcanic center, Cascade Range, California. The composition of olivine and chromian spinel in primitive calc-alkaline and tholeiitic lavas from the southernmost Cascade Range, California: a reflection of relative fertility of the source. Magma mixing and the devastating eruptions of May at Lassen Peak, California abstract. Petrology of basaltic xenoliths in andesitic to dacitic host lavas from Martinique Lesser Antilles : evidence for magma mixing.
Vesiculation of mafic magma during replenishment of silicic magma reservoirs. Compositional and dynamic controls on mafic-silicic magma interactions at continental arc volcanoes: evidence from Cordon El Guadal, Tatara-San Pablo Complex, Chile. The entrainment of high-viscosity magma into low-viscosity magma in eruption conduits. A model for mixing basaltic and dacitic magmas as deduced from experimental data.
Effect of stirring on crystallization kinetics of basalt: texture and element partitioning. Evidence for two-stage mixing in magmatic inclusions and rhyolitic lava domes on Niijima Island, Japan.
Origin of mafic enclaves: constraints on the magma mixing model from fluid dynamic experiments. Journal of Geophysical Research. Magmatic inclusions in the Holocene rhyolites of Newberry Volcano, central Oregon. Jorullo volcano, Michoacan, Mexico — : the earliest stages of fractionation in calc-alkaline magmas. On the crystallinity, probability of occurrence, and rheology of lava and magma. Amphiboles and pyroxenes: characterization of other than quadrilateral components and estimates of ferric iron from microprobe data abstract.
Magma ascent rates from amphibole breakdown: an experimental study applied to the — Mount St. Helens eruptions. Thermal and mechanical constraints on mixing between mafic and silicic magmas. Analysis of geologic materials by wavelength-dispersive X-ray fluorescence spectrometry.
Dissolution kinetics of plagioclase in the melt of the system diopside—albite—anorthite, and the origin of dusty plagioclase in andesites. An experimental investigation of volatile exsolution in evolving magma chambers. The age of Lassen Peak, California, and implications for the ages of latest Pleistocene glaciations in the southern Cascade Range.
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