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MT. HOOD ERUPTIONS.
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Technical discussion of geological history of the volcano. Atmospheric affects.... More...
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Paper Abstract:
Technical discussion of geological history of the volcano. Atmospheric affects.
Paper Introduction:
Analysis: Effect of Mt. Hood Eruption on Atmosphere
Introduction
Each of three major eruptive periods at Mount Hood (12,000-
15,000), 1,500-1,800, and 200-300 years ago) produced dacite
domes, pyroclastic flows, and mudflows, but virtually no pumice.
Most of the fine lithic ash that mantles the slopes of the volcano and the adjacent mountains fell from ash clouds that accompanied the pyroclastic flows. Widely scattered pumice lapilli that are present at the ground surface on the south, east, and north sides of Mount Hood may have been erupted during the mid-1800's, when the last known activity of the volcano occurred (REPORT: Recent Eruptive History..., 1998).
The geologically recent history of Mount Hood suggests that
the most likely eruptive event in the future will be the formati
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Crater Rock is a dacite dome 3 -4 macross at its base and about 17 m high on its south side. 1982. 4. & Sigurdsson, H. & Hoyt, D. 198 . Journal of Volcanology andGeothermal Research 8:69-94. Isotopicstudies of the stratospheric aerosol layer. Analogous to intensity but meant forinstantaneous rather than sustained eruptions. The lower stratosphere waswarmed by approximately 6°C (Parker & Brownscombe 1983), but seasurface temperatures exhibited a maximum cooling in the order of .15°C which is not a significant temperature departure from thenormal (Parker 1985). Generation and dispersal of fine ash and dust byvolcanic eruptions. M.R., Stothers, R.B. Journal of GeophysicalResearch. 199 . 1981. Widely scattered pumice lapilli that are present at the groundsurface on the south, east, and north sides of Mount Hood may have beenerupted during the mid-18 's, when the last known activity of the volcanooccurred (REPORT: Recent Eruptive History..., 1998). Volcanol. The Lewis andClark expedition of 18 4-18 5 saw the Sandy River within a few years of themain Old Maid events,while the river was still clogged with eruptivedebris. Can rapid climate changecause volcanic eruptions? Geophysical Monograph 52. 199 . Analysis: Effect of Mt. 1988. (Eds).1982. Walker, G.P.L. Determined by the extent of devastation,actual or estimated, caused by an eruption. Determined by the volume of ejecta per unit timecalculated from column height and velocity. 1985. Determined from the volume of volcanic ejecta. (eds).Understanding Climate Change. 1982. & Manowitz, B. Bibliography Ardanuy, P.E., Lee Kyle, H. Global Relationshipsamong the Earth's Radiation Budget, Cloudiness, Volcanic Aerosols andSurface Temperature. Hood, thus indicating a vent ator near the summit.* Timberline Eruptive Period (1,8 -1,4 yrs ago). St. Other hazards that could result from such eruptions includelaterally directed explosive blasts that could propel rock fragmentsoutward from the sides of a dome at high speed, and toxic volcanic gases.The scarcity of pumiceous ash erupted during the last 15, years suggeststhat explosive pumice eruptions are not a major hazard at Mount Hood; thus,there seems to be little danger that such an eruption will significantlyaffect the Portland (Oregon) metropolitan area in the near future (REPORT:Recent Eruptive History..., 1998).Background Mount Hood is a Quaternary stratovolcano composed of lavaflows, domes, and volcaniclastic deposits. Hood Eruption on AtmosphereIntroduction Each of three major eruptive periods at Mount Hood (12, -15, ), 1,5 -1,8 , and 2 -3 years ago) produced dacitedomes, pyroclastic flows, and mudflows, but virtually no pumice.Most of the fine lithic ash that mantles the slopes of the volcano and theadjacent mountains fell from ash clouds that accompanied the pyroclasticflows. The use of eruption chronologies to prove that volcaniceruptions are the major independent variable in the generation of transientbut powerful "fluctuations" such as cooler temperatures, damaged tree-rings, disturbed atmospheric circulation patterns, famine and socialunrest, remains problematic. Tellus 26:222-234. 5. In Berger, A., Dickinson, R.E., & Kidson, J.W. 1998. The Taupo pumice: product of the mostpowerful known (ultraplinian) eruption. Ash clouds which accompany pyroclastic flows may depositas much as a meter of fine ash close to their source, and as much as 2 centimeters at a distance of 11 kilometers downwind from the pyroclasticflows. The meteorological Magazine 114:149-161. Electronmicroprobe analysis of volcanic tephra allowed estimates ofsulphur and halogen yields of specific eruptions to be madecorrelated observed mean surface temperature decrease with sulphur yield.They found a direct link between the sulphur output and temperatureresponse (Palais and Sigurdsson, 1989). 11:81-92. Theprincipal hazards that could accompany dome formation include pyroclasticflows and mudflows moving from the upper slopes of the volcano down thefloors of valleys. On-line. Walker, G.P.L. Fumarolic activity is vigorous nearCrater Rock and some hot-spring activity occurs lower on the volcano flanks(REPORT: Recent Eruptive History...,1998).Atmospheric Affects from Mt.Hood Eruption? 1992. 12:221-243. Parker, D.E. Hood would affect the atmosphere.Almost without question, particles and dust thrown into the atmospherewould have a short term cooloing affect. Vol 87, No C2:1231-1238. SP 458. Similar results wereobtained following the 1982eruptions of El Chichon, in Mexico. 1985. Palais, J.M. & Lacis, A.A. The mere association of an eruption and anenvironmental phenomenon thought worth recording in the past is not proofof a dependent relationship. Following the 1963 eruption of Agung, peak sulphuraerosol levels were not reached until one year after the eruption[Castleman, 1974]and following the 1983 eruption of El Chich-n maximum opticaldepth perturbation did not occur for 11 months (Ardanuy, 1992). 198 . Hood might create atmospheric affects is an unqualified yes.The degree of affect is strictly hypothetical, since everyeruption is unique, and ever volcano has its own character.Historical records suggest that there is some justificationfor the assumption that a volcanic eruption may bring aboutclimate change, but the often loose (and convenient) correlationof historical events and volcanic eruptionscan be challenged (THR DYNAMICS....). and Self, S. Parker D.E. Deepak, A. Journal of Climate 5:112 -1139. An older pre-Mount Hoodvolcano, the Sandy Glacier volcano (basaltic andesite and andesite), isexposed on the west side of Mount Hood and is partly covered by Mount Hoodrocks. Stratospheric warmingfollowing the El Chichon volcanic eruption.Nature 3 1:4 6-4 8. The Taupo pumice: product of the mostpowerful known (ultraplinian) eruption. J. Petrologic evidence ofvolatile emissions from major historic and pre-historic volcaniceruptions. The monitoring of sea-surface cooling wasobscured by a particularly strong El Nino event which resulted in warmerthan average sea temperatures (Parker 1988).Conclusion Based on almost no evidence, accept volcanic rock and ash, it isimpossible to guess if an eruption of Mt. 3. Destructive potential. Determined from column height. Eruptions whichintroduced sufficient volatiles into the stratosphere, to increase opticaldepths by .1, appeared to be climatically significant (Deepak & Newell,1982). The aerosol cloud impedes and backscatterssolar radiation which may result in surface cooling and stratosphericwarming (Sigurdsson, 199 ). REPORT: Recent Eruptive History of Mount Hood, Oregon, andPotential Hazards From Future Eruptions. The deposits of this periodinclude a dome (Crater Rock), a pyroclastic flow, and many debris flows inthe White and Sandy River valleys. & Newell, R.E. Agungeruption a stratospheric warming in the order of 6°C was observed (Hansen,1978) and a fall in northern hemisphere surface temperature ofapproximately .3°C (Rampino & Self 1982) is reported. Castleman, A.W., Munkelwitz, H.R. & Fairbridge, I.W. Violence of the eruption. Science 2 6:826-829 Rampino. The VEI depends on an estimation of fiveparameters of magnitude first suggested by Walker [198 , 1981]. Some debris flows,however, extend down the Sandy River or 8 km to its mouth at the ColumbiaRiver.* Zigzag Eruptive Period (6 -4 yr ago). Sigurdsson, H. The impact of an aerosol cloud of this naturecan be detected for up to three years following an eruption, but it isunlikely to generate cooling in excess of normal temperature fluctuationafter year one (Bradley, 1988). The bulk of the volcano is builtof andesite and is younger than 78 , yrs old. The summit area of Mount Hood is comprised of several andesite ordacite domes and has been weakened by alteration and is susceptible to apotentially catastrophic slope failure. Mt. The introduction of volcanic gases into the stratosphere will alsodisturb the global albedo. Sun and dust versus greenhouse gases:an assessment of their relative roles in global climate change. Evidence of volcanic loading of theAtmosphere and of climate response. In the stratosphere the residence time of acid volatile material isenhanced beyond that of silicate dust. The geologically recent history of Mount Hood suggests thatthe most likely eruptive event in the future will be the formation ofanother dome, probably within the present south-facing crater. 2. IUGGVolume 7.:31-53. Available:http://bigmac.civil.mtu.edu/home/classes/ce459/public/p14/clouds.html Hansen, J.E. The use of eruption chronologies to prove that volcaniceruptions are the major independent variable in the generation of transientbut powerful "fluctuations" such as cooler temperatures, damaged tree-rings, disturbed atmospheric circulation patterns, famine and socialunrest, remains problematic. Helens eruptions of198 . & Browscombe, J.L. (Atmospheric effects and potential climatic impact.) NASA. 1989. Beyond that, only an accurateaccount of an actual eruption could bring any clarification to thequestion. Acid gases are oxidised into an aerosol form,primarily from SO2 to H2SO4. Geotherm. The most recent pumice and ash eruption occurred in the 1865 andwas centered at the Crater Rock dome. Nature. THE DYNAMICS AND IMPLICATIONS OF VOLCANIC ERUPTION PRODUCTS IN THEATMOSPHERE. Res. Major volcanic mudflowdeposits of altered Mount Hood andesite are know to occur up to 4 km fromthe volcano (REPORT: Recent Eruptive History..., 1998).Four eruptiveperiods have been recognized at Mt. The past provides no answers, only clues. Intensity. That being said, the only way to hazard aguess is to inspect the recent records of similar volcanic types andconsider the aftermath of their eruptions in light of atmospheric change. Nature346:713-719. & Self. In the wake of the 1963 Mt. 1974. 1988. S. Dispersive power. Journal of Climatology 8:87-9 . Stratospheric aerosols and sea surfacetemperatures. An alternative measure of volcanic eruptions was proposed by Newhalland Self [1982] who suggested that it was the volume of dust injected intothe stratosphere which determined the climatic effectiveness of an eruptionand advanced a Volcanic Explosivity Index [VEI], as a measure of thiseffect (Newhall and Self, 1982). Bradley, R.S. Palaeo', Palaeo', Palaeo'.89:277-289. Walker, G.P.L. This periodproduced between .7 and 1.1 cu km of pyroclastic flows and debris flows.The vents for Timberline and all subsequent eruptions were high on thesouthwest flank, perhaps at the vent now filled by the Crater Rock dome.Deposits of Timberline age are mostly restricted to the southwest flank ofthe volcano, where they form a broad volcaniclastic fan. ClimaticChange. As a result the lower atmosphere should be cooledand the stratosphere warmed. Climatic impact of explosive volcaniceruptions. The volcanic explosivity index (VEI):An estimate of explosive magnitude for al volcanism. The photochemical conversion of volcanic gases to an aerosol formwithin the stratosphere has been observed to be relatively slow, andmaximum perturbation of global albedo may not occur until several monthsafter an eruption. Deposits ofthis period were formed chiefly by pyroclastic flows and debrisflows, occur on all sides of Mt. A sulphur-aerosol cloud will reduce transmittance of solarenergy to the ground and back scatter solar energy within thestratosphere. These deposits are oflimited volume and have been found only in the valleys of theSandy and Zigzag Rivers.* Old Maid Eruptive Period (25 -18 yr ago). Hood during the last 15, yrs. 313:272. Journal ofVolcanology and Geothermal Research 8:69-94. Magnitude. Climatic effects ofvolcanic eruptions. On-line.Available:http://vulcan.wr.usgs.gov/Volcanoes/Hood/Hazards/Bulletin1492Rampino, M.R., Self, S. 1983. These are: 1. Parker D.E. The short answer to the question of whether an eruption ofMt. The explosive volcanic eruption signal in NorthernHemisphere continental temperature records. Recent large eruptions failed to increase Optical Depthsufficiently to cause major climate perturbation. Sulphur gases are converted tosulphuric acid aerosols, these acid aerosols are the main inhibitor ofincoming solar radiation (Rampino and Self, 1985). Newhall, G.C. The mere association of an eruption and anenvironmental phenomenon thought worth recording in the past is not proofof a dependent relationship. Theeruptive periods have spilled pyroclastic block and ash flows and laharsinto the four river systems that drain the volcano.* Polallie Eruptive Period (15, -12, yrs ago).
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