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This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. CHRONOLOGY OF ERUPTIONS IN CALIFORNIA * DURING THE LAST 2,000 YEARS Richard T. Kilbourne, Geologist California Division of Mines and Geology San Francisco, California ABSTRACT lichenometry, palynology, tephrochronology, amino-acid racemination, thermolumines- The length of time since the latest cence, and electron spin resonance. Utiliza- eruption, along with the periodicity and tion of these dating methods will undoubted- temporal pattern of eruptions, is a useful ly add to the list of active volcanoes. tool in assessing the risk of future volcanic events. Using occurrence of an eruption in the latest 2,000 years as the definition for INTRODUCTION active volcanoes, California has at least 27 active vents with 93 dated ash- or The primary aim of this study was to lava-producing eruptions. Ten of these compile California's age-dated volcanic vents are in the Cascades; three are in the eruptions -- that is, gather age-dates -- to Mojave Desert; and the remaining 14 are in obtain a picture of eruption frequency and the Mono Basin-Long Valley region. eruption patterns in California. The time period of interest was the latest 2,000 years. Chronological data suggest that a peak eruptive period occurred in both the Cas- Definition of Active. In looking at the cades and Mono Basin provinces near the latest 2,000 years of volcanic activity in year A.D. 900 (+100). Smaller peaks of California, we are seeing the truly "active" activity are apparent in the Cascades for volcanoes. The term "active" is more of a the years A.D. 100 (+100) and A.D. 1750 lay than a scientific term, but it behooves (+100). On a scale of centuries, the us to use it when we speak of volcanic eruption-frequency pattern in California hazards because it is one for which the seems to follow a Poisson distribution. This public already has a general understanding. pattern suggests that future volcanism This conference has shown that communi- would occur as a major period of eruption, cation is essential for scientists to properly lasting perhaps more than a century, serve the public in predicting hazards. followed by centuries of decreasing activity until the next cycle is started. More data The term "active volcano" has been are needed before any periodicity for this defined in a recent edition of the American cycle can be established. Geological Institute Glossary (1980) as: "A volcano that is erupting, also, one that is Dating methods most useful in this study not now erupting but is expected to do so. are: dendrochronology, historic observation, There is no precise distinction between an Carbon-14, rate of formation of obsidian active and a dormant volcano." This hydration rinds, stratigraphic tephrochron- definition is imprecise and, as stated, and ology, and declination magnetostratigraphy. can be confused with dormant volcano, Other methods deserving wider application which has been defined as: "A volcano that for dating youthful volcanic events include: is not now erupting but that has done so 4-76977 29 within historic time and is considered likely these questions, they are usually from the to do so in the future." chronological records of its previous eruptive history. The author feels there is a need for a revision of the definition of the term Just as confirmation of the theories of "active volcano." As used in this article, continental drift and the astronomical and as I would propose its standardization theories of glaciation has been advanced (Kilbourne and Anderson, 1981 ): An active with age-dating, so too may age dating volcano is one that is now erupting or one someday enable us t& recognize a period- that is known to have erupted within the icity to volcanic eruptions. This would be a past 2,000 years. The term "potentially major breakthrough in the science of active volcano" is proposed to replace the volcanic predictions. term "dormant" and to represent a volcano that has erupted in the past two million Dating methods found to be most useful in years but that is not known to have erupted this compilation and listed in order of in the last 2,000 years. estimated reliability are: dendrochronology, historic observation, Carbon-14, rate of These definitions have the advantage of formation of obsidian hydration rinds, strat- being precise, mutually exclusive, and not igraphic tephrochronology, and declination overburdened with loosely defined concepts magnetostratigraphy. Other methods such such as "historic time" and "considered as lichenometry, palynology, amino-acid likely to erupt in the future." racemization, thermoluminescence, and electron spin resonance have the potential Uses of Age-Dating in Volcanology. The to date young volcanic eruptions but have usefulness of chronological studies of vol- not been used to date any of the specific canic eruptions lies primarily in volcanic eruptions compiled in this study. Volcanic eruption prediction. Simply stated as a events dated only by K-Ar methods or by modified converse of Hutton's principle of their general youthful appearance were not uniformitarianism: The past is a key to the added to the list. The margin of error in future. Probably nine of ten of the world's these methods is considered too great to eruptions this century were from volcanoes accurately date events of the past 2,000 that have previously erupted within the past years. 2,000 years. It is also reasonable to esti- mate that 99 of 100 volcanic eruptions this Correction Factors. Authors using century were from volcanoes that have various dating methods commonly use erupted in the last two million years. Age- diff erent notations of age ("years ago," dating of the products of eruption identifies 1776, B.P., etc.). For this study, all dates these active and potentially active have been adjusted to calendar years A.D. volcanoes. Uncertainties, if expressed in the original date, have been transferred to the A.D. Age-dating can also be used to discrim- date. Due to lapses in communication be- inate between volcanoes that erupt often tween this author and an original source of a from those that erupt seldom. The particular date, a correction factor may identification of Mount St. Helens as the have been incorrectly applied. The original most active of the Cascade volcanoes date source has been quoted verbatim so (Crandell and Mullineaux, 1978 ) was based that any such error(s) could be adjusted in primarily on age-dating and led to their the future. forecast that Mount St. Helens was likely to erupt before the end of the century. Radiocarbon dates have consistently involved the most "correction," so the Another use of age-dating in volcanology methods used will be detailed here. Before is to distinguish patterns of eruptions. When changing to calendar years, C-14 dates were a volcano erupts, does this mean it will be first standardized to B.P. (before 1950) followed closely by further eruptions? larger notation and the Libby half-life ( 5568 eruptions? smaller eruptions? eruptions of years). Ages were then corrected for the other volcanoes? When we have answers to atmospheric variation with time of C-14 30 levels according to the curves of Seuss California is not a single volcanic pro- ( 1970, 1979) and Stuiver ( 1978). This last vince. Study of temporal patterns of vol- mentioned step adjusts the dates to A.D. canism is most productive through the study calendar years and recorrects for the of individual tectonic regions. California's presently accepted half-life of C-14 ( 5730 Cascade Range is such a region. A years). At this point, a few radiocarbon histogram of dated volcanic eruptions for dates on wood were further adjusted for the the Cascades (see Christiansen, 1982, this annual rings to the outside ring (presumed volume) is presented in Figure 2. This year of death). histogram suggests three periods of relatively high volcanic activity in the Source of Error. Not all eruptions of the Cascade in the past 2,000 years. It also past 2,000 years are datable nor, in some suggests a Poisson distribution for eruptive cases, even identified by geologic study. events with major periods of activity Lava flows and ash falls in active volcanic followed by centuries of decreasing regions must bury and obscure earlier flows activity. It may be significant that, at least and ash falls. It is a reasonable estimate in the most recent two periods of high that perhaps 40 to 50 percent of the major volcanic activity ( 800-1000 A.D. and tephra eruptions during the first millennium 1600-1800 A.D.), all three of California's have been recognized, dated, and included in major Cascade volcanoes, Mt. Shasta, the count presented in this paper. For the Lassen Peak, and Medicine Lake Highlands, second millennium, we recognize perhaps 75 erupted.