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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. POTENTIALLY ACTIVE VOLCANIC ZONES IN CALIFORNIA Charles W. Chesterman, Senior Scientist (retired) California Division of Mines and Geology and Curator of Mineralogy, California Academy of Sciences San Francisco, California ABSTRACT zones and 18 other volcanic zones whose latest eruptions are known (or presumed) to Volcanic activity has played a dominant be Quaternary in age, but had not erupted in role in California's long geologic history. the last 2,000,000 years, constitute the Records in rocks of many ages demonstrate known potentially active volcanic zones in that volcanic activity commenced in the California. Precambrian as early as 1.7 billion years ago, and has continued almost continuously into the present century. A review of the INTRODUCTION Geologic Map of California is sufficient to indicate the widespread distribution of Volcanism has been very active and volcanic rocks and source areas, except in prominent almost continuously throughout the Great Valley, where the cover of soils California's geologic history, commencing as and sedimentary rocks may mask other early as 1.7 billion years ago in the Pre- possible volcanic centers. cambrian and extending into the twentieth century. It has manifested itself through Advances in geophysics and geochemistry the extrusion of tremendous volumes of have demonstrated the close relationships volcanic rocks as lava flows and fragmental between zones of volcanic activity and deposits, and the intrusion of dikes, domes, movements of tectonic blocks. Zones of plugs, and sills. volcanic activity in the Cascade Range and the Coast Range provinces in California can Time and the erosive forces of running be explained through a subduction mech- water, moving ice and air, and of gravity anism, whereas similar zones in the Mono have obliterated much of the evidence of Basin-Long Valley region of Mono County, volcanic activity, but thick deposits of the Modoc Plateau Province, and elsewhere metamorphosed volcanic rocks, of which in the Mojave Desert and Colorado Desert some are greenstones and others, schists, provinces owe their origins to the combined form prominent geologic units ranging in effects of plate tectonics and block faulting age from early Paleozoic through late of thin continental crust. Mesozoic (an interval of some 500 million years) in the Sierra Nevada, the Klamath Five active volcanic zones in California and Siskiyou mountains, the Coastal Ranges include Mount Shasta, Lassen Peak, and of California, and the Mojave Desert. Medicine Lake Highland, all in the Cascade Range Province, the Mono Basin-Long In order to illustrate in a modest way the Valley region of Mono County, and the Cima extent of volcanic activity in California, volcanic field in San Bernardino County. A one need only examine carefully the significant number of documented eruptions northeastern part of the state, which has occurred in these areas in the last 2,000 includes all of Modoc County and significant years. The above named active volcanic parts of Lassen, Plumas, and Siskiyou 9 counties. This is an area of approximately lacking for certain areas of the state; and 12,000 square miles, one-thirteenth of the (3) techniques of geochronology, although area of California, which is underlain by gaining stature and becoming more reliable volcanic materials ranging in age from all the time, are still not sufficiently Miocene to Holocene. Much of these advanced as to allow clear and reliable volcanic rock materials, totaling approxi- dating of all rocks. Because of these mately 2,400 cubic miles, were deposited factors, deposits of quaternary volcanic through explosive volcanic eruptions. rocks may be omitted or incorrectly dated on the map, and in some cases rocks of Very rarely indeed is volcanic activity an other ages may be labeled Quaternary. isolated earth process unrelated to other forces at work in the Earth's crust. On the A significant amount of data, however, is contrary, it can be demonstrated that vol- readily available, and many deposits of canic activity is and has throughout the volcanic rocks in California can be Earth's long geologic history been closely identified as being of Pleistocene and associated with earthquakes, mountain Holocene ages. Holocene is here defined to building processes, and lastly and more include events since the latest glacial recently, through the interaction of gigantic (Tioga) stage. This is a variable time span plates that comprise segments of the Earth's representing, perhaps, 6,000 to 10,000 crust. years. The traditional 1,000,000 year date for the beginning of the Pleistocene has Volcanic activity in the Cascade Range been pushed back to 2,000,000 years, and in and Coast Range provinces in California is some cases further still. For rock units of best explained through the interaction approximately this age, I am using here, for between the Pacif ic plate and the North convenience, a Plio-Pleistocene time American plate, in which portions of the interval. This is a convenient slot for many former being subducted beneath the North volcanic rock units in California which are American plate are remelted and rise along not yet precisely dated. zones of weakness to the Earth's surface to result in volcanic activity. Volcanic Following Kilbourne and Anderson ( 1981), activity in the Basin and Range Province in zones of volcanic activity in California can California, which includes the Modoc be considered in three categories: (1) active Plateau-Warner Range area in Modoc -- one in which there is current volcanic County, the Mono Basin-Long Valley region activity, or which has erupted in the last of Mono County, and the Mojave Desert and 2,000 years; (2) potentially active -- one Colorado desert regions in southeastern known (or presumed ) to be Quaternary in California, is probably due to a combination age, but the most recent eruption has not of the interaction of Earth plates and the been dated as being less than 2,000 years block faulting of thin continental crust on ago; and (3) inactive -- one in which there the margin of an area undergoing extension. has been no eruption in the last 2 million years. Since our interest for this workshop lies in ZONES OF VOLCANIC ACTIVITY IN the active and potentially active volcanic CALIFORNIA zones in California, discussion will be confined to 23 areas in the state in which Another look at the Geologic Map of rocks of Pleistocene and especially California tells one that the state has been Holocene ages occur. Frequent reference mapped geologically one way or another, but will be made to Figure 1, a map of it is not possible to identify all deposits of California which shows the areas of known quaternary volcanic rocks since: (1) geo- deposits of Quaternary volcanic rocks, and logic process do not stop conveniently at also several major tectonic features, temporal or spatial boundaries; (2) geologi- particularly faults, as they may relate to cal mapping is incomplete or even totally zones of Quaternary volcanic activity. 10 MAP OF CALIFORNIA SHOWING AREAS UNDERLAIN BY QUATERNARY VOLCANIC ROCKS-THE POTENTIALLY ACTIVE VOLCANIC ZONES c-11',4149£•IT'*# ·•:·· Volcanic areas-zones : ,¢'1 3129 YA••f•.1, 1. Mount Shasta r0 \ ./ 1• 0 \ 46. '..- .- 'gtill#/95;WA:WEHEawbilflitili 2. Medicine Lake Highlands 2 B. 1.- LAEr. 3. Modoc Plateau 1 ( ', '1,4-.6 > 4..,$- :1 - /'isffl •:1 5-=.4 4. Mount Lassen a Vicinity 4» f-//.=•--,« 5. Eagle Lake 6. Lousetown - •r . /0 'ft'j 7. Clear Lake , ,. '• 8. Mono Basin-Long Valley \ 036" 9. Aberdeen « 1-» :7 -,1 -1., », t ./ '29 10. Coso Mountains A- '\' Fi 11. Cima I b 'fl f,•.» 12. Amboy-Pisgoh 'I042'r•t•13. Turtle Mountains V , L ,<' -, r . <·....r 4.... I . - I.\' - = .,-c:*• 14*. 14. Pinto Basin-Salton Creek = 3 « - '1&«'I .«44•/•-B., 15. Obsidian Buttes 1 2 / \ '. 12 ··\. / \'.% ..is: '-Al'<"•1£,Ex, 16. Miscellaneous ,'. ,» . - -. 2<116)87 \.: \ ./ --I 33*11 &*.>ey \. : + I -, 1---- -t ···. .r·-F ,. - Faults tAL•, . \. • T U 9.6,4..,•2•••• ---A,- .4 1 l Aii'Oi:•8 i - merN - \ 7. 0 1' --- -.---.--. -163••••4•----•%6 ,--tt·•...• E AN LUIS OBISPO e 19 . Ifi'•4:*©'eaurib ' .4%8 *- -':L.f -# 4#J./.-':FLW.b:. '.. a3», 1. .- - 6 - f -i --1 •te) 1:1.:.:.:.:.I.P.3...''.'. ''.''.. \ ij•.E:.ES k.., .-C. 461 -3 \...: F > tf 49%%7.- -:. X•<:15-9. f.. ) Figure 1 - 11 POTENTIALLY ACTIVE VOLCANIC Holocene time ( 660 A.D. + 240 years) ZONES IN CALIFORNIA resulted in the mantling of-the area by rhyolitic pumiceous tephra -- deposition of The following discussion will consider the which was followed by the emplacement of potentially active volcanic zones in obsidian domes and the development of California, their geographic locations, types cinder cones. More than 100 cinder cones of volcanic products, and physical features, developed during the Holocene volcanic and will commence with those zones in the episode, and at least 20 cones during the northern part of the state and conclude with Pleistocene.