Cover 1Cover 2Title Pagebacks title pageForewardContentsList of FiguresList of TablesIntroductionTerminology and LimitationsFigure 1Volcanoes and VentsDistribution of Quaternary Cascade VolcanoesGaribaldi Volcanic BeltFigure 2Table 1Table 2Figure 3Mount BakerFigure 4Glacier PeakFigure 5Mount GaribaldiGaribaldi Lake Volcanic FieldMount CayleyFigure 6.Mount MeagerFigure 7.Garibaldi Volcanic Belt—OverviewMount Rainier to Mount Hood SegmentFigure 8.Cascade AxisGoat RocksFigure 9Mount AdamsMount AdamsFigure10Mount HoodFigure 11ForearcMount RainierMount St. HelensFigure 12Indian Heaven Volcanic FieldFigure 13Diffuse Forearc VolcanismContrasts with Garibaldi Volcanic BeltOregon Cascades SegmentFigure 14Figure 15Jefferson Reach (I)Figure 16Mount JeffersonSisters Reach (II)Figure 17Tumalo Volcanic Field (Bend Highland)Broken TopFigure 18Three SistersFigure 19Mount BachelorWickiup Reach (III)Thielsen Reach (IV)Mazama Reach (V)Figure 20Mount MazamaMcLoughlin Reach (VI)Contrasts Between Oregon and Rainier-to-Hood SegmentsShasta SegmentMount ShastaFigure 21Figure 22Lassen SegmentLassen Volcanic CenterCalifornia ComparisonsContemporaneous Volcanic Fields Behind the ArcSimcoe Mountains Volcanic FieldNewberry Volcanic FieldMedicine Lake Volcanic FieldCharacteristics of Quaternary Volcano DistributionTypes of VolcanoesFigure 23Table 3.Vent RedundancyFigure 24.Distribution Patterns of 2,050 Independent VolcanoesBreadth of the Cascade ArcVolcanic Front and Forearc VolcanoesSpacing of Major Volcanic LociTable 4.Vent DensityAlong-arc GapsTable 5.Why are Rear-Arc Volcanic Fields Few and Clustered?Figure 25.Eruption Rates and EpisodicityOutput of Distributed Mafic Vents vis-à-vis Long-Lived Evolved CentersEruption Rates at Individual LociTable 6.Postglacial Eruptions in the CascadesTable 7.Start-up Times and Major Growth EpisodesTable 8.Table 9.Outstanding QuestionsProductivity of the Cascades vis-à-vis Other Continental ArcsTable 10.Inferences Concerning Future Eruptive ActivityParental MagmasHigh-Alumina Olivine Tholeiite (HAOT)Intraplate BasaltArc BasaltHigh-Mg Basaltic AndesiteShoshonitic Arc MagmaRhyolite and DaciteSr-Rich AndesiteInferences About Mantle MagmatismIntracrustal Evolution of Arc MagmasMASH ModelShifts of FocusFigure 26.Figure 27.Basaltic AndesiteAndesite and DaciteRhyodacite and RhyoliteTable 11.Arc Plutons and Upper Crustal Magma ChambersFigure 28.Tectonic and Geophysical InsightsConcave Curvature of the MarginTranslation and Rotation of the ForearcSlab Depths and DiscontinuitiesYoung Warm Slab and Influences on MagmatismCascadian Basement and Sr-Isotope OverviewFigure 29.Vent Distribution and Basement InfluencesK2O Asymmetry Across the Cascade Arc?Figure 30.Continuing Quest for ClarityAcknowledgmentsReferencesQuaternary Magmatism in the Cascades— Geologic PerspectivesProfessional Paper 1744U.S. Department of the InteriorU.S. Geological SurveyG U S SThis page intentionally left blankQuaternary Magmatism in the Cascades— Geologic PerspectivesBy Wes HildrethProfessional Paper 1744U.S. Department of the InteriorU.S. Geological SurveyU.S. Department of the InteriorDIRK KEMPTHORNE, SecretaryU.S. Geological SurveyMark D. Myers, DirectorU.S. Geological Survey, Reston, Virginia 2007 This report and any updates to it are available online at: http://pubs.usgs.gov/pp/pp1744/ For product and ordering information: World Wide Web: http://www.usgs.gov/pubprod Telephone: 1-888-ASK-USGS For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment: World Wide Web: http://www.usgs.gov Telephone: 1-888-ASK-USGS Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted material contained within this report. Cataloging-in-publication data are on file with the Library of Congress (http://www.loc.gov/).Produced in the Western Region, Menlo Park, CaliforniaManuscript approved for publication, July 19, 2007Text edited by James W. Hendley IILayout and design by Judy Weathers Cover—Distributed arc volcanoes in the Cascades of central Oregon. At least 23 separate vents are discernible in this southwestward view from low on the southern slope of South Sister. Vent belt in this continuous reach of the north-south arc is about 40 km wide, east to west. The numerous small volcanoes here consist predominantly of basaltic andesite, but many are basaltic, a few are dacitic, and (in contrast to most of the arc) several dozen are rhyolitic. In foreground, Rock Mesa rhyolite coulee and an adjacent rhyolite dome, both about 2,200 years old, each contain 73.5 percent SiO2. Partly bare cone just beyond the rhyolite flow is early postglacial LeConte Crater, a scoria cone at the source of an extensive apron of mafic lava flows (54.5% SiO2). The Cascade arc includes more than 2,300 Quaternary volcanoes, at least 2,050 of them independent but fewer than 30 of them the andesite-dacite or mafic stratovolcanoes conventionally thought to represent the arc. (Photograph by author taken in July 2001.Frontispiece—Burney Mountain, 37 km north-northwest of Lassen Peak, is by far the largest of ~340 Quaternary lava domes recognized in the Cascades. The middle Pleistocene composite dacite dome has a volume of ~9 km3, which is more than three times as large as Lassen Peak or (Shasta) Black Butte (each ~2.5 km3) and exceeds the amount of dacite erupted from such andesite-dacite stratovolcanoes as Mount Adams or Mount Baker. Along with 110 shield volcanoes and ~1,850 monogenetic (mostly mafic) vents, individual lava domes greatly outnumber the 30-odd Quaternary stratovol-canoes and major silicic complexes in the Cascades. Relief visible in the image is ~850 m. (Photograph by L.J.P. Muffler, taken in May 1981 from near the summit of Doyle Butte, 4 km northeast of the summit of Burney Mountain.)iii ForewordThe Cascade magmatic arc is a belt of Quaternary volcanoes that extends 1,250 km from Lassen Peak in northern California to Meager Mountain in Canada, above the subduction zone where the Juan de Fuca Plate plunges beneath the North American Plate. This Professional Paper presents a synthesis of the entire volcanic arc, addressing all 2,300 known Quaternary volcanoes, not just the 30 or so visually prominent peaks that