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Quaternary extrusion rates from the , northwestern United States and British Columbia David R. Sherrod and James G. Smith U.S. Geological Survey, Menlo Park, California 94025

ABSTRACT abruptly along the length of the . The The Quatemary extrusion rate and style of eruption rate north of is about 0.21 for the Cascade Range of thePacific Northwest changes km3/km/m.y.; in southern and northern 13d 125• 120• - \ 1'.X 1 ..1-1• '' Mt. Garibaldi - 50' Explorer .0-9.RM&£14'Wl,6 . Plate BRITISH ------COLUMBIA / 11<13» 0,41 ------0 Mt Baker WASH 00-0 . C 0 1"('.. 4 '\QK-/ t. Pk e / 4# ·' -'i:-:* E;/ t. ; 94 . ..(4. 01., 4-.P 00490•,2 ... /1 1iF• Mt. Rainier 6 f Juan de Fuca 2 Plate I. 1'•St. Helens 1,J-- 4 ys i .-_-!.1--'·«'- Convergent ,, I Mt. Hood 4140 Margin e Mt JeffersoL9.1•10 - 45' 6/-"00o ;: k..04 U /'C)404. 4,6 2- Three Sisters11 0 0 ** 4394, Pacific ge Newberry . ip . Plate /) ««'2, • Ae, 0 150 km l 1 / r ('100 mW/m2OREG bC, --,-- C< --- 3 CALIF. | NEV. ,* Mt. Shasta • Medicine Lake S,1 1 /! S. Lassen Pk.1 I 1 Figure l. General setting ofthe QuaternaryCascadevolcanic arc inwesternNorthAmerica, modifiedfromGulfanti andWeaver (1988). Dots locatemajorQuaternaryvolcanoes. Arrow on the JitandeFucaplate indicates directionof present-day convergence betweentheJuande FucaandNorthAmericanplates. Dashedlinesare depthcontoursdrawn on top oftheJuan de Fucaplate underpart ofthe Coast rangesas locatedbyWeaver andBaker(1988): showndotted whereinferred. Linewithsaw teethshowslocationofsubductionzone marking the boundary betweenthe Juan de Fuca andNorthAmericanplates: sawteethOn upperplate. Line A-A'fromWeaver andMichaelson(1985) divides the arc in Washington state into two segments. Heat-flow contours of 70 and 100 mWim• from Blackwell and others (1989). Brothersfault zonefrom Walker and King (1969); its postulatedextension shown dotted through CascadeRange(fromSherrodandConrey, 1988). 94 as far south as , the rate is about newly published and unpublished mapping and radio- 1.6 km3/km/m.y. In contrast, the rate is 3 to 6 metric determinations completed since 1980. km3/km/m.y. in central Oregon and 3.0 km3/km/m.y. We report here some surprising conclusions about in northern California. For central Oregon, short- and the rates and volumes of Quaternary volcanism in the long-term Quaternary extrusion rates have not varied Cascade volcanic arc. In comparing different parts of significantly over intervals as short as 0.25 m.y. the volcanic arc from British Columbia to California, Volcanic style changes at different latitudes than we find that the extrusion rate changes abruptly along rate. Volcanic style at the ends of the arc is character- the arc's length, diminishing by an order of magnitude ized by volcanism focused near isolated intermediate to northward from Mount Rainier across a zone that central volcanoes. These central volcanoes are matches an apparent seismic boundary suggested by -30 percent of the total volcanic production along the Weaver and Michaelson ( 1985). Consequently, arc arc. volcanic fields partly ring some central vol- segments north and south of this boundary may have canoes, more so in the south than in the north, but they different resource potential for igneous-related geother- contribute less volume than central volcanoes along a mal energy. corresponding length of arc. In contrast, diffuse volcanism characterizes the middle of the arc, a 260-km- GEOLOGIC SETTING long segment in central Oregon stretching from north of The Cascade volcanic arc reaches from British to soutli of . Columbia to northern California. It parallels the Pa- Numerous overlapping mafic shields and a few central cific coastline and convergent margin between the North volcanoes have built a broad . American and Juan de Fuca plates (fig. 1). In northern Quaternary extrusion rate correlates closely with Washington and British Columbia, the arc lies 300 to regional heat flow. That part of the arc in Washington 400 km east of the margin and is built largely upon and Oregon where extrusion rate is greater than 1.6 lower and preTertiary sedimentary and crystalline km3/km/m.y. lies within the 70-mW/m2 regional heat- rocks. In northern California, Oregon,+ and southern flow contour. Different extrusion rates likely indicate Washington, the Quaternary Cascade arc lies 250 to 300 zones of differing heat input. Contrasting volcanic km east of the convergent margin and is built upon up- style may signify diffuse versus focused heat sources or per Eocene and younger volcanogenic rocks. crustal changes in permeability to ascending In Washington, the most important components of along the arc. Both extrusion rate and volcanic style Quaternary Cascade volcanism are the and may be significant in the search for exploitable southern part of the Simcoe volcanic fields and six geothermal energy. composite stratovolcanoes (, , Mount Rainier, , , and Mount INTRODUCTION St. Helens). In the Oregon part of the arc, Quaternary In igneous-related geothermal systems, the rate of volcanic rocks form a continuous outcrop belt (High upper crustal magmatism is an important component of ) that extends from Crater Lake northward to the shallow-level heat budget. Although the balance near Mount Hood (fig. 2). The northern and southern between volcanism and intrusion is difficult to assess, parts of Oregon have only a few small, discrete volcanic the extrusion rate may be a first-order approximation of centers. In California, Quaternary volcanic rocks are the overall rate of magmatism. areally extensive in a roughly triangular area whose Until the middle 1980's, reliable estimates of vol- corners are formed by , Medicine Lake ume, timing, and extrusion rate for the Cascade Range volcano, and . volcanic arc of the United States could not be made for In the following discussion, the extrusion rate is lack of sufficient isotopic ages and detailed geologic normalized to cubic kilometers per kilometer of arc maps from which to draw the necessary stratigraphic and length per million years (km3/km/m.y.). Except for structural details. Our rate calculations are based on low-density air-fall , we did not adjust volumes of volumes measured while constructing 1:250,000- and different eruptive products to dense-rock-equivalent 1:500,000-scale geologic compilation maps of the magma; all volumes are shown as 1 or 2 significant volcanic rocks of the Cascade Range in figures. Thus the calculated extrusion rates are only Washington, Oregon, and California (Smith, 1987, in approximate. However, as even these approximate rates press-a, in press-b; Sherrod, 1987; Sherrod and Smith, differ by close to an order of magnitude for different 1989). These maps are not merely reworkings of segments of the arc, we predict our conclusions will not previously published data but instead rely largely on be modified greatly as more detailed mapping,

95 geochronology, andrefined volume calculations become 124 1220 available. 'f -6 BRITISH E---27 . COLUMBIA Quaternary volcanism in northern Washing- Vancouver ton and southern British Columbia: Volume Island . Mt. Baker and extrusion rate 1...C Quaternary volcanogenic rocks are not volumetri- " cally abundant in this segment of the Cascade Range, Glacler Pk..•3 which contains Mount Baker, Glacier Peak, Mount + 4 -48• Garibaldi, and a few flows and cinder cones be- tween Glacier Peak and Mount Rainier. All known Quaternary volcanic rocks were erupted less than 1 Ma; volcanic rocks 1 to 2 Ma are unknown (table 1). This distribution suggests that volcanic production in this arc I.. '1».7 segment is episodic over intervals less than one million gv«9¢,. years. Volumes for some volcanoes are published. For 65 »taner·..,2.-F others we estimated volumes by conventional methods, either subdividing the volcano into approximate regular geometric solids or using a planimeter to measure areas yASH. ,») »0*, at successive elevations (Church, 1981). T••-•t==-St Hele•n•9A . »ro•\,,• Mt Adams The present cone of Mount Baker and underlying OREG.7• «-''' ''fir...,1 - 46 volcanoes erupted 72 km3 of mainly an- 0 4#up 0 desite flows during the last 700 ka. The summit forms L-. -- an impressive snow-covered peak over 3,200 m high, D»<-3 15 7 but the volcano is perched on a high bedrock ridge and 161 7Mt Hood thus its volume is much less than its summit elevation 3 09 rj 200 2 would suggest. A few flank flows, volcanic debris 0 +B#,3 + flows and tephra layers contribute less than 2 km3 to g • ' Mt Jefferson the total volume. The next south along 0 tl-f ...,-, the arc, Glacier Peak, also sits on a bedrock ridge. The af 2 main cone, comprising domes and short flows, 5 0 Three Irt. 2 contains only 6 km3 of (Tabor and Crowder, 1969; + Ssters A, N 417 R.L. Christiansen, U.S. Geological Survey, written Avt . YA"7\\ 1/Y , 44 commun., 1987). However, vigorous explosive activ- 311Newberryvocano ity resulted in deposition of about 20 km3 of down- i stream debris- and pyroclastic-flow deposits (Beget, eC.1 /Z-- 1982) and about 3.4 km3 of downwind tephra (reduced •, f•/ 1 -I»- to an equivalent volume of dacite magma) (Porter, Crater 1978). Lake -7-1 .b•-4 EXPLANATION A few cinder cones and short basalt flows between £ 0 012 Ma Glacier Peak and Mount Rainier account for 1.3 km3 at E] 012 073 Ma --«t , most. The in southern EJ 073 20 Ma British Columbia has a volume of approximately 26 km3 (Mathews, 1958). OREG. The Quatemary extrusion rate for the Cascade arc in 0 60 km CALIF 42 northern Washington and southern British Columbia is -0.21 km3/km/m.y. (volume = 130 km3; length from Figure 2. Generalizedgeologic map showing extent of Mount Garibaldi to line A-A' (see figure 1) of Weaver Quaternaryvolcanic deposits in the Cascade Range of and Michaelson (1985) = 310 km). Because all known Washington and Oregon, generalizedfrom Smith (in volcanic deposits in this segment are younger than 1 pressa) andSherrodandSmith(1989). Dashedcontacts Ma, the rate for the last million years is twice the Qua- indicate downstreamor distal debris-flow andpyroclas- ternary rate or -0.42 km3/km/In.y. tic-flow deposits from Glacier Peak and Mounts Rainier, St. Helens, HoodandMazama. 96 Table 1. Amount ofmaterial extruded in Washington and southern British Columbia during the Quaternary period. Only tephra corrected to an equivalent volume of magma. Dash indicates no volcanic activity. Allvolumes and to- tals rounded to two significantfigures.

Volcano or field name Volume (km3)

0 to 1 Ma l to 2 Ma

1 Mount Garibaldi, B.C. 26 2 Mount Baker 72 3 Glacier Peak 29 4 Mafic flows between Glacier Peak and Mount Rainier 1.3

total for northern Washington segment 11Q

5. Mount Adams 210 6. Small volcanoes north and south of Mount Adams 10 4.9 7. Mount Rainier 140 8. Goat Rocks volcano 10 30 9. Hornblende near Goat Rocks 15 10. Basaltic shields north of Goat Rocks 20 11. Mount St. Helens 79 12. Indian Heaven volcanic field 34 27 13. Small volcanoes in southern Washington 7 3.8 14. Mount Hood 50 15. Small volcanoes in northern Oregon 5 5

total for southern Washington- northern Oregon segment 5BQ 11

Quaternary volcanism in southern Washing- Oregon, Quaternary volcanism in southern Washington ton and northern Oregon: Volume and and northern Oregon is limited to these centers and does extrusion rate not form a continuous outcrop belt of volcanic rocks Quaternary volcanism in southern Washington and (fig. 2). northern Oregon contrasts in style and timing with vol- Mount Adams, with a volume of about 210 km3, canism in segments of the range to the north and south. is the most voluminous Quaternary stratovolcano in Mafic and intermediate Quaternary volcanic vents are Washington. Widespread tephra layers and debris-flow common. Andesitic and dacitic activity is concentrated deposits are not common in the volcano's history at five stratovolcanoes: Mount Adams, Mount Rainier, (Hildreth and others, 1983;Vallance, 1986), and 99 per- Mount St. Helens, Goat Rocks, and Mount Hood. cent of the volume is contained in the main cone. Goat Rocks is the only stratovolcano with rocks older Mount Adams is flanked on the north and south by than 1 Ma. Quaternary basaltic activity is found pri- several smaller basalt and andesite volcanoes. Their to- marily in the Tumac Mountain area GO km southeast of tal volume is about 15 km3 (Hammond and Korosec, Mount Rainier), the Indian Heaven volcanic field, 1983; and also our volume calculations). basaltic fields peripheral to Mount Adams, and in nu- Mount Rainier ( 140 km3) is the second most vo- merous, small isolated volcanoes located in the area be- luminous stratovolcano in Washington. The volume of tween Vancouver, Wash., Mount St. Helens, Indian the main cone is about 86 km3; large intracanyon lava Heaven, and Mount Hood. Many vents in these areas flows that extend beyond the main cone contribute erupted 1 to 2 Ma. However, in contrast to central about 40 km3; volcanic sedimentary deposits (

97 and Pleistocene) contribute about 14 km3 (Smith, in volume of Quaternary mafic volcanic rocks in press a). Tephra contributes less than 0.3 km3. Washington (Church and others, 1983). Hammond Volcanism in the Goat Rocks area includes the (unpublished data) estimated its volume at just over 70 largely eroded Goat Rocks stratovolcano, small-volume km3, of which 34 km3 has erupted in the last 1 Ma homblende andesite volcanoes, and basaltic shields 10- (P.E. Hammond, Portland State University, written cated between Goat Rocks and Tumac Mountain commun., 1985). (Clayton, 1983). Goat Rocks volcano was active from More than 30 small-volume volcanoes, mostly 3 to 0.7 Ma, with probably two-thirds of its activity basalt and , have been mapped between taking place in thq-Quaternary. It erupted approxi- Vancouver, Wash., Mount St. Helens, the Indian mately 40 km3 during the Quaternary (Clayton, 1983, Heaven volcanic field, and Mount Hood (Phillips, his table 2) of which we estimate 30 km3 erupted be- 1987a and b; Korosec, 1987a and b; Sherrod and Smith, tween 1 and 2 million years ago. Hornblende andesite 1989; Smith in press a). The largest volcano contains volcanoes north of Goat Rocks are all less than 2 Ma less than 3 km3 and all these volcanoes add only some and erupted about 15 km3 of lava (Clayton 1983, his 20 km3 to the total volume in southern Washington table 2). Several basaltic shields between Goat Rocks and northern Oregon. volcano and Tumac mountain are less than 1 Ma and The Quaternary extrusion rate for the Cascade arc in erupted 20 km3 of lava and (Clayton, 1983, his southern Washington and northern Oregon is about 1.6 table 1). km3/lan/m.y. (volume = -648 km3; length from line Mount St. Helens (79 km3), which began erupting A-A' of Weaver and Michaelson (1985) to Mount Hood only about 40 ka, is the youngest, most active, and = 200 km). The extrusion rate for just the last million most explosive stratovolcano in the Washington Cas- years is higher, 2.9 km3/km/m.y., because the major cade Range (Crandell and Mullineaux, 1978). The main stratovolcanoes are relatively young. The rate for 0 to 1 cone contains about 25 km3. Flank flows beyond the Ma in this segment is approximately equal to the rate in main cone total less than 0.5 km3. Extensive debris central Oregon, but the Quaternary rate is less. and pyroclastic flows, which choke the surrounding valleys, account for 6 to 8 km3 (Crandell and Quaternary volcanism in central Oregon: Mullineaux, 1973). There are no published volumes of Volume and extrusion rate tephra layers erupted before May, 1980. However, In central Oregon, the volcanic arc has built a broad Sarna-Wojcicki and others (1981) compared volume and ridge that extends for 260 km from north of Mount Jef- thickness at arbitary distances from vents for modern ferson to south of Crater Lake (fig. 2). Basaltic andesite ash-fall deposits and constructed an empirical curve for and basalt generally predominate; they were erupted estimating volume from thickness known at a given from cinder cones and shield volcanoes, some as large as distance. Their results for May 18th deposits from 15 km3. Andesite, dacite, and are concentrated Mount St. Helens agree well for volume calculated from near the composite volcanoes Mount Jefferson ( 14 the curve and from isopach maps. Using this curve, km3 ), - (36 km3), and Mount they calculated the volume of Mount St. Helens tephra Mazama ( 130 km3). layer Yn: 4.5-5 kin3. Since Yn forms 45 percent (by For the central Oregon High Cascades from Mount thickness) of tephra set Y (all thickness measurements Jefferson to Crater Lake, we calculated long- and short- made 8 to 10 km downwind and reported in Mullineaux, term extrusion rates by different methods. We calcu- 1986), then tephra set Y is 10-11 km3, which agrees lated the long-term rate by drawing numerous cross sec- well with the estimate made by Crandell and tions that represent the thickest and thinnest parts of the Mullineaux ( 1973). Applying this teChnique to the volcanic arc. Each cross section was assigned a width 1.46-m total thickness of Mount St. Helens tephra at 8- of one kilometer to obtain a representative cross sec- 10 km from source (Mullineaux, 1986), we calculate 73 tional volume. For stratigraphic and structural reasons, km3 of uncompacted tephra or 47 km3 of dense-rock- the base of the modern Cascade arc includes rocks as old equivalent dacite. as 3.5 Ma (Sherrod, 1986). The long-term extrusion Mount Hood (50 km3 ) is a Quaternary volcano rate ranges from 3 to 6 km3/km/m.y. (table 2). built chiefly of andesite (Wise, 1969). Dacite is limited We calculated short-term extrusion rates for the in- to the products of the last 15,000 yrs (Crandell, 1980). tervals 0.73 to 0.25 Ma and 0.25 to 0 Ma in the central Its cone overlaps a few basaltic andesite shields. Oregon Cascade Range by using the conventional geo- The Indian Heaven volcanic field, located between metric method of cones, frustums, and prisms because Mount Adams and Mount St. Helens, forms the largest the rocks are sufficiently well exposed, mapped and dated (Sherrod, 1986; Taylor and others, 1987). The 98 Table 2. Extrusion ratefor upper andQuaternary volcanic rocks, Cascade Range of central Oregon

Age Volume per km Extrusion rate km3/km/m.y.

3.5-0 Ma 10 to 20 km3 3 to 6

0.73-0.25 Ma 1.8 3.8

0.25-0 Ma 1.0 4.0

7500-0 yrs ago 0.01 1.3

rate is constant at about 4 km3/km/m.y. for those two few cinder cones and small-volume lava flows too small time intervals, which indicates no discernible lessening to show at the scale of figure 2. of volcanic rate (table 2). Within the error of the method, we consider the long- and short-term rates to be Quaternary volcanism in northern California: the same. Volume and extrusion rate Rates over shorter intervals are difficult to assess, Volume calculations for California are not as de- however, because single eruptive events can bias the tailed as for other parts of the arc because existing map- very short-term rate. Between latitudes 43°-44° N, a ping is not as detailed. However, the two volcanic cen- very short-term rate of 1.3 km3/km/m.y. was calculated ters of Mount Shasta and Lassen Peak dominate this for rocks younger than Mazama ash, a widespread air- section of the arc. fall tephra deposited during climatic eruptions at Crater Mount Shasta's cone has a volume of about 350 Lake (3 km south of lat. 43° N) about 6845+50 14C km), all of which erupted in the last 0.73 Ma. Its as- yrs ago (7500 calendar years ago) (Bacon, 1983). We sociated 300-ka landslide adds an additional 45 km3 put no significance in this low rate because we believe (Crandell, in press) to the total. Smaller Quaternary that the time interval is too short to be statistically volcanoes that ring Mount Shasta on 3 sides account for meaningful. Indeed, the Holocene extrusion rate exceeds another 121 km3; approximately 70 km3 erupted 1 to 2 50 km3/km/m.y. when Crater Lake-related volcanic Ma and 51 km3 erupted 0 to 1 Ma. The Lassen Peak rocks (south of latitude 43°) are combined with High volcanic center contains approximately 200 km3 of Cascades lava erupted between latitudes 43°-44° N. This lava, all erupted in the last 0.73 Ma (M.A. Clynne, conclusion echoes that of Sample and Karig ( 1982), U.S. Geological Survey, oral commun., 1988). Exten- who found from their study of the Mariana arc that sive areas of calc-alkaline Quaternary volcanic rocks, "pulses of igneous activity at individual centers... can indistinguishable from volcanic rocks elsewhere in the dominate the extrusion rates for periods of perhaps Cascade Range, crop out between Lassen Peak and Su- 100,000 years." sanville, Calif. A conservative estimate of their vol- The Quaternary extrusion rate diminishes slightly ume is about 350 km3. in northern and southern Oregon. North of Mount Jef- Using the above volumes, the Quaternary extrusion ferson, Quaternary volcanic rocks are mostly older than rate for the Cascade arc in California is about 3.0 0.73 Ma, and even these rocks pinch out south of km)/km/m.y. (volume = 1066 km3; length from lati- Mount Hood (fig. 2). Similarly the rate falls off south tude 42° to Susanville, Calif. = 180 km). of Crater Lake in southern Oregon as the style of arc volcanism changes from numerous small-volume DISCUSSION basaltic andesite shields to two large-volume strato- cones: Mount Shasta and Lassen Peak. Cones in the Along-arc variation in extrusion rate Mount McLoughlin area of southern Oregon contain The extrusion rate diminishes by a factor of 5-15 less than 40 km3 of lava and near-vent tephra (Mount north of Mount Rainier. This geographic division co- McLoughlin, 13 kn13; , 20 km3; Browns incides with a seismically defined boundary between Mountain, 5 km3). Eruptions farther south comprise a segments of the Cascade arc (Weaver and Michaelson, 1985). Weaver and Michaelson, who reported on pre-

99 sent-day seismicity patterns in northern Oregon and Washington, drew their boundary parallel to the direc- tion ofconvergence between the Juan de Fuca and North American plates. Guffanti and Weaver (1988) recog- nized the same segment boundary on the basis of vent 1200- distribution; substantially fewer vents are located north 500 of Mount Rainier. The abrupt change in extrusion rate 2 Mt. Garibaldi »,30/4,VAffee&[·British indicates that the segment boundary may have signifi- 1100- ·.···»A.U6.:, -1 \1...1.r- cance beyond the location ofearthquake hypocenters. .,: 1 11_----11 W.# This boundary also separates areas of contrasting '1 01:ZI-1 heat flow (Duffield, 1983). Heat flow ranges from 50 1000- :1:.... to 70 mW/m2 north of Mount Rainier; regional heat - Mt. Baker -15:K-•« flow is greater than about 100 mW/m2 in the Cascade 400 arc of central and southern Oregon (fig.1; Blackwell and 900-Glacier- Pk. others, 1989). 800- Along-arc variation in style of volcanism 1 Mt. Rainier Figure 3 shows extruded volume versus arc length •] Goat Rocks 700=----1 Mt. St. Helens for the Cascade Range. The contributions from the alongDistancearc 1 Mt. Adams major composite volcanoes form pronounced spikes on (km) 3 indian Heaven 1 Simcle the volume-latitude plot (e.g., Mounts Adams and 600 Rainier in Washington), whereas the volume of the - Mt. Hood numerous basalt and basaltic andesite shields in the • (Mt. Jefferson) central Oregon part of the arc is distributed over a wide 500- latitude. Thus, the volume-latitude plot shows a change 3 (Three Sisters) in volcanic style from large, localized centers in south- 400- Newberry ern Washington and northern Oregon to a broad, exten- sive accumulation of lava flows in central Oregon and back to localized centers in California. 300- (Mt. Mazama) In northern Oregon, the transition in volcanic style occurs between Mounts Jefferson and Hood, androughly coincides with the projection of the Brothers zone, 200 which may pass through the Cascade Range north of --j Mt.Shasta MedicineLake Mount Jefferson (Smith, 1986; Sherrod and Conrey, 100- 1988) (fig. 1). The Brothers fault zone (Walker, 1969) accommodates differential extension and faulting be- 7 Lassen Pk. tween the Blue Mountains province to the north and the 0I0 10 20 30 0 10 20 Basin-Range province to the south (Lawrence, 1976; km3 per km of arc length per 2 m.y. Walker and Nolf, 1981). Presumably the change in style of Quaternary volcanism in the Cascade Range re- Figure 3. Graph showing extrusion rate per km per 2 sults from relatively greater deviatoric stress that pro- m.y. (Quaternary)for Cascade arc from Lassen Pk to motes extension south of Mount Hood. In such an en- Mount Garibaldi. Vertical axis is distance along arc, measured northfrom lat 40°N (0 km) to Glacier Peak vironment, magma would be expected to rise rapidly (890 km) and northwestfrom Glacier Peak to Mount along dikes, with less chance of differentiating to inter- Garibaldi (see inset). Medicine Lake, Newberry, and mediate and silicic compositions (Hildreth, 1981). Simcoe volcanoes shown with separate vertical axis A similar transition from extensive volcanic plat- (same horizontalaxis) because they are east ofthemain form to localized large stratocones occurs south of arc. Boxesfor individual volcanoes drawnso that base Crater Lake. In this location, however, there is no evi- of box equals basal diameter of volcano along arc dence for a structural boundary east ofthe arc that sepa- length, and area of box equals Quaternary volume. rates regions of contrasting extensional deformation, as Mount Jefferson, Three Sisters-BrokenTop, andMount is the case across the Brothers fault zone farther north. Mazamalabeledparenthetically becausetheirvolumeis Guffanti and Weaver (1988) postulated a segment partofthe continuous beltofQuaternaryvolcanic rocks boundary at about the Oregon-California boundary on in the High Cascades of central Oregon; their vol- umetric contributionis shown. 100 Table 3. Comparison ofextrusion and convergence rates from volcanic arcs.

Arc Extrusion rate Convergence rate km3/km/m.y. cm/yr

Lesser Antilles 3 to 5 2 to 3.7

Cascade Range 3 to 6 2 to 3 (Jefferson to Shasta)

Marianas 12

Central America 31 8.1

the basis of a changing ratio of andesitic and basaltic (1984) suggested that convergence rate at a plate bound- vents as defined by Luedke and Smith ( 1981, 1982). ary might be broadly correlated with volcanic vigor, and This putative compositional change has no basis in the Cascade data generally support this contention. published chemical analyses, but results from the arbi- Within the Cascade arc, however, the geometry seems trary age and composition categorization of rocks by contradictory: extrusion rate is lowest where conver- Luedke and Smith. gence presumably should be highest-that part of the The Quaternary volcanoes of Medicine Lake (= 600. arc oriented normal to the convergence direction (north km3, Donnelly-Nolan, 1985) and Newberry (= 450 of Mount Rainier). One explanation is that relatively km3, MacLeod and Sherrod, 1988) are two large shield- greater crustal contraction in the Cascade arc of northern Washington and British Columbia accounts for the like volcanic landforms east of the Cascade Range. lesser amount of volcanism (Rogers, 1985; Weaver and Though not included in this calculation of extrusion rates, each of their volumes exceeds the volume erupted Michaelson, 1985). If so, the crustal stress regime of along an equivalent length of the axis of the Cascade the overriding plate becomes a first-order control on ex- Range to the west. The Simcoe volcanic field, which trusion rate (though perhaps not intrusion rate), at least lies east of the Cascade Range in Washington, is domi- in arcs with low convergence rates. nantly basaltic in composition and Pliocene in age ACKNOWLEDGMENTS (Anderson, 1987a, b); it contains about 5.2 km3 of lava This work results partly from regional mapping and estimated to be 1 to 2 Ma (Smith, in press a; our vol- compilation done in conjunction with the U.S. ume calculations). The genetic relation of these volca- Geological Survey's Geothermal Research Program. noes to the Cascade Range is unknown. We thank C. Dan Miller for his review of the In our opinion, the contrasting style of Quaternary manuscript. volcanism along the Cascade arc led previous workers to an erroneous conclusion concerning changes of volume REFERENCES CITED and rate. McBirney and White (1982) suggested that the Anderson, J.L., 1987a, Geologic map of the Klickitat total volume of Cascade Quaternary rock in Washington 15-minute quadrangle, Washington: Washington and California is small compared to Oregon. According Department of Natural Resources, Division of to our calculations, however, the southern Washington Geology and Earth Resources Open File Report and northern California volumes are only slightly 87-14, scale 1:38,400, 1 sheet, plus 13 p. smaller than the central Oregon volume, and the differ- Anderson, J.L., 1987b, Geologic map of the Goldendale ence may further diminish with more detailed calcula- 15' quadrangle, Washington: Washington De- tions. Clearly, the large stratocones of Mounts Adams partment of Natural Resources, Division of Ge- and Rainier in Washington (347 km3 combined vol- ology and Earth Resources Geological Open File Report 87-15, scale 1:38,400, 1 sheet, plus 9 p. ume) or Mount Shasta in California (395 kn13) contain Beget, J.E., 1982b, Recent volcanic activity at Glacier as much Quaternary volcanic rock as 30 to 70 km of arc Peak: Science, v. 215, no. 4538, v. 215, p. length in central Oregon. 1389-1390. Bacon, C.R., 1983, Eruptive history of Mount Rates at other arcs Mazama, Cascade Range, U.S.A.: Journal of For comparison, extrusion rates from the Cascade and Geothermal Research, v. 18, p. and other volcanic arcs are listed in table 3. Wadge 57-115.

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