Journa l o1 D)lcanology a n d G e o t h e r m a l Research, 56 ( 1 {)93 ) 253-266 253 Elsevier Science Publishers B.V., Amsterdam

Deformation of the Wineglass Welded Tuff and the timing of caldera collapse at Crater Lake, Oregon

Hiroki Kamata ~-~, Keiko Suzuki-Kamata "-2 and Charles R. Bacon ~ ~('a.wade,~ I 'olcano Ob,servatory, { ~.S. Geo/o~:ica/.%~rvey, 5400 ~lac IrltH~r Blvd., 1 }am'~,ztvcr, [111 9,%%f~ l, ~ .S" I

b ~ ".S. Geological Surv{% 345 . l l tddl~jiNd Road. ~1,S-910, ,ll~,t~/¢~ Park, ( ' t 94025. ~ S" I

(Received September 28, 1992: revised version accepted December 23. I ~*t~2 )

ABSTRACT

Four types of deformation occur in the Wineglass Welded Tuffon the northeast caldera rim olCrater Lake: la ) ~ertical tension fractures: (b) ooze-outs of fiamme: (c) squeeze-outs of fiamme: and (d) horizontal pull-aparl structures. The three Dpes of plastic detormation (b -d ) de~ eloped in the lower parl of the Wineglass '~ clded I uffwhcre degree of welding anti density are maximum. Deformation originated flom concentric normal faulting and landsliding as the caldcra col- lapsed. The degree of deformation of the Wineglass Welded Tuff increases toward the northeast part of the calder& where plastic deformation occurred more easily because ova higher emplacement temperature probabl} due to proximit> to lhc vent. The probable glass transition temperature of the Wineglass Welded Tuff suggests that its cmplacemenl temperature was > 7 5 0 C where the tuffis densely welded. Calculation of the conducti\ c cooling histor> of the Wineglass Welded Tuff and the preclimactic Cleetwood (lava) flow under assumptions of a in itiall~ isothermal sheet and uniform properties suggests that (a) caldera collapse occurred a maxmmum of 9 da>s aflcr emplacement of tile Wineglass Welded Tuff, and that (b) the period between effusion of the ( 'leet~ood (lava) flo\~ and onset of the climactic eruption ~vas - l I)0 years. 11 cooling is controlled more by precipitation during quiescent periods than h3 conduction, these intcrxals must hc shortcJ than tile calculated times.

Introduction

Plastic deformation of glass/supercooled liquid is the fundamental welding process of pyroclastic-flow deposits. After primary em- placement of a pyroclastic flow, plastic defor- mation may occur during, or subsequent to, welding as "secondary flowage" (Smith, 1960). Large-scale secondary flowage has been reported in which welded tuff flows subparal- lel to the inclination of its basement surface (Aramaki, 1964; Watanabe, 1974; Wolff and Wright, 1981 ). Tilting and folding of still-mol- ten intracaldera tuff have been shown to occur

( 'or rcspondence to H. Kamata. Present addresses: ~Osaka Office, Geological Surxcx ,.)f Japan, Government Bldg., No.2, Bekkan, 4-1-67. Ole- mac, Chuo-ku, Osaka 540, Japan: - 'Department of Earth Sciences. Kobe University, Nada, Kobe 657, Japan.

in association with caldera subsidence (Har- grove and Sheridan, 1984). Heterogeneous volume change gives rise to local secondary flowage during welding due to irregular base- ment topography beneath welded tuff (Chapi n and Lowell, 1979; Watanabe et al., 1983). Small-scale flowage producing folds and drag structures within welded tuff is widely recog- nized (Smith, 1960; Schmincke and Swanson, 1967; Ragan and Sheridan, 1972; Ono and Watanabe, 1974). Descriptions of features such as squeeze (or ooze) out o f f i amme into open cracks are fewer (Ono et al., 1977; Wa- tanabe et al., 1983: Bacon, 1983: Suzuki, 1984). Reports of brittle pull-apart structures related to compaction of the interior of a densely-welded tuff are rare (Lipman and Aramaki, 1966 ).

The Wineglass Welded Tuff (Williams,

0377-0273/93/$06.00 g) 1993 Elsevier Science Publishers B.V. MI rights reserved.

254 H. KAMATA ET AL.

1942; Bacon, 1983) at Crater Lake caldera in the Cascade Range of central Oregon exhibits four types of plastic deformation (vertical ten- sion fracture, ooze-out of fiamme, squeeze-out of fiamme, and horizontal pull-apart struc- ture) related to caldera collapse. The plastic deformation occurs in the densely-welded part of the Wineglass Welded Tuff on the caldera rim. We have estimated the minimum em- placement temperature of the tuff by assuming that dense welding would be effective only at temperatures above that of the glass transition of the Wineglass Welded Tuff ( ~ 750°C). Be- cause plastic deformation took place before the Wineglass Welded Tuff cooled to a tempera- ture of ~500°C (Riehle, 1973), we have cal- culated the maximum period between em- placement of the tuff and plastic deformation associated with caldera collapse. In addition, calculation of the cooling history of the rhyod- acitic Cleetwood (lava) flow, which was em- placed just before the plinian phase of the cli- mactic eruption and whose interior was still capable of flow when the caldera collapsed (Bacon, 1983 ), suggests the maximum elapsed time between lava effusion and onset of the cli- mactic eruption.

Geologic setting

The 8- by 10-km-diameter Crater Lake cald- era was formed in a stratovolcano, now known as Mount Mazama, during eruption of ~ 50 km 3 of compositionally zoned magma 6845___50 ~4C yr B.P. (Bacon, 1983). The cli- mactic eruption was preceded by emplace- ment of rhyodacitic lava domes, the youngest of which is the Cleetwood (lava) flow (Bacon and Druitt, 1988) (Fig. 1 ). Bacon (1983) di- vided the climactic eruption into two phases: (1) a single-vent phase, which began with a plinian pumice-fall eruption and evolved into the pyroclastic-flow eruption that deposited the Wineglass Welded Tuff; and (2) a ring-vent phase in which a compositionally zoned pyro- clastic-flow deposit, called "'the climactic ig-

nimbrite", was emplaced by flows originating from eruption columns associated with multi- ple vents as the caldera collapsed. Postcaldera eruptions on the floor of the caldera con- structed andesitic cones and lava flows and a small rhyodacitic dome; there is no resurgent dome.

The plinian-fall deposit is locally 20 m thick on the caldera rim. Several pumice-fall beds are separated by < 1 m of nonwelded pyroclastic flow and surge layers that become more nu- merous toward the top of the deposit. These layers record instability and marginal or par- tial collapse of the plinian eruption column prior to bulk collapse and emplacement of the Wineglass Welded Tuff (Bacon, 1983). The distribution of coarse pumice blocks and large lithic fragments suggests that the vent for the plinian eruption was situated slightly north- east of the center of the caldera (Bacon, 1983 ) (Fig. 1 ).

The Wineglass Welded Tuff is an orange to ocher, incipiently to densely-welded rhyodaci- tic pyroclastic-flow deposit (Williams, 1942: Bacon, 1983). Although it is generally <8 m thick, caldera-rim exposures of the Wineglass Welded Tuff commonly are densely welded from a few tens of cm above its base. The Wi- neglass Welded Tuff is thickest (maximum ~ 10 m on the caldera rim) and most densely welded in paleo-valleys, and thin or absent on topographically high areas. It is present mainly in topographic depressions from Pumice Point clockwise around the caldera rim to Skell Head and, locally, south of Pumice Castle and west of Llao Rock (Fig. 1 ). On the north and east flanks of Mount Mazama, the Wineglass Welded Tuff is generally buried by the climac- tic ignimbrite, but local outcrops are present in stream valleys and north of the area of Figure 1. Its total volume is probably several km 3 (Bacon, 1983).

The Wineglass Welded Tuff consists of as many as 4 flow units that form a single cooling unit. In many places its top has been eroded down to rather densely-welded tuff, presum-

DEFORMATION OF TUFF AN[) TIMING OF CALDERA C()I_LAPSE, CRATER LAKE. ()REG()N ~.5 5

I I . . . . . . . . . . .

122°10'W 122*00'W o Sk i

i L ~ q

-43°00'N ~ ~ Cleetwood flow ~Q i

"Ip~Llao Point ~ - / Rock fTWineglass ~ I

j Wizard (~ . . . . ) )_ ! / ~ l a n d ~ ~

EXPLA NA T/ON \ ~ _ j ~ ~ [~Wineglass Welded Tuff ~ i ~Pre-c l imact ic rhyodacite lava o [

_~ _ _ _ ! J

Fig. 1. Location map and distribution of outcrops of the Wineglass Welded Tuff and pre-climactic rhyodacile lavas, after Bacon (1983) and Nelson ct al. (1988). keucr I in a circle shows the estimated venl ['or the plinian and the Wineglass Welded Tuff eruptions, alter Bacon (1983). Exposures of the Wineglass Welded Tuff on caldera rim exaggerated in size t'or clarity. The Wineglass Welded Tuff is also exposed north of the area of this figure,

ably by subsequent ash flows. Locally, the Wi- neglass Welded Tuff grades upward into prox- imal-facies ignimbrite (lag breccia) of the ring- vent phase (Bacon, 1983).

The Wineglass Welded Tuff has several lithic-rich layers in which welding is less in- tense. Like the lag breccia in the climactic ig- nimbrite, these layers typically are clast-sup- ported. A 30- to 100-cm-thick lithic layer is continuous in the Wineglass Welded Tuff around its main outcrop arc of the caldera rim. Bacon (1983) reported that the apparent uni- formity of lithic-clast lithologies and their re- semblance to those of the plinian-fall deposit are evidence that the Wineglass Welded Tuff was erupted from the single plinian vent (Figs. I and 2). Suzuki-Kamata et al. (1992) dem- onstrated by component analysis of lithic clasts that the Wineglass Welded Tuff was effused from the single vent of the plinian eruption as the vent enlarged at a relatively shallow level within the precaldera edifice of Mount Mazama.

The ring-vent phase ignimbrite (climactic

ignimbrite) represents a magmatic volume of > 10 km 3 (Bacon, 1983 ). Near the caldera rim and on the flanks of Mount Mazama, the cli- mactic ignimbrite has a proximal facies (lag breccia) consisting mainly of accidental lithic fragments (Bacon, 1983: Druitt and Bacon, 1986; Suzuki-Kamata et al., 1992). The cli- mactic ignimbrite, emplaced on top of the Wineglass Welded Tuff, flowed down all mttjor drainages around Mount Mazama.

Deformation of the Wineglass Welded Tuff

Four types of deformation are observed in the Wineglass Welded Tuff:

( 1 ) Vertical tension, or gash, fractures visi- ble in horizontal exposures of the welded inte- rior of the tuff:

(2) ooze-out of fiamme on overhanging crack surfaces;

( 3 ) squeeze-out of fiamme on vertical open cracks: and

(4) horizontal pull-apart structures.

2 5 6 H. KAMAIA ET At..

SOl2) SO00) \

,7o(14)

PU~i.ce • :::. e~:xm Point__/> ~ ~ Round*""

P A ( I O ) . . . . . ~ T w ~ ' - ' ~ . . • " z~ t ' ~ l ~ : ; " SQ(3)--SQ(29)

H q l ~ ) . . . . . ~ ' t / +OZ P A ( 3 5 ) ~ t ~

Crater Lake Wineglas~ elO

0 1 2kin

Skell ~11

: i . . . . . . . Fig. 2. Location of outcrops showing ooze-outs offiamme (OZ), squeeze-outs offiamme (SQ), and horizontal pull-apart structure ( 1:4 ). Bar indicates strike ofvertical tension fracture. Circles and associated figures indicate outcrops and their locality numbers where density profiles of the Wineglass Welded Tuffwerc obtained (closed circle) and were not obtained (open circle). Numbers in parentheses with SQ show extruding length (mm) of fiamme in the open crack of a vertical tension fracture. Numbers in parentheses with PA show maximum vertical width (opening) of horizontal open cracks (cm). Circled I rshows the estimated vent for the plinian and the Wineglass Welded Tuff, after Bacon (1983). Index map of this figure shown in Fig. 1.

Vertical tension fractures

Smooth caldera-facing joint surfaces are present in densely-welded caldera-rim expo- sures of the Wineglass Welded Tuff (Bacon, 1983, p. 102). Such joints are mostly vertical, cut the horizontal flattening plane defined by fiamme, and are generally parallel to the cald- era walls. These joints are similar to those re- ported in the Aso-3 welded tuff, where differ- ential subsidence of thick tuff in valleys led to formation of pull-apart structures subparallel to valley walls (Watanabe et al., 1983 ).

These joints, which we interpret as "vertical tension fractures", differ from columnar or other cooling joint surfaces in the following respects:

( ! ) Vertical tension fractures in the Wine-

glass Welded Tuff are in many cases character- ized by squeezed fiamme.

(2) Strikes of vertical tension fractures are subparallel to the caldera walls (Fig. 2), whereas strikes of cooling joints appear to be random.

(3) Vertical tension fractures have smooth surfaces except at their ends, which are jagged. In contrast, cooling joints have monotonous, rough or slightly sandy surfaces probably due to fracturing in a more viscous condition (e.g., Watanabe et al., 1983 ).

(4) Vertical tension fractures undulate slightly in a vertical sense with long wave- lengths and are in some places oblique in dip to the underlying caldera wall. Cooling joints typically are planar and are perpendicular to the surface of greatest heat loss.

(5) Widths of open cracks bounded by ver-

DEFOR M-XTION OF T[ :FF A N D T I M I N G OF C-k[.DERA C()I I.a, PSI:., CRATER LAKE. ( )RE(; ( )N _! 57

tical tension fractures may be up to 10 cm, which exceed those of cooling joints. In the Aso-3 welded tuff the open width of vertical tension fractures is as much as 1.5 m (Watan- abe et al., 1983).

(6) The length of segments of vertical ten- sion fractures is as much as 5 m and exceeds that of cooling joints. In the Aso-3 welded tuff the length of vertical tension fractures is > 15 m, whereas that of cooling joints is mostly < 1- 5 m (Watanabe et al., 1983).

( 7 ) Cooling joints cut vertical tension frac- tures and in some places are bent perpendicu- lar to them.

In a few places (e.g., Loc. 3 in Fig. 2), sev- eral parallel caldera-facing vertical tension fractures are spaced a few meters apart. West of Roundtop (Loc. 6 in Fig. 2), vertical en echelon open cracks, several tens of cm long and similar to vertical tension fractures, can be seen in the upper surface of the tuff. The strike of these gash fractures is subparallel to the caldera rim and the local vertical tension frac- ture on the caldera side of the outcrop. The surfaces of these cracks are slightly wrinkled and have many fins that show drag indicating

ca-l-dera side

E u O

t! o u t e r caldera side

Fig. 3. Sketch of profile of vertical open crack with drag- ging fins west of Roundtop (koc. 6 in Fig. 2).

downward movement (Fig. 3 ) of the tuff on the caldera side of the fracture. An analogous situation may be present in the Aso-3 welded tuff, where a vertical tension fracture extends vertically for a few meters and terminates as a jagged pull-apart surface: another vertical ten- sion fracture occurs en echelon (Watanabe et al., 1983 ). The open cracks west of Roundtop may be the local end of a longer vertical ten- sion fracture, where fins and wrinkled surfaces were formed by slower disruption compared with the central, smoother part of the vertical tension fracture. All of these occurrences sug- gest that vertical tension fractures formed as the caldera walls failed along concentric nor- real faults and landslide headwalls during cald- era collapse.

Ooze-outs

At Wineglass (Loc. 9 in Fig. 2 ), f iamme were still capable of flow and oozed downward on an overhanging "vertical" tension fracture be- fore the block on the caldera side had fallen away (Bacon, 1983): oozing of f iamme oc- curred before the block fell, because ooze-out material devitrified at high temperature dur- ing degassing of the tuff. We call this structure an "ooze-out" of fiamme, because viscous melt protruded downward gravitationally as much as 3 cm from a slightly overhanging joint sur- face (Fig. 4), in contrast to vesiculation-dri- ven extrusion described later as "squeeze-out" of fiamme. Bacon ( 1983 ) reported that ooze- outs of supercooled liquid occurred after the vertical tension fracture formed, and therefore indicated that caldera was present before the Wineglass Welded Tuff had completely cooled: the block on the caldera-facing side subse- quently fell away.

The widths of ooze-outs are 1-15 cm. Alter some melt oozed out from fiamme, the spaces originally occupied by the f iamme formed cav- ities (Fig. 4). Where f iamme were > 10 cm wide, the cavities apparently are larger than can be accounted for by the amount of drained

2 5 8 H. KAMAYA ETAL.

!0 - = r 7 . . . . . . . : u - - -

". i:." "..' .ca.

~ ' c a v i t ~ . i f ia . m ".

Fig. 4. Photograph and sketches of ooze-outs of f iamme showing devitrified drained glass from flattened pumicc lens on vertical tension fracture at Wineglass (Lot. 9 in Fig. 2 ).

melt, suggesting that some melt broke loose or that open cavities were enlarged vertically. Lensoid cavities were reported in the Aso-3 welded tuff, where f iamme cut by vertical ten- sion fractures were vertically pulled apart to form cavities in which vertically elongated sec- ondary vesicles formed (Watanabe et al., 1983).

Ooze-outs are found only on the caldera-fac- ing, overhanging "vertical" tension fracture at Wineglass (Loc. 9 in Fig. 2 ). Squeeze-outs oc- cur nearby, suggesting that ooze-outs may have originated as squeeze-outs. The ooze-outs in- dicate that melt lenses were less viscous than the matrix of the tuff during cooling. This character has also been observed on a micro- scopic scale; f iamme in densely-welded tufts are commonly elongated and squeezed into the

matrix along the flattening plane (Smith, 1960; Watanabe et al., 1983 ). F iamme probably re- tain heat more efficiently than the matrix, which is also more viscous because of its par- ticulate, multiphase nature.

Squeeze-outs

Forcibly squeezed fiamme 1-20 cm wide oc- cur on vertical tension fractures (Fig. 5) at 6 locations (Fig. 2). Such features were first re- ported in the Aso-3 (densely) welded tuff and named "squeeze out of obsidian lens" (Wa- tanabe et al., 1983). Material protrudes 1 to 3 cm from the walls of tension fractures (Loc. 9 in Fig. 2 ). The top surface of a squeeze-out is flat and may have a bread-crusted cap (Fig. 5C), which is similar to the trap-door bud at the front of a lava pillow (Moore, 1975; Su- zuki, 1984). Streaks may develop on the sides of squeeze-outs in a manner similar to corru- gations on side surfaces of subaquaous pillow lobes (Moore, 1975). Spherical vesicles are present inside squeezed-out melt lenses (Fig. 5D), presumably as a result of pressure de- crease on fracturing, whereas ooze-outs do not have such vesicles. This re-vesiculation forced melt to extrude into open vertical tension frac- tures. Density profiles of the Wineglass Welded Tuff show that squeeze-outs and ooze-outs oc- cur near the zone of most intense welding in the lower part of the tuff (Fig. 6). Both the fre- quency of occurrence of squeeze-outs and the amount of extruded melt decrease above and below the most densely-welded horizon:

Horizontal pull-apart structures

Horizontal pull-apart structures 1-30 cm high and 1-3 m long commonly occur parallel to the flattening plane in the most densely- welded part of the Wineglass Welded Tuff (Fig. 7A). Many smaller horizontal cracks occur either en-echelon or parallel to the main pull- apart. The surfaces ofpull-aparts are rough and in many places have needle and crown features

D E F O R M A T I O N OF T U F F A N D T I M I N G OF ( 4 L D E R ~, COIA. kPSE, ( ' R 4 T E R LAKE. ( )RE(; ( )N 159

/ ~ / b r e a d ~ "-~7-~ / %. ~ c r u s t ~. ~ ~ ,

~'~ I;; .. "~v'es, culati0~t i

"-v \,'~ . . ~ " - J v e r t i c a l \. \~ ~ ~ '~~ t e n s i o n \_ i~' i

~ ~ ) ) f r a c t u r e ~ I . . . . . . . . . . /~____JL_ ~-_~ . . . . . ~i . _t

Fig. 5. Photographs and sketch of squeeze-outs of f i amme on vertical tension fracture cast of Palisade l 'oint ( l_oc. 5 in Fig. 2 ). (A) Blocks with the original surface of a vertical tension fracture (left) and the cut surt'acc short ing its pr, Hllc ( r ight) . Scale in cm. (B) Sketch of photograph A. (C) Bread-crusted surlace of squcc/ed l i amme on xertical tc~!~ion fracture. ( D ) Re-vesiculat ion inside the f i amme lens, shown m profilc.

10 i t 0

3 8 9

o ~ . . . . . . . I . . . . . . . . ~ . . . . ~ . . . . ~ . . . . > 7 / - -- ~ .... ]o

. i . . . . ~ . [ . . . . J . . . . . L , , ' ' 2 1 2 g / c m 3

Fig. 6. Density profiles of the Wineglass Welded Tuff with locations of squeeze-outs (.SQ), ooze-outs ( O / ) . and horizon- tal pull-aparts (I~4). Numbers correspond to locations in Fig. 2, Vertical scale sho~s height ( m ) abo~c the base of the Wineglass Welded Tuff. Horizontal scale shows densi ty ( g / c m 3 ) measured in the laboratory. Dashed lines sho~ flou -unit boundaries .

760 H. KAMATA ET AL.

Fig. 7. (A) Open crack formed by horizontal pull-apart structure. Photograph west of Wineglass (Lot. 8 in Fig. 2). Hammer is 35 cm long. (B) Photograph of a typical pull-apart texture showing re-vesiculation and needle and crown shapes at same locality as (.A ).

(Fig. 7B). Vertically elongated secondary ves- icles occur along pull-aparts, as in the Aso- welded tuff (Watanabe et al., 1983 ) . In some places close to pull-aparts in the Wineglass Welded Tuff, voids are present on specific sides of lithic fragments, similar to those found by Suzuki ( 1984) in the Ata welded tuff, suggest- ing that the lithic fragments were rotated by differential movement associated with forma- tion of pull-aparts. All of these features indi- cate that the pull-aparts were formed by verti- cal extension associated with caldera collapse.

Order offormation of structures

At Wineglass (Lot. 9 in Fig. 2), horizontal

l-aparts are cut by vertical tension frac- ‘tures, which are in turn cut by cooling joints. Small columnar joints ( <a few cm wide) commonly develop perpendicular to vertical tension fractures. Therefore, horizontal pull- aparts formed first when the matrix of the tuff was plastic enough to make rough tear-off sur- faces. The vertical tension fractures formed second when the welded matrix had cooled enough to fracture under a high strain rate when the caldera collapsed (Bacon, 1983 ), al- though fiamme were still fluid. Columnar cooling joints formed last, when both matrix and fiamme were hardened.

Lateral variation of deformation

The occurrence of squeeze-outs, ooze-outs, and pull-aparts varies along the caldera rim, s ing that the degree of deformation of the Wineglass Welded Tuff is greatest on the northeast part of the caldera rim even though the tuff is thicker elsewhere. Evidences includes:

(1) The extent of extruded material in squeeze-outs increases toward Wineglass (Lot.

9 in Fig. 2 ) . (2) Ooze-outs are observed only at

Wineglass. (3) The width of pull-aparts is maximum

( - 35 cm) west of Wineglass (Lot. 8 in Fig. 2). Near Wineglass, plastic deformation is consid- ered to have occurred more easily owing to higher temperature and hence lower rigidity of the tuff. The tuff on the northeast rim of the caldera may have been emplaced at the highest temperature owing to its proximity to the vent.

Emplacement temperature

Some densely-welded tuffs occur in units of only a few meters thickness and are thought to have been emplaced at temperatures high enough to induce complete welding without load being an important factor (Ross and Smith, 1961, p.26). Smith ( 1960, p.825) re-

D E F O R M A T I O N O F T I J F F , \ N D T I M I N G O F ( ' ~ k L I ) E R ~ , ( ' ( ) L I M ' S E , ( R A T E R I - ~ , K f I)RE(;IIN -~(~1

ported from experimental results that for ash flows of the order of ~ 30 m thickness, temper- atures above 735 °C probably are necessary for the formation of a densely-welded facies. Boyd (1961) suggested that a minimum welding temperature for degassed rhyolitic magma is ~700°C. Druitt and Bacon (1989) reported that the pre-emptive temperature of rhyoda- cite magma of the Wineglass Welded Tuff was about 880°C.

On continuous cooling and in the absence of crystallization, the structure of a liquid be- comes frozen in at the glass transition temper- ature, which is itself a function of thermal his- tory' (Bacon, 1977; Richet and Bottinga, 1986). The glass transition commonly is as- sumed to occur near the temperature where the viscosity is 10 ~3 poise. Assuming that dense welding without significant loading is unlikely at temperatures below that of the glass transi- tion, we can estimate the minimum emplace- ment temperature of the Wineglass Welded Tuff as the appropriate glass transition tem- perature. The glass transition temperature de- pends on water concentration in the glass (Taniguchi, 1981). Measurements of H,O content of glass in four vitrophyre samples of the Cleetwood flow, which has the same anhy- drous composition and pre-emptive tempera- ture as the Wineglass Welded Tuff ( Bacon and Druitt, 1988; Druitt and Bacon, 1989), aver- age 0.10 wt.% (Bacon et al., 1992). We assume that the H20 content of the degassed melt of the Wineglass Welded Tuff is 0.1 wt.%, which is close to the 1 atmosphere solubility at mag- matic temperature (Friedman et al., 1963, p. 6534). This leads to a glass transition temper- ature of ~ 750°C (Taniguchi, 1981 ).

Cooling history of the Wineglass Welded Tuff and the Cieetwood flow

The cooling history of the Wineglass Welded Tuff and that of the preclimactic Cleetwood (lava) flow provide constraints on the timing of caldera collapse and maximum duration of the climactic eruption. Three pieces of evi-

dence have been reported previously: ( 1 ) The vertical tension fractures, ooze- and

squeeze-outs, and horizontal pull-apart struc- tures in the Wineglass Welded Tuff indicate that thick densely-welded tuff ( > 10 m) was still plastic at the time of caldera collapse 1 Ba- con, 1983).

(2) Bacon ( 1983, p. 101 ) noted that partly- welded Wineglass Welded Tuff'of ~< 2 m thick- ness, locally present on the Cleetwood flow, had cooled to the point of brittle failure when rifts formed on the Cleetwood flow as the lava slid north, and also down the caldera wall at Cleet- wood Cove, during caldera collapse. Faulting of the Wineglass Welded Tuff occurred before the end of the climactic eruption, because the climactic ignimbrite ( lithic breccia ) is banked against the Wineglass Welded Tuff in rift ,aalls. Post-depositional slumping cannot explain the mapped extent of breccia in the broadest rift, where the breccia has bedforms and is banked against rifted Wineglass Welded Tuff.

(3) Bacon (1983, p. 100) reported thai the entire climactic eruption and collapse of the caldera took place before complete cooling of the Cleetwood flow. At Cleetwood Cove where the Cleetwood flow is thickest ( - 150 m thick ), the interior of the flow was sufficiently hot when the caldera formed that lava oozed down the caldera wall. Also, the plinian deposit is oxidized and sintered only on the surface o | the Cleetwood flow, and all deposits of the climac- tic eruption are cut by fumarolie alteration ap- patently related to degassing of the lava.

Calculation q/coolin~ period

The transition from plastic to brittle defor- mation in a felsic lava flow or in a densely- welded tuff depends on its temperature, strain rate, chemical composition, and hydrostatic pressure. Most welded tufts become rigid at a temperature between about 500 and 550°C (Riehle, 1973). Ash-flow tufts that are em- placed at higher temperatures develop a rigid crust at their tops and bases as they cool. This

2 6 2 H. KAMATA ET AL,

crust increases in thickness with time until the entire cooling unit becomes rigid and defor- mation ceases. If we assume that plastic defor- mation of a sheet depends on the relative thickness of the ductile region and the rigid zones, cooling and welding models can be de- vised that will set limits on the length of time allowable for this deformation.

Hargrove and Sheridan (1984) estimated the duration of welding deformation by as- suming the thermal flux at the top and base of a tuff as that of semi-infinite sheets with rigid crusts developing inward from the tuff inter- faces. Following Hargrove and Sheridan (1984), we have assumed that the Wineglass Welded Tuff became rigid as the temperature dropped below about 500 ° C. Various thermal profiles develop with time as the cooling front moves into the interior of a sheet. We treat a conductive cooling process of an ideal semi-in- finite sheet of homogeneous rhyodacite cooled by both overlying air and underlying base- ment. Our calculations use an algorithm based on a thermal conduction equation for a semi- infinite sheet (Carslaw and Jaeger, 1959, p.62, eq. 14 ) and modified to include a temperature factor for two cooling interfaces--ambient air and basement:

T=O.5( T o - Ts) [ 2erf{x/2( at ) ~/2}

-erf{ ( x - d ) /2 ( at ) L/2}

-ef t{ (x +d) /2( at)~/2} ] + T,

where: T= temperature ( ° C ) at depth x (m) To = emplacement temperature of deposit Ts = temperature of ambient air and basement (20°C) x = depth from tuff surface (m) a = thermal diffusivity (m2/yr) t = time from onset of cooling (yr) d = thickness of a sheet (m)

Results are predominantly affected by em- placement temperature. We assume that the highest and the lowest possible emplacement temperatures for the densely-welded part of the

Wineglass Welded Tuff are 880 °C and 750 ° C, respectively.

The same calculation also applies to cooling of the Cleetwood flow. Because the Cleetwood flow is thick, it must have been emplaced at something close to its preeruptive temperature of ~ 880°C (Druitt and Bacon, 1989).

We used a thermal diffusivity for the Wine- glass Welded Tuff of 14.8 m2/yr as calculated for rhyolitic ash by Bailey (1974) and Riehle ( 1973, p. 2198 ). Thermal diffusivity tbr the Cleetwood flow (a =K/pCp) is calculated as 22 m2/yr by using the mean value of thermal conductivities (K) of rhyolite for the temper- ature range of 500-880°C as 4.6× 10-3cal/ cm s °C (Murase and McBirney, 1973, com- position "NRO") , density (p) for rhyolitic glass as 2.2 g /cm 3 (Murase and McBirney, 1973, composition "NRO") , and mean spe- cific heat (Cp) of rhyolitic glass and super- cooled liquid as 0.299 cal/g°C (Bacon, 1977, composition 5 ). For thickness (d), we used 10 m and 2 m for the Wineglass Welded Tuff and 150 m for the Cleetwood flow. Results of the calculations are presented in Figure 8.

Caldera collapse occurred after the 2-m-thick partly-welded Wineglass Welded Tuff on the Cleetwood flow cooled to the point o f brittle failure. The vertical tension fracture at Wine- glass formed in response to caldera collapse be- fore fiamme, which formed ooze-outs, became rigid in the 10-m-thick densely-welded Wine- glass Welded Tuff.

The conductive cooling model suggests that the time interval between emplacement of the Wineglass Welded Tuff and caldera collapse was 9 days on the basis of cooling of the 2-m- thick partly-welded tuff on the Cleetwood flow from 750 to 500°C (Fig. 8A). This result, however, gives a maximum time, because ini- tial temperature probably was not uniform, nor perhaps as high as assumed, and because stud- ies of Hawaiian lava lakes and a dacite dome at Mount St. Helens indicate significantly more rapid cooling there owing to percolation of rain water down cooling joints (Peck et al., 1977,

D E F O R M A T I O N O F T U F F A N D T I M I N G OF C A L D E R A COLL. \PSE , C R A T E R L A K L ( ) R E G O N 263

Wineglass Welded Tuff Wineglass Welded Tuff Cleetwood ( lava) f low 2 m thick 10 m thick 150 m thick

A T°=75°°c B To =88°°c C T°=880°0 o o o 0 o o o o 0 o o o o O

0 0 0 0 0 0 0 0 0 0 0 0 o

bo,,om,I -3° F oo - \ 9oY 9o I / / LL2T - LU-bt '°°Y

,o.o. [,.o 12",rf I ~ 12oY

' 1 ' t / X / / I / ' / / I 1/

01 =o+~ V) , IV I / / o, IrA/ "°rig~~ o' o o , s ~ a o o ~ m m

Fig. 8. Temperature-with-time profiles of the Wineglass Welded Tuff and the Cleetwood llow, based on a conductive cooling model. Vertical axis shows thickness in meters. To is the assumed emplacement temperature. I)= days: l ! = weeks: ,U=months: Y=years. (A) Wineglass Welded Tuff. 2 m thick. (B) Wineglass Welded Tuff, 10 m thick. (( ' ) ('leetwood (lava) flow, 150 m thick.

Dzurisin et al., 1990). Following Dzurisin et al. (1990, pp. 2777-78) and using physical properties given above, 1 m of rain water would cool a 3.3-m thickness of partly-welded tuff (p= 1.0 g /cm 3, Fig. 6) from 750 to 100°C. Thus, << 1 m of rainfall, which could have been associated with the climactic eruption itself, may have cooled the partly-welded tuff on the Cleetwood flow sufficiently to satisfy the con- straint imposed by field observations. Note also, that at Pumice Point and west of Llao Rock the Wineglass Welded Tuff grades up- ward into the proximal climactic ignimbrite (Bacon, 1983; Druitt and Bacon, 1986), indi- cating no break in deposition. These occur- rences of lithic breccia (proximal ignimbrite) argue for onset of caldera collapse during the transition from the Wineglass Welded Tuff to the climactic ignimbrite emplacement.

Results of modeling conductive cooling sug- gest that the ooze-outs at Wineglass formed in 10-m-thick densely-welded tuff cooling from 880 to 500°C within 11 months of deposition (Fig. 8B). The result for ooze-outs is consis- tent with other information, because exactly

when after caldera collapse the vertical tension fracture formed at the Wineglass locality is un- known. Moreover, a vertical tension fracture propagating rapidly might allow brittle behav- ior in fiamme (and tuff matrix) that would flow under a lower strain rate. If this were the case, the 500°C requirement underestimates the temperature of fracturing and the cooling time would be accordingly decreased. The ef- fect of rain on cooling of thick densely-welded tuff is somewhat less dramatic than the case of partly-welded tuff: 1 m of rain water cools 1.7 m of densely-welded tuff (p= 2.0 g /cm 3, Fig. 6) from 750 to 100°C. But this is also ade- quate to significantly decrease cooling time in comparison to a pure conduction model.

Similar models can be applied to the Cleet- wood (lava) flow. Backflow of the Cleetwood flow occurred from the central pasty layer of lava that had a total thickness of ~ 150 m ( Ba- con, 1983 ). Where thinner and nearer its mar- gins, the Cleetwood flow fractured to form the cliffs above either side of Cleetwood Cove (Bacon, 1983). Our calculations suggest that it would have taken a maximum of ~ 100 years

264

for the central l/3 of the total thickness of the Cleetwood flow to reach 500°C by conductive cooling (Fig. 8C). The interior of the flow in the cliff faces on either side of Cleetwood Cove is devitrified from a very few meters above the base, and this high-temperature crystallization must have taken place before caldera collapse. Because most of the crystallization would have occurred above 500’ C, devitrification may have slowed the cooling of the Cleetwood flow relative to our calculations (Fig. 8C) owing to liberation of latent heat. However, precipita- tion would have tended to offset this effect.

A limit on the effect of precipitation can be approximated using the model of Dzurisin et al. ( 1990, p. 2777) and incorporating latent heat of crystallization of rhyodacitic liquid, here estimated to be 50 Cal/g. The result is that 1 m (as liquid water) of snow could have cooled a 1.2-m thickness of rhyodacite lava to 100°C. It is uncertain to what depth cracking would have allowed melt-water circulation to keep pace with cooling. Nevertheless, if we as- sume a precipitation rate we can constrain the cooling time of the lava flow in the simple case of precipitation being the rate limiting process. The long-term average precipitation rate mea- sured by a gauge on the Cleetwood flow is 12 1 cm/yr (Nathenson, 1992), most of which falls as snow. Although climatic conditions likely were different in the years before the climactic eruption, we employ this rate as a suitable ap- proximation for the present purpose. At 1.2 m/ yr, it would have taken N 80 years to cool the top 100 m of the Cleetwood flow to 100 o C. The effects of precipitation and of nonuniform heat loss during emplacement, which is impossible to quantify, could therefore reduce the con- ductive cooling time significantly.

The observation that the climactic plinian deposit is oxidized and sin&red only on the surface of the Cleetwood flow (Bacon, 1983)) a fact particularly well displayed in road cuts at Cleetwood Cove, suggests that the calcu- lated maximum elapsed time between em- placement of the lava flow and caldera col-

lapse, whether by conduction or evaporation of surface precipitation, is too long. Possibly, superheated steam escaping from fractures in the lava and trapped by the pumice blanket was responsible for the oxidation and sintering. Elsewhere near the caldera rim, the lower part of the plinian deposit is slightly pinkish and in- cipiently welded, as though only a slight in- crease in its deposition temperature would have been sufficient to cause the oxidation and sintering present on the Cleetwood flow. “A few tens of years” may be the most defensible esti- mate of the time elapsed between emplace- ment of the Cleetwood flow and onset of the climactic eruption.

Acknowledgments

The authors (H.K. and KS-K.), who car- ried out this study as Visiting Scientists at the Cascades Volcano Observatory (CVO ) of the U.S. Geological Survey during 1988- 1990, wish to express their sincere gratitude for this opportunity and for the hospitality of the CVO staff. Brent Hetzler, Maura Hanning, and Si- mon Young helped us safely collect from the caldera-rim exposures. Marvin Couchman and Richard P. Hoblitt helped us measure densi- ties of samples. We thank Tsukasa Nakano, Naoyuki Fujii, Hiromitsu Taniguchi, and Ka- zunori Watanabe for constructive discussions of this study. Superintendent Robert E. Ben- ton permitted us to work in Crater Lake Na- tional Park. The manuscript was improved as a result of reviews by Rick Hoblitt and Judy Fierstein and journal reviews by Michael Ort and Don Swanson.

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