Geophysical Research Letters

Transcription

1 Geophysical Research Letters SPTMBR 4, 1992 Volme 19 Nmber 17 AMRICAN GOPHYSICAL UNION

2 COOLING RAT OF AN ACTIV HAAIIAN LA VA FLO FROM NIGHTTIM SPCTRORADIOMTR MASURMNTS Lke P. Aynn and Peter J. Moginis-Mark Planetary Geosciences, Dept. of Geology and Geophysics, SOST University of Haaii Abstract. A narro-band spectroradiometer has been sed to make nighttime measrements of the Phase 50 erption of P' 0'0, on the ast Rift Zone of Kilaea Volcano, Haaii. On Febrary 19, 1992, a GR spectroradiometer as sed to determine the cooling rate of an active lava flo. This instrment collects 12-bit data beteen 0.35 to m at a spectral resoltion of 1-5 nm. Thirteen spectra of a single area on a pahoehoe flo field ere collected over a 59 minte period (21:27-22:26 HST) from hich the cooling of the lava srface has been investigated. A to-component thermal mixing model [Flynn, 1992] applied to data for the flo immediately on emplacement gave a best-fit crstal temperatre of 768 C, a hot component at 1150 C, and a hot radiating area of 3.6% of the total area. Over a 52-minte period (ithin the time interval beteen flo resrfacings) the lava flo crst cooled by 358 C to 410 C at a rate that as as high as 15 C min-i. Or observations have significance both for satellite observations of active volcanoes and for nmerical models of the cooling of lava flos dring their emplacement. Introdction On Febrary 17, 1992, a ne 150 meter long fissre opened jst p-rift of P' 0'0, on the ast Rift Zone of Kilaea Volcano, Haaii (Fig. 1). Dring the next to days, this vent as erpting at -1.2 x 106 m 3/day and fed an active pahoehoe flo field -I km 2 in size. This erption represented or first sccessfl attempt to collect radiative temperatre data for active lava flos sing a narro-band field spectroradiometer. e visited the Phase 50 erption site on the night of Febrary 19, 1992 to collect temperatre measrements of the lava flo field. Nighttime data ere collected to ensre that the data ere not contaminated by reflected solar radiance, even thogh data redction techniqes have been developed to remove this contamination from mltispectral measrements [Flynn, 1992]. Nmeros investigators have tried to se either spacecraft sensors [e.g., Rothery et ai., 1988; Pieri et ai., 1990; Oppenheimer, 1991] or field instrments [Jones et ai., 1990; Oppenheimer and Rothery, 1991; Oppenheimer 1991] to measre the radiative temperatre of a lava flo. Particlarly noteorthy have been attempts to se daytime data from the Landsat Thematic Mapper (TM) to gain an overvie of the spatial distribtion of flo temperatres [Pieri et ai., 1990; Oppenheimer, 1991]. These Landsat measrements have nevertheless sffered both from the poor spectral resoltion of the TM and from the limited range of temperatres for hich dal-band fits are possible [Oppenheimer, 1991]. Sch information has great potential relevance to the attempts to model nmerically the cooling of a lava flo [Crisp and Baloga, 1990], becase the data provide qantitative information on the skin temperatre of a flo as a fnction of time. Copyright 1992 by the American Geophysical Union. Paper nmber 92GL /92/92GL-01577$ cf' v, v, '0, KlLet.4TRS Fig. 1. Location map for the Phase 50 erption of P' 0'0. Asterisk marks the approximate location here or spectroradiometer data ere collected on Febrary 19, Also shon (cross-hatched) are the lava flos from P' 0'0 and (dotted) the lava flos from Kpaianaha lava pond. Inset shos the location of the stdy area on the Big Island of Haaii as ell as the locations of the smmits of Kohala (Ko), Haalalai (H), Mana Kea (MK), Mana Loa (ML), and Kilaea (Ki) Volcanoes. Instrment and Field Techniqe e sed a commercial GR (Geophysical nvironmental Research) visible/infrared dal-beam spectroradiometer to collect or spectra of the cooling lava flos. The GR instrment collects 1,000 measrements of spectral radiance in the m range ith a spectral resoltion of 1-5 nm (AIdA = ). The advantages of this instrment over previosly sed single-channel radiometers [e.g., Jones et ai., 1990; Oppenheimer and Rothery, 1991; Oppenheimer, 1991] are the many data points (-200 sable channels ithin atmospheric indos), a ide dynamic range (12- or 15-bit data can be obtained), very high spectral resoltion (l - 5 nm), and the ability to collect to simltaneos measrements at 3.0 anglar separation. Or measrements ere made -1 km est of the active Phase 50 rift of P' 0'0, looking at a 5 m-high steep (-70 ) slope that as the site of active lava flos. The orientation of the target slope ith respect to the spectroradiometer afforded s near-normal vieing geometry hich as important to accrately qantify the area of narro cracks radiating at near magmatic temperatres. e positioned the spectroradiometer -20 m from this slope, reslting in a beam area of m 2 on the target srface. Sixty-five spectral pairs (a total of 130 spectra) ere collected at this location beteen 20:40 to 23:00 Haaii Standard Time (HST), hich as 2 hors and 9 mintes after local snset at 18:31 HST. Of these data, 13 pairs ere collected in a period (21:27 22:26 HST) dring sccessive episodes of lava flo emplacement. De to the operating characteristics of the spectroradiometer, each pair of measrements took 30 seconds to collect. 1783

3 Time-lapse photographs taken of the target area for the 20 mintes before the spectral measrements shoed that the spectroradiometer's field of vie as the site of to episodes of sheet flo emplacement at 21:22 HST and 21:25 HST. The second flo as -2 m ide, 3 m long, and as visally estimated to be -30 cm thick. This flo remained visible for 4 mintes on sccessive images obtained ith 400 A.S.A. color film taken ith an ato-exposre camera. At 54 mintes, the video record shos lava flos advancing at the base of the rise belo the target area. At 58 mintes, separate flos cascaded over the top of the rise and, by 60 mintes, had advanced into the target area; conseqently, spectral measrements made after 54 mintes may inclde radiance from resrfacing flos. Ths, for the prposes of cooling history, e ill stdy only the measrements made before 54 mintes. Spectral Observations Fig. 2 presents three representative spectra of the lava flo. These spectra ere collected at the start of a single episode of flo emplacement, and after 29 mintes and 52 mintes of cooling of the flo srface. From these data, not only is it evident that each spectrm is extremely "clean" (nighttime data obviosly mean the lack of featres from snlight reflecting off the lava srface), bt also there as a prominent decrease in crstal radiative temperatre and in the fractional hot area ith time. The shape of each spectrm shos that the thermal flx as first detectable by the spectroradiometer at -0.9 m. Sch observations old not be possible dring the daytime, hen the reflected solar component from the target srface old samp these measrements. Atmospheric absorption bands occpy the m and m regions of the spectra and are easily identified by decreases in flx. The radiance from the crstal component is manifested in the spectra as the rapid increase in radiative flx ith avelength in the 2.0 to 2.5 m range. Fig. 2 also shos that the slope of this part of the collected spectrm shifts frther toards the infrared ith sccessive observations, hich (by Planck's La) sggests sccessively cooler srface temperatres. The flx measred in the atmospheric indo shortard of 1.27 m is primarily dominated by radiators of magmatic or near-magmatic temperatre. Assming a niform magmatic temperatre for the cracks in the cooling flo, this temporal decrease in radiated flx shortard of 1.27 m signifies that the fractional area of the hot component is also decreasing. The rapid increase in flx ith avelength in the 1.20 to 1.27 m range sggests that the percentage of the hot component as large compared to earlier observations of the Kpaianaha lava lake [Flynn, 1992]. Using the same tocomponent thermal model developed by Flynn [1992], the best fit reslts.for the spectrm at the start of or observations (Time = 0) are for a hot component temperatre (Th) of 1150 C, a crst temperatre (Tc> of 768 C, and a fractional radiating area, Ah, of 3.6% of the total area. As a fnction of time, for sccessive spectra, the magnitde of the hot component shos a decrease in the 1.66 to 2.15 m region. At Time = 0, the flx at 1.66 m is -55% that at 2.35 m. After cooling for 29 mintes, the lava flo has a spectrm that exhibits a radiance at 1.66 m that is -28% that at 2.35 m. This trend contines as the flo cools, so that at 52 mintes after emplacement the 1.66 m radiance is -20% that at 2.35 m. Cooling of the Flo Srface Thirteen of or spectra permit the cooling and resrfacing history of the flo field to be observed over a 59 minte period. Visal observations of the flo over this period verified that no srface activity took place in the spectroradiometer's field of vie for the first 54 mintes, althogh it is likely that sbsrface flo or inflation as taking place for part of the time. Table 1 presents the best fit 60 A. >:::: () 30 Z 20 > T""R :i B. >:::: 20 () Z 10 > AVLNGTH () 40 >:::: C. 30 () z > AVLNGTH () AVLNGTH (11m) Fig. 2. Spectra of lava flo taken immediately after its emplacement (Time = 0), after 29 mintes, and after 52 mintes. Note that the Y-axis is in relative radiance. De to the atoscaling of the GR spectroradiometer, these spectra cannot be directly compared for total flx per channel. Hoever, e ere able to calibrate the channel to channel spectral response of the instrment sing blackbody simlators [Flynn, 1992], and this is necessary since the calclation of the component blackbodies depends on the slope (i.e., relative response beteen channels) of the spectral crve as opposed to the absolte radiance. These data represent measrements taken at 21:27 HST, 21:56 HST and 22: 19 HST respectively (see Table 1).

4 Table I. Smmary of to-component thennal model best fit reslts for spectra collected beteen 21 :27 and 22:26 HST. No. TIM (HST) Th (OC) TcC C) Ah(%) 1. 21: : : : : : : : : : : : : Th is the temperatre of the hot component, Tc is the temperatre of the crst, and Ah is the percentage of the field of vie that comprised the hot area. reslts for Th, Tc, and Ah as derived from the tocomponent thennal model of Rynn [1992]. A 20 minte gap in data collection occrs beteen the acqisition of the first and second spectrm becase spectra ere being collected elsehere on the flo field. Or data sho that all of the best fit reslts, except for the first, forth, and seventh spectra in the series, occrred for a hot component temperatre of C. Flynn [1992] notes that this model is insensitive to the hot component temperatre de to the small area of the hot component and the spectral distribtion of flx for srfaces over this temperatre range. Note that spectra 2 to 6 do not sho any significant cooling over a 5 minte interval, and that short-term (-2 min) increases in temperatre can occr. The cooling history of the flo field (Fig. 3) provides an insight into the thennal properties of the lava flo. Dring the first 20 mintes, the crstal temperatre dropped by 164 C or -8 C min - 1. The fractional area of the hot 800 -r, r' ;:;:; 700 CRUST TMP :::>..c <{ f <{ 600 / <i <{ a5 500 f f- a I f (j) 0.01 :::> I ' I ' i ' I ' I ' i ' I 0.00 o LAPSD TIM (mintes) Fig. 3. Cooling crves for the lava flo investigated here. Temperatres are the crstal temperatres (Tc) and percent hot areas (Ah) given in Table 1. lapsed time starts at 21 :27 HST, hich as 2 hors and 9 mintes after local snset. component (Ah) also decreased by a factor of 3, sggesting that cracks on the srface of the lava flo had solidified dring the intervening 20 mintes beteen measrements, althogh e recognize that a thennal model ith 3 or more components [Flynn, 1992] cold be sed to identify intennediate components having temperatres beteen that of the hot and crst components. The cooling rate of the crst as greatest (-15 C min-i) beteen 22:16 and 22:19 HST. Slightly sloer crstal cooling rates ere measred beteen 21:52 and 21:56 HST (11 C min-i) and 21:59 and 22:08 HST (lo C min-i). Isothennal conditions existed for 8 mintes (22:08 to 22: 16 HST). Increases of 22 C min- 1 and 52 C min- 1 started at 21:47 HST and 22:19 HST, respectively. The latter increase marks the arrival of a ne flo into the target area. Discssion The data presented here represent, to or knoledge, the first mltispectral measrements of the cooling rate of an active lava flo. Use of nighttime data not only made the analysis far simpler than for measrements collected dring the day, bt this approach also pennitted shorter avelengths (don to 0.9 m) to be sed for determination of the blackbody crve. In this regard, e note that this particlar erption as also detected at 1 km/pixel spatial resoltion in band 2 (near infrared; m) of the NOAA-ll AVHRR images collected by P. Flament, University of Haaii. Sch AVHRR observations of volcanic thermal anomalies are nsal, bt in this case are the conseqence of the large size of the active P' 0'0 flo field and the high temperatres. In trn, these AVHRR observations point to the vale of collecting nighttime Landsat TM data [e.g. Oppenheimer et ai., 1992] of volcanic thennal anomalies at avelengths beteen 0.76 to 0.90 m (TM band 4) and 2.08 to 2.35 m (TM band 7). Using the model reslts for or spectral data, it is also possible to calclate the energy bdget (Q) of the lava flo in the field of vie. Table 2 gives the vales for the energy radiated by the hot and cooler portions of the lava flo, and shos that the hot component contribtes only a small amont of the emitted energy (11 % on initial emplacement, bt < 4% after 20 mintes) for the total radiant energy bdget. The observation that the crstal component radiated most of the energy from the flo is interpreted to be the reslt of the limited size of the hot component. This observation implies that Landsat TM investigations hich Table 2: Temporal variation of emitted energy from the hot (Qhot) and crst (Qcrst) components of the cooling lava flo. No. Time (Mins) Qhot() Qcrst() Qtotal() Qtotal gives the total energy emitted by the flo from an area of 0.55 m 2. Spectra nmbers are the same as Table I.

5 provide estimates of the total energy bdget of volcanic erptions by solving for the temperatre and area of the hot component (e.g. Glaze et ai., 1989; Pieri et ai., 1990) cold potentially have large errors since it is the temperatre of the crst component hich dominates Q [Oppenheimer, 1991; Flynn, 1992]. Or determination of the hot fractional areas of the Ho are on the order of those calclated by Crisp and Baloga [1990], ho calclated fractional areas of -0.5% for the Phase 13 and 18 P' 0'0 lava flos erpted in ell constrained field spectroradiometer measrements of active lava flos cold therefore assist the formlation of more detailed nmerical models of flo emplacement. In addition, nighttime thermal measrements cold aid in the field identification of active tbe systems and the processes related to the post-emplacement inflation of Haaiian lava flo fields [alker, 1991; Hon and Kaahikaa, 1991]. The thermal evoltion of the Phase 50 P' 0'0 lava flos also provides some insight into the internal strctre of the flo. e believe that the most likely case for the isothermal conditions for at least 5 mintes beteen 22:08 to 22: 16 HST (spectra 2 to 6; Table I) as the sbsrface injection of ne material into the interior of the flo. This injection as not observed, bt the area of the flo p-slope of or stdy area as experiencing mltiple rapid «1 minte) episodes of flo advance in directions other than the spectroradiometer's field of vie. The rapid drop in crstal temperatre beteen spectra 6 and 7 may ths be the conseqence of the isolation of a tbe system and the lack of intrsion of ne magma into the flo. e conclde that high spectral resoltion data, particlarly measrements taken at night, offer major advances over radiative measrements made from Landsat or single-channel radiometers. Crrently lacking from this approach is imaging spectrometer data, sch as the kind advocated by Oppenheimer and Rothery [1991] and Flynn [1992]. The spatial context of the flo that e stdied here cold only be approximated from visal observations. Imaging spectrometers operating at avelengths beteen 0.4 to m, sch as the one nder constrction at the University of Haaii by Lcey et al. [Po Lcey, pers. comm. 1992], therefore offer great potential for frther qantitative measrements of the radiative properties of lava flos. Acknoledgments. Or thanks go to Harold Garbeil, Jonathan Gradie, Henning Haack and Marc Norman, ho all labored ith s day and night over many miles of lava flos and rain forest, helping to carry the field eqipment sed to collect these data. Frank Trsdell (U. S. Geological Srvey, Haaii Volcano Observatory, Haaii) provided mch appreciated timely information on the ne erption. e are also gratefl to personnel at STS Technology, Inc. (Mililani, Haaii) for making the spectroradiometer available to s (Jonathan Gradie) and for sefl discssion abot the instrment's response fnction (Pamela Oensby). This ork as spported by grant NAG-1162 from NASA's Geology Program, and NAG-2468 from NASA's OS Project Office. This is Planetary Geosciences Paper #689 and SOST Contribtion #2964. References Crisp, J., and S. Baloga, A model for lava flos ith to thermal components. J. Geophys. Res., 22., ,1990. Flynn, L. P., Radiative Temperatre Measrements of the P 0'0 - Kpaianaha rption ith Implications for Satellite Remote Sensing, Ph.D. thesis, 262 pp., University of Haaii, May Glaze, L., P.. Francis, and D. A. Rothery, Measring thermal bdgets of active volcanoes by satellite remote sensing. Natre, ill, , Hon, K., and 1. Kaahikaa, The importance of inflation in the formation of pahoehoe sheet flos (Abstract). os, 72,p.557, Jones, A. C., L. ilson, and H. Pinkerton, Srface temperatre measrements of active Haaiian lava flos (abstract). Paper presented at International Volcanological Congress, Int. Assoc. Volcano!. Chem. arth's Interior, Mainz, Germany, Sept. 3-8,1990. Oppenheimer, C., Lava flo cooling estimated from Landsat Thematic Mapper infrared dat The Lonqimay erption (Chile, 1989). J. Geophys. Res. 96,21,865-21,878,1991. Oppenheimer, C. M. M., and D. A. Rothery, Infrared monitoring of volcanoes by satellites. J. Geo!. Soc. Lond., 148, , Pieri, D. c., L. S. Glaze, and M. J. Abrams, Thermal radiance observations of an active lava flo dring the Jne 1984 erption of Mont tna. Geology, l8.., , Rothery, D. A., P.. Francis, and C. A. ood, Volcano monitoring sing short avelength infrared data from satellites. J. Geophys. Res., 93, , alker, G. P. L., Strctre, and origin by injection of lava nder sbsrface crst, of tmli, "lava rises", "Iavarise pits", and "lava-inflation clefts" in Haaii. Bll. Volcanol., 53, , L. P. Flynn and P. J. Moginis-Mark. Planetary Geosciences, Department of Geology and Geophysics, SOST, University of lliaii at Manoa, 2525 Correa Road, Honoll, HI (Received May 5, 1992; accepted Jne 27, 1992.)