Satellite observation of active carbonatite volcanism ...

int. j. remote sensing, 1998 , vol. 19 , no. 1 , 55± 64

Satellite observation of active carbonatite volcanism at Ol Doinyo Lengai, Tanzania C. OPPENHEIMER Department of Geography, Downing Place, Cambridge CB2 3EN, England, U.K. ( Received 20 December 1996 ) Natrocarbonatite lavas extruded by Ol Doinyo Lengai volcano, Tanzania, exhibit the lowest known magmatic eruption temperatures, ranging between #500 and 600ß C. Nevertheless, as shown here, the near-infrared bands 5 and 7 of the Landsat Thematic Mapper ( TM), and the mid-infrared channel 3 of the spaceborne Advanced Very High Resolution Radiometer (AVHRR) are able to detect thermal emission from active carbonatites. Laboratory-measured visible-to-near-infrared re¯ ectance spectra of both silicate and carbonatite rocks from Ol Doinyo Lengai are used to infer spectral emissivities, enabling interpretation of satellite measurements. Given the remote location of this unique volcano, satellite remote sensing could play a valuable role in its future surveillance, and o ers a potential means for distinguishing between silicate and carbonatite eruptions. Abstract.

1.

Introduction

Ol Doinyo Lengai volcano, located in the Gregory Rift of northern Tanzania, is justly famous for its unique demonstration of active carbonatite volcanism. It also 1 emits copious quantities of CO2 , releasing some 80 kg sÕ of the gas into the atmosphere ( Brantley and Koepenick 1995). The centre developed in the last 0´37 Ma ( Dawson et al . 1995a) and now presents an elegant stratocone constructed predominantly of phonolites, nephelinites and melilites. Recent studies have focused on the petrogenesis of its historic lavas which are dominated by natrocarbonatite mineralogies but which also exhibit mixed carbonate-silicate compositions (e.g., Dawson 1995b). Following several e usive and explosive phases in the early- to mid-1960s, Ol Doinyo Lengai entered a period of quiescence ( Dawson et al . 1990). This ended in January 1983, with a number of small magnitude ash eruptions that resulted in ® ne tephra falls several tens of kilometres distant ( Nyamweru 1990 ). Eruption clouds were identi® ed in NOAA Advanced Very High Resolution Radiometer (AVHRR) images of February 1983 ( Smithsonian Institution 1989). Since this time, the active crater has been progressively in® lled by the accumulation of low volume but mobile lava ¯ ows (up to 200 m long, 30± 40 m wide) emitted from hornitos and spatter cones (up to 10 m high) on the crater ¯ oor. Similar phenomena have persisted up to the present, with an interval of explosive activity in 1993 that heralded extrusion of particularly viscous carbonatite lavas ( Dawson 1994). Table 1 gives a brief summary of the typical active volcanic manifestations of Ol Doinyo Lengai observed during the last century. The volcano is occasionaly visited by ® eld parties and tourists but observations 0143± 1161/98 $12.00

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56 Table 1.

C. Oppenheimer Recorded temperatures and dimensions of thermal features at Ol Doinyo Lengai.

Feature Ash plumes Lava ¯ ows

Dimensions < 100 km extent 10± 200Ö 1± 40 m

Lava ponds < 10 m diameter Fumaroles

few cm

Temperatures /ß C Ð 491± 576± 544 593 78± 72±

506 593 143 312

References Nyamweru 1988, Dawson et al. 1995a Kra t and Keller 1989 Pinkerton et al. 1995 Kra t and Keller 1989 Pinkerton et al. 1995 Kra t and Keller 1989 Koepenick et al. 1996

Figure 1. Spectral radiances calculated for a surface at temperature 550ß C and broad-band emissivity of 0´9, occupying various proportions, f , of the area of a hypothetical image pixel.

are comparatively scarce (see Dawson et al . ( 1995a) and Nyamweru ( 1988, 1990 ) for records of historic activity). The aim of this study, therefore, was to assess the potential role of satellite remote sensing in surveillance of this exceptional volcano. Several previous studies have shown the capabilities of satellite infrared data for detection and monitoring of active silicate lavas (e.g., Oppenheimer et al. 1993; Harris et al. 1995 ). However, Ol Doinyo Lengai’s natrocarbonatite eruption temperatures ( 0´9. Using this result, it is straightforward to predict spectral radiances from hot lavas, at any given wavelength, for di erent surface temperature-distribution scenarios (e.g., Glaze et al . 1989, Oppenheimer 1991 ). Figure 6 does this for a range of two-thermal component models in which a fraction f of a hypothetical image pixel is at temperature T c , and ( 1 Õ f ) is at T s . For example, T s= 1000ß C; T c= 20ß C could represent a nephelinite lava but not a carbonatite ¯ ow (curve i in ® gure 6). From ® gure 6 it is evident that Landsat TM bands 5 and 7 are capable of distinguishing active silicate lavas (eruption temperatures >800ß C) from carbonatites, so long as su cient surface areas at near magmatic-temperatures are exposed. The same cannot be said for discrimination of carbonatites from silicates since partly cooled silicate lavas could exhibit the same ranges of temperature (#500± 600ß C) as active carbonatites. Unfortunately, the Landsat TM image presented here was geometrically resampled with a cubic-convolution ® lter kernel, rendering it unreliable for precise

Remote sensing of Ol Doinyo L engai

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(a)

(b)

(c)

Figure 5. Calibrated spectral re¯ ectances measured with a GER IRIS Mk IV dual ® eld of view spectroradiometer. Samples illuminated by 100 W lamp at zenith angle of 45ß . (a ) LEN12Ð ® ne-grained nephelinite from ¯ ank, LEN 39Ð dark ® ne-grained ijolite, LEN40 Ð ijolite with phlogopite phenocrysts, LEN 46Ð nephelinite block erupted in 1966, LEN 47Ð ijolite; ( b) freshest samples: LEN12 Ð ® ne-grained nephelinite from ¯ ank, LEN36Ð carbonatite ¯ ow erupted in 1993; (c ) progressively altered carbonatite lavas erupted in 1993 and 1995 ( 1993 samples are less altered since they had been stored under vacuum). Note the similarity of carbonatite and silicate spectra in ( b).

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C. Oppenheimer

Figure 6. Spectral radiances in Landsat TM bands 5 (L 1´65 mm ) and 7 (L 2´2 mm ), calculated from the Planck function for the following two-thermal-component surfaces: ( i ) T c= 1000ß C, T s= 20ß C; ( ii ) T c= 1000ß C, T s= 200ß C; ( iii ) T c= 800ß C, T s= 20ß C; ( iv ) T c= 800ß C, T s= 200ß C; ( v) T c= 600ß C, T s= 20ß C; ( vi ) T c= 600ß C, T s= 200ß C; ( vii ) T c= 500ß C, T s= 20ß C; ( viii ) T c= 400ß C, T s= 20ß C. Diamonds represent `hot’ pixels from 1 July 1984 Landsat TM image; circle represents summed spectral radiance for whole thermal anomaly.

radiometry of such small features (Oppenheimer et al . 1993 ). The consequence of such pre-processing is to clip the high DN values of the thermal anomaly due to their high spatial frequency. This is particularly likely to a ect the band 7 response and this is almost certainly why the bands 5 and 7 spectral radiances for the two hottest pixels plot close to curves for silicate lavas (® gure 6 ). Alternatively, if the bands 5 and 7 thermal responses for the whole anomaly (i.e., all six hot pixels) are 2 1 1 summed, then the spectral radiances are 0´36 and 1´56 mW cm Õ srÕ m mÕ , respectively. This total anomaly then falls within the carbonatite ® eld on ® gure 6. Assuming a two-component thermal distribution with an ambient background temperature 2 suggests a lava temperature of #540ß C for an area of about 70 m , entirely consistent with typical ¯ ow temperatures and dimensions. The corresponding radiative heat ¯ ux obtained from the Stefan-Boltzmann equation is of the order of 1´6 MW (the combined radiative and sensible heat ¯ uxes amount to #2 MW). Given the uncertainties already mentioned, this calculation is intended merely to demonstrate the potential for measuring temperatures and heat ¯ uxes. 3.

Conclusions

(i )

(ii )

(iii )

Satellite infrared sesnors such as the Landsat TM and NOAA AVHRR are able to detect natrocarbonatite lava ponds and ¯ ows, and active hornitos (and to track Ol Doinyo Lengai’s ash plumes). It is feasible to identify active silicate lavas at Ol Doinyo Lengai with Landsat TM data but discrimination of active carbonatites from cooled silicate lavas is di cult. The sensors’ dynamic ranges, radiometric resolution, and optical performance determine their suitability for quantitative analysis of subpixel thermal features. AVHRR imagery o ers sign® cantly greater imaging frequencies ( hours or days) compared with Landsat TM (weeks or months), and is readily available. However, the optimal 1 km spatial resolution of Local Area Coverage


(iv )

(v)

63

and High Resolution Picture Transmission data precludes sensititve thermal analysis, particularly of daytime data where subpixel cloud re¯ ectance can contaminate the signal. High-spatial resolution infrared data (e.g., Landsat TM ) could provide valuable information on the thermodynamics of carbonatite ¯ ows, particularly in the event of future larger magnitude e usions that feed ¯ ank lavas. Future surveillance could be used to characterise the thermal ¯ ux from Ol Doinyo Lengai through time and possibly to determine its correlation with CO2 ¯ uxes ( Brantley and Koepenick 1995), and magma discharge rates. A signi® cant inference of the detection of thermal emission from active carbonatites in Landsat TM bands 5 and 7 is that these channels must therefore be sensitive, as predicted on theoretical grounds, to heat emitted from the partially cooled and crusted surfaces of silicate lava ¯ ows.

Acknowledgments

I am grateful to David Pyle for loan and descriptions of the 1966 and 1993 eruptive samples, David Emery and the NERC Equipment Pool for Spectroscopy in Southampton for spectrometer time and assistance, EOSAT for Landsat data grant 134 through which the TM image was purchased, and Celia Nyamweru and Bob Symonds for comments on the manuscript. References B rantley, S . L . , and K oepenick, K . W . , 1995, Measured carbon dioxide emissions from Oldoinyo Lengai and the skewed distribution of passive volcanic ¯ uxes. Geology , 23, 933± 936. D awson, J . B ., G arson, M . S ., and R oberts, B . , 1987, Altered former alkalic carbonatite lava from Oldoinyo Lengai, Tanzania: inferences for calcite carbonatite lavas, Geology , 15, 765± 778. D awson, J . B ., P inkerton, H ., N orton, G . E . , and P yle, D . M . , 1990, Physicochemical

properties of alkali carbonatite lavas: data from the 1988 eruption of Oldoinyo Lengai, Tanzania, Geology , 18, 260± 263. D awson, J . B ., K eller, J ., and N yamweru, C . , 1995 a, Historic and Recent eruptive activity of Oldoinyo Lengai. In Carbonatite V olcanism, edited by K. Bell and J. Keller (Berlin: Springer-Verlag ), pp. 4± 22. D awson, J . B ., P inkerton, H ., N orton, G . E ., P yle, D . M ., B rowning, P ., J ackson, D ., and F allick, A . E . , 1995 b, Petrology and geochemistry of Oldoinyo Lengai lavas extruded in November 1988: magma source, ascent and crystallisation. In Carbonatite V olcanism, edited by K. Bell and J. Keller, Berlin: Springer-Verlag, pp. 47± 69. D awson, J . B ., P inkerton, H ., P yle, D . M . , and N yamweru, C . , 1994, June 1993 eruption of Oldoinyo Lengai, Tanzania: exceptionally viscous and large carbonatite lava ¯ ows and evidence for coexisting silicate and carbonate magmas. Geology , 22, 799± 802. G laze, L . S ., F rancis, P . W . , and R othery, D . A ., 1989, Measuring thermal budgets of active volcanoes by satellite remote sensing. Nature , 338, 144± 146. H arris, A . J . L ., V aughan, R . A ., and R othery, D . A ., 1995, Volcano detection and monitoring using AVHRR data: the Kra¯ a eruption, 1984. International Journal of Remote Sensing, 16, 1001± 1020. K oepenick, K ., B rantley, S . L ., T hompson, J . M ., R owe, G . L ., N yblade, A . A ., and M oshy, C . , 1996, volatile emissions from the crater and ¯ ank of Oldoinyo Lengai volcano, Tanzania. Journal of Geophysical Research, 101, 13 819± 13 830. K oberski, U ., and K eller, J ., 1995, Cathodoluminesce nce observations of natrocarbonatites and related peralkaline nephelinites at Oldoinyo Lengai. In Carbonatite V olcanism, edited by K. Bell and J. Keller ( Berlin: Springer-Verlag), pp. 87± 99. K rafft, M ., and K eller, J ., 1989, Temperature measurements in carbonatite lava lakes and ¯ ows from Oldoinyo Lengai, Tanzania. Science, 245, 168± 170.

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