Thermal Remote Sensing Characteristics of Basaltic Lava Flow Surface Units: Implications for Flow Field Evolution

J. M. Byrnes, D. A. Crown, and M. S. Ramsey, Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, PA, 15260-3332, jmbst102@pitt.edu

Introduction: Compound lava flow fields are emplaced as a complex sequence of interfingering and overlapping flow units. The texture and morphology present at the flow surface are indicative of local emplacement conditions and provide important constraints for understanding flow field evolution [1-16]. Lava flow surface units have been identified and mapped within the 1969-1974 Mauna Ulu flow field (Kilauea Volcano, Hawaii) using visible-wavelength remote sensing datasets [17]. Previous analyses [17] indicate that the distribution of surface units is related to the pre-eruption topography and lacks a correlation to the location of major lava tube segments mapped earlier [18]. This study utilizes field observations to interpret the thermal remote sensing characteristics of the identified surface units to incorporate additional parameters related to cooling and flow emplacement into the classification methodology.

Study Area: The study area is within the Mauna Ulu lava flow field, emplaced during a period of prolonged volcanic activity along Kilauea Volcano's east rift zone from May 1969 to June 1974 [10-12,18-20]. The area consists primarily of tube-fed pahoehoe flows along with some channel-fed a'a. Observations were made within flows emplaced during 1970-1972 on and below Holei Pali, 7 to 9 km from the Mauna Ulu vent [18-19].

Background: Compound flows are emplaced in complex distributory networks and display a wide range of surface unit morphologies and textures. The development of a lava flow field is represented by the distribution of and relationships between these surface units. Characteristic pahoehoe unit morphologies include toes, sheets, and channels. Pahoehoe toes typically form where flow front advance is slow, sheets result from emplacement that is unconfined or from the coalescence of toes and/or small sheets, and channels form where local flow rates are sufficiently high. Pahoehoe surfaces display textures that result from deformation of the surface crust and are related to viscosity, cooling, and other factors [7-8].

Pahoehoe surface units within an initial study site at Mauna Ulu have been described previously [17]. Four pahoehoe units are distinguished that each display a dominant morphology and characteristic surface texture and color. Unit I has a smooth glassy surface and displays a dominant sheet morphology; unit II also has a smooth glassy crust, though is dominated by toes; unit III lacks a smooth crust and is emplaced as local late-stage toey breakouts of relatively higher viscosity lava; unit IV has a highly disrupted surface transitional to a'a and displays a dominant channel morphology.

Field Observations: Field observations were made at 44 sites in the Mauna Ulu flow field in November 1999 to document small-scale surface parameters affecting thermal remote sensing data, and to compare our observations with georeferenced, atmospherically-corrected NASA TIMS (Thermal Infrared Multispectral Scanner) data. Specifically, we characterized surface units by estimating the degree of spallation and the abundance of vesicles and phenocrysts (primarily olivine) on the original surface crust and spallation surface [Table 1]. Values are not reported for unit IV and a'a because the unit IV surface is highly variable and a'a surfaces do not typically preserve an original crust. Spallation is most significant for unit III surfaces, which contributes to the high degree of small-scale roughness of this unit. Vesicles on the surfaces of units I and II are stretched and generally lack surface expression, consistent with emplacement of hot, fluid lava; unit III tends to preserve surface vesicles, consistent with emplacement of a higher viscosity fluid. Phenocrysts are also most significant for unit III, further supporting the interpretation that this Mauna Ulu unit is emplaced from lava that has experienced prolonged storage in the subsurface distributary network [1,17].

Remote Sensing Datasets: High resolution color aerial photographs were acquired for the study area in April 1985 in conjunction with TIMS data. The aerial photographs were taken with a Zeiss camera system and scanned, mosaiced, and classified to allow mapping of the pahoehoe surface units [17].

TIMS data, with six channels between 8 and 12 µm, have been used previously to map: 1) volcanic units of similar composition and varying age [21], 2) volcanic units of similar composition and varying vesicularity [22-24], and 3) thermal anomalies associated with lava tubes [25]. TIMS acquires thermal infrared radiance, which is a function of both surface temperature and composition. Calibrated radiance data for the study area were rectified and coregistered to the aerial photographs. Average radiance values for each pahoehoe unit and a'a were calculated from a preliminary sampling of units in the study area. No atmospheric correction was used for data reported herein, therefore sample values were taken from a localized area to minimize error introduced from elevation differences.

Thermal Remote Sensing Characteristics: Average radiance values for the sampled surface units were calculated [Table 2]. Thermal radiance tends to decrease with increasing small-scale surface roughness, from smooth, glassy-surfaced units I and II to rough, vesicular unit III to disrupted unit IV surfaces to clinkery a'a. Temperature differences are not expected to be significant except for a'a, which is cooler due to self-shadowing. Field observations and previous work [23] indicate that emissivity differences are expected in the pahoehoe units due to variations in phenocryst abundance, vesicles/micron-scale roughness, and microlite abundance.

Conclusions: Preliminary analysis of TIMS data combined with field observations indicates that: 1) basaltic surface units each have a characteristic thermal radiance signature; 2) this signature is primarily controlled by variations in phenocryst abundance, vesicles/micron-scale roughness, and microlite abundance, all of which are functions of emplacement and cooling; and finally 3) thermal radiance data may therefore be used to map the identified surface units at Mauna Ulu. Further analysis of thermal data is required to document the full range of thermal radiance for units within the Mauna Ulu flow field, and to separate the effects of temperature and emissivity on radiance spectra.

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