Radar Observations of Basaltic Lava Flows and Fissure Vents: Craters of the Moon, Idaho

Martel, L

Evaluated use of radar to detect fissure vents in CRMO (Craters of the Moon National Monument) lava fields. Near vent flows is no unique radar backscatter & that stratigraphy is too complex to trace flows to sources. Fissure vents are detectable by radar because of their structure. Geographic location: CRMO Detection of fissure vent areas in basaltic plains using radar images requires a) characterizing surface roughness of the effused lavas and b) delineating vent surfaces. The utility of this approach is highly dependent on the geologic character of the particular area and on resolution constraints of the radar system. This study tested the hypothesis that smooth-to-rough textural transitions of effused lavas can be detected in radar images, and that these transitions occur with distance from a fissure vent. Although smooth and rough surface textures can be successfully differentiated in X band (3-cm wavelength) and L band (25-cm wavelength) dual polarization images, there are constraints in tracing textural changes back to a particular fissure vent. First, in the Craters of the Moon lava field, near-vent shelly pahoehoe has identical backscatter to older pahoehoe with smooth mantles of Aeolian material. Thus, near-vent flows do not have unique radar backscatter. Second, the stratigraphy is sufficiently complex that flows rarely can be traced to their sources. Textural transitions seen in radar images did not result from single flows emanating from fissure vents. Rather, lavas of different textures, degrees of weathering, flow paths, and source vents combined to form complex textural patterns. Small-scale, individual flows with textural transitions were not seen clearly in the radar images due to resolution restrictions of the imaging systems. Fissure vents are detectable in the radar images by virtue of the structures that form along their lengths. Spatter ramparts paralleling the open fissures have diagnostic, strong backscatter in the X band images. These vent structures were not delineated in the L band images. The agglutinate texture of the ramparts has an average roughness of several centimeters, which may be responsible for the appearance of the ramparts in X band, but not in L band images. Differences in system design governing resolution, such as signal to noise ratio and number of looks may also contribute to the differences in visibility of the ramparts in the radar image. The fissure vents illustrate the importance of using multiple wavelengths in radar studies.