APPLYING GEOPHYSICAL METHODS TO DEPTH-TO-BEDROCK MAPPING FOR HAWAIIAN VOLCANIC TERRAINS AND OBTAINING RATES OF PEDOGENESIS

When studying soil formation, the five factors to consider are climate, parental material organisms, relief, and time. Within the Pu’U Maka’ Ala 7.5” quadrangle on the island of Hawaii, the volcanic soils are an ideal environment to study soil formation rates, given: the stability of the climate, the parent material composed only of volcanic basalt flows and vent pyroclastics from Mauna Loa, the long-term homogeneity of organisms living in isolation, the <10° eastern ground slope angle, and the existing database of geochronology for the extrusive parent material. Consequently, four of the five soil forming factors are constant or known, and the only variable controlling soil thickness is the relative contributions of weathered basalt and pyroclastics to the parent material. Preliminary results indicate that a ground penetrating radar (GPR) system with a 160 MHz and 450 MHz antenna can identify the contact between weathered and unweathered bedrock, thus determining the soil thicknesses. The 450 MHz antenna imaged horizontal reflectors above the bedrock contact that may be individual soil horizons. A dual-frequency GPR system with 170 MHz and 600 MHz antenna and a 24-electrode electrical resistivity system (ERT) are being used to corroborate and refine the preliminary results of this ongoing work. Furthermore, continuous soil cores to the bedrock surface provide representative samples of the soil profile for detailed descriptions, the solum thickness, and geochemical analyses. Published historical lava flow maps provide a lower bound on constraints for ages of bedrock. This USGS EDMAP project will benefit land management decisions related to agriculture and future residential and commercial development as it encroaches onto active volcanic terrains.