Morphology of inflated pahoehoe lavas and spatial architecture of their porous and permeable zones, eastern Snake River plain, Idaho
Welhan JA, Johannesen CM, Reeves KS, Clemo TM, Glover JA, Bosworth KW. 2002. Morphology of inflated pahoehoe lavas and spatial architecture of their porous and permeable zones, eastern Snake River plain, Idaho. Special Paper - Geological Society of America. 353:135-150
The internal vesicular structures of individual pahoehoe lava flow lobes observed in drill cores and in Holocene lava fields on the eastern Snake River Plain indicate that emplacement by inflation has operated throughout the Pleistocene and is dominant in Holocene monogenetic lava flow groups. This suggests that Holocene lava fields are representative analogs for understanding the morphology and internal elements of buried lava flows that determine spatial distribution of porosity and permeability in the eastern Snake River Plain aquifer. Inflated pahoehoe lava flows are characterized by a hierarchical growth pattern of smaller lobes breaking out of larger lobes in a fractal pattern. Lobes <300 m long on the Hell's Half Acre and Wapi flow groups have a mean aspect ratio of approximately 3.2:1 (length: width) and mean length of the order of 85 m. The fractal dimension of lava flow units was analyzed with a modified box-counting method and found to be remarkably similar (1.3 to 1.6) in seven flow units from three different areas, and in flow units mapped at two very different spatial scales. These results suggest that information about lava morphology obtained from large-scale mapping can be extrapolated to understand inflationary lava flow features at smaller scales. Our data suggest that in excess of 15% of interflow zone fracture porosity may survive burial by younger lava. Bridging by viscous lava may be the preservation mechanism, becoming effective at a mean fracture aperture threshold of approximately 10 cm. Because more than 50% of fractures measured on inflated lava flows have apertures of 10 cm or less, a significant amount of fracture porosity may be retained after burial, particularly along the margins of inflated lava flow units where fractures are concentrated in long networks. If sufficiently interconnected, such fracture porosity could support permeable zones extending over distances of kilometers.
- Type
- Journal Article
- Authors
- Welhan, John; Johannesen, Chad; Reeves, Kelly; Clemo, Thomas; Glover, John; Bosworth, Kenneth
- Units
- CRMO
- Keywords
- aquifers, Bingham County Idaho, Butte County Idaho, Cenozoic, Clark County Idaho, Columbia Plateau, ground water, Hell's Half Acre lava field, Holocene, hydrodynamics, Idaho, INEEL, Jefferson County Idaho, lava, lava fields, lava flows, pahoehoe, permeability, porosity, Quaternary, Snake River plain, Snake River Plain Aquifer, United States, volcanic features, volcanism