New Mexico Geological Society Annual Spring Meeting — Abstracts

Based on mapping of the McCartys lava flow, comparison with flow-fields in Hawaii, and observations of actively inflating flows, we are beginning to untangle the sequence of emplacement mechanisms on large flow-fields, particularly those with characteristics attributed to inflation. These results are providing insights that we believe will enable us to map the sequence of events during the emplacement of the McCartys lava flow.

Mapping efforts have focused on determining the dimensions, geometry, and timing of lava surface characteristics, and structure of vertical sections along transects of the McCartys flow field at several places along its length. We have done similar measurements at the distal ends of the 1859 lava flow, Hawai’i, although the 1859 lava flow has proven more complicated. Both lava flows are pahoehoe flows similar in thickness and length, and consist of extensive sheeted surfaces with micro-spiny surface textures and deformed “skins” of variable thickness (from millimeters to meters). Central platy pahoehoe-bearing platforms with V-shaped pits and marginal clefts are commonly elevated from one to over 10 meters above surrounding pahoehoe sheet lobes. These sheet lobe surfaces were subjected to differential surface stresses causing immediate plastic and subsequent brittle shear, compressional, and extensional deformation zones defined now by thin platy rubble zones. Based on timing estimated from thickness of deformed pahoehoe crusts, the lavas of these platforms were emplaced and deformed in less than a few hours. Subsequent deformation of the margins, squeeze ups in deep clefts, and marginal tilting and apparent elevation of the platforms occurred as late as several weeks after the sheet lobe.

As a first order model, the platform-sheet lobe characteristic appears to have been strongly influenced by the balance of lava margin strength and lava hydrostatic head. Stress in excess of the critical yield strength necessary for breakouts and advance may be governed largely by regional slope or, on low angle slopes, the timing of excess pressure from local inflation.