THERMAL, DYNAMIC, AND HYDRAULIC SEQUENCE IN THE EMPLACEMENT OF LARGE LAVA FLOWS: MAPPING THE MCCARTYS LAVA FLOW, EL MALPAIS, NEW MEXICO

The pathway of a given partial of mantle melt to the surface and its distribution in large lava flow fields is not well understood. Mapping of young lava flow fields is providing evidence that the basic hydraulic, thermal, and structural events during the emplacement of lava flows is surprisingly complex, yet the events can be unraveled. Our work shows that the distribution of magma partials during the lifetime of an eruption and during the corresponding flow field development may be determined. Mapping of the McCartys lava flow, El Malpais, New Mexico, focused on the time-transgressive thermal and mechanical details of emplacement, preserved in the physical details of its surface and interior, is yielding new information on the sequence of emplacement events in large flow-fields. This mapping is being compared with the results of similar mapping on historic lava flows and actively inflating lava flows in Hawai’i.

One of the largest sources of complexity is a result of lava inflation, a platform-sheet flow characteristic of large flows that is 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. Both the McCartys and 1859 Hawai'i lava flows are extensive sheeted pahoehoe 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.