INFLATION FEATURES ASSOCIATED WITH THE MCCARTYS LAVA FLOW, ZBVF, NM

Detailed mapping of the Lava Falls area of the 3900 year old McCartys Lava flow, part of the Zuni-Bandera volcanic field in New Mexico, reveals a complex emplacement history. The southern part of the flow is dominated by pahoehoe flows with features that have been interpreted as formed by lava inflation processes (Mabery et al. 1999). A series of at least 4 plateaus, are separated by steep escarpments with numerous deep cracks formed as lava injection inflated the flow from below. The plateaus also include numerous circular and elongate depressions , with approximately concentric crack patterns surrounding pits between 5 and 15 m deep. These pits are interpreted to be inflation features formed above topographic highs as the surrounding lava preferentially thickened by inflation.

Crack depths suggest a minimum of several months for the emplacement of the flow, but depending on the whether the whole flow was one more or less continuous flow or a series of discreet pulses or eruptive phases, the actual duration of the emplacement may have been much longer. This raises an important issue regarding the origin of terracing within pahoehoe flow fields. Are the plateaus, the result of successive flows stacking on top of one another in a “layer cake” fashion? Are they the result of a single sheet-like lobe with differential inflation occuring due to the development of preferred lava emplacement? Or is the construction process more complicated, with initial lobes helping to confine subsequent flows that in turn invert the topography through inflation and create new barriers for later flows? Mapping of flow units exposed in the Lava Falls area provides evidence in support of the latter scenario, with at least 10 phases of lava emplacement that can be distinguished on the basis of cross-cutting relationships, crack patterns, and surface textures. The later phases of lava emplacement produced small flows and lobes that originated from different levels within the extensive plateaus implying a complex emplacement and cooling history of the overall flow. This implies that partial burial of plateaus margins by younger breakouts may obscure the true thickness of the each plateau and that the overall duration of emplacement was much longer than the crack depths at any one location would suggest.