Abstract:
In Hawaii, basaltic lavas can be readily divided into 2 major types: aa and pahoehoe [Macdonald, 1953]. Aa flows are defined by having angular, spinose, breccia at both the top and base of the flow and a massive interior. This massive interior typically contains angular vesicles and partially melted breccia clasts. Pahoehoe flows are characterized by piece-wise continuous surfaces that often include folds (ropes). Larger pahoehoe flows are inflated [Hon et al., 1994], producing a diagnostic three-part cross section with a vesicular flow top and dense core of roughly equal thickness and a thin vesicular base [Self et al., 1998]. The transition between the two flow types is controlled by a combination of lava viscosity and strain rate [Peterson and Tilling, 1980]. In Hawaii, transitional lava types are relatively uncommon and have been described as slab pahoehoe and spiny/sharkskin/toothpaste pahoehoe [e. g., Rowland and Walker, 1987]. However, none of these terms adequately describe a lava type found on the surfaces of many basaltic lava flows found in Iceland, the Kerguelen Plateau, the Columbia River Basalts, and elsewhere. These flows have been previously described as "brecciated", "scoriacious", and "aa". Unlike true aa flows, these flows are characterized by a flow top breccia dominated by fluidal fragments of pahoehoe lava. The base of the breccia typically grades into a coherent vesicular top. Many of these flows have dense interiors that contain partially re-melted vesicular breccia clasts. Flow base breccias are often absent. The vesicle shapes are generally distorted but sub-rounded. Details of this type of lava flow is reported from the Kerguelen Plateau as seen during ODP Leg 183 and from field work in this summer on the 1783 Laki flowfield in Iceland. We propose the name "rubbly pahoehoe" for this lava type.