Constructional Talus: Formed during Eruption, Not By Later Tectonism
Abstract:Talus does not always indicate mass wasting due to destructive faulting, landslides, or caldera collapse. That talus slopes might be constructed during submarine volcanic eruptions was first recognized in 2005 from ROV observations and samples of lava that dripped over a cliff of a steep pillow mound of the 1996 eruption on the Gorda Ridge. Such talus was also found in 2013 along the steep pillow ridge that formed on the South Rift of Axial Volcano during the 2011 eruption. At each site, wedges and stubby cylinders of broken glass-rimmed pillow lava lie at the base of a vertical cliff of truncated pillows, as they do in other places where faults may have broken them. However, more telling evidence of this process is slender, elongate glassy rods of lava, which we call “lavacicles” from their resemblance to icicles, which were lying below the overhanging cliffs from which they had dripped and fallen. The drips can be phyric and aphyric lava, primitive to evolved in composition. Therefore they don’t indicate a particular basaltic composition or viscosity; they probably result from very slow eruption rate that builds a steep stack of pillows.
In 1 meter resolution AUV bathymetric maps, these talus slopes appear much like talus found at the base of fault-block slices at other parts of those ridges and at the highly faulted Endeavour Segment, however these pillow ridges are too young to have been tectonically faulted. Their orientations are not necessarily aligned with the dominant strike of faults in the area, but instead with the outline of the pillow mounds. We now have identified numerous other constructional talus slopes at Gorda and Juan de Fuca Ridges, Alarcon Rise, and Davidson Seamount off California that meet this criterion so are likely to be constructional also. At intermediate rate ridges constructional talus and cliffs contribute a small amount of the area, but perhaps significant permeability pathways for hydrothermal circulation.