A New Look At The 1996 Gorda Ridge Eruption

Lava flows of known age are rare on the mid-ocean ridge system. The only such flow on the Gorda Ridge erupted during an intense seismic swarm in Feb.-March 1996. We revisited the 1996 eruption site on the northern Gorda Ridge with the ROV Tiburon in August 2005. The lava flow, although less than 10 years old, has accumulated noticeable sediment, the glossy sheen on the pillow flow is now a dull finish, and sessile fauna have begun to colonize it. With more extensive surveys, we found that the flow is significantly smaller than originally proposed, consisting of a northern mound extending from 42.688 °N to 42.675 °N and a small southern mound centered at 42.67 °N, separated by about 1 km of older flows. A Tiburon dive in 2002 also shows that the hypothesized flow 5-7 km farther south is all older, so the 1996 eruption was limited to the two small pillow mounds. The northern mound is unusual in that it consists of a narrow 80-90m tall pillow ridge surrounded by talus at the base. In places the tall narrow pillow ridge has vertical to overhanging slopes consisting of elongate and truncated pillow flows. On the northern edge of the flow, the 1996 talus directly overlies older flows. The talus and surrounding older flows are peppered with angular glassy debris spalled from the pillow fragments as they tumbled down the steep slopes. The top of the pillow mound consists of lobate to rounded pillow forms. The eruption produced little pyroclastic debris, as limu o Pele (bubble-wall fragments) is rare in all sediment and suction samples from on and adjacent to the 1996 flow. Some of the talus at the base of the 1996 pillow ridge is elongate 'lavacicles' that formed when elongate pillows disgorged over vertical or overhanging slopes, forming cylindrical stalactite-like lava forms up to 2-3 m long, with glassy rinds that are wrinkled on a millimeter scale, in contrast to smooth glassy rinds on pillow toes. A tall narrow pillow mound with abundant talus at its base would usually be interpreted as tectonically modified by subsequent faulting, and would therefore be considered an older flow. In this case, however, the talus formed during the eruption and is therefore a constructional landform. Exploration of the flows adjacent to the 1996 flow found other similar structures that are also interpreted to be relatively young based on their thin sediment cover and poorly developed faunal communities. Such narrow, tall pillow mounds most likely form during very low-flux extrusion of lava and may be a common constructional landform on slow- to moderate-spreading ridges, such as the northern Gorda Ridge.