Monterey Bay Aquarium Research Institute

Gorda Ridge Cruise
August 5 - 21, 2002

August 17, 2000: Day #13

Dave Clague writes: Dive 198 explored an extensive young lava flow at the northern Escanaba Trough site. We dove into an elongate crater at the summit of a lava shield, only to find the entire shield was extensively fractured and fissured, with most of the surface consisting of talus. Reddish-orange hydrothermal sediment partially filled the depression at the summit. As we traversed down the northwest slope, the surface was offset by numerous faults, in each case with the upthrown block towards the summit.

Near the base of the shield, the faulted terrain was covered by younger pillow lavas, apparently erupted from west of the summit of the shield. The shield is mainly a tectonic feature that has been uplifted after eruptions that formed the shield. Continuing down slope to the northwest, the flows become increasingly lobate and eventually sheet-like, commonly with folded surfaces. The edges of the flow in the proximal part of the flow are pillow lavas whereas the distal edges are chaotically jumbled sheet flows. The younger flows have no fissures or fractures, even near the eastern edge of one of the uplifted sediment hills, indicating that the late parts of the eruption postdate the tectonic uplift that formed the hills.

Beneath the unfractured flow is another flow that now drapes hummocky sediment hills. This bottom flow is heavily fractured and broken up, indicating that it predates the tectonic activity that formed the hills. However, the sediment cover on both flows is similar, suggesting that tectonic uplift and volcanism occurred in the same timeframe. The sediment cover on the flow suggests that it is several hundred years old, which in turn suggests that the hydrothermal activity (most of which is now completed) started several hundreds of years ago.

We collected 15 lava samples from the early and late flows and cored massive sulfides at two sites. During the evening we also did three gravity cores on the central hill at the northern Escanaba hydrothermal site, recovering sediments that smelled slightly of hydrocarbons. On Saturday and Sunday we will explore the southern Escanaba Trough hydrothermal site and an unexplored region just south of the known hydrothermal deposits. Both areas have thick shallow sills of volcanics intruded into the sediments. We hope to recover lavas from the margins of these sills and sulfides from inactive hydrothermal deposits.

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