Monterey Bay Aquarium Research Institute

Gorda Ridge Cruise
August 5 - 21, 2002

August 15, 2000: Day #11

Dave Clague writes: Dive 196 explored a cluster of small volcanic cones at the northernmost end of Escanaba Trough. The eastern cones are relatively young, as seen by their thin sediment cover. In contrast, the western ones have thick sediment cover and are inferred to be much older. Most of the cones have pillow lavas exposed on their lowest flanks, talus along the steep slopes halfway to the summit, and truncated or elongate pillows near the top of the slope.

The cone tops tend to be fairly flat and to consist of lobate flows with less pillows. Many of the pillows are unusual in being decorated with numerous small glassy buds that resemble small elephant trunks. These are extruded from cracks in the smoother pillow surface. Many of the lobate flows, particularly on the flat summits, have drained and collapsed.

We recovered 33 rocks and 5 pushcores from seven cones. The pushcores we used were modified by addition of core catchers and were highly effective in collecting fine volcanic rubble on the lower flanks to obtain as wide a range of lava compositions as possible. Nearly all the volcanic fragments recovered in the cores are glass. One surprise is that two samples from the last, largest, and only cratered volcanic cone are moderately vesicular. Such high abundances of magmatic volatiles suggest that these are alkalic basalts-previously unknown from the Gorda Ridge axis. Many of the recovered samples are glassy pillow buds and thin roofs from collapsed lobate flows. Recovery of these delicate samples would not have been possible but for the dexterity of the Tiburon vehicle and the skill of the ROV pilots.

During the night, we switched from the rock crusher to the gravity corer. The rock crusher was deployed 30 times and recovered good samples 26 times and traces of glass 3 times. Our first two gravity cores recovered 127 cm and 36 cm of sediments from the northern Escanaba Trough hydrothermal vent region. Today we dive at the northern Escanaba Trough hydrothermal field to collect additional water samples and to explore the area to the north of the known active vents.

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