Gorda Ridge Cruise
August 5 - 21, 2002
August 9, 2000: Day #5
Dave Clague writes: Dive 189 explored the youngest volcanoes in the President Jackson Seamount chain. This chain consists of a linear array of large volcanoes with multiple, offset, large calderas. At the southeastern end of the chain is a region of scattered small cones that represent the early stage of one of the larger volcanoes. We examined and collected samples from 9 of these cones.
The ocean crust beneath these small cones is about 1.9 million years old and the cones are thought to be about 1.2 million years old. We hope these samples, erupted directly from the mantle with no shallow storage in the ocean crust, will help us understand magma generation under the seamount chain and under the nearby ridge crest.
Despite the geologically young age estimates for these cones, they have thick sediment cover and the exposed pillow lavas have manganese-oxide crusts up to several centimeters thick. These crusts made sample collection a challenge, since the manganese cements the lava fragments together. We still managed to collect 24 lava samples, many with fresh glass rinds beneath the manganese. We also collected some benthic animals including a different large, predatory tunicate than those found on the seamounts offshore Monterey; a small Venus-flytrap anemone, also different from the ones seen on the seamounts offshore Monterey; a worm with brass-colored bristles (nicknamed the "golden cockroach"); a gorgonian; and a few small sponges.
The weather continues to subside and the seas are beginning to flatten out. Tomorrow we are diving near the base of another of the President Jackson Seamounts to collect a suite of generated-generated basalts exposed on a 200-m tall ridge-parallel fault. We will then proceed to examine and sample some additional small cones near the base of the easternmost large seamount. The wind continues to subside and the seas are slowly flattening out-a trend we hope continues.