Monterey Bay Aquarium Research Institute

West Coast Expedition
July 20 - August 30, 2002
West Coast of North America

July 26, 2002: Day #7

Log Entry: Our dive today at the President Jackson Seamounts finished a couple hours early due to the weather. We're in gale-force conditions that are expected to last through tomorrow. The ship is taking some good slams. But we've all got our sea-legs, so even though we staggered around, our push-core extrusions, rock processing and equipment turn-around went like clock-work.

As we were unloading the samples from the vehicle following our dive, Buzz was rather dampened by a water jet that erupted when a wave slapped the underside of the closed moon-pool doors, upon which he was standing. Buzz anticipated it, dancing away from the doors' seam when the wave hit, but the motion of the ship placed him squarely back over it. You should have seen the look on his face! The part of the geyser that didn't get stopped by Buzz rose higher than the top of the vehicle, perhaps 4 meters.

- Jenny Paduan

Dive 453 Report
Today's dive was on one of the eastern submarine volcanoes in the President Jackson Seamounts, a linear chain of volcanoes formed adjacent to the Gorda Ridge on ocean crust between 2 and 4 million years old. These seamounts are striking because they have complex nested calderas and lava shields built in their summit regions. The nested calderas are younger to the east, towards the ridge axis, so each exposes a series of lavas in their steep inner walls. By looking at several adjacent calderas of different ages we get a time series of the composition of the lavas that constructed the volcanoes. Several of these volcanoes were sampled in the 1980's, but no dives have been done here nor have there been any camera tows to examine the types of flows that make up the seamounts. The dive we did progressed up the steep south-facing wall of one of the youngest calderas, drove along the surface of a large lava shield, dropped down into a slightly older caldera and progressed up its north-facing wall. We then crossed over a small ridge separating these two calderas from an even older, circular caldera with a deep collapse pit inside. The walls of each caldera were different. The first started with pillow basalt at the base, then moved upwards through a thick deposit of talus consisting of basalt fragments and blocks. Most of the fragments are angular pieces of lava flow interiors rather than pillow basalt fragments. As we moved above the talus, a series of thick lava flows were found interbedded with volcanic breccia and pillow basalt. However, most of the section, consists of thick lava flows. Such thick flows usually form when lava ponds, commonly inside a caldera. The second caldera wall is mostly thin sheet flows and small pillow lavas, once we got above the talus slope. The final caldera wall again consists mostly of thick lava flows with some fragmental rocks. Most of the lava on the surface of the shield is sheet-like flows that drape the irregular surface and form channels, but ropey surfaces could only rarely be seen through the thick crusts of manganese oxides. These lava flows are very fragile and difficult to sample. The morphologies suggest that they were very fluid and had high eruption rates. We also observed and collected some fragmental volcanic rocks that may have formed during explosive eruptions when the calderas collapsed and water could enter the magma system and cause steam-driven explosive events. Such eruptions clearly occurred on Loihi Seamount, offshore Hawaii, at depths as great as 1300 m, so we were searching for evidence on this seamount for similar explosive activity. In order to sample fine-grained fragments like those produced during such eruptions, we collected a number of short sediment cores and separated the sand-sized particles from the mud. More work on these samples, once we are back on shore, should enable us to determine what the eruption styles were like when this seamount formed about 2 million years ago. There were also a few biologic surprises on the seamount. One was that gorgonians were exceedingly rare and occurred only at the shallowest depths. This is in contrast to seamounts offshore central California where they are a major component of the ecosystem at all depths. Another surprise was that we encountered a clam that we had only seen previously on Davidson Seamount. Yet another was the scarcity of sponges, but the abundance of stalked crinoids. The seamount is a veritable desert compared with some of the seamounts offshore central California. During the ascent at the end of the dive we also briefly observed a large jellyfish, mainly known from a dozen or so sightings on seamounts near Monterey Bay.

--Dave Clague

StarT452.jpg (45218 bytes)
From yesterday's dive, here is a composite image of a seastar eating a sea-pen. The seastar crawled up the stem, eating the polyps as it went.
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Part way up the >500m high wall of the youngest caldera surveyed on dive 453. Fragmental material (volcanic breccia) is at the bottom of this image, overlain by pillow lavas (which were truncated when the caldera collapsed, so we see them in cross section), then by thin sheet flows, then by a massive flow. This sequence was repeated numerous times up the wall of this caldera.
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Sample being collected by the manipulator arm from a sheet flow below a massive flow.
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Ropy sheet flow on the surface of the shield. Sheet flows in the deep sea can only occur where eruptions are effusive enough to counter the chilling effect of seawater.


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