April 10th, 2003; Leg 4, Day #7

We did a very extensive visual survey (about 6 kilometers over the bottom) at the site of the hydrothermal plume, but found no sign of a hydrothermal vent or its associated community. The axial valley of this mid-ocean ridge is an asymmetrical ridge with steep faulted terrain to the northwest within 50 meters of the axis, whereas the southeast side extends with only small fault scarps to at least 700 meters from the axis. Along the axis, the youngest flow in the survey area extends for at least 1.8 km. However, it is not a flow erupted within the last 50-100 years since it has substantial sediment cover. On the other hand, it is a very interesting flow since it consists of sheet flows that have been extensively folded and broken during emplacement to form a jumbled sheet flow. 

For those who are familiar with on-land lava flows, this is most similar in morphology to slabby 'a'a or the thin-crusted pahoehoe slabs formed near eruptive vents. The flows in the rest of the survey area range from flat sheet flows with total sediment cover, to lobate flows, to pillow flows. We collected a few samples of the lava for future chemical analysis to accompany the interpretation of eruption style. The axial flow is both extensive and, based on the observed morphology, erupted at high volume rates compared to most other mid-ocean ridge basalt. 

We also collected several push cores and vacuumed some sediments to search for fragments of tiny glass bubbles that would indicate that eruptions have a minor component that is mildly explosive. Similar fragments, called limu o Pele (Pele's seaweed) were first found in Hawaii where lava pours into the sea and forms steam explosive secondary eruptions. Since then, similar fragments have been described from a seamount near the East Pacific Rise, Loihi Seamount, and the Gorda mid-ocean ridge offshore northern California. Our cores recovered similar fragments for the first time from the East Pacific Rise. 

The more widespread distribution of such particles suggests that mid-ocean ridge eruptions may usually have mildly explosive activity at the erupting fissures. There is an ongoing debate whether these explosions are driven by steam derived from water-saturated sediments covered by the flows or by release of volatiles (mainly carbon dioxide) that is released from the magma during eruption. I think they are driven by release of volatiles from the magma, and the discovery of such particles along a ridge crest that has almost no sediment supports the idea that magmatic volatiles drive the mild explosions.  

In addition to the exciting volcanological results outlined above, we also discovered a few animals that were new to us all. In particular, we found a sea cucumber that had attached numerous brachiopod shells (similar to small clam shells) to the outside of his body, perhaps to avoid detection (although it actually made them rather conspicuous to us) or perhaps to make them less palatable. In any case, none of us had ever seen a cucumber that agglutinated particles to itself. We also saw several 8 cm long worms that have lovely golden 4 cm needles projecting from their bodies. When we first saw these, or a closely related worm on the Gorda Ridge several years ago, the geologists on board nicknamed him the "gold lamé worm" for his striking color.

To sum up, the search for hydrothermal vents along the Alarcon Rise did not find the vents nor any associated animal communities, but the dive still succeeded in narrowing the search by eliminating a fairly large region where we now know the vents do not occur. We also were quite successful in mapping and sampling the lava flows in the region and in finding evidence that mildly explosive eruptions occur at the Alarcon Rise as well as along the Gorda Ridge. At the end of the day, the geologist (me) was happy but the biologists were not.

Dave Clague

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