![]() |
![]() |
|
![]() |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Logbook | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
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 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. |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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.
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. |