Alvin Dive 3364 - Summary


Feb 25, 1999
Tracey Gregg (Transcript) and Anji Shah (Transcript)
Pilot: Steve Faluotico

Summary: When we first saw bottom, approximately 1.3 km west of the axial trough near 17°27'N, it was composed of flat-lying, Macdonald age 1.5 lobes. In the process of landing, we drifted onto a constructional slope (30-40°) of decorated pillows and lobes. The combination of macro- and microtopography made it appear that the constructional slope had less sediment cover than the flat-lying lobate lavas, although no contact was observed. We traveled upslope, and we observed a jumbled sheet flow which had been dramatically autobrecciated while flowing down the steep pillow slope. This jumbled flow also contained clots of lava that appeared to have been accreted while rolling in a lava channel; these clots were roughly 3 m in diameter. We crossed the contact into the "young lava" approximately 1.1 km west of the west wall of the axial trough at this latitude.
We identified the young lava on the basis of it's spotty sediment cover and fresh, black glass. The western margin of this flow is characterized by decorated pillows and buds. There are lineated and jumbled channels observed approximately 800 m west of the axis. These channels originate in small collapse features, and are onlapped by lobate lavas. This suggests that the first outpouring of lava during the eruption were high-effusion rate channeled flows, which were buried by subsequent lobate flows as the effusion waned.

Approximately 500 m west of the west axial wall (near the 2610 m SeaBeam contour line) we crossed a contact into Macdonald age 1.5-2.0 pillows and lobes. This increase in sediment cover corresponded with an observed increase in vent fauna (especially crabs, fish, and dandelions) and increased turbidity of the water. However, the superposition relations are obvious, and there is no doubt that the Macdonald age 1.5-2.0 pillows and lobes are a kipuka of older material, and not merely the result of increased hydrothermal sedimentation. The eastern boundary of this kipuka is not clearly defined, however. It appears that the sediment cover gradually decreases until ~200 m west of the axis (near the 2600 m SeaBeam contour), where the young lava can be observed clearly.
We followed a lineated, smooth, and jumbled channel up to the axis. The channel is bounded by collapse features at either margin, and ends in a ~2-4 m high collapse feature (capped by lobate lavas) located 10 m west of the axial wall.

We crossed the axis, and found the Gumbo black smoker, where we sampled tube worms for Cindy. The axial floor is filled with pillars, rubble, and lineated and chaotically folded sheet flows. The wall of the axial trough have an irregular planform, and are coated with bathtub rings. The axial wall locally projects several meters into the trough, in narrow "peninsulas" that may be 5 or more meters long but only 1-2 m wide. The axial trough is 8-10 m deep both here and farther to the south.

We followed a channel down the eastern flank of the axis, marked by folded and lineated lavas. This channel disappears by diving underground at an ~2-m-high collapse in lobate lavas, roughly 350 m east of the axial wall. A short distance farther east (~75 m), we encountered a series of elongate, aligned collapses floored with rubble, as well as folded and lineated lavas, which we interpret to be the surface expression of a lava tube.

As we turned back west to go back to the axis, we encountered 2 more kipukas of older pillows and lobes. These were not associated with any increased hydrothermal activity, and it is possible that these kipukas represent 2 different lava ages.

Lava channels are not as common on the eastern side of the axis as they are on the western side, as suggested both by our dive observations and by the DSL-120 data. However, we did follow a lineated channel a short distance up to the eastern wall of the axis. It ended in a ~3-m-high collapse feature within lobate lavas approximately 10 m away from the axial wall. Unfortunately, we could not see through the collapse to the axial trough.

Within the axis near 17°27.2'S, we collected portions of 2 pillars, approximately 100 m apart, as well as a portion of chaotically folded sheet flow from the axial floor. Although hydrothermal sediment was common on the pillars and especially on the piles of talus at the base of the axial wall and at the bases of pillars, we did not observe any present hydrothermal activity. Spaghetti worms were common all along the axial walls, and serpulid shells were common along the axial floor. Finally, orange and white hydrothermal staining was visible in the cracks and pockets of lava flows along the entire dive track, varying slightly in abundance.


Sample Locations:

Time	Number X	  Y	Depth	Lat(-17)	Lon(-113)
957	1	-1245	-3603	2709	27.033	13.556
1029	2	-760	-3617	2631	27.041	13.282
1154	3	367	-3462	2592	26.957	12.646
1243	4	1232	-3624	2601	27.045	12.157
1253	5	1247	-3841	2607	27.162	12.149
1323	6	288	-3951	2567	27.222	12.690
1415	7	208	-3929	2567	27.210	12.736
1423	8	245	-4030	2572	27.265	12.715
1432	9	245	-4030	2572	27.265	12.715
1453	10	225	-4050	2577	27.276	12.726


Navigation (GMT)