Summary: The objective for the first part of the dive
was to locate and sample a mussel bed at Animal farm for biodiversity
comparisons. We returned to the coordinates of the mussel bed
sampled on Dive 3343 (Animal Farm Site 1; x=3525, y=8433), measured
temperature (ambient=1.74°C; within mussels=1.84°C), and
collected one large (2-liter) box of mussels using a scoop. Subsequent
to this sampling, we undertook a short survey of the area in search
of a mussel bed with more vigorous water flow and healthier mussels.
Not finding this, we returned to within 20m of our site (Animal
farm Site 2; x=3530; y=8420) where there was a larger (10m vs
4m max dimension) mussel bed. Maximum temperature measured in
this mussel bed was 2.6°C. We tested one mussel pot sampler
but it failed due both to the single layer of mussels making up
the bed and to a snag in the cinch line. Five scoop samples (each
of ~2 liter volume) were collected from this site as well as an
incidental rock sample which was given a sample number post-dive.
The objective for the second part of the dive was to survey the
system of right-stepping en echelon fissures visible in the DSL-120
side scan record along the ridge axis. We proceeded southward
from the Animal Farm site and zig-zagged across the fissures to
assess their nature, width, and dimensions. All the lava flows
encountered were very glassy, with only a light dusting of sediments;
sessile macrofauna was present almost continuously, and anemones
were a common sight. We estimate a relative age of the lavas of
1.0 on the Macdonald age scale, similar to previous age estimates
for the Animal Farm lava flow. Each crossing of the fissures revealed
similar features. The depths of the fissures estimated from Alvin's
pressure sensor and down-looking sonar vary between 5 and 9 m.
Their widths are estimated to be 40-60m (although all crossings
were oblique to their strike, DSL 120 records suggest a strike
of N010° near 18°36'S). Untectonized lobate flows occurred
a short distance from the fissures, sometimes grading into sheet
flow and jumbled sheet flow near their rims. The walls of the
fissures never exposed fresh break (= these fissures are eruptive
fissures). Sheet flows draping down the walls of the fissures
like waterfalls were observed on several occasions. The upper
crust of these cascading sheet flows is fragmented into imbricated
pieces in the upper reaches of the wall (= the fragments are still
plastered on the cascading sheet flow, frozen-in-action). Down
in the face of the walls, fragmented sheet flows were common,
and behind the draped flow, "bathtub-ring-like" layering
a few cm thick. Lava pillars have been observed only near 18°36.8'-36.9'S.
Most of the time, the floor of the fissure was lined by lineated
sheet flow (with lineations striking sub-parallel to the trend
of the fissure), and jumbled, fragmented sheet flow. These observations
suggest a high eruption rate which infilled the fissure and flooded
the flanks with lobate flows, followed by a drainback so rapid
that the lavas just erupted on the rims cascaded back down to
the bottom of the fissure.
Samples collected
# Time Latitude(S) Longitude(W)Depth (m) Site Description 3349-1 10:37 18°36.439' 113°24.025' 2677 Animal Farm mussel bed sampled with "mussel pot" (turned out to be empty) 3349-2 10:48 18°36.433' 113°23.993' 2677 Animal Farm ~2 liters of mussels; fragments of jumbled sheet flow 3349-3 11:51 18°36.433' 113°23.993' 2677 Animal Farm ~2 liters of mussels 3349-4 12:06 18°36.433' 113°23.993' 2677 Animal Farm 2 liters of mussels 3349-5 12:20 18°36.433' 113°23.993' 2677 Animal Farm 2 liters of mussels 3349-6 12:59 18°36.431' 113°23.997' 2677 Animal Farm 2 liters of mussels 3349-7 13:02 18°36.431' 113°23.993' 2676 Animal Farm 2liters of mussels 3349-8 13:37 18°36.431' 113°23.993' 2676 Animal Farm 2 liters of mussels 3349-9 13:54 18°36.664' 113°23.958' 2672 east of ASC large slab of lobate flow 3349-10 14:39 18°37.081' 113°24.152' 2679 bottom of ASC 3 pieces of sheet flow 3349-11 15:03 18°37.165' 113°24.167' 2679 bottom of ASC small piece of sheet flow
Plots:
