Alvin Dive 3351 - Dive Summary

Lava Lake 18 08-18 18S first dive in area.

Macdonald (transcript) Boyle, Heintz;

LAUNCH POINT: 18º 08.30' S, 113º 20.5' W

Rock Czar: Bergmanis

Objectives:

  • To map out northern extent of lava lake
  • Map out possible more recent lava flow north of lava lake.
  • Estimate flow thickness and extent.
  • Estimate amount of extension versus collapse.
  • Inspect inner and outer walls of axial trough.
  • Keep Matt under control.

    • Notes:

  • Between waypts 1 and 6 there is a transition in the DSL records between a smooth relatively unfissured inner floor and an intensely fissured and faulted inner floor similar to that seen on John's dive on the so-called Auzende flow.
  • DSL side scan records indicate a really neat overflow of the inner wall by lava near waypts 8, 9
  • Near waypt 9-10, there is a regional change in depth of 30-50 m, shallower to the S. This may be the N limit of the lava lake flow, it may also be a significant discontinuity even though the offset is nil. Existing rock chemistry seems to change here too.

    • waypts:

    1  -18 08.30, -113 20.5
2  -18 08.47 -113 20.67
3  -18 08.54 -113 20.30
4  -18 08.85 -113 20.53
5  -18 09.20 -113 20.90
6  -18 09.20 -113 20.70
7  -18 09.20 -113 20.30
8  -18 09.65 -113 20.60
9  -18 09.70 -113 20.80
10 -18 10.00 -113 20.80
11 -18 10.31 -113 20.95
12 -18 10.40 -113 20.70
13 -18 10.70 -113 20.10
14 -18 10.70 -113 19.50

     

    Dive Summary: We explored the floor of the axial trough from ~18 08.5S to 18 10S as well as one traverse up the east wall at ~18 09.5S. The age range of rocks observed was 2.0-3.5; nowhere did we see young lavas in the age 0-1.5 range. The northern part of the area was oldest and was intensely fissured. The southern end of the dive was the youngest with age 2.0 elongate pillow lavas on the flank of a pillow mound near 18 10S This is also the location of a ridge axis discontinuity of some kind. Three different kinds of local linear topographic highs were observed within the floor of the axial trough. One, which is commonly seen on many dives here, is a horst, 1-20m wide and about ten times as long as it is wide. These horsts are bounded by vertical tensional faults. They are about 5-10m high. Another kind of topographic high is the lava lake remnant. The boundaries are very sinuous and glassy selvidges or bathtub rings are often seen on the walls. The third kind is a new one to me and were first observed during our 1996 ARGO runs: it is a volcanic edifice, perhaps a lava lake remnant or a linear array of lava pillars, which is topped by and often entirely masked by sulfide chimney deposits. These "hydro-volcanic constructs" are similar in dimensions to the horsts and are often linear with a strike parallel to that of the overall axial trough floor.

    Many dead sulfide chimneys were observed in addition to one active black smoker which belched out effluent from a crack in a dead sulfide field rather than from a chimney. No extensive biological communities were observed, but there were a few crabs and Alvinella worms. Samples were collected here.

    The east wall is characterized by narrow faulted slivers which appear to by calving off of the main trough wall into and toward the axial trough floor.

    The dive was called up about 1 1/2 hrs early due to weather concerns at the surface as the bow thruster was broken.

    Rock sample locations [taken from final dive map]:

    1. -118° 08.29, -113° 20.53
2.       08.44 S      20.59W
3.       08.44        20.59
4.       08.44        20.59
5.       08.44        20.59
6.       08.67        20.59
7.       08.86        20.52
8.       09.21        20.60
9.       09.21        20.50
10.      09.63        20.57
11.      09.67        20.62
12.      09.88        20.71
13.      09.93        20.72

    Plots:

    Navigation (GMT)