21 February, 1999

John Sinton (transcript), Linda Popels (transcript), Bob Waters
What:
1. The EPR just north of 17° 29'S has an axial trough, whereas to the south it does not.
Why would it do that?
- younger to south, hence not yet collapsed???
- more voluminous to north, hence more drainage, more contraction etc. ??
2. The "troughed" axis is segmented into left-stepping en echelon bits. Do individual bits erupt lavas that differ in chemistry, volume or age?
3. Where are the contacts of the younger lava units in this region?

How:
1. Start and go far enough off axis to address question #3.
2. Visit and sample lavas from each en echelon trough bit, and from the uncollapsed axis.
3. Visit one of the larger channel-like structures, visible in the DSL 120 record.

Why:
It will be Sunday.

Way Points [Net origin = 17° 25.08'S, 113° 12.853'W]

1. -1420 -6500
2. -0700 -6410
3. -0300 -6220
4. -0310 -6450
5. +0500 -6770
6. -0520 -7110
7. -0580 -7380
8. -0340 -7470
9. -0450 -8130
10 +0190 -8300

Summary: The main objectives of this dive were to map contacts bounding the youngest flow along the axis near 17° 29'S, and to determine the eruptive characteristics in two contrasting areas: (1) where the axis has a deep collapse trough, and (2) south of 17° 29'S where the axis is relatively uncollapsed. Contacts were found on both the west and east sides of the axis north of 17° 29'S. In addition a deep collapsed channel structure was visited on the west side of the axis. This structure shows characteristics of lava lakes, including the presence of pillars and bathtub rings, but it obviously is part of a lava distribution system off axis. At one time lava must have run deep or ponded all along the length of this structure. It may represent a partially collapsed lava tube system. The roof of the structure is lobate lava, but the floor contains sheet flows. It is identical in structure, although somewhat less deep than the axial trough north of 17° 29'S.
A traverse to the east discovered a sheet lava channel running about 110 in otherwise lobate lava. This channel disappeared beneath lobate lavas where it apparently became roofed over. The eastern contact with older, more sedimented lobate lavas occurs at a depth of about 2630 m along this track, but then rises to about 2580 m just to the south. The presence of thin lobate lava cascading into a collapse pit (sample locality 8) where both the collapsed lava and the lava cascade have a generally similar sediment cover indicates that at least some of the collapse is syn-eruptive.
That part of the axis without an axial trough is covered with essentially untectonized lobate and especially pillow lavas. The correspondence of axial troughs with lobate and sheet lavas suggests that these troughs are closely related to eruption character and not to post-eruptive extension.

Sample Localities

1 17° 28.65'S 113° 13.76' W
2 17° 28.59   113° 13.46
3 17° 28.54   113° 13.30
4 17° 28.43   113° 13.12
5 17° 28.64   113° 13.11
6 17° 28.81   113° 12.62
7 17° 28.90   113° 13.02
8 17° 29.13   113° 13.13
9 17° 29.58   113° 13.20