Dive started from
the western margin of a sheet flow which covers the north to east floor of the
dive area and went to the south along the flow. The sheet flow is thin (less
than 3 m) and show flat surface with ropy wrinkles. Pressure ridge is often
observed. Pillow mound is often recognized in the surface. Pillow lavas are
distributed near the flank of the cone. These lava flows are covered by thin
mud layer, and the lava samples are coated by thin Mn layer (1 mm). These
suggest that the flows are much younger than previously investigated flows and
cones of the SW Oahu volcanic field. The submersible went up to lower ridge of
the composite cone. At the flank, platy or flat layered sedimentary rocks are
exposed widely. Also talus deposits of brecciated sedimentary rocks are
distributed at the flank. Pillow lobes are rarely found at the slope. The slope is covered by thick mud, compared
with the surface of the sheet flow. The submersible tried to go to the upper
part of the cone. However, we could not reach there. The structure of the cone
could not be understood.
SWオアフ火山地域の西部に分布するシート溶岩と複合火山丘の調査を行った。潜水艇はシート溶岩流西方の基底部に着底し，溶岩流の西縁に沿って南下した。シート溶岩流は薄く（3m以下）、表面は溶岩しわが発達し、プレッシャーリッジも認められる。tumulusやピロー丘も認められた。南方の火山丘の麓に近い部分では枕状溶岩流も認められた。これらの溶岩を覆う泥は薄く、採取されたサンプルは薄くMn被覆されていた(＜1.5mm）。これらのことからこのシート溶岩流は、これまで調査されたSWオハフ火山地域の噴出物と比較すると、その年代はかなり若いと考えられる。複合火山丘のうち下部の火山体斜面には、平板状の泥岩が広く分布している。またそれらの堆積岩を主体とする崖錐堆積物が覆っている。まれにピローローブも認められた。これらはシート溶岩の表面より厚く泥に覆われている。火山体下部の調査後、山頂部に向かったが時間切れとなったため、複合火山丘の構造は明らかにはならなかった。
Three dives by
2001 Kaiko and S701 have investigated newly recognized SW Oahu volcanic field.
Also, many samples were collected during the dives. However, these dives
focused on central to eastern area of the field. In order to reveal the
constituent and variations of the lavas of the SW Oahu volcanic field, we have
to carry out dives for other areas.
This dive is the
first dive for western area of the volcanic field. The main targets are widely distributed
sheet flows and a composite cone. The sheet flow can be recognized by side scan
sonar images. The western edge of the sheet flow locates at around the dive
area. A composite cone is 4 km X 1.5 km and elongates NW-SE directions. The
cone is composed of lower bench overlined by the upper cones. The lower bench
is about 140 m high from the floor. There exist two overlying cones which have
260 – 320 m in height.
We plan to
investigate the sheet flow at first, and then try to go up the upper cone.
Collecting samples would give us the information of regional variations of both
age and chemical comopostions of the SW Oahu volcanic field.
We landed muddy
bottom, and performed push core sampling. Using sonar scanning, we looked for
the out crops of the sheet flow. We reached the western edge of the sheet flow,
and moved southward along the flow. During moving, we often see the sheet flow
in thin, flat muddy surface. Small mounds of pillow lobes (tumulus) are also
exposed on mud. The sheet flow is covered by thin mud, suggesting the flow may
be young. A lobate sheet flow is composed of jointed lava crust with wrinkles
surface. Slightly inflated dome-like surface and collapsed hollow lobe-like
structure can be observed. Also, pillow lobes decorate a up lift of the surce,
and form tumulus. After sampling, we move southward along the possible edge of
the sheet flow. In some case, some bulbous pillow lobes and a collapsed hollow
lobe can be seen. Sporadic exposures of lava surface beneath mud, suggesting
thin mud cover. Collected samples are coated by thin Mn layer (less than 1.5
mm). These thin mud cover and thin Mn-coating that eruptive age of the sheet
flow is quite young, possibly < 1 Ma.
After
collecting sample #6 from lobate sheet flow, we turned toward the composite
cone. At the lower slope of the cone, mud stone layer is exposed. Blocks of mud
stone are also scattered on the muddy slope. We collected a big block of mud
stone from a veneer mantling the slope. The slope of these mud stone would be
formed by uprifting due to injection of magma. Talus of rounded blocks can be
also seen at the slope. It could be suggested that many of the blocks are soft
mud stones, because these could not be handled to crush. However, blocks of
pillow lobe are sporadically found in rubble. We skipped sampling these pillows
to approach the upper part of the cone. We reached at the top of the NE lower
ridge of the cone. The surface is covered by thick sediment. Then, we left the
bottom. Unfortunately, we could not reveal whole constituent of the composite
cone.
For sheet lava flow: Loc. 1, 12:06:50-12:19:15, pressure ridge (?) of sheet flow; 12:20:41-12:33:55, tumulus, lobe with wrinkle; 13:05:50- 13:06:14, Pahoehoe lava with ropy wrinkles; Sample 5, 13:35:00-13:44:35, tumulus, pillow lobe; Sample 6, 14:11:38- 14:15:10, Sheet flow, pressure ridge, pahoepahoe lobe ; No sample, For a composite cone: 14:22:00-14:23:50, Talus of mud stone; Sample 7, 14:28:45-14:37:00, Talus of lavas (?) and mud (?); No sample, 14:40:29-14:56:40, Talus of mud stone sometimes with broken pillow
Dive log
Time Depth (m) Heading (X) (Y) Descriptions Sample
11:45 4381 1730 830 On bottom, mud floor
11:46 Try push core at landing site.
Dropped black PC
11:53 Sampled mud Blue PC
11:53 4381 180 Start moving
11:58 4379 179 Heading to souwth
12:01 4376 152 Turned heading
12:06 4378 96 1575 909
12:07 4378 1577 905 Jointed lava crust. Large pit ahead.
Try sampling #1
Some inflated dome-like lava surface
and collapsed hollow lobe-like structures
can be seen. Small tubular lobes outgrow
from the dome-like surface. Apparent pillow
lobes decorating a domal uplift (tumulus)
on a lobate sheet flow.
12:19 Sampled couble-sized lava clast #2A
12:24 Second sample of lava #2B
12:31 1580 920 Third sample of wrinkled crust #2C
12:34 4377 118 Start moving. Heading 210 degree.
Flat muddy bottom
12:38 4377 120 1536 1018 Small mound of lobes of lava exposed on mud
12:41 4373 118 1512 1071 Landed on an outcrop of lava lobes
with a rugged surface. Some bulbous
pillow lobes and a collapsed hollow
lobe can be seen.
Sampled boulder-sized lava block #3A
Sampled an elongate lava clast #3B
1500 1070 Sampled lava #3C
12:56 4372 157 Start moving to 180 degree
12:58 4373 182 Moving on a flat land covered with mud.
13:06 4373 192 1210 1050 Passed an inflated lobate sheet
13:37 4375 181 1020 1009 Mooving on muddy flat land.
Blue fish
13:15 4373 182 981 1001 Stop for sampling in front of
outflows of fractured pahoehoe lobes
and pillows.
13:21 Sampled lava #4A
13:23 Sampled elongate pieace of lava #4B
13:23 Sampled lava #4C
13:25 980 1010 Start moving to 180 degree
13:28 4374 180 Moving over collapsed lobes?
Embedded in mud
13:32 4373 177 827 976 Wrinkled sheet flow. Stop for sampling
13:41 4372 820 970 Sampled a big ropy wrinkled crust #5
13:42 Start for 190 degree
13:48 4372 188 Heading to souwth on muddy floor
Trouble in no.2 battery
14:00 4371 191 470 890 Battery recovered. Moving on muddy floor.
Wrinkled surface underneath the mud?
14:06 4371 191 220 870 Still mud
14:07 4368 184 170 858 Lobate sheet flow. Stop for sampling
14:16 170 860 Sampled a pillow lobe #6
14:16 Start moving to 210 degree
14:18 4364 211 Heading to 210 degree. Collapsed lobes
14:23 4363 230 123 771 Wrinkled sheet flow on the bottom of a pit?
14:26 4368 231 50 690 Sporadic exposures of lava surface
beneath mud, suggesting thin mud cover.
14:29 4352 231 Fragmented lava surface?
14:32 4349 -8.7 607 Sampled a big block from a veneer
mantling the slope. #7
14:35 Start moving to 230 degree
14:38 4340 231 -29 585
14:40 4338 231 Rubble?
14:43 4348 231 -76 421 Rubble? of subround blocks
14:44 4354 233 Stop for sampling in front of rubble
of subround blocks
14:53 4350 -100 480 Tried sampling but abandoned
15:00 4302 228 Elongate pillow on rubbly slope
15:02 4287 229 Slope becomes gentle and pahoehoe
lobes appear.
15:04 4270 229 -170 350 Muddy floor with occasional exposures
of lava.
15:06 4247 230 Pillows
15:09 4221 230 -256 224 Turn to 190 degree on the ridge, covered
with flat lying bed? or pahoehoe lava,
covered with mud.
15:16 4217 107 Going upslope
15:20 4229 -390 260 Lost sonar reflection
15:22 4230 Heading to 230 degree
15:27 4226 231 -496 267 Moving on flat mud
15:32 4206 231 Left bottom