This dive focused
on the structure of the flat-topped cone at Kauai Channel and collected lava
samples. In addition, survey of the ridge which is partially covered by the
cone. Dive started from NE ocean floor of the cone, and went to west toward the
flank of the ridge. Although moderate backscattered image is recognized at the
floor, thick mud is covered it entirely. However, considering the sea
condition, we abandoned the survey of the ridge, and climbed up the NE slope of
the cone. The lower slope of the cone is covered by blocks of subrounded lavas
and pillow lobes. These blocks are coated by various thickness of Mn, which is
much less than the coating of the sample from the outcrop. Pillow lavas are
sporadically found in mud at middle to upper part of the slope. These lavas are
covered by relatively thick mud. Near the top, lobate pillow lava is widely
distributed. Although we reached near the rim of the summit crater, we had to
leave the bottom without entering into the crater. In summary, the cone is one
of the typical flat top cone, in which pillow lava is effused the summit crater
and overflow outside of the crater. The slope is covered by lavas of relatively
later stage of the formation of the cone and by talus.
オアフ島とカウアイ島の間のKauai海峡に点在する火山のひとつである、平頂火山丘の構造調査と溶岩試料の採取を目的として潜行は行われた。そのほかに平頂火山丘に続くリッジの調査も目的とした。潜水艇は平頂火山丘北東部の海洋底に着底し、リッジを目指した。後方反射像によるとやや強めのイメージが着底地点周辺で認められたが、実際には泥が覆われ露頭は確認できなかった。海況が悪いことも考慮して、リッジの調査は断念し、平頂火山丘の調査に向かった。平頂丘の山麓部は、やや角のとれた角礫やピロー破片からなる崖錐が広く分布し、それらは中腹部まで続く。崖錐の合間にはピロー溶岩の露頭と考えられる部分を散見できた。これらは表面を泥で覆われている。崖錐のブロックのMn被覆の厚さは様々であった。中腹からはピロー溶岩の露頭が泥の合間に散在している。平頂部近くの肩にはロベート溶岩流が分布していた。海況の変化のため潜行は約1時間短縮され浮上を開始したため、山頂火口内に入ることはできなかったが、地形から判断して溶岩湖が形成されていたと考える。今回の潜行で調査した平頂火山丘は、山頂火口から溶岩が溢れ出すことによって山体が成長するともに山麓部に崖錐が発達するという、これまで考えられてきた典型的な火山丘の成長史を辿ったと考えられる。
Gloria images have revealed that there exists the
volcanic field which consists of sheet flows and volcanic cones. These could
provide important information not only on rejuvenated volcanism of Ohahu and Kauai
islands but also on the presence of the SW Oahu volcanic field. However, the
volcanic field (Kauai channel volcanic field) had not been investigated until
2001 Hawaii cruise. 2001 Kalikow dive (K204) focused on the cone of the
volcanic field. The dive revealed the structure of the cone, and collected
samples. Collected samples are alkaline, and show similar chemical features of
rejuvenated volcanism, such as Honolulu volcanics. Preliminary K-Ar age of one
of the sample is about 0.4 Ma.
Beside the geochemical features, K204 revealed that
samples become gradually differentiated with increasing the height. This cannot
be explained by a simple growth model of the volcanic cone, in which lava
overflows from the summit crater toward the flank. In the simple model, the
cone is covered by lavas of relatively later stage, and by talus.
The main purpose
of the Shinkai dive (S706) is to collect samples to know chemical variations of
the rocks of the Kauai Channel volcanic field. In addition, we try to
investigate whether stratigraphy controlled compostional variations in the K204
cone is common in the volcanic field.
The flat-topped cone of the target of the dive S706
locate southwest of the K-204 cone. The cone is situated at southern end of N-S
trending ridge, and is XX km in diameter and YYY m in height. The cone has the
flat top of ZZZ m in a diameter. According to back-scattered images, the
possible outcrops may be exposed at the flamk of the cone, eastern flank of the
N-S trending ridge, and ocean floor northeast of the cone.
This dive was planned at first as follows: The Shinkai
will land at the ocean floor and go to eastern slope of the ridge. During this,
we will find the outcrop possibly shown by the back-scattered images. Then, we
will climb up the ridge to investigate the constituent of the ridge. We will
turn to southwest toward the cone, and will go up along the northeastern slope
of the cone. We expected to find outcrop of pillow lavas and/or talus. In final,
we will go into the summit crater, and then leave the bottom.
We landed muddy bottom, and performed push core
sampling. Using sonar searching, we looked for the outcrops. However, we could
not find both sonar images and outcrops. We need to go to main target as soon
as possible, because the sea condition had not been so good. Thus, we abandoned
to go to the ridge, and turned the course to go to the cone directly.
When we reached the deepest part of the northeastern
flank of the cone (4500 m in depth), we found the outcrop of pillow lavas which
is partially covered by the talus. After we collected sample, we climbed up the
flank. The slope is covered widely by talus. The talus continued until middle
part of the slope (4420 m in depth). The talus is usually composed of
subangular lava block with broken pillow. However, outcrops of pillow lavas
might be sporadically exposed among the talus. We collected samples from the
talus. We continued to swim along the slope. The middle part of the slope of
the cone ( ~ 4400 m in depth) consists mainly of pillow lavas with the talus. Some
pillow lobes are partially broken, showing radially jointed crusts with holes.
These features may indicate that the talus was formed by the collapse of
advancing pillow front that were emplaced on the steep slope. We collected one
of these broken pillow. After that, commander decided to leave the submersible
from the bottom 30 minutes later. So we swam toward the summit crater. During
this flying, we recognized sporadically exposed pillow lavas with mud. We
landed at near the shoulder of the summit crater. Pillow lavas with relatively
thick mud were widely distributed. It was hard to collect sample, because
visibility became worse by mud due to followed current. It took nearly 15
minutes to take a sample, and the submersible leaft the bottom.
All the collected samples are similar olivine basalt
with poor vesicularity. Although size and modal volume of olivine phenocrysts
are variable among the samples, mineral assemblage of these did not change. This
indicates that the surface of the flat-topped cone is covered by pillow lavas,
which effused and overflowed from the summit crater. Overflowed lavas were
collapsed to form talus at the lower part of the steep slope. Less variations
in collected samples are inconsistent with petrological variations in a single
cone, K-204. Maximum thickness of Mn-coating is 5-6 mm, suggesting that age of
the lavas can be estimated to be about 2 Ma, much older than that (~0.4 Ma) of
K-204. The samples of the dive S706 could provide important information of
variations of both active age and magmatic compostions in volcanism at Kauai
Channel.
(1) 12:35:00-12:43:00,
cliff of pillow covered by talus of broken pillow,
(2) 12:44:40-12:47:00,
rubble of broken pillows,
(3) 12:55:45-12:58:50,
talus of angular blocks on the lower slope of the cone,
(4) 13:04:10 -13:07:45,
lobate pillow block,
(5) 13:21:01
-13:25:00, pillow lavas partially covered by talus,
(6) 13:45:15 -
13:46:40 pillow lavas,
(7) 14:15:44 –
14:18:00, 14:20:10 – 14:24:40, pillow lava on the shoulder of the cone
Time angle depth X Y description Sample Push core
(m)
11:19 158 3642 descending
11:23 178 3766 10 540 descending
11:34 336 4227 descending
11:38 82 4399 descending
11:50 303 4504 ocean floor apear; mud
11:53 74 4505 -130 530 on bottom
11:55 57 4504 headding to 45
12:03 59 4511 muddy bottom
12:09 77 4512 21.3 673 Push Core (green) green core
12:15 300 4511 15 681 changecourse
12:17 300 4507 50 620 muddy bottom
12:26 304 4504 150 400 muddy bottom
12:29 224 4500 muddy bottom
12:35 278 4498 outcrop; intact pillow
12:40 313 4496 talus with thick mud
12:47 261 4487 20 160 sampling from talus #1
12:50 269 4490 pillow
12:52 286 4488 talus with thick mud
12:57 282 4481 -5 125 sampling from talus #2
13:00 343 4480 0 130 abandon sampling
from pillow lava or talus
13:05 80 4462 lobate flow
13:07 4 4461 0 90 sampling from lobate flow #3
13:11 220 4471 headding to 220
13:13 220 4479 -30 60 muddy bottom
13:21 277 4453 -122 3 talus with thick mud
13:40 6 4432 -120 -20 sampling from lobate flow #4A, #4B
13:43 220 4429 headding to 220
13:46 220 4403 knoby pillow (botryoidal)
13:48 220 4400 -234 -123 muddy bottom surrounding
pillow
13:52 220 4373 -310 -170 muddy bottom
13:56 185 4355 pillow lava outcrop
covered by mud
14:05 272 4333 abandoned sampling
14:14 354 4331 attempt sampling
14:28 20 4328 -390 -270 sampling from pillow lava #5
14:29 30 4320 -362 -240 leave from the bottom