Date: 11 Sept. 2001
Place: Southern flank of the Hana Ridge
Observer: Takeshi Hanyu

Abstract

The dive track was designed to investigate vertical structure of the Hana Ridge. We recognized three different structural and magmatic features in the ridge by video observation and collected rocks. The lower part of the ridge (below -3400m) consists of pillow lavas, indicating submarine eruption to build this part of the ridge. The surface lavas in this part demonstrate different rock types; picrites and olivine basalts dominate below and above the depth of -4200m, respectively. This may suggest sequential or secular change of the magma supply system in the ridge. On the other hand, no in-situ pillow basalts were found above -3400m, and the slope was filled with talus blocks. Since they were rounded rocks, the upperpart of the ridge is interpreted to be brecciated subaerial rocks. Along the dive track we observed a mound structure on the flank; it is composed of sheet and pillow lavas. The ridge body would be built by repeated eruption to form this structure on the ridge flank.

概要

ダイブK214では、ハナリッヂの垂直方向の構造を調べるために、このリッヂの南側斜面を垂直に上るルートを設定した。カメラによる観察と採取した岩石から、この斜面は大きく３つの異なる構造により成り立っていることが分かった。水深３４００ｍより下部では主として枕状溶岩よりなる溶岩流で構成されており、海底で起こった火成活動でリッヂが形成された。この部分はさらに水深４２００ｍあたりを境界にして、それより下部ではピクライト、上部ではオリビンバサルトと岩石タイプが異なることが分かった。一方水深３４００ｍより浅い部分では、その場に噴出した溶岩は確認されず、斜面は破砕された岩石で埋められていた。これらの岩石は丸みを帯びていたことから、リッヂ上部は陸上の破砕された溶岩が堆積して作られたものと考えられる。また、リッヂ斜面上に点在するマウンド地形を観察した結果、これらは溶岩で構成されており、このような小規模な活動が繰り返されたことによりリッヂが成長したことが示唆される。

Location

Southern flank of Hana Ridge..

Objectives

The dive track was designed to transect several steep slopes and a small mound-like structure on the southern flank of the Hana Ridge. The steep slopes along the vertical track will provide good outcrops that are not covered by sediments to observe the structure of the ridge. Samples from them will be used to investigate possible secular change of chemical composition of the volcanic activity. Small mound-like structures are generally found on the flank of the ridge, and the dive will evaluate what they are. This dive shall be combined with two other dives on the Hana Ridge (K#212, 216) to demonstrate not only vertical variation but also lateral change of chemical composition of magmas that formed the ridge.

Dive summary

We landed on sediment-covered flat floor with scattered rocks. We went north along a shallow slope and saw a lobate sheet flow covered by thin sediments. The lobes are 20-50cm high and have wave length of 2-5 m. Two collected rock samples (#1, #2) from the floor were picritic basalts. The flow stream of the lavas was perpendicular to depth contours. When encountering shallow slopes, we saw talus fragments of pillow lavas scattered on the mud floor, then followed by pillow lava outcrops on relatively shallow slopes. In the first half of the dive track, pillow lava outcrops and sediment-covered slopes appeared repeatedly, and rock samples were collected from each lava flow. Both elongated and rounded pillows were seen, but the former ones were generally found on steep slopes. At the depth of 4250m autobrecciated lavas appeared. A relatively long and continuous pillow outcrop was recognized from the depth of 4100m to 4010m. The rocks from this lava flow (#7, #8) contain less olivine than the lower lava flows that consist of picrites. From the depth of 3900m to 3700m we passed over a very shallow slope on a bench of the ridge where rock fragments were occasionally scattered on the sedimentary floor. From the depth of 3700m we encountered the flank of the mound-like structure on a steep slope. We saw rounded pillow lavas at the edge of the flank, and the central slope was formed by sheet lava flows covered by thin sediment. Elongated pillow lavas were often seen between the central sheet lavas and rounded pillow lavas at the edge of the flow. At the top of the mound-like structure we saw big pillow lavas that are 2-3 m size. Then we traced a nearly flat surface above the mound-like structure. This surface was entirely covered by sediment with ripple marks that demonstrates strong water currents at this place. From the depth around 3400m some rock fragments appeared on the floor, increasing as we moved further. After passing the initially scheduled finishing point, we kept on climbing the uppermost slope of the ridge to the depth of 3130m. The slope was covered by brecciated lava fragments, which look like debris flow, but we could not find in-situ pillow lavas. We collected several loose rocks (#14, #15) and finished the survey at the depth of 3127m.
We collected 23 rock samples from 15 sites and two push core samples. From #1 to #13 the samples are pillow lavas or their fragments. #14 and #15 samples are rounded rocks. Rock types are picrites (#1-5, 9, 14-15) and olivine basalts (#6-8, 10-13) basalts. Samples #7 and 10 were plagioclase bearing. Some rocks have glassy rims up to 15mm thick on the pillow surface. Mn coating was recognized in all the samples, but the thickness was less than 1 mm thick except for sample #12 with 2.5 mm thick Mn-coating. Most of the rocks were moderaterly vesiculated (maximum 15%).

Dive interpretations

We recognized different lava morphology at the bottom, lower, and upper parts of the ridge. A relatively flat surface beneath the ridge consisted of sheet lavas. The flow direction of the lavas was perpendicular to the depth contours, suggesting that the lavas were supplied from the ridge. Outcrops on the lower part of the ridge flank were in-situ pillow basalts and pillow basalt breccias, demonstrating that the lower part is mostly composed of submarine erupted lavas. Several outcrops along the dive track were isolated each other, and flow direction was parallel to the dipping angle of the slope. These lava flows therefore erupted and reformed the surface repeatedly to build the ridge structure. In contrast we did not see any in-situ lavas on the upper part of the ridge (above -3400m); rather the slope was talus covered with rounded blocks, suggesting that the upper part of the ridge was formed by debris from subaerial erupted lavas that moved down the submarine slope. This might be consistent with the observation that mound-like structures produced by lava eruption (see below) are rare at the upper part of the southern flank of the ridge.
Regarding rock types, picrites dominated in the lowest part of the ridge (sample #1-5). In contrast, olivine basalts were generally found above it (#6-13 except for #9); then picrites appeared again in the talus blocks at the uppermost part of the ridge (#14-15). This suggests that (a) rock types changed from picrites to olivine basalts as the ridge was established or (b) the magma supply system is layered in the ridge, supplying different type of magmas.
The mound-like structure on the ridge flank was composed of sheet and pillow lavas, demonstrating this structure was produced by lavas erupted at this place and hanging on the flank. Since we see many structures of this kind on lower parts of the ridge flank, it seems magmas supplied through inner ridge occasionally reach the flank to cause flank eruption. It is not yet known if mound-like structure on the flank and flat-top cone structures on the ridge axis have similar origins, but this will be examined by dive #216, in which we will survey the uppermost part of the flank and flat-top cones on the ridge axis.

Video Highlights

9:33-9:46; sheet lava flows at the bottom of the ridge
10:12-10:15; pillow outcrop in the lower part of the ridge
10:24-10:29; autobrecciated lava
11:01-11:22; very long pillow lava flow
12:36-13:02; mound structure on the middle part of the ridge
13:57-14:35; talus blocks at the upperpart of the ridge