NT06-21 LEG 2-1 CRUISE REPORT - · PDF file10:08 2109.5 69.7sea anemone? 10:10 2100 68.7pillow ridge (passed over) 10:13 2095.5 56.7back on floor, pillow (large, with concentric joints)

NT06-21 LEG 2-1 CRUISE REPORT

Yoshihiko Tamura (IFREE, JAMSTEC)

Richard S. Fiske (Smithsonian Institution)

Alison Shaw (Woods Hole Oceanographic Institution)

Hiroshi Shukuno (IFREE, JAMSTEC)

Kenichiro Tani (IFREE, JAMSTEC)

Taketo Shimano (Fuji-Tokoha Univ.)

Fukashi Maeno (Earthquake Research Institute, University of Tokyo)

Koji Ito (Japan Coast Guard)

Satoshi Nakagawa (JAMSEC)

Maki Ito (Nippon Marine Enterprises, LTD)

1

Leg2-1 Dives HD616 and HD617: Steep cliffs along the eastern margin of Torishima

and Sumisu rifts. Objective: to observe cross-sections of small silicic volcanoes, where

the underlying middle crust is much thinner than that below the large arc volcanoes.

Fig. Dive areas HD616 and HD617 (enclosed by black rectangles). The arc-front

volcanoes of Sumisu (31°30 N, 140°E) and Torishima (30°30 N, 140°20 E), in

the central Izu-Bonin arc, are similar in size and rise as relatively isolated

edifices from the seafloor.

2

Introduction

Kodaira et al. (in press) conducted an active-source wide-angle seismic survey to

examine along-arc structural variations of the Izu intra-oceanic arc over a 550-km-long

profile along the volcanic front from Sagami Bay to Torishima. Low average crustal

seismic velocities (~6.7 km/s), due to thick middle crust, were obtained beneath basaltic

volcanoes, whereas higher average velocities (~7.1 km/s) were obtained beneath

rhyolitic volcanoes. They concluded that continental crust grows predominantly beneath

the basaltic volcanoes of the Izu arc, and the rhyolitic volcanism may be indicative of a

more juvenile stage of crustal evolution, re-melting of pre-existing continental crust, or

both (Kodaira et al., in press).

Fig. Simplified from Kodaira et al. (in press). Upper panel shows average wt. %

SiO2 of volcanic rocks sampled and dredged from Quaternary volcanoes. Lower

panel shows average crustal velocities. The horizontal axis indicates distance

from the northern end of the profile.

Nishizawa et al. (2006) showed across-arc structural variations from Torishima

toward the Shikoku basin to the west. Their profile also shows that the middle crust is

thick below Torishima volcano, and that it decreases in thickness westward toward the

Torishima rift. It thickens again and increases toward Horeki volcano, a basaltic

volcano located in the rear-arc.

3

Tamura & Tatsumi (2002) examined rock samples from 17 Quaternary volcanoes

of the Izu-arc and concluded that the rhyolitic magmas may have been produced by

dehydration melting of calc-alkaline andesite at a level within the upper to middle crust.

They also suggested that a lateral influx of hot basaltic magma, probably from adjacent

basaltic volcanoes, could have provided the heat source to re-melt the middle crust.

Hypothesis to be tested

The thinning of the middle crust beneath rhyolitic volcanoes may be the result of

re-melting of the middle crust by a lateral influx of basaltic magmas from nearby

large arc volcanoes.

Areas south of Sumisu and southwest of Torishima may be the two best places to

test the hypothesis, because both areas contain small volcanoes, and both are cut by

very high rift scarps that expose thick upper-crust sections. Thus, the rift walls could

show some evidence of lateral intrusions of basaltic magmas from Sumisu and

Torishima, respectively, as well as “country-rock” piles of rhyolites and/or dacites, that

were produced by remelting of the middle crust. The middle crust of these areas is

thinner as a result of this remelting process, which changed the middle crust into a more

dense residual lower crust, and was the source of erupted rhyolites and/or dacites.

There is a dramatic change in middle crustal thickness along the 30-km distance

from Sumisu to South Sumisu volcano. Basalt-basaltic andesite (<55 wt % SiO2) and

dacite-rhyolite (66-74 wt % SiO2) are the predominant eruptive products in the Sumisu

caldera volcano. Thus the average SiO2 content of the Sumisu volcanic rocks is about

64 wt %. On the other hand, South Sumisu is dominated by rhyolites having ~75 wt %

SiO2. Torishima, at the other end of the spectrum, is a large volcano with

predominantly basaltic compositions. There is a small knoll southwest of Torishima,

which is linked to the Torishima volcano by a linear topographic high.

4

SCIENTIFIC SUMMARY:

Dive 616

We collected pillow lavas of Torishima rift basalts from the bottom of the rift valley

(2120 m) to a depth of ~1890 m on the rift scarp. At a depth of 2056 m, we found

outcrops of lithologies that are similar to the volcaniclastic deposits of the Mio-Pliocene

Shirahama Group, Izu Peninsula, Japan. We collected dacite breccia from these

outcrops, which consist of pumice- and dacite-rich clastic rocks in thin and massive

beds. Upward from a depth of 1830 m to 1130 m, dacite breccias replace basaltic pillow

lavas as the dominant lithology.

Technical information:

Location: Torishima rift, southwest of Torishima volcano

Objective: sample rocks from Torishima rift to Torishima volcano. How is the knoll

southwest of Torishima related to the Torishima magma? What is the origin of dacites

and rhyolites that erupt near large basalt-dominant volcanoes vs. those erupted

between large basaltic volcanoes?

DIVE 616 On bottom: Off bottom:

Time (local) 09:48 15:47

Latitude: 30°24.33' 30°24.924'

Longitude: 140°05.484’ 140°07.825’

Depth (m): 2130m 1132m

Samples returned: 18 rocks (one lost on ascent), 2 scoop samples

Fig. Numbers on the right figures are locality numbers of the dive 616.

5

Dive #: NT0621HD-616Date: November 29th (local)Location: Torishima riftObjectives: Investigate lateral dikes from TorishimaLogger:

Recorders: note in water and out of water timesnote time and depth at regular intervals (10-15 minutes)any particular event, record time, positionfor sampling, note type, location, latitude, longitude and description of materialssamples are noted NT0621HD-DDD-#-X-YY

where DDD is dive number, # is the station numberX is S for scoop, R for rock, C for push core, W for water, YY is number

Time (local) Depth (m) Vehicle Heading Notes Sample #

8:16 0 - into water

9:05 800

9:48 2130 52.5 Arrive at sea floor, flat

9:51 2123.6 53 pillow lava (basket bottom)616-1-R01 freshbasalt lava

10:00 moving on

10:03 2114.8 70 pillow lava (same basket position to R01)616-2-R02 freshbasalt lava

10:07 2112 69.8 moving on

10:08 2109.5 69.7 sea anemone?

10:10 2100 68.7 pillow ridge (passed over)

10:13 2095.5 56.7 back on floor, pillow (large, with concentric joints)616-3-R03 freshbasalt lava

10:17 2092.5 55.6 moving on

10:27 2081.2 64.6 flat accelerated

10:28 2057.1 67.5-106 vertical outcrop 616-4

10:32 on the floor

10:38 2056.5 87 sample from breccia outcrop (Shirahama Group?)616-4-R04 dacitebreccia

10:40 2050 79.8 moving on

10:45 2034.6 74.5 flat bottom

10:47 2028.9 91.6 lava flow; structures infdicate motion to left

10:51 2029.4 105.4 pillow lava (front-right basket)616-5-R05 freshbasalt lava

10:53 2025.3 75.1 moving on

10:55 2021 74.7 rubbly pillow surface

11:00 2000 74.6

11:03 1991.6 73.7 living things (many), collect pillow lava (yellow basket)616-6-R06 freshbasalt lava

11:14 1957.5 77.2 talus blocks of pillow lava; collect (yellow basket)616-7-R07 freshbasalt lava

11:24 1887.9 64.8 blocks of pillow lava (yellow basket)616-8-R08 freshbasalt lava

11:26 moving on

11:30 1874.5 70 traveling in open water (floor not visible), near spot 4

11:36 1929.1 68.8 lavas covered thickly with sediment

sporadic pillow lava (tumulus-like)

11:40 1927 79.4 flat bottom (with ripple marks)

11:47 1911.3 69.4 collect snow-like sediment (in jar) 616-9

11:53 1909.2 69.5 moving on

12:00 1880 71 flat bottom

6

12:09 1830 70 vertical wall (alternation of coarse blocks, each unit thick)

12:11 1831 71.3 angular boulder from lava breccia outcrop616-10-R09 dacitebreccia

12:17 1826 71.3 departing breccia outcrop - moving over sandy slope

12:30 1734 70.2 landing in front of rock wall (massive breccia dyke? Then moving

12:47 1729 71 sampling the breccia (in front-right black box)616-11-R10 andesitebreccia

breccia is unconsolidated and fragile

12:53 1726.6 72 moving on; more breccia; collect a second sample616-11-R11 dacitebreccia

12:59 1718.6 70.2 see breccia dyke (scoriaceous) w/weakly stratified facies 616-12

13:08 1713 78.5 depart brecciea dyke & moving over sandy slope

13:23 1628.3 79.2 lower scoria breccia overlain by upper stratified deposit 616-13

13:27 1622.6 159.2 blocks on surface (collect detached rock; front-right basket)616-13-R12 brokenand lost

13:30 1619 74.5 moving on

13:51 1522.8 90.6 flat surface

13:56 1516.7 90.2 surface deposit (fine grained) Sample (black box front-left) 616-14-S01

14:05 1510.9 90.1 moving on

14:14 1439.9 80.7 low outcrop of sediment

14:15 1431.1 90.3 outcrop (upper part) breccia with some white blocks 616-15

14:22 1429.7 92.5 lava blocks (detached; angular)616-16-R13 dacitelava

14:30 1402 75.7 surface block (basket left backside)616-17-R14 andesitebreccia

14:33 1383.4 88.2 breccia (angular blocks) loosely consolidatd616-18-R15 dacitebreccia

14:41 1334.7 116.5 breccia (poorly sorted; massive)

14:45 1335.2 144 breccia (large block) collect (front-right basket)616-19-R16 dacitebreccia

14:59 1251.5 108.5 loose sediment, talus; sampled with scoop (cylindrical box) 616-20-S02

15:20 1238.2 76.7 lapilli patches on surface, sampled w/scoop (cylind. Box) 616-21-S03

15:25 1215 99 observe rock wall (pumice breccia); move over it, then departing

15:33 1174 74.7 Again, lapilli concentrations on surface (patches), viewing

15:40 1142 80 base-rock, dacite (angular shape)616-22-R17 dacitepumice

15:48 1132.5 80.3 large lava block; sample (in rear black box)616-23-R18 heavilyaltered dacite lava

15:49 1120 78.5 end of dive; depart sea floor

16:31 0 out of the water

7

2124 m

9:51 Pillow lava on the floor

of the Torishima rift. Sample:

HD616-1-R01, 616-2-R02

2115 m

10:02 Young pillow lava on

the floor of the Torishima rift.

Tube structures are well

developed.

2057 m

10:32 Coarse volcanic breccia

overlying undisturbed layers

of dacitic tuff and lapilli tuff

on the east floor of the

Torishima rift at a depth of

2057 m.

8

2057 m

10:32 Slightly higher view of

vertical section in outcrop

near the base of the

Torishima rift. Basement

rocks of the Izu-Bonin arc are

similar to those of the

Mio-Pliocene Shirahama

Group, Izu Peninsula.

2057 m

10:33 Closer view of dacitic

volcaniclastic deposits that

resemble those of the

Shirahama Group.

2057 m

10:37 Sampling a clast from

a breccia deposit similar to

those in the Shirahama Group

(HD616-4-R04).

9

2029 m

10:49 Young pillow lava

extrusion. Sample

HD616-5-R05.

1992 m

11:03 Pillow lava; note radial

fractures in pillow

cross-section. Sample

HD616-6-R06.

1957 m

11:14 Talus blocks derived

from pillow lava.

HD616-7-R07.

10

1831 m

12:14 Lava breccia locally

covered with flaming-red

bacterial film(?).

1729 m

12:44 lava breccia; note

homogeneity of clasts.

1729 m

12:47 Sampling the lava

breccia shown above.

HD616-11-R10.

11

1719 m

12:57 scoriaceous breccia

dyke with weak lineation.

616-12

1628 m

13:25 well-stratified

pumice-rich deposits resting

on lower scoria breccias.

616-13

1628 m

13:25 Close-up view of the

bedded outcrop shown above

reveals small holes – possibly

the result of burrowing

organisms or gas bubbles?

12

1517 m

14:02 fine surface volcanic

sediments collected by scoop

(HD616-14-S01).

1431 m

14:21 monolithologic dacitic

lava breccia. 616-15

1430 m

14:22 detached angular blocks

of dacite lava flow

(616-16-R13).

13

1402 m

14:27 Surface blocks (talus)

(616-17-R14).

1383 m

14:33 volcanic breccia, semi

consolidated

(HD616-18-R15).

1335 m

14:44 monolithologic volcanic

breccia (616-19-R16).

14

1251.5 m

15:00 talus, sampled with

scoop (616-20-S02).

1132 m

15:42 large block, sampled

just before the end of dive

#616 (616-23-R18).

15

SCIENTIFIC SUMMARY:

Dive 617

Dive 617 began on the Sumisu Rift floor, and the goal was to observe the lithologies as

we ascended one of the steepest cliffs from the Rift floor up to a point just west of

South Sumisu caldera. The bottom of rift is thickly covered by fine, light-colored

sediment, perhaps volcanic ash. No pillow lavas (as seen in dive 616) were observed on

the floor of the rift valley. When we approached the east wall of the rift, some rocks

were visible. However, most of these were difficult to collect since they were firmly

attached to the outcrops, as opposed to being loose boulders. 617-2-R02 may be the

only rock sampled that truly represents the Sumisu Rift basalts. Sumisu rift basalts

continued from 2110 m up to a depth of 1940 m. The Sumisu Rift wall, which is

thought to show the dissected South Sumisu volcano, consisted mostly of massive

dacite dikes, which display beautiful columnar joints. The observed layering in parts of

the section was similar to those of the Mio-Pliocene Shirahama Group, Izu Peninsula.

These rocks were possibly the basement of South Sumisu volcano, which were

subsequently intruded by series of dacitic dikes. Alternatively, given that the layered

section seems to overlie the dacitic unit, it is possible that these stratified volcanic rocks

are related to eruption of South Sumisu volcano after emplacement of the dacite.

Technical information:

Location: Sumisu rift, west of South Sumisu volcano

Objective: to sample rocks from the floor of the Sumisu rift to near South Sumisu

volcano. How is South Sumisu volcano related to the Sumisu magmatic system? What is

the origin of dacites and rhyolites that erupt near large basalt-dominated volcanoes or

between large basaltic volcanoes?

DIVE 617 On bottom: Off bottom:

Time (local) 09:42 14:22

Latitude: 31°15.663' 31°15.427'

Longitude: 139°56.271’ 139°57.284’

Depth (m): 2110 m 1386 m

Samples returned: 10 rocks

16

Fig. Numbers on the right figures are locality numbers of the dive 617.

17

Dive #: NT0621HD-617Date: November 30th (local)Location: Sumisu riftObjectives: Investigate lateral dikes from minami-Sumisu calderaLogger:

Recorders: note in water and out of water timesnote time and depth at regular intervals (10-15 minutes)any particular event, record time, positionfor sampling, note type, location, latitude, longitude and description of materialssamples are noted NT0621HD-DDD-#-X-YY

where DDD is dive number, # is the station numberX is S for scoop, R for rock, C for push core, W for water, YY is number

Time (local) Depth (m) Vehicle Heading Notes Sample #

8:13 0 - into water

9:42 2110 90 on sea floor, begin survey, white sandy mud

9:47 2110 110 swimming shell

9:54 2108 110 observe white sediment beneath sand layer

10:00 2106 110 flat, flat, and flat, 31°15.624, 139°56.447

10:04 2105 110 dusts (nets?) in hollows

10:09 2101 110 flat surface, observe swimming fishes

10:15 2079 110 gentle slope, 31°15.585, 139°56.615

10:16 2070 110 arrive at point 3 (event mark)

10:23 2043 113 see blown jellyfish

10:30 1994 110 white sandy mud, 31°15.535, 139°56.795

10:32 1983 90 black, tube-shaped rock, unable to sample, 31°15.531, 139°56.805

10:40 - - angular lava blocks resting on sand

10:44 1978.5 88.3 sampling glassy lava block (in back black box, next to yellow)617-1-R01 phyricdacite lava

10:52 1977.7 102.1 repeated attempts to sample but fail; depart the outcrop

10:57 1938.4 97 exposure discovered (fragmented glassy pillow lava), stop and observe

11:02 1940.3 121.5 sampling pillow breccia (glassy & angular); in back-side black box617-2-R02 two-pxandesite lava

11:04 1937 112 Depart this exposure

11:10 1887 111.6 small fishes sometimes appear

11:10 1874 110.2 location No 4 (event mark), rock surface is brecciated and jointed

11:13 1861 110.3 fragments of rock, and large massive rock (lava?)

11:16 1864.1 83.9 sample rock fragment, covered by somewhat brown-color617-3-R03 dacitelava

11:18 1858.2 114 depart outcrop

11:26 1809 75.9work temporarily stopped because wind became very strong, thedistance between Natsushima and HPD is readjusted

11:54 1848.7 93.8 HPD returns to sea floor, moving on to Mark 5

11:57 1835.4 71.1 view steep-wall composed of massive and fragmented rock

12:02 1834.4 76.7sampling elongate rock, but it breaks!! Only small fragment recovered;placed in front-side basket 617-4-R04 dacite tuff

12:03 1829.7 90.5 depart outcrop; move up slope

12:04 1824.6 88 massive lava or sediment layers with stratified facies

12:06 1815.1 90.4 continuing outcrops of massive lava

12:13 1806.7 89.5 sample (altered?) fragile lava block (in rear black box)617-5-R05 dacitelava

12:18 1806.4 88.9second sampling at the same site, large lava fragment placed in yellowbox

617-5-R06 dacitelava

18

12:20 1781 95 climbing across massive lava outcrop with columnar joints

12:23 1761 95stop at lower part of talus, the talus continues upslope, sample a lavacrust


12:28 1747 95 talus (gentle slope), attempted sample falls back to talus

12:33 1743 80 sample rock from same talus mass617-7-R08 dacitelava

12:40 1718 95arriving a top of talus (rock-wall), where lava fragments are dispersedin flat (white sandy mud)

12:42 1715 100sampling a lava fragment exposed on a hollow, near 31°15.470, 139°57.057


12:45 1711 110 arriving at lava-wall with columnar joints;ascend over it

12:48 1704 83 gentle slope, unable to sample, continue ascent

12:52 1701 100 arriving at flat area, unable to sample

12:54 1696 92

stop in front of massive lava-wall (step), unable to sample,again workwas temporally stopped because wind became very strong, and thedistance between Natsushima and HPD is adjusted.

13:03 1643 200 HPD returns to work

13:07 1687 111HPD returns to sea floor (31°15.463, 139°57.101), climb up and overlava-wall with colmunar joints

13:14 1626 111 observe outcrop (stratified ash layers); then ascend

13:20 1616 110 observe outcrop (tephra?); then ascend

13:22 1604 110 observe outcrop (lava surface); then ascend

13:24 1598 111 observe outcrop (layering structures in lava); then ascend

13:29 1582 111sample overhanging part of lava; place in left-side of yellow box, 31°15.438, 139°57.132

617-9-R10 rhyolitetuff

13:36 1576.4 109.4 depart sampling site

13:44 1537.1 110.7 observe fractured lava (dyke?) with columnar joints

13:52 1528.1 105.7 white stratified layers (unknown)

14:02 1494 101.4 observe fractured rock (platy or slab shaped)

14:07 1447.8 111.7 observe massive lava, see local flow-banding

14:11 1422.4 111.2 end of massive lava, see loose sand or silt-covered basement

14:15 1400.7 112.8 continuing sand or silt-covered basement

14:18 1394.6 53.4 massive tuff breccia (older strata), Shirahama group?

14:22 1380 47.5 depart sea floor

15:18 0 - hoisted out of water

19

2110 m

9:47

We moved along the floor of

Sumisu rift for about one

hour without seeing anything

except a thick cover of

sediment that looked like

powdered snow.

1979 m

10:46

After an uneventful trip

across the Sumisu rift basin,

we finally found some

angular lava blocks. A

sample was collected

(HD617-1-R01, aphyric

dacite).

1940 m

10:59

Most rocks were firmly

attached to outcrops and were

therefore difficult to collect,

but we found one glassy

angular pillow breccia –

likely the only sample related

to the Sumisu rift

(HD617-2-R02, basalt or px

andesite lava).

20

1864 m

11:17

We saw many kinds of

uniform brecciated and

jointed lavas and/or dikes

from ~1870 m up to the end

of the dive. Most outcrops

were covered by snow-like

fine volcanic sediment. A

lava sample was collected

(HD617-3-R03, dacite lava).

1834 m

11:59

A large block of dacitic tuff

was collected from the

massive steep wall. It broke

into small pieces during

sampling (HD617-4-R04).

1824 m

12:04

We observed massive rocks

with layering, but no samples

of this unit could be

collected. These may be

outcrops of the Mio-Pliocene

basement of this area (the

Shirahama Group), which

were densely lithified by heat

from nearby intrusions.

21

1806 m

12:10

Rocks from talus adjacent to a

massive dike were collected

(HD617-5-R05, dacite).

1806 m

12:13

Dacitic dikes with

well-developed columnar

joints were observed up to

1700 m and beyond. A large

lava sample was collected

from this unit (HD617-5-R06,

dacite).

~1750 m

12:27

Columnar joints of a massive

dacitic intrusion.

22

~1750 m

12:27

Well-developed columnar

joints in a massive dacite dike.

~1750 m

12:27

Columnar joints in a massive

dacite dike.

~1750 m

12:28

Columnar joints in a massive

dacite dike.

1743 m

12:33

A sample from the dacite dike

was collected at 1743 m

(HD617-7-R08).

23

1743 m

12:33

Outcrop of massive dacite

1715 m

12:42

A sample was taken from

tumbled blocks derived from a

nearby large dike

(HD617-8-R09).

1711 m

12:46

Near vertical wall of lava wall

showing well-developed

columnar jointing. The

snow-like sediment covering

is ubiquitous.

24

1696 m

12:55

A vertical wall of lava

showing classic columnar

joints.

1626 m

13:16

Cliff containing gently dipping

layers. These are likely

volcaniclastics of a

Shirahama-like basement. We

were unable to collect samples

from these steep cliffs.

However, at a depth of 1582

m, we sampled rhyolitic tuff

(HD617-9-R10).

1537 m

13:45

We again encountered a

massive dacitic unit with

blocky jointing.

25

1528 m

13:50

Light colored beds of tuff

were observed beneath the

‘snow’.

1528 m

13:51

A close-up view of the

light-colored tuff beds

revealed small lithic fragments

in addition to pumice clasts.

1394 m

14:19

Massive, poorly-sorted tuff

breccia, resembling those of

the Shirahama Group, was

encountered towards the end

of the dive. Unfortunately we

were unable to collect samples

of this unit.

26

27

28

HD616 Sample Photos (1/5)

HD616-R01

HD616-R02 HD616-R02 cut

HD616-R01 cut

HD616-R04 HD616-R04 cut

HD616-R03 HD616-R03 cut

29


HD616-R05

HD616-R07

HD616-R08 HD616-R08 cut

HD616-R05 cut

HD616-R06

HD616-R07 cut

HD616-R06 cut

30


HD616-R11 HD616-R11 cut

HD616-R10 HD616-R10 cut

HD616-R09 cut

Sample LostHD616-R12

Sample LostHD616-R12

31

HD616-R16 cut


HD616-R16

HD616-R13

HD616-R14 HD616-R14 cut

HD616-R15 HD616-R15 cut

HD616-R13 cut

32

HD616-R18 cut

HD616-R17

HD616-R18


33

HD617-R01 HD617-R01 cut


HD617-R02 HD617-R02 cut

HD617-R03

HD617-R04

HD617-R03 cut

34


HD617-R08 HD617-R08 cut

HD617-R05 HD617-R05 cut

HD617-R06 HD617-R06 cut

HD617-R07 HD617-R07 cut

35

HD617-R09 HD617-R09 cut


HD617-R10 HD617-R10 cut

36

Bathymetric surveys R/V Natsushima completed additional Seabat surveys in two areas near the dive sites

(see Fig. below). The new data acquired will be merged with existing multi-beam data

to produce revised maps of each study area.

Fig. NT06-21 Leg 2-1 MNBES Survey areas. Bathymetric map of study area with

new Seabat survey fields shown by yellow boxes.

37

38

39

NT06-21 Cruise

Dynamics of submarine eruption of felsic magmas at

Myojinsho caldera

Cruise Report

From Dec. 1 to Dec. 3, 2006

NT06-21 Leg 2-2

Research Leader

Taketo Shimano

1

CONTENTS

MEMBER LIST.................................................................................................................. 3

1. INTRODUCTION........................................................................................................ 4

1-1 DYNAMICS OF RECENT ERUPTIONS AT MYOJINSHO ................................................................ 5 1-2 DYNAMICS OF CALDERA FORMING ERUPTION AT MYOJINSHO CALDERA .................................. 7 1-3 EVOLUTION OF VOLCANIC EDIFICE AND MAGMA AT MYOJINSHO CALDERA .............................. 8

2. PAYLOADS ................................................................................................................. 9

2-1. SCOOP.............................................................................................................................. 9

3. DIVE LOG ................................................................................................................ 10

3-1. DIVE #618 ...................................................................................................................... 10 3-2. DIVE #619 ...................................................................................................................... 12 3-3. DIVE #620 ...................................................................................................................... 13 3-4. DIVE #621 ...................................................................................................................... 15

4. RESULTS .................................................................................................................. 16

4-1. DIVE SCHEDULE .............................................................................................................. 16 4-2. DIVE TRACKS.................................................................................................................. 16 4-3. DIVE POINTS ................................................................................................................... 19 4-4. REPRESENTATIVE COLUMNAR SECTIONS............................................................................. 29 4-5. NEW TOPOGRAPHIC MAP .................................................................................................. 33

5. FUTURE SCHEDULE ............................................................................................... 34

5-1. RECENT ERUPTIONS OF MYOJINSHO .................................................................................. 34 5-2. CALDERA FORMING ERUPTION OF MYOJINSHO CALDERA ..................................................... 34 5-3. EVOLUTION OF MYOJINSHO CALDERA ............................................................................... 35

6. SAMPLE LIST .......................................................................................................... 36

6-1. ROCK SAMPLE LIST.......................................................................................................... 36 6-2. SAMPLE PHOTOS.............................................................................................................. 37

2

MEMBER LIST Participants aboard Research Group

Taketo Shimano Fuji-Tokoha Univ.

Koji Ito Japan Coast Guard

Kenichiro Tani

JAMSTEC

Fukashi Maeno

Earthquake Research Institute, University of Tokyo

3

1. INTRODUCTION Understanding the mechanism of explosive eruption has been one of the major subjects in volcanology. It

is now fairly understood for those occurred on land; for example, degassing of magma during ascent in a

conduit controls the eruption style. However, it is still unclear what kind of fundamental processes really

control the degassing and how deep they work during the course of an eruption, and how effusive eruption

could result from similar magma to those of explosive eruptions, or visa versa. On the other hand, violent

explosive eruptions are also resulted from sudden large volume expansion of seawater due to magma-water

interaction. It is highly destructive

generating jets (called “cock’s tail

jets”) and base surges. Some

models have been proposed for

these phenomena (ex. FCI theory

for water-magma interaction).

However, the mechanisms of

magma-water interaction and

mode of pyroclast deposition on

the sea floor are still unclear.

Fig. 1-1 Locality map in the Izu-Bonin Islands near Myojinsho caldera (after Fiske et al., 1998)

In addition, the mechanism of

caldera forming eruption in the sea is much

less understood. This type of eruptions rarely

occurs (almost no direct observation), but are

known as the most violent and destructive one

on earth that generates large scale pumice fall,

pyroclastic flows and surges, and tsunamis.

Although the deposits on land formed by these

caldera eruptions (including subaquaeous) are

documented in detail due to their

well-preserved and accessible exposures, the

counterparts in the sea have not been

investigated well enough. Fig. 1-2 Topographical map near Myojinsho caldera (Tamaki et al., unpublished data). Myojinsho caldera is at 31-53N, 139-57E (WGS84). Caldera with ca. 10 km diameter at 32-07N, 139-52 is Myojin Knoll caldera.

4

The evolution of the edifice and magmatic system for these caldera forming volcanoes is one of the

subjects that we should solve to understand how magma ascends from depth and resides at shallow level in

the crust. Recent topographical mapping by Sea beam scanning in the Izu-Bonin arc revealed many

submarine calderas which consist of several topographical peaks around each caldera in this area. So the

investigation of stratigraphy with its petrological evolution and the correlation among these calderas should

give constraints on the evolution of submarine calderas in this island arc.

In this research project, there are three main subjects to be solved; (1) dynamics of recent eruptions, (2)

dynamics of caldera forming eruption, and (3) evolution of volcanic edifice and magma for caldera forming

volcano. In the following sections, the

objectives of each main subject are

described.

Table 1-1 Representative recent

eruptions at Myojinsho volcano

1-1 Dynamics of recent eruptions

at Myojinsho

The 1952-53 eruption of Myojinsho

volcano is one of the most violent and

best documented phreatomagmatic

eruptions in Japan. The eruption is

characterized by the alternation of

dome formation and its collapse

accompanied by phreatomagmatic

eruption. The lava dome morphology

changed from rubbly lava in early 1952

to spine after October 1952 to October

1953. Some explosion events are

investigated during eruption and the

pressure sources of the explosions were

estimated by using these documents.

Fiske et al. (1998) proposed a model of

the mode of emplacement for the

products from bathymetric data; Large

dense blocks and pumices ejected in

water flew down the slope as density

currents. In contrast, pumices ejected in

the air should float to be dispersed

broadly, or descend as vertical density

currents as a consequence of

5

hyperconcentration, if huge amount of

pumices fell on the sea surface.

There are some reports on discolored

seawater and floating pumices in

1970-80’s. As the locality of this area is

far from the main land of Japan, however,

much more small scale eruptions might

have occurred until present.

In this cruise, we are planning to survey

the slope of Myojinsho. Detail

bathymetric data have been obtained

recently around this area (Fig. 1-1). From

direct observation of the topography and

structure of the deposit, we will discuss

the mode of emplacement of pyroclastics

on the slopes of submarine volcano

accessing the Fiske’s model. We are also

planning to survey summit area to

investigate present activity of Myojinsho

if possible.

Table 1-2 The sequence of the 1952-53

eruption (after Fiske et al., 1998).

Fig. 1-3 The phreatomagmatic

explosion on September 23, 1952 (after

Ossaka, 1990).

We will also collect samples of recent

eruptions. Shimano and Nakada (2006)

investigated vesicular texture of

submarine scoria from submarine fissure

in the 2000 eruption at Miyakejima

volcano in Izu-Bonin arc. They all have

identical composition (basalt), but

different density (vesicularity) and different groundmass water content with a certain systematics (less

vesicular scoria has more water). The results show that vesiculation of magma proceeds by nucleation

dominantly with minor growth of bubbles, and beyond vesicularity ~ 0.5, vesiculation mode changes to

growth dominant. Detail investigation of the texture shows that bubble coalescence occurred significantly at

6

snapshots of vesiculation so that we can discuss

esiculation processes, and we are expecting in this cruise low density pumices to investigate early stage of

lati

a and large amount of pumices were not ingested by the air but by

eawater with precipitation of volcanic vapor. The bathymetric survey showed that most ejecta were

Fi

deposit was observed near

th

will search for the essential pumices of caldera forming eruption. Then, we will correlate the unit

vesiculariy ~ 0.5. This is the first observation that the mode changes with vesiculation for basaltic magma,

and is very important because coalescence is an essential process to promote degassing of magma. As in this

example, pyroclasts that quenched at different timing provide

v

vesicu on for silicic magmas.

1-2 Dynamics of caldera forming eruption at Myojinsho Caldera

There are many submarine caldera in the Izu-Bonin island arc. Among them, Fiske et al. (2000) proposed,

for Myojin Knoll caldera, that most pumices ejected from the caldera sank and formed caldera-fill deposit

because the volcano is deep in the se

s

deposited near the rim of the caldera.

g. 1-4 Topographic map of Myojinsho caldera (Japan Coast Guard, 2000).

In contrast, Tani et al. (submitted) investigated systematically the deposits of Sumisu Caldera. They

showed that pumices were deposited far from the source and only small amount of

e source. They concluded that this is because the caldera was at near the sea level and the air could ingest

into pumices so that they could float and had been transported far from the source.

Myojinsho Caldera is at relatively shallow depth but deeper than Sumisu calderas. Thus, pumices by

caldera forming eruption may have floated but brought not so far as those of Sumisu caldera. In this research

cruise, we

7

at veral localities and investigate the distribution of pumices that are considered to have erupted at caldera

tion.

acitic magmas are

do

the caldera wall at some localities to reconstruct the evolution

istory of Myojinsho caldera. We will also collect sequential samples to investigate chemical evolution of

magmatic system at Myojinsho caldera.

se

erup

1-3 Evolution of volcanic edifice and magma at Myojinsho Caldera

Myoujinsho caldera volcano consists of several edifices which have some peaks in the sea one of which is

above sea level called Beyonnaise rocks. There are many caldera volcano in Izu-Bonin island arc, and they

have some similarity in its topography. They consist of several edifices with topographical peak around the

main caldera. Chemistry of the rock of these caldera forming volcanoes seem to have similar variation

although data are not sufficient at present. As one of the examples, Tani et al. (submitted) show the

stratigraphical relationship of the edifices of Sumisu caldera and defined there stages of evolution history; (1)

stratovolcano stage, (2) caldera forming stage, and (3) post-caldera stage. They also suggested systematic

change in chemistry with the edifice evolution. At stratovolcano stage, basaltic and d

minant. At caldera formation stage, large amount of rhyolitic magma was ejected as pumices. Then, at

post-caldera stage, lava domes with dacitic composition were formed around the caldera.

In this cruise, we are planning to survey

h

8

2. PAYLOADS 2-1. Scoop

Samples of loose pyroclastic materials are collected by manipulator using a steel scoop in Fig. 2-1. To

sample, hold the handle (yellow) with hand, scoop the ground by rotating the wrist of the manipulator.

Scooped samples are poured into box or cylinder with cap to prevent lost during ascent to the sea surface.

Fig. 2-1 Scoop

9

3. DIVE LOG 3-1. Dive #618

10

11

3-2. Dive #619

12

3-3. Dive #620

13

14

3-4. Dive #621

15

4. RESULTS 4-1. Dive schedule

Dive 1 (#618) Dec. 1, 2006

From (31-52.500N, 140-00.600E; 940 m) to (31-51.975N, 140-01.051E; 380 m)

Payloads: Scoop, square box, cylinder box (2), baskets

Dive 2 (#619) Dec. 1, 2006

From (31-53.00N, 139-58.00E; 1000 m) to (31-53.500N, 139-58.500E; 350 m) Payloads: Scoop, square box, cylinder box (2), baskets

Dive 3 (#620) Dec. 2, 2006


Dive 4 (#621) Dec. 2, 2006


4-2. Dive tracks

Dive 1 (#618)

From (31-52.511N, 140-00.594E; 958 m) to (31-51.823N, 140-01.184E; 291 m)

Samples: HD618-R01-R15, S01

1: 09:10 on floor

09:13 rock sampled

2: 09:24 D = 918 m, rock sampled




10:03 pumice sampled


7: 10:28 D = 652 m, pumices sampled


9: 10:45 D = 579 m, pumice sampled



12: 11:45 D = 291 m, rock sampled

11:48 pumice sampled

11:50 D = 291 m, off the floor

16

Dive 2 (#619)

From (31-53.00N, 139-58.00E; 1000 m) to (31-53.500N, 139-58.500E; 350 m) Samples: HD619-R01-R08

1: 14:36 D = 1016 m, on floor

14:39 D = 1014 m, rock sampled






7: 15:31 D = 650 m, observation



10: 15:53 D = 516 m, rock sampled

11: 15:55 D = 481 m, off the floor

Dive 3 (#620)

From (31-55.720N, 140-00.813E; 500 m) to (31-55.000N, 140-00.983E; 110 m) Samples: HD620-R01-R08, S01-S02

1: 08:58 D = 561 m, on floor






09:45 rock sampled




10: 10:39 D = 133 m, rock sampled

10:44 pumices sampled

11: 10:55 D = 95 m, off the floor

17

Dive 4 (#621)

From (31-52.700N, 139-57.200E; 1000 m) to (31-52.200N, 139-56.800E; 300 m) Samples: HD621-R01-R12, S01-02

1: 13:49 D = 999 m, on floor

13:54 rock sampled






7: 14:53 D = 669 m, rocks sampled

8: 15:06 D = 600 m, 2 rocks sampled





13: 16:03 D = 335 m, rock sampled

16:03 off the floor

18

4-3. Dive points

Dive 1 (#618): Dive at the SE rim of Myoujinsho caldera (Dec. 1, 2006)

9:11 HD618-1 (958 m bsl)

Angular blocks are scattered on the caldera

floor which is covered with “snow-like” white

fine particles.

Pumice fragment with columnar or radial joints

were sampled (R01).

9:21 HD618-2 (918 m bsl)

Angular blocks are scattered on the surface

which is covered with “snow-like” white fine

particles.

Two altered but angular dense lava fragments

were sampled at the same time (R02 and R03).

9:31 HD618-3 (885 m bsl) Unsolidified massive deposit was covered

directly by black lava-like homogeneous rock. The deposit is matrix supported (white fine particles) with abundant black angular to subrounded blocks (decimeter size).

An altered subrounded lava block was sampled from the deposit (R04).

9:40 HD618-4 (833 m bsl) Massive lava-like rock crops out on the

sub-vertical wall with “snow-like” sediments. An altered but subrounded dense lava fragment was sampled from near the outcrop (R05).

19

9:51 HD618-5 (760 m bsl) Pale brown rounded large blocks of pumices are

scattered on the surface here and there. Some of them are broken into pieces and show beautiful radial joints. A broken pumice fragment and a part of large rounded pumice scratched by manipulator were sampled (R06 and R07, respectively).

10:11 HD618-6 (680 m bsl) Black lava-like massive crops out and many

fragments with similar lithology (R08) are scattered on the surface with “snow-like” covers. Many pumice blocks with beautiful radial joints are also scattered on the surface (R09).

10:28 HD618-7 (652 m bsl) Pyroclastic massive unconsolidated deposit is

matrix supported with some blocks larger than some decimeters.

The matrix consisting of pumice fragments were sampled by manipulator by scratching the outcrop and throwing matrix pumice fragments into cylinder box (S01).

10:36 HD618-8 (589 m bsl) Angular to subrounded black blocks are

scattered on the surface which consist mostly of pumiceous particles. One of the fragments wes sampled (R10).

10:42 HD618-9 (579 m bsl) Relatively well sorted, grain supported reddish

brown pumice deposit covers ca. 10 cm thick white pumiceous unit including some black lithics (?).

Foreground is snow-like surface fine deposit with ripple marks around here.

A piece of the reddish brown pumice was sampled (R11).

20

10:58 HD618-10 (465 m bsl) The surface from HD618-9 to 10 is mostly

covered with loose brown pumices the size of which increases upwards.

A part of a vesicular brown pumice was sampled (R12; request from R. F.).

11:05 HD618-11 (445 m bsl) Pumices are scattered on the surface of flat fine

particle deposit. Some of them are clearly banded.Here only a small part of large banded pumice

(center) was sampled (R13).

11:46 HD618-12 (291 m bsl) Light gray and pale brown blocks of pumices

(decimeters) are almost entirely cover the surface here. Some of them are broken into pieces. Gray pumice was sampled (R14). In addition, a large pale brown pumice (~1 m) was sampled (R15).

21

Dive 2 (#619): Dive on the W side of the central edifice in Myojinsho caldera (Dec. 1, 2006)

Objective: Understanding generation mechanism of central edifice; resurgent dome (cript-dome) or

remnant of pre-caldera edifice?

14:37 HD619-1 (1016 m bsl)

Angular blocks are scattered on the caldera floor


particles.

An altered blocky fine grain pyroclastic rock was

sampled (R01).

14:49 HD619-2 (937 m bsl)

Surface around here is thickly covered by

“snow-like” white fine particles. Some subangular

to rounded blocks are scattered on the surface

which is covered with “snow-like” particles.

An altered blocky pyroclastic rock was sampled

(R02).

14:58 HD619-3 (895 m bsl) Lava-like rock crops out on the wall with

fracture. Angular blocks are scattered under the

wall as talus.

An altered massive block was sampled (R03).

15:05 HD619-4 (848 m bsl) Lava-like rock crops out on the wall with

fracture. Angular blocks are scattered under the

wall as talus.

An altered blocky pyroclastic rock with distinct

pumice shape in the cross section was sampled

(R04).

22

15:15 HD619-5 (787 m bsl) Talus deposit under the wall of massive lava-like

rock.

An altered blocky tuffacious rock was sampled (R05).

15:24 HD619-6 (714 m bsl) Alternation of banded and fractured lava-like

rocks crop out on the wall. Banded part lacks

significant fracture so we cannot sample the rock.

An altered angular tuffaceous block was sampled

from the fractured part (R06).

15:31 HD619-7 (641 m bsl) Distinct banding structure like sediments crops

out on the wall (~650 m bsl). On top of this wall

subangular to rounded blocks are scattered on the

surface. An altered blocky pyroclastic rock was

sampled (R07).

15:47 HD619-8 (524 m bsl) Alternation of massive black part and beautiful

banding structure crops out on the wall. The talus around here is white color and white lapilli size blocks are scattered on the surface.

15:51 HD619-9 (516 m bsl) Angular blocks are scattered on the caldera floor


particles.

An altered blocky pyroclastic rock was sampled (R08).

23

Dive 3 (#620): Dive on the NW slope up to the summit of Myojinsho (Dec. 2, 2006)

Objectives: Understanding the mode of emplacement of pyroclastic deposit on the slope of submarine

volcano. Sampling fresh pumices to correlate with those sampled at the time of eruption in 1952-53 and

1970.

9:00 HD620-1 (553 m bsl)

Some decimeter size blocks are scattered on the

surface with lapilli-size pyroclasts.

A piece of pumice was sampled (R01).

9:07 HD620-2 (534 m bsl)

The surface is covered with relatively larger size

white pumices (>10 cm). The distribution shows

lobate topography and some of them have large

pumice concentrated rims.

A rounded fresh pumice block but with reddish

part was sampled (R02).

9:14 HD620-3 (509 m bsl) Around here, alternation of two distinct areas;

large white pumice-rich part and gray lapilli-rich part. Black dense angular blocks are scattered on the surface of gray part.

A black dense lava block was sampled from the gray part (R03).

9:32 HD620-4 (373 m bsl) Alternation of white pumice and gray lapilli areas

continues, but less pumice area which clusters every several meters. Banded pumices are getting increased. We could not sample the banded pumice in this figure.

24

9:45 HD620-5 (345 m bsl) Most large blocks on the surface are banded

pumice. A part of the large pumice (right) was sampled (R04). A dense black block held by manipulator was also sampled (R05).

9:49 HD620-6 (319 m bsl) Alternation of white pumice and gray lapilli areas

ceased. Most part of the surface was covered with red scoriaceous lapilli and dense gray angular blocks. Gray dense rock was sampled (R06).

10:00 HD620-7 (293 m bsl) The surface was covered with red scoriaceous

lapilli and dense gray angular blocks. Almost no pumice blocks. Red scoriacious and gray dense lapilli were sampled in cylinder box with scoop (S01).

10:20 HD620-8 (156 m bsl) Red scoriaceous lapilli decreased and surface

was covererd mostly with gray lapilli or sand. Then, here, surface was covered with blocks with angular to subangular blocks. One of the fragments was sampled (R07).

10:46 HD620-9 (589 m bsl) Angular to subrounded reddish brown blocks are

scattered on the surface with finer matrix of similar lithology and gray angular lapilli. One of the large block and matrix was sampled (R08 and S02, respectively).

25

Dive 4 (#621): Dive on the SW caldera wall from 1000 m bsl up to ca. 500 m bsl (Dec. 2, 2006)

Objectives: Collecting sequential samples, reconstructing evolution history of the edifice and correlating

the deposit with those at other locality, looking for syn-eruptive products of caldera formation.

13:50 HD621-1 (999 m bsl)

Angular blocks are scattered on the caldera floor

which is thickly covered with “snow-like” white

fine particles.

An altered lava fragment was sampled (R01).

14:05 HD621-2 (910 m bsl)

Angular blocks are scattered on the surface

which is thickly covered with “snow-like” white

fine particles.

An altered dense lava block was sampled (R02).

14:18 HD621-3 (845 m bsl) On the wall, lava-like massive rocks with widely

spaced fractures are exposed. Angular blocks are in

the talus under the wall covered thickly with

“snow-like” white fine particles

An altered dense tuff block was sampled (R03).

14:26 HD621-4 (806 m bsl) Massive lava-like rock crops out on the

sub-vertical wall with “snow-like” sediments. An altered but subangular dense lava fragment

was sampled from near the outcrop (R04).

26

14:38 HD621-5 (723 m bsl) Massive lava-like rock with many fractures crops

out on the sub-vertical wall with “snow-like” sediments.

An altered subrounded dense homogeneous fragment was sampled from near the outcrop (R05).

14:43 HD621-6 (713 m bsl) Black lava-like massive rock with many parallel

fractures crops out on the wall. One of the fragments with laminae structure was sampled (R06).

14:51 HD621-7 (669 m bsl) Similar to HD621-6, black lava-like massive rock

with fracture crops out. One of the blocks was sampled (R07).

15:02 HD621-8 (600 m bsl) Massive yellowish lava-like rocks with many

fractures are observed on the surface. One of the blocks was sampled but broken into

two pieces (R08 and R09).

15:13 HD621-9 (565 m bsl) Lithology changed from sub-vertical lava-like

wall to the slope of “snow like” white sediment and cluster of lava-like rocks and blocks. Roundness of blocks seems to change from cluster to cluster.

A subangular dense block of lava was sampled (R10).

27

15:21 HD621-10 (546 m bsl) Many pale gray pumice blocks are on the surface

of coral sand. The coral sand was sampled in cylinder box by scoop (S01).

15:31 HD621-11 (475 m bsl) Reddish brown pumice fall deposit mantles the

welded spatter like deposit above another pumice fall deposit which produces talus blocks on the slope.

Some pieces of pumice fragments were sampled from the reddish brown pumice deposit (S02)

15:43 HD621-12 (423 m bsl) On the surface, large (meter size) blocks of lava

like lithology with sticky surface are scattered. There are some light brown-white pumices on the surface, too.

A piece of pumice with elongated vesicles (inside color was white) was sampled (R11).

16:00 HD621-13 (334 m bsl) On the surface, large blocks of lava like lithology

with sticky surface are scattered. Some of them might be outcrops. There are some light brown-white pumices on the surface, too.

A piece of lava-like block with sticky surface was sampled (R12).

28

4-4. Representative columnar sections

Dive 1 (#618): SE wall of Myojinsho Caldera

29

Dive 2 (#619): W side of central edifice

30

Dive 3 (#620): NW slope of Myojinsho up to the summit area

31

Dive 4 (#621): SW wall of Myojinsho caldera

32

4-5. New topographic map

West of Myojinsho Caldera There is a swarm of small conical structures and crater like depressions trending in N-S

direction. Each of these structures may be an eruptive center from a N-W trending dike. The direction implies that the dike might have been generated by rifting activity of Izu-Bonin back arc.

33

5. FUTURE SCHEDULE 5-1. Recent eruptions of Myojinsho

This is the first submersible operation of Myojinsho volcano. First of all, the present state of activity is not so quiet, although discolored seawater has not been witnessed for years. When the ship crossed above the summit of Myojinsho, echo gram showed two anormaly above the summit (Fig. 5-1). This would have shown bubbles rising up from the vent or crater by hydrothermal activity. We should be watching for the activity in the future.

Fig. 5-1 Echo gram of the ship

Natsushima which shows anormaly above the summit of Myojinsho (top left).

As for the topography and the deposit on the slope of Myojinsho, There have been

some characteristic features. On the slope, there were some lobate topographic structures with a lot of decimeter size pumices covering the surface. They may be a deposit by some types of gravity currents. In addition, the pumices in these deposits changed from white pumice at depth to banded pumice at shallower level. Then near the summit, most lobate deposits consist of angular lava blocks, and some of the blocks are red color implying oxidation in the air. These observations may indicate the distance which possible gravity currents travel is controlled by density. Although we cannot constrain on what mechanism works, we are going to investigate physical characteristics of pumices we collected in this cruise.

We are also planning to carry out chemical analysis to identify the essential products of the 1952-53 and the 1970 eruption, and textural analysis to investigate vesiculation processes of these eruptions.

5-2. Caldera forming eruption of Myojinsho caldera

Through this cruise we had two dives for caldera wall survey; at SW wall and at SE wall. As a result we have found some candidate pyroclastic fall units at this two sites that might have erupted during caldera formation. We will correlate them in terms of sequential relation, chemical composition, and morphological characteristics of the pyroclasts that we sampled in this cruise.

34

On the other hand, the mechanism by which central edifice was generated is one of cs of caldera formation. In this cruise, we had one

di

tte that the central edifice is a remnant of a. On the other hand, the alteration of lavas

heat source below the bottom of the caldera, and cript-dome may exist beneath the caldera floor. We cannot tell at present which model is true, but are going to investigate the samples obtained in this cruise to check if the sequence of

l can be really correlated in terms of chemical

5-3. Evolution of Myojinsho caldera

the subjects to understand dynamive at the western side of the central edifice. What we found at the cliff were very

similar to those found at two sites on the caldera wall as described in the following section. The deposit was almost totally altered and stratified from the caldera bottom up to ca. 300 m below the top. Some altered rocks have pyrite. This means that this edifice is not a lava dome icaldera wall rather seems to indicapre-caldera edifice of Myojinsho caldermay imply the

self, and stratified structure similar to the

the central edifice and the caldera walanalysis and stratigraphy.

As stated in section 5-2, we had two dives for caldera wall survey. At SW wall, vertical section of the edifice of Beyonnaise rocks is expected. At SE wall, vertical section of the edifice of SE peak at ca. 300 m below sea level. These two wall show similar lithology and sequence; alternation of altered lava and pyroclastic rocks. We will correlate these two walls by chemical and stratigraphical analyses. In addition, the chemical and petrological relationship among the rock samples collected in this cruise will be carried out to reconstruct chemical evolution of magma system in the whole history of Myojinsho caldera in conjunction with caldera formation.

35

6. SA

MPL

E L

IST

6-1. R

ock sample list

36

6-2. Sample photos

37

38

39

40

41

42

43

44

45

46

47

48

Documents

NT06-21 LEG 2-1 CRUISE REPORT - · PDF file10:08 2109.5 69.7sea anemone? 10:10 2100 68.7pillow ridge (passed over) 10:13 2095.5 56.7back on floor, pillow (large, with concentric joints)